Iman Shakeri (a), Marc Seidel (b), Trayana Tankova (a), Milan Veljkovic (a)

(a) Department of Engineering Structures, Delft University of Technology, 2628CN Delft, the Netherlands
(b) Siemens Gamesa Renewable Energy Deutschland GmbH, Hamburg, Germany

International Journal of Fatigue, Volume 213, 2026

Abstract

In wind turbine towers, bolted flange connections are subjected to repeated cyclic loading, making fatigue a critical concern for structural integrity. This study experimentally and numerically investigates the fatigue behaviour of M48 bolted L-flange connections subjected to high preload introduced by tension-tightening. The preload initially reached approximately 95% of the bolt yield strength and stabilised at approximately 79% after tensioner removal. A total of 12 full-scale fatigue tests were conducted to quantify the influence of combined axial and bending loading, as well as preload relaxation during cyclic loading. The results show that high preload significantly improves fatigue life, particularly in the endurance-limit region, where existing design standards provide overly conservative predictions. Cyclic loading was found to accelerate preload relaxation, resulting in an increase in the tensile stress range in the bolt. To account for both bending effects and preload loss, an equivalent effective stress range was proposed, which reduced scatter in the fatigue data and provided a more consistent fatigue-damage parameter. In addition, a three-dimensional fracture-mechanics-based fatigue crack growth framework was developed to predict fatigue life. Bolt preload was introduced in the finite element model using a thermo-mechanical approach, and the model was validated against experimental results. Fatigue crack growth parameters were obtained from dedicated tests on specimens extracted from the bolt material. The numerically predicted S-N curves showed good agreement with experimental results and standard-based predictions in the finite-life regime. The proposed methodology improves fatigue-life prediction and supports more reliable design guidelines for preloaded wind turbine flange connections.

Keywords

Fatigue life; Bolt, S-N curve; Fatigue crack growth; L-flange connection; Preload; Stress intensity factor.

Iman Shakeri, Hagar El Bamby, Trayana Tankova, Milan Veljkovic : Delft University of Technology
Rajab Said, Chris Timbrell : Zentech International Limited

NAFEMS UK Conference, Coventry, 18-19 May 2026

Offshore wind turbines are increasingly required to operate beyond their original design life, placing renewed emphasis on structural integrity and fatigue performance. Bolted L-flange connections used to assemble tower sections are a key structural detail:

• Complex combinations of axial and bending load induced by wind, wave action and rotor dynamics
• Bolt preload helps mitigate against fatigue damage but actual preload often deviates from the design value due to uncertainties in the tightening process and geometric imperfection

The work summarized in this presentation is recent research carried out at Delft University of Technology, adopting a fracture-mechanics framework:

• Realistic bolt and thread geometries are explicitly represented, enabling accurate evaluation of load transfer function (LTF), crack evolution and stress intensity factor (SIF)
• The methodology combines 3D FE modelling in Abaqus with automated fracture-mechanics analysis using Zencrack
• Parameterisation of key FEA results allows creation of synthetic S-N curves accounting for effect of preload loss and crack development

Ramesh Chandwani, Zentech International Limited

ESIA18 – ISSI2026 International Conference, Strathclyde, 18-20 May 2026

Ramesh Chandwani¹, Chris Timbrell¹, B.R.K. Blackman², Rhys Jones³ and Anthony J. Kinloch²

¹Zentech International Limited, 590B Finchley Road, London, NW11 7RX, UK.

²Department of Mechanical Engineering, Imperial, Exhibition Road, London, SW7 2AZ, UK.

³Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria, 3800, Australia.

Theoretical and Applied Fracture Mechanics, Volume 143, Part 1, April 2026, 105487

Abstract

The Hartman-Schijve methodology offers a direct route to calculating various fatigue crack growth (FCG) rate curves that are associated with delaminations growing in fibre polymer-matrix composites. However, up to the present, only a ‘manual’ method has been described to deduce the values of the Hartman-Schijve constants that are needed for such calculations. Whilst this manual method may give reasonably acceptable results for calculating the Hartman-Schijve constants, it is a difficult and tedious implementation route. Thus, it may, if the operator is inexperienced, give relatively large errors in the values of the key constants that are needed to calculate the FCG rate curves. Furthermore, for very large data sets there is great difficulty incurred in manipulating such a large amount of data using the ‘manual method’.

Therefore, the main aim of the present paper has been to introduce a novel computer model and the associated software, ‘Zencrack’ from Zentech International Limited, UK, to obtain automatically the Hartman-Schijve constants. The aim has been to deduce the ‘worst-case upper-bound’ FCG curve for small, naturally-occurring, delaminations in the composite material, or component. To achieve such an ‘automatic’ method, the current work (a) has taken previously-published algorithms that employ a ‘Total Least Squares’ method for the fitting process of the experimental input data to the Hartman-Schijve equation and (b) has investigated the effect of a normalising scaling factor. It is shown that an automatic calculation that includes all the input data gives the best representation of the Hartman-Schijve constants and is the recommended approach.

Keywords

Automating fatigue predictions; Carbon-fibre composites; Fatigue; Total least squares method.

Rhys Jones [1][2], Andrew Ang [1], Nam Phan [3], Michael R. Brindza [4], Michael B. Nicholas [5], Chris Timbrell [6], Daren Peng [1][2], and Ramesh Chandwani [6]

[1] ARC Industrial Transformation Training Centre on Surface Engineering for Advanced Materials, School of Engineering, Swinburne University of Technology, John Street, Hawthorn, VIC 3122, Australia
[2] Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia
[3] Structures Division, Naval Air Systems Command, Patuxent River, MD 20670, USA
[4] Air Warfare & Weapons Department, Air Platforms Division, Office of Naval Research, Arlington, VA 22203-1995, USA
[5] US Army Research Laboratory, U.S. Army Combat Capabilities Development Command Weapons and Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD 20852, USA
[6] Zentech International Limited, 590B Finchley Road, London NW11 7RX, UK

Materials 2026, 19(2), 372

Abstract

This paper reviews the fracture mechanics parameters associated with the variability in the crack growth curves associated with forty-two different tests that range from additively manufactured (AM) steels to cold spray additively manufactured (CSAM) 316L steel. As a result of this review, it is found that, to a first approximation, the effects of different building processes and R-ratios on the relationship between ΔK and the crack growth rate (da/dN) can be captured by allowing for changes in the fatigue threshold and the apparent cyclic toughness in the Schwalbe crack driving force (Δκ). Whilst this observation, when taken in conjunction with similar findings for AM Ti-6Al-4V, Inconel 718, Inconel 625, and Boeing Space Intelligence and Weapon Systems (BSI&WS) laser powder bed (LPBF)-built Scalmalloy^®, as well as for a range of CSAM pure metals, go a long way in making a point; it is NOT a mathematical proof. It is merely empirical evidence. As a result, this review highlights that for AM and CSAM materials, it is advisable to plot the crack growth rate (da/dN) against both ΔK and Δκ. The observation that, for the AM and CSAM steels examined in this study, the da/dN versus Δκ curves are similar, when coupled with similar observation for a range of other AM materials, supports a prior study that suggested using fracture toughness measurements in conjunction with the flight load spectrum and the operational life requirement to guide the choice of the building process for AM Ti-6Al-4V parts. The observations outlined in this study, when taken together with related findings given in the open literature for AM Ti-6Al-4V, AM Inconel 718, AM Inconel 625, and BSI&WS LPFB-built Scalmalloy^®, as well as for a range of CSAM-built pure metals, have implications for the implementation and certification of limited-life AM parts.

Keywords

Additively manufactured steels; CSAM 316L steel; fatigue crack growth; fracture mechanics; limited-life replacement parts; build quality.

Iman Shakeri, Hagar El Bamby, Trayana Tankova, Milan Veljkovic
Department of Engineering Structures, Delft University of Technology, 2628CN Delft, The Netherlands

Thin-Walled Structures, Volume 217, Part B, December 2025, 113893

Abstract

Bolted flange connections in wind turbine towers are subjected to cyclic loading, making fatigue a critical concern for their structural integrity. Bolt preload helps mitigate fatigue damage, but actual preload levels often deviate from design values due to uncertainties in the tightening process and geometric imperfections. This study evaluates the fatigue life of bolts l-flange connections under varying preload levels using a numerical fracture mechanics approach. A comprehensive three-dimensional finite element analysis (FEA) is conducted to assess the effects of preload on the stress intensity factor (SIF), crack propagation behaviour, and load transfer function (LTF). Additionally, the influence of thread helix angle, as well as combined axial and bending loads, on SIF and crack front evolution is examined. Experimental validation of the numerically obtained LTF is performed. A methodology for predicting S-N curves is proposed by deriving normalised solutions for LTF and SIF. The results indicate that increasing preload up to 90 % significantly reduces the SIF range, thereby decelerating crack growth and enhancing fatigue life. However, beyond 90 %, the improvement in fatigue life becomes less pronounced. Furthermore, the findings suggest that Eurocode 3 provides conservative fatigue life predictions, as it neglects bending effects, which are less detrimental than axial loading. Notably, even minor preload loss considerably shortens fatigue life, an effect that becomes more pronounced at higher preload levels. This research contributes to the development of predictive fatigue models for the bolted L-flange connection, providing insights into incorporating preload effects into fatigue life assessments.

Keywords

Fatigue life; S-N curve; Fatigue crack growth; Bolted flange connection; Preload; Stress intensity factor.

Shareen Chan [1], Daren Peng [1] [2], Andrew S. M. Ang [1], Michael B. Nicholas [3], Victor K. Champagne [3], Aron Birt [4], Alex Michelson [4], Sean Langan [4], Jarrod Watts [5] and Rhys Jones [1][2]

[1] ARC Industrial Transformation Training Centre on Surface Engineering for Advanced Materials, Faculty of Science, Engineering and Technology, Swinburne University of Technology, John Street, Hawthorn, VIC 3122,
[2] Australia Centre of Expertise for Structural Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia.
[3] US Army Research Laboratory, U.S. Army Combat Capabilities Development Command Weapons and Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD 21005, USA
[4] Solvus Global, 104 Prescott Street, Worcester, MA 01605, USA
[5] Rosebank Engineering Australia, 836 Mountain Highway, Bayswater, VIC 315, Australia^*Author to whom correspondence should be addressed.

Crystals 2025, 15(6), 544

Abstract

MIL-STD-1530D and the United States Air Force (USAF) Structures Bulletin EZ-SB-19-01 require an ability to predict the growth of naturally occurring three-dimensional cracks with crack depths equal to what they term an equivalent initial damage size (EIDS) of 0.254 mm. This requirement holds for both additively manufactured and conventionally built parts. The authors have previously presented examples of how to perform such predictions for additively manufactured (AM) Ti-6Al-4V; wire arc additively manufactured (WAAM) 18Ni 250 Maraging steel; and Boeing Space, Intelligence and Weapon Systems laser bed powder fusion (LPBF) Scalmalloy^®, which is an additively manufactured Aluminium-Scandium-Mg alloy, using the Hartman-Schijve crack growth equation. In these studies, the constants used were as determined from ASTM E647 standard tests on long cracks, and the fatigue threshold term in the Hartman-Schijve equation was set to a small value (namely, 0.1 MPa √m). This paper illustrates how this approach can also be used to predict the growth of naturally occurring three-dimensional cracks in WAAM CP-Ti (commercially pure titanium) specimens built by Solvus Global as well as in WAAM-built Inconel 718. As in the prior studies mentioned above, the constants used in this analysis were taken from prior studies into the growth of long cracks in conventionally manufactured CP-Ti and in AM Inconel 718, and the fatigue threshold term in these analyses was set to 0.1 MPa √m. These studies are complemented via a prediction of the growth of naturally occurring three-dimensional cracks in conventionally built M300 steel.

Keywords

Naturally occurring 3D cracks; fatigue crack growth; additive manufacturing; MIL-STD-1530Dc; Hartman-Schijve crack growth equation.

Rhys Jones [1,2], Andrew Ang [2] and Daren Peng [1,2]
[1] Centre of Expertise for Structural Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria, 3800, Australia. 
[2] ARC Industrial Transformation Training Centre on Surface Engineering for Advanced Materials, Faculty of Science, Engineering and Technology, Swinburne University of Technology, John Street, Hawthorn, Victoria 3122, Australia.

DRMS2025, 2nd International Conference on Durability, Repair and Maintenance of Structures, Faculty of Engineering, University of Porto, 13th -14th March 2025

• The objective of this presentation is to delineate the State of The Art in the Durability Assessment of Additively manufactured (AM) and Cold Spray Additively Manufactured (CSAM) parts and Cold Spray Repairs to metallic airframes.
• It also highlights the unique potential of Boeing Space, Intelligence and Weapon Systems (BSI&WS) laser powder fusion (LPBF) built Scalmalloy® to build limited life aircraft parts and drones that are both durable and corrosion resistant.

Ramesh Chandwani, Zentech International Limited, UK

Engineering Excellence: Simulation for Fatigue Strength and Durability, Berlin, 13-14 November 2024

Ramesh Chandwani, Zentech International Limited, UK

IOM3 - Defence Safety & Security Group Technical Seminar: Additive Manufacturing in Defence, London, 21 Nov  2024

Iman Shakeri [1 & 2], Weijian Wu [3], Alexander Michel [4], Martin A. Eder [2]
[1] Faculty of Civil Engineering and Geosciences, Department of Engineering Structures, Delft University of Technology, Delft, Netherlands
[2] Department of Wind and Energy Systems, Technical University of Denmark, Roskilde, Denmark
[3] Department of Civil and Mechanical Engineering, Technical University of Denmark, Lyngby, Denmark
[4] Department of Environmental and Resource Engineering, Technical University of Denmark, Lyngby, Denmark

Fatigue Fract Eng Mater Struct. 2024; 1-19

Abstract

The present study proposes a novel model to modify master S-N curves of components according to their load redistribution capability reflected in different boundary conditions (BCs) based on the fracture mechanics analysis. To that end, a comprehensive numerical study was conducted on a Single Edge Notch Bend (SENB) specimen constrained with different kinematic BCs using discrete fatigue crack growth (FCG) simulation. It was observed that BCs indeed can have a significant effect on the crack growth behavior and consequently on the resulting fatigue life under the same nominal loading conditions. The proposed model was applied to the S-N curve of a T-welded joint, and the predicted fatigue life was validated against 3D FCG simulations. Finally, FCG tests were conducted on SENB specimens to experimentally corroborate the effect of BCs on the FCG rate.

JIAO Guangchen [1], ZHAN Yong [1], WEN Jianfeng [2]
[1] Shanghai Huice Information Technology Co., Ltd., Shanghai 201108, China
[2] Key Laboratory of Pressure Systems and Safety (MOE), School of Mechanical and Power Engineering, East China University of Science & Technology, Shanghai 200237, China

Transactions Of The China Welding Institution, Vol.44(11):52-58, November 2023

Abstract

Simulation method of fatigue crack growth considering material discontinuity was developed based on Abaqus, Zencrack and relevant user subroutines to accurately describe the fatigue crack growth behavior in welded structures. For initial embedded cracks located in weld region of a welded plate, fatigue crack growth simulations with different matching strengths between weld and base metal were carried out, and the influence of different matching strengths on fatigue crack growth life and characteristic for the welded structure were investigated. It is found that distinct differences of SIF calculation results are obtained once the crack propagate from weld to base metals. Also, the difference gradually increases with increasing the crack size. As the values of crack growth parameters in weld are increasing, the relative crack growth rate of the crack located in the base metal decreases gradually and the crack shape transits from round to flat. In contrast, the relative crack growth rate in the base metal increases gradually, and the crack shape transits from round to ellipse. The present simulation method can provide an effective way to accurately predict crack evolution in multi-materials of welded structures.

Keywords

Finite Element Method, Welded Structure, Fatigue Crack Growth, Stress Intensity Factor (SIF).

Rhys Jones [1][2], Ramesh Chandwani [3], Chris Timbrell [3], Anthony J. Kinloch [4], Darren Peng [1][2]
[1] Centre of Expertise for Structural Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia
[2] ARC Industrial Transformation Training Centre on Surface Engineering for Advanced Materials, Faculty of Science, Engineering and Technology, Swinburne University of Technology, John Street, Hawthorn, VIC 3122, Australia
[3] Zentech International Limited, 590B Finchley Road, London NW11 7RX, UK
[4] Department of Mechanical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK

Aerospace 2023, 10(11), 946

Abstract

Adhesively bonded doublers and adhesively bonded repairs are extensively used to extend the operational life of metallic aircraft structures. Consequently, this paper focuses on the tools needed to address sustainment issues associated with both adhesively bonded doublers and adhesively bonded repairs to (metallic) aircraft structures, in a fashion that is consistent with the building-block approach mandated in the United States Air Force (USAF) airworthiness certification standard MIL-STD-1530D and also in the United States (US) Joint Services Structural Guidelines JSSG-2006. In this context, it is shown that the effect of biaxial loads on cohesive crack growth in a bonded doubler under both constant amplitude fatigue loads and operational flight loads can be significant. It is also suggested that as a result, for uniaxial tests to replicate the cohesive crack growth seen in adhesively bonded doublers and adhesively bonded repairs under operational flight loads, the magnitude of the applied load spectrum may need to be continuously modified so as to ensure that the crack tip similitude parameter in the laboratory tests reflects that seen in the full-scale aircraft.

Keywords

Bonded structures; bonded repairs; flight loads; sustainment; crack growth; Zencrack.

Arvinthan Venugopal 1,2, Roslina Mohammad 2, Md Fuad Shah Koslan 1, Ashaari Shafie 3, Alizarin bin Ali 4 and Owi Eugene5
1 RMAF Centre of Aerospace Engineering Services Establishment, Subang Airbase, Shah Alam 40000, Malaysia
2 Razak Faculty of Technology and Informatics, Universiti Technology Malaysia, Jalan Sultan Yahya Petra, Kuala Lumpur 54100, Malaysia
3 Aerospace Technology Systems Corporation Sdn Bhd, ATSC Corporate Centre, PT 192, Jalan Lapangan Terbang, Subang 47200, Malaysia
4 CAIDMARK Sdn Bhd, Damansara Utama, Petaling Jaya 47400, Malaysia
5 RMAF Combat Training School, TUDM Bukit Ibam, Muadzam Shah 26700, Malaysia

Metals 2021, 11(9), 1453 (Special Issue : Fatigue Crack Propagation Micromechanisms of Metallic Materials)

Abstract

The critical aircraft structure, being the load-bearing members, is a vital component for any aircraft. The effect of fatigue loading, operating conditions, and environmental degradation has caused the structural integrity of the airframe to be assessed for its airworthiness requirement. Using the fatigue design concept of Safe Life, the RMAF adopts the Aircraft Structure Integrity Program (ASIP) to monitor the structural integrity of its critical components. RMAF has produced the task card using the engineering analysis concept on the aircraft’s critical structure. Various Computer-Aided Engineering (CAE) methods were used, and for this analysis, the Crack Growth Prediction method was used to determine the crack growth behavior and its ultimate failure point in case of any crack occurrences. Although there are six critical locations, the wing root is chosen since it has the highest possibility of fatigue failure. The analytical methods which were discussed are Crack Growth Analysis and Low Cycle Fatigue. For the numerical method, NX Nastran was used for the simulation of crack growth. The result from the crack growth analysis was validated with the numerical result. The conclusion is that, based on the fatigue life cycle, the wing root structure condition is not affected by severe damage, and its failure is approximately around 30 to 100 years for both the through hole and through side crack. Thus, its structural life can be extended. The research outcome will be on the extension of the structure life of the aircraft wing.

Keywords

Fatigue critical location; wing root; crack growth prediction; structure life extension; aircraft structural integrity program.

Chao Liu [1], Guang-chen Jiao [2], Ramesh Chandwani [3], Chris Timbrell [3]
[1] School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, PR China
[2] Shanghai Hui Ce Information Technology Co., Ltd, Shanghai, PR China
[3] Zentech International Limited, London, UK

International Journal of Pressure Vessels and Piping, Volume 193, October 2021

Abstract

Stress intensity factors (SIF) of a nozzle corner crack in a pressure vessel subject to pressure and thermal loadings are evaluated using finite element method (FEM) and compared with the method described in ASME XI appendix G (2013 section XI). The formula in ASME XI appendix G is defined for the crack corner at a specific location of the nozzle/vessel connection which may limit the structural integrity assessment of the pressure vessel under ever changing pressures and temperatures duty loads during heat-up and cool-down phases. Using FE method SIFs at corners of similar cracks assumed at other nozzle locations are then evaluated and compared with SIFs using ASME formula to assess the degree of conservatism.

Keywords

Stress intensity factor (SIF); FEM; Nozzle corner crack; ASME XI code; Pressure vessel.

Defu Nie, Xuedong Chen, Qiaoguo Wu, Yan Liu
National Safety Engineering Technology Research Center for Pressure Vessels and Pipelines, Anhui Province Safety Technology Laboratory for Pressure Vessels and Pipelines, Hefei General Machinery Research Institute, Hefei, Anhui 230031, China

Engineering Failure Analysis, Volume 116, October 2020, 104701

Abstract

Stress corrosion cracking (SCC) in compressor impellers made of FV520B stainless steel frequently occurred, and hence was investigated by slow strain rate tests (SSRTs) and fracture toughness tests, combined with finite element analysis (FEA) based on ABAQUS and ZENCRACK software. SSRTs revealed that the FV520B stainless steel had slight susceptibility to aqueous brine solution, which increased with increasing temperature, and fracture surface exhibited ductile feature with dimples similar to that in nitrogen; however, this material showed significant susceptibility to NACE solution A at various temperatures, and fracture surface consisted of initial quasi-cleavage region and subsequent dimple region; both tempered martensite matrix and localized δ ferrites were susceptible to SCC. Fracture toughness KIC (JIC) of this steel was 150MPam1/2 (102KJ/m2) in air, but threshold stress intensity factor (KISCC) dramatically reduced to 48MPam1/2 in NACE solution A, and Boltzman equation was modified to describe SCC propagation rate (da/dt-K) curves. Comparison with FEA results indicated that the fracture of SSRT specimen was initially affected by environmental factor and then mainly attributed to mechanical factor, whereas the whole fracture process of impeller was closely related to corrosive environment. This study was useful for SCC evaluation of materials and failure analysis of compressor impellers.

Keywords

Stress corrosion cracking; Fracture toughness; Finite element analysis; FV520B stainless steel; Compressor impeller.

Jianhua Pan [1,2], Mingjie Guo [1]
[1] State Key Laboratory of Nuclear Power Safety Monitoring Technology and Equipment, China Nuclear Power Engineering Co., Shenzhen, 518172, China
[2] School of Mechanical Engineering, Hefei University of Technology, Hefei, 230009, China

SN Applied Sciences (2019) 1:1085

Abstract

This paper examines the numerical simulation on dynamic fracture toughness values derived from SHPB testing programs with three-point bending specimens using RKR criterion. Three-dimensional and two-dimensional finite element analyses are performed by using software ABAQUS and 3D fracture mechanics code ZENCRACK. The finite element model includes the bullet, incident bar, specimen and supports. Three-point bending specimens with different ratios values of initial crack lengths to width are utilized for the simulations under the identical initial conditions. The fracture initiation time is explored with the RKR local stress fracture criterion. The dynamic fracture toughness value of the material is determined by the combination of response curve of the dynamic stress intensity factor and fracture initiation time. The experimental data are taken as a benchmark to verify the results of three-dimensional and two-dimensional finite element computations. It is found that the computational results from 3D finite element are more accurate.

Jianbin Li1 [1], Zhange Zhang [2], Zhichao Meng [2], Junzhou Huo [2], Zhaohui Xu [2], Jing Chen [3]
[1] China Railway Hi-Tech Industry Co., Ltd, Beijing, People’s Republic of China
[2] School of Mechanical Engineering, Dalian University of Technology, Dalian, People’s Republic of China
[3] School of Navigation and Naval Architecture, Dalian Ocean University, Dalian, People’s Republic of China

Advances in Mechanical Engineering, 2019;11(6)

Abstract

Fatigue damage is one of the most common failure modes of large-scale engineering equipment, especially the full-face tunnel boring machine with characteristics of a thick plate structure bearing strong impact load. It is difficult to predict the location and propagation life of crack of cutterhead under strong impact load. Unseasonal maintenance of equipment caused by inaccurate prediction of life cycle of cutterhead seriously affects the construction efficiency of the equipment and the life safety of the operators. Determining the crack location of tunnel boring machine cutterhead structure under strong impact load and predicting the crack propagation life are difficult scientific problems. To solve them, first, the location of the stress concentration of the cutterhead is determined by using finite element analysis method of statics. Second, prediction model for crack propagation life of tunnel boring machine cutterhead characteristic substructure based on time integration is built. And the test of crack growth of cutterhead characteristic substructure is performed. The feasibility and accuracy of the prediction model are verified by contrasting crack prediction models and the results of the test. Finally, the life prediction of tunnel boring machine cutterhead of water diversion project in Northwest Liaoning Province is carried out by using crack propagation model based on time integration. Results show that the maximum error of theoretical prediction and experimental results of crack propagation is 16%. So the feasibility of crack propagation model based on time integration in predicting the crack growth of cutterhead is verified. It is predicted that the tunnel boring machine cutterhead panel can work normally for 5.9 km under the condition of ultimate load. Building the crack propagation model considering the influence of plate thickness and strong impact load has important research value for improving the working efficiency of engineering equipment, prolonging service time, and improving the working safety.

M. Lepore [1], F.Berto [2], D.Kujawski [3]
[1] Department of Industrial Engineering, University of Salerno, Salerno, Italy
[2] Department of Mechanical and Industrial Engineering, NTNU, Trondheim, Norway
[3] Department of Mechanical and Aerospace Engineering, WMU, Kalamazoo MI, USA

Theoretical and Applied Fracture Mechanics, Volume 100, April 2019, Pages 14-26

Abstract

This work addresses FEM-based numerical simulations of fatigue crack propagation in a cruciform specimen under biaxial loading and small scale yielding (SSY) conditions. Three non-linear models are used as fatigue crack propagation laws. The cruciform specimens are made of aluminium alloy D16T and modelled with non-linear material properties. In the experimental tests, starting from a surface flow mechanically created in the centre of the cruciform specimen, biaxial loading conditions are applied to create various pre-cracked configurations that correspond to different values of initial crack aspect ratios. In the numerical simulations, the material behaviour is modelled with a low hardening bilinear law. Each simulation is carried out with one of the established non-linear laws. At the end of the fatigue crack propagation simulations, several crack aspect ratios are obtained. Finally, these aspect ratios are compared with the corresponding experimental tests available in the literature.

Keywords

SSY; FPZ; Non-linear model; Cruciform specimen; Fatigue crack growth.

Marcello Lepore (a), Filippo Berto(b)
(a) Department of Industrial Engineering, University of Salerno, Via G. Paolo II 132, 84084 Fisciano (SA), Italy
(b) Department of Mechanical and Industrial Engineering, NTNU, Richard Birkelands vei 2b, 7491 Trondheim, Norway

Engineering Fracture Mechanics, Volume 206, 1 February 2019, Pages 463-484

Abstract

In this work, the propagation life of a friction stir-welded sample made of ductile materials is estimated by employing linear elastic fracture mechanics (LEFM) and small-scale yielding (SSY) conditions. The purpose is to demonstrate that by considering the SSY, the prediction of the propagation life of the welded sample can be improved when compared to the traditional LEFM. The process of friction stir welding (FSW) of an AA2024-T3 butt joint is then simulated by using the finite element method. Hence, a thermal analysis of the numerical model is performed, and the calculated temperature field is subsequently subjected to thermo-mechanical analysis. In the latter model, the two defects located in the most critical position, detected experimentally by performing fatigue tests on the same component, are introduced into the model by using the constrained crack faces technique. Furthermore, to enable the thermo-mechanical simulation of the FSW process, temperature-dependent non-linear material properties, material softening, and isotropic hardening are considered. Concerning fatigue crack growth analysis, three simulations of the fatigue crack propagation are performed by using three different propagation laws. The first is performed by considering linear elastic material properties and Vasudevan’s law on fatigue crack propagation; the second is by employing non-linear material properties and Kujawski–Ellyin law; the third takes into account the non-linear material properties and UniGrow law. Thereafter, appropriate constraints and a remote fatigue load are applied to the specimen to allow residual stress redistribution and fatigue crack growth, respectively. The constraint effect is also evaluated by the calculation of the T-stress parameter. Finally, a comparison between the numerical and experimental results is presented; consequently, a better agreement with the case of the non-linear model under the SSY conditions is found.

Keywords

FSW; Residual stress; LCF; SSY; LEFM; Fatigue crack growth.

Chris Timbrell [1], Ramesh Chandwani [1], Ma Chunlei [2]
[1] Zentech International Limited
[2] Consys Group Limited

ISSI 2018 - Nanjing, China, 2-5 November 2018

Abstract

Since the industrial revolution when a German mining engineer August Wohler first studied the frequent breaking of chains causing several casualties and developed the concept of what we now know as the S-N curve, many experimental, theoretical and software-aided simulation techniques have been developed to study ageing material behaviour and to design new materials. Over time the demands placed on new materials have required operation under more severe temperatures and loads in order to conserve natural resources and minimise emissions.
Fracture mechanics based finite element algorithms to simulate 3D cracks in components / structures have proved very useful in assessing the residual life and developing repair and maintenance strategies as mandatorily required by various licensing authorities for the continuous operation of infrastructure projects in Aerospace, Power, Transportation, Oil and Chemical industries under the ever more demanding operating conditions. Here one such software tool for crack simulation of industrial applications is presented with examples including combined fatigue and time dependent crack growth under thermo-mechanical loading including hold-time and weld defect assessment with inclusion of dis-similar materials.

Wei Sun, Jun-Zhou Huo, Xiao-Hong Chen
Dalian University of Technology, Dalian, China

Engineering Failure Analysis, Volume 93, November 2018, Pages 224-240

Abstract

Practically, there is multiple cracks interpenetrating in TBM cutterhead, which gives rise to its failure prematurely. While the existing theoretical formulas and calculating methods cannot calculate its fatigue failure process under dynamic loading. In view of such situation, crack cutting sampling of TBM cutterhead after service and its fracture failure analysis were performed to clear that its fracture failure is mainly brittle fracture, based on which, A three-dimensional finite element crack propagation model was established to calculate the crack propagation process under dynamic loading. Stress intensity factor of compact test specimens is calculated by standard formula, and the growth paths of crack are got from multiple cracks fatigue tests, compared the two results obtained above to verify respectively the accuracy of the proposed method in calculating stress intensity factor and its growth path. It is confirmed that the two results about the change of stress intensity factor is basically match, and the maximum error of the growth path of the multiples crack model and the test result is about 3.7% within the permissible range. It is proved that the proposed method is feasible to calculate the multiple cracks failure process under the dynamic loading. Finally, the growth processes of collinear multiple cracks, parallel multiple cracks, nonparallel multiple cracks, and penetrating multiple cracks were calculated. The results show that the stress intensity factor at the intersection of collinear cracks decreased by about 26%, compared with a single crack. Parallel and non-parallel multiple cracks are attracted to each other as the cracks propagate in the process of growth and the stress intensity factor gradually decreases. Although the growth path and stress intensity factor do not change, the crack stops expanding, with the crack tip penetrates through another crack. The failure criterion of TBM cutterhead and the rule of interpenetration of multiple cracks proposed in this paper are the theoretical basis and technical support for the its life prediction and risk prediction.

Keywords

TBM cutterhead; Crack propagation; Three-dimensional crack; Multiple cracks; Stress intensity factor; Crack propagation path.

R. Citarella [1], V. Giannella [1], M. Lepore [1], G. Dhondt [2]
[1] Department of Industrial Engineering, University of Salerno, Salerno, Italy
[2] MTU Aero Engines AG, Dachauer Strasse 665, 80995 Munich, Germany

Fatigue Fract Engng Mater Struct, 2018; 41:84-98

Abstract

Three-dimensional mixed-mode crack propagation simulations were performed by means of the dual boundary element method code BEASY and 2 finite element method-based crack propagation codes: ZENCRACK (ZC) and CRACKTRACER3D (CT3D). The stress intensity factors (SIFs) along the front of an initial semielliptical crack, initiated from the external surface of a shaft, were calculated for 4 different load cases: bending, press fit, shear, and torsion. The methods used for the SIF assessment along the crack front were the J-integral for BEASY and ZC and the quarter point element stress method for CT3D. Subsequently, crack propagation simulations were performed, with the crack growth rate evaluated by using Paris' law, calibrated for the material at stake (American Society for Testing and Materials A469 steel). The kink angles were evaluated by using the minimum strain energy density and maximum tangential stress criteria for BEASY, the maximum energy release rate and maximum tangential stress for ZC, and the maximum principal asymptotic stress for CT3D. The results obtained in terms of SIFs and crack propagation life show very good agreement among the 3 codes. Also, the shape of the propagated crack, which is significantly out-of-plane for the shear and torsion loading, matched very well.

Mahyar Asadi[1], Majid Tanbakuei Kashani[1], Mathew Smith[1], Chris Timbrell[2], Ramesh Chandwani[2], Arasch Rodbari[2]
[1] SKC Engineering (A division of Applus)
[2] Zentech International Limited

ESIA 14 – ISSI 2017 (Engineering Structural Integrity Assessment 14 in conjunction with the International Symposium on Structural Integrity 2017), Manchester, UK, 16-17 May 2017.

Abstract

A limited number of automated algorithms and software are available that predict the 3D evolution of crack fronts in a mixed loading condition in welds in particular in the interface of weld and base metal i.e. fusion line. In this paper, the authors present a study of common low carbon steel pipe weld joints containing a crack detected in radiography films and embedded into a 3D FE pipe model that is constructed with different weld and base metal properties including different crack growth laws and fracture properties. Evolution of the detected crack front is predicted in 3D under mixed fatigue loading. This paper shows that a 3D model of crack growth captures the transient change of stress intensity factor along the crack front and therefore the immediate change in the direction of crack growth and the dynamic shape of crack can be predicted. A solution is also presented for handling the stress intensity factor on the boundary of weld metal and base metal when the crack front reaches the weld fusion line. From the structural integrity management viewpoint, the number of fatigue cycles, time for the crack to start growing, time to break to surface and leak-before-break, and the total time to final fracture are calculated. This paper shows that a fracture critical region such as welds with a high likelihood of service cracking or welding flaw can precisely be analyzed and life can be estimated to avoid early life failure in welded structures.

A. R. Maligno [1], R. Citarella [2], V. V. Silberschmidt [3]
[1] Institute for Innovation in Sustainable Engineering, University of Derby, Derby, UK
[2] Department of Industrial Engineering, University of Salerno, Fisciano, SA, Italy
[3] Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, UK

Fatigue Fract Engng Mater Struct, 2017; 40:1484-1500

Abstract

Fatigue data are generally derived under constant-amplitude loading conditions, but aircraft components are subjected to variable-amplitude loading. Without interaction effects, caused by overloads and underloads intermingled in a loading sequence, it could be relatively easy to establish a crack growth curve by means of a cycle-by-cycle integration. However, load-spectrum effects largely complicate a crack growth under variable-amplitude cycling. In this paper, fatigue crack growth behaviour of aeronautical aluminium alloy 2024-T3 was studied. Effects of various loading conditions such as stress ratio and amplitude loadings were investigated. In particular, the effect of different overloads on the fatigue crack growth was simulated using Zencrack code. Preliminary analyses on Compact Tension (CT) specimens proved that the numerical results generated were in agreement with the results provided by an Afgrow code for the same conditions. A case study was carried out on a helicopter component, undergoing repeated overloads, to compare numerical results obtained implementing yield zone models in Zencrack.

Jian-Feng Wen [1,2], Yong Zhan [1], Shan-Tung Tu [1], Fu-Zhen Xuan [1]
[1] Key Laboratory of Pressure Systems and Safety (Ministry of Education), School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, China
[2] Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843, USA

AIMS Materials Science, 2016, Volume 3, Issue 4: 1649-1664

Abstract

A plate under cyclic loading, containing two coplanar surface flaws with both identical and dissimilar sizes, is considered in the present study. By conducting detailed step-by-step finite element analyses, the conservatism contained in different combination rules for multiple coplanar flaws provided by fitness-for-service codes (ASME, BS7910, API579 and GB/T19624) have been quantitatively assessed for the fatigue failure mode. The findings show that the re-characterization guideline provided by ASME and BS7910 may cause non-conservative estimations when two crack sizes are similar, whereas API579 and GB/T19624 lead to excessively pessimistic predictions for almost all the cases. Based on the fatigue crack growth life, we suggest a new combination rule and conclude that it always yields a reasonable estimation with necessary conservatism, for various initial crack depths, material constants and relative sizes of two cracks.

R. Suresh Kumar [1], K. Velusamy [2], P. Puthiyavinayagam [3], P. Selvara [4]
[1] Head Structrual Mechanics Laboratory, Indira Gandhi Centre for Atomic Research, Kalpakkam
[2] Head Mechanics & Hydraulics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam
[3] Director, Reactor Design Group, Indira Gandhi Centre for Atomic Research, Kalpakkam
[4] Director, Fast Reactor Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam

Proceedings of First Structural Integrity Conference and Exhibition (SICE-2016), Bangalore July 4-6, 2016

Abstract

One of the most desirable safety features of a sodium piping system to be ensured during service condition is the structural integrity during in-service condition. Leak-Before-Break (LBB) is one of the necessary criteria to be respected towards achieving the required level of structural integrity for the pressure retaining the piping system and vessels. It is necessary to ensure the inherent safety capability of providing a clear warning before breakage. The term refers to a demonstration of the system to leak before catastrophic failure. The breakage of the system pressure boundary can cause large sodium leakage. It can lead to rapid disruption and the shutdown of the plant function.
The LBB approach requires application of fracture mechanics methodology to demonstrate that the sodium piping system will ensure detectable leakage and demonstrates that the crack will be stable under the maximum credible loading conditions. The size of this crack should be large enough so that leakage is assured of detection with a margin using the installed leak detection equipment under in-service conditions. This paper discusses the design methodology adapted to Fast Breeder Reactor (FBR) piping system to ensure the structural integrity of a through wall cracked sodium piping system under service conditions. RCCMR A16 & CEGB R6 design methodology is adopted in this paper towards demonstrating the structural integrity of a typical secondary sodium piping system. Failure Assessment Diagram is used for computing the critical crack length. Using the fracture mechanics concepts the crack growth has been computed against fatigue loading. It is demonstrated that the typical 500 NB FBR pipe bend critical crack length is much greater than that the minimum crack length responsible for the detectable leak rate.

Keywords

Structural Integrity; Leak-Before-Break; Detectable Crack length; Failure Assessment Diagram.

Zhi-jie Li [1], Geng-yu Zhou [2], Guang-chen Jiao [1], Chun-lei Ma [1]
[1] Consys Group Limited
[2] Suzhou Nuclear Power Research Institute

18th National Conference on Fatigue and Fracture (NCFF2016), Zhengzhou, China, April 2016

Abstract

The reactor pressure vessels are important parts in nuclear power plants, the integrity of which is a barrier for the reactor safety. Small cracks would generate with the reactor running and the complex conditions in the reactor would cause fatigue crack growth. The pressure vessel cannot be replaced during its service life, so the fatigue crack growth should be considered in an accurate or conservative way to ensure the size of the crack are under limit. In addition, the calculation of fatigue crack growth is necessary in life extension program for old nuclear power plants. Meanwhile, simulating the whole process from a surface crack to a through-wall crack is helpful and useful in LBB evaluation. In a word, crack growth analysis is very important.
In this paper, the whole growth process of internal surface crack in the circumferential direction on the pressure vessel under fatigue loads is simulated on the basis of Zencrack and parameters in the whole process are monitored such as stress intensity factor, J integral. The specific value of the parameters in the whole growth process are achieved. This analysis provides a practicable method for the design and safety evaluation when taking cracks of the pressure vessel into consideration, the life extension evaluation of old power plant and LBB evaluation.

Guang-chen Jiao[1], Ming Cao[2], Rui Shen[2], Zhi-jie Li[1], Chun-lei Ma[1], Chris Timbrell[3]
[1] Consys Group Limited
[2] Shanghai Nuclear Engineering Research & Design Institute
[3] Zentech International Limited

18th National Conference on Fatigue and Fracture (NCFF2016), Zhengzhou, China, April 2016

Abstract

Stress intensity factor (SIF) of nozzle corner in pressure vessel was calculated by using FEA and method of ASME appendix G (2013 section XI) under condition of thermal and mechanical loads. Because the computational formula in ASME appendix G can only be explained in analysis of nozzle structure with specific crack position, but engineering application is limited by using this formula. So the Stress intensity factors of nozzle corner with different crack positions were calculated, the application condition of this method and safety assessment of vessel nozzle were investigated in this paper.

Paulo Chambel [1], Rui F. Martins [1], Luís Reis [2]
[1] UNIDEMI, Department of Mechanical and Industrial Engineering, Faculty of Science and Technology, Universidade NOVA de Lisboa, Campus de Caparica, 2829-516 Monte de Caparica, Portugal
[2] IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal

Procedia Structural Integrity, Volume 1, 2016, Pages 134-141
XV Portuguese Conference on Fracture, PCF 2016, 10-12 February 2016, Paco de Arcos, Portugal

Abstract

The main purpose of the research presented herein was to study the fatigue crack propagation under loading modes I, II or III, either for plane strain or plain stress state. Therefore, several finite element analyses (FEA) were carried out and some experimental tests were performed in a bi-axial servo-hydraulic machine. J-integral values were determined through numerical simulations and stress intensity factors, KI, KII and KIII were inferred at the crack tip of both thick and thin C(T) specimens, assuming several crack’s lengths, and either for plane strain or plane stress state. Load opening-mode I shown to be the predominant mode of crack propagation under plane-strain state, leading to the highest J-integral values calculated, followed by Mode III. In addition, shearing load (mode II) induced the lowest stress intensity factor values at the crack tip both for plane stress state and for plane strain state. Finally, some fatigue crack growth rates (FCG) were determined under loading modes I or III, for thin specimens, at room temperature. C(T) specimens used in the experimental tests were made of two austenitic stainless steels, namely the AISI 316L and the Cr-Mn stainless steel, which are commonly used in several specific engineering applications. Results were compared and some conclusions could be drawn.

Keywords

Fracture mechanics; Loading modes I, II, III; Plane strain or plane stress state; Fatigue Crack Growth Rates (FCGR); Experimental tests.

J.H.Jia, Z.Q.Zhang, C.Zhang
Key Laboratory of Pressure Systems and Safety, Ministry of Education, East China University of Science and Technology, Shanghai, 200237, China

Procedia Engineering, Volume 130, 2015, Pages 150-157

Abstract

Vibration fatigue of small branch pipes usually induces weld toe cracks. Some cracks will cause advertently shutdown of the Nuclear Power Plants (NPPs). Moreover, to judge the vibration fatigue failure is the bottle neck in structure health monitoring of the small branch pipes in NPPs. In order to solve this problem, a socket weld pipe was simplified as the cantilever beam model, and the stress intensity factor (SIF) was calculated by the ABAQUS software and ZENCRACK software in this paper. The effects of crack depth, circumferential crack size and working load on SIF were discussed. The analysis results show that the SIF for outer surface circumferential crack distributes as an upside down “U” alphabet. And the maximum value of SIF is at the deepest point of the crack and is a linear increasing function of external load P. Basing on the plane strain fracture criterion, critical crack sizes bearing different external loads were calculated. With the increasing of loads, the critical crack size decreases. These critical values can be applied as the benchmark for evaluating the vibration fatigue failure in the structure health monitoring of small branch pipes in NPPs.

Keywords

Nuclear power plant; Stress intensity factor; Vibration fatigue; Weld toe crack; Small branch pipe.

Qiaoguo Wua [1,2], Xuedong Chena [1], Zhichao Fana [1], Defu Niea [1,2], Jianhua Pana [1]
[1] National Safety Engineering Technology Research Center for Pressure Vessels and Pipelines, Anhui Province Safety Technology Laboratory for Pressure Vessels and Pipelines, Hefei General Machinery Research Institute, Hefei 230031, China
[2] National Key Laboratory of Compressor Technology, Hefei General Machinery Research Institute, Hefei 230031, China

Engineering Fracture Mechanics, Volume 146, September 2015, Pages 210–223

Abstract

The dynamic fracture assessment method applied to a cracked centrifugal impeller subjected to dynamic loading is the focus of this study. J-integral histories and loading rates at crack tips were evaluated by numerical simulation through ABAQUS and Zencrack software. True stress–strain relations, fracture mechanism and fracture toughness of the impeller material FV520B steel were investigated over a wide range of loading rates by experimental and theoretical methods. Fracture assessment with consideration to rate effects was then conducted based on the CEGB R6 procedure. The present method demonstrated advantages over existing methods evaluated by an equivalent static load.

Keywords

Impeller; FV520B steel; Loading rate; Dynamic fracture; Failure assessment

R. Srilakshmia [1], M. Ramjia [1], Viswanath Chinthapentab [2]
[1] Engineering Optics Lab, Department of Mechanical and Aerospace Engineering, IIT Hyderabad, India
[2] Micro-Mechanics Lab, Department of Mechanical and Aerospace Engineering, IIT Hyderabad, India

Engineering Fracture Mechanics, Volume 134, January 2015, Pages 182–201

Abstract

In this work, the fatigue life of unrepaired and repaired Al-2014-T6 panels with an inclined center crack is investigated. Cracked panels are repaired through single- and double-sided adhesively bonded carbon fiber reinforced polymer (CFRP) patch. The fatigue crack growth is monitored experimentally using digital image correlation and numerically using 3D finite element analysis. The adhesive-interface between the panel and patch is modeled using bilinear cohesive law. The CFRP/Al-2014-T6 adhesive-interface properties are obtained from the baseline tests. Fatigue life of double-sided repaired panel is observed to be twice that of single-sided. And non-uniform crack front is observed in single-sided repaired panels.

Keywords

Aluminium alloys; Cohesive zone modeling; Digital image correlation; Fatigue crack growth; Bonded joints.

R.Citarella [1], M.Lepore [1], A.Maligno [2], V.Shlyannikov [3]
[1] Dept. of Industrial Engineering, University of Salerno, Fisciano (SA), Italy
[2] Institute for innovation in Sustainable Engineering, University of Derby, Quaker Way, DE1 3HD, Derby, UK
[3] Researches Center for Power Engineering Problems of Russian Academy of Sciences, Lobachevsky Street, 2/31, 420111, Kazan, Russia

Frattura ed Integrità Strutturale, 31 (2015) 138-147

Abstract

An edge crack propagation in a steel bar of circular cross-section undergoing multiaxial fatigue loads is simulated by Finite Element Method (FEM). The variation of crack growth behaviour is studied under axial and combined in phase axial+torsional fatigue loading. Results show that the cyclic Mode III loading superimposed on the cyclic Mode I leads to a fatigue life reduction. Numerical calculations are performed using the FEM software ZENCRACK to determine the crack path and fatigue life. The FEM numerical predictions have been compared against corresponding experimental and numerical data, available from literature, getting satisfactory consistency.

Jianfu Hou, Ron Wescott, Marco Attia
Aerospace Division, Defence Science and Technology Organisation (DSTO), 506 Lorimer Street, Fishermans Bend 3207, Australia

Engineering Failure Analysis, Volume 131, November 2014, Pages 406–418

Abstract

This paper presents a study on residual life assessment of turbine discs in a military aircraft engine that were found to be susceptible to fatigue cracking failure due to forging flaws formed in the original manufacturing process. As these flaws were not considered in the original life assessment, it is important to predict the residual lives of affected turbine discs and to determine the safe inspection intervals in order to prevent possible failures during service. The examination of the cracked disc revealed that the flaw was formed during the hammer forging. A systematic analysis approach was developed to analyse all four turbine discs and to predict the fatigue crack growth (FCG) rates by using advanced finite element (FE) and numerical FCG predictions. The critical locations for these discs were found to be on the aft neck face of disc web. The predicted FCG for the cracked disc correlated reasonably well with the striation counting from the cracked disc. The residual lives for representative discs at the critical location and associated inspection intervals are determined for life management of the affected turbine discs.

Keywords

Turbine disc stress analysis; Turbine disc cracking; Forging flaw; 3D crack growth prediction.

Ramesh Chandwani, Zentech International Ltd.

NAFEMS UK Seminar: Current Best Practices in Fatigue Analysis using FEA, London, UK, 16 September 2014

Abstract:
Fatigue failure is a complex physical process, encompassing both crack initiation and crack propagation phases. Using fracture mechanics, fatigue crack growth can be characterised and residual fatigue life evaluated. In the case of a failure, the cause of fracture can be determined forensically. Indeed, fracture mechanics, In conjunction with finite element analysis techniques, has become a very powerful tool for the assessment of the safety and durability of new and legacy based infrastructure projects, the failure of which could have enormous economical and loss of human life consequences. This paper highlights some of the techniques available and demonstrates their application to a range of fatigue life assessments.

Saranath K.M, M.Ramji
Indian Institute of Technology Hyderabad, Andhra Pradesh, India, 502205

International Conference on Computer Aided Engineering (CAE-2013), Department of Mechanical Engineering, IIT Madras, India

Abstract

Prediction of crack propagation or postulated flaw of structures is important in the assessment of their life, performance and reliability. Aerospace structures, gas turbine engines, pressure vessels and pipelines are some examples where crack propagation prediction plays an important role in estimating the system reliability, life and safety. Life of any component can be evaluated by means of experiments which are very time consuming and difficult to carry out on complex structures. In order to eradicate the difficulties associated with experiments one can use finite element modeling of the given system to estimate its life under different loading conditions. Cracks propagate in structures as a result of dynamic or cyclic loading. In the proposed work, a 3D finite element based life assessment study is carried out for an interacting parallel crack specimen under constant load amplitude cyclic loading and it is compared with experiments for assessing the accuracy of finite element modeling technique in case of complex structures. The commercially available finite element analysis (FEA) package ANSYS is used for the finite element modeling and is coupled with Zencrack package for crack propagation studies. Virtual Crack Extension (VCE) method is used for crack growth prediction. In VCE technique, strain energy release rate (ERR) is calculated for a series of virtual crack extensions along the normal plane at the crack tip and the direction of crack propagation is estimated along the direction in which maximum energy release rate is obtained. Stress intensity factor (SIF) at each crack extension is obtained using FEA and number of cycles is estimated using Paris law. Fatigue experiments are also conducted for estimating the life of the same interacting crack system along with digital image correlation (DIC) technique for obtaining strain fields surrounding crack tip. SIF obtained from FEA are compared with the analytical solution. Plot relating number of cycles and crack length is obtained from both FEA and experiment.

Keywords

Interacting crack, Finite element method, Fatigue life, Stress intensity factor.

M.J. Stevens [1], R.J. Dennis [1], I.J.M. Bottomley [2], R.A.W. Bradford [3]
[1] Frazer–Nash Consultancy Limited, 1 Trinity Street, College Green, Bristol BS1 5TE, UK
[2] Frazer–Nash Consultancy Limited, Stonebridge House, Dorking RH4 1HJ, UK
[3] EDF Energy Nuclear Generation Limited, Barnett Way, Barnwood, Gloucester GL4 3RS, UK

International Journal of Pressure Vessels and Piping, Volumes 108–109, August–September 2013, Pages 13–27

Abstract

The work reported in this paper describes a simulation of the manufacture, through-life operation and limiting defect size assessment of a pipework joint in a nuclear powerplant boiler. The objective of this work is to understand the critical factors that influence the integrity of the joint in-service and support accurate predictions of service life. This work differs from typical structural integrity assessments in that advanced modelling techniques have been used throughout the assessment process and include detailed simulations of the manufacturing process, a simulation of the entire in-service operating history including predictions of creep-fatigue damage and cracked body analysis to determine limiting defect sizes.
Residual stresses resulting from the manufacturing process can be a key driver for creep and creep-fatigue damage. The calculation of creep-fatigue damage for assessment purposes is typically undertaken within the framework of an appropriate assessment code, such as EDF Energy's R5. The standard assessment approach usually requires calculation of stresses using elastic finite element analysis followed by hand calculations to calculate the damage.
A combination of explicit and implicit finite element methods are employed to simulate a range of manufacturing processes which influence the in-service structural integrity of a branched pipework joint. Where available, test data have been compared to the results to assess the validity of the simulation. The simulation results then feed into a finite element based structural integrity assessment. The methods follow the principles outlined in the EDF Energy R5 assessment code but use the inelastic strains calculated directly from analysis. The methods are based around the general purpose finite element code Abaqus.
The residual stresses generated during manufacture may adversely affect the critical defect sizes for the pipework joint. However, the complex geometry and loading complicate the assessment of the cracked body. Therefore, a finite element analysis representing the cracked body has been carried out on the pipework joint to evaluate the J-integrals at the locations of interest and hence calculate the critical defect sizes. The analysis considers the residual stresses determined from the finite element analysis of the manufacturing processes.

Keywords

Creep-fatigue damage; Finite element analysis; Residual stress; J-integral; Cracked body mesh.

D.Sommerville, Structural Integrity Associates, Inc., Seattle, WA, USA
M.Qin, M.Walter, Structural Integrity Associates, Inc., Centennial, CO, USA.

Proceedings of the ASME 2013 Pressure Vessels & Piping Division Conference PVP2013 July 14-18, 2013, Paris, France; PVP2013-97727

Congratulations to Structural Integrity Associates for their award for Outstanding Technical Paper under the Computer Technology & Bolted Joints Technical Committee at the 2013 PVP Conference.

Abstract

As part of license renewal activities, many Boiling Water Reactor utilities must assess the integrity of bolted components within the reactor pressure vessel. Since inspection techniques capable of identifying cracking in the threaded regions of the core plate bolting do not currently exist it is important for flaw tolerance evaluations to be performed with as much accuracy as possible in order to obtain realistic inspection intervals and to assess the degree of redundancy in the bolted joint designs. Both 3-D finite element analysis and handbook linear elastic fracture mechanics solutions are used to assess the flaw tolerance of a core plate bolt design. Consideration of the relevant degradation and crack growth mechanisms is given for both initial crack configuration(s) and crack growth calculations. The 3-D finite element analysis is performed to investigate the effects of various simplifications in the available handbook solutions presented in the literature. The results of the flaw tolerance evaluations using both methods are compared and conclusions drawn regarding the applicability of the available handbook solutions for similar work in the future.

Chris Timbrell, Ramesh Chandwani, Zentech International Ltd., Steve Jacques, Lee Waterhouse, Andrew Wisbey, Amec Technical Services (was Serco), Steve Williams, Rolls-Royce plc

13th International Conference on Fracture, Beijing, China, 16-21 June 2013

Abstract

As the need for improved prediction of component life and development of new materials for use at high temperatures becomes more demanding, there is increasing requirement for a detailed understanding of thermo-mechanical fatigue (TMF) behaviour including the combined effects of fatigue and dwell on the overall crack growth rates under such conditions. To generate experimental TMF crack growth test data, a conventional servo-electric load frame, used in combination with a radiant lamp furnace, has been employed. The method for the measurement of crack growth under the TMF load cycle is also described. The performance of the experimental method is demonstrated with trials on an advanced nickel-based superalloy, RR1000. To reduce future testing requirements, simulation via the finite element method provides a means for crack growth prediction. This clearly requires validation with real test data at the outset. A method is described whereby separate fatigue and time dependent growth data can be combined and applied to TMF load cycles ranging from simple test cycles to full flight cycles. Results of this method used in conjunction with finite element based crack growth simulation are compared with experimental data, using the test method described above, from several TMF load cycles for RR1000 specimens.

Chris Timbrell, Angelo Maligno, Zentech International Ltd.
David Stevens, BLOS International

NAFEMS World Congress 2013, Austria, 9-12 June 2013

Abstract

An improved numerical technique for complex shaped non-planar three-dimensional crack growth simulations is proposed. This technique couples the adaptive re-meshing method used during crack growth simulation in the FE- based fracture mechanics code Zencrack with mesh relaxation using radial basis functions. This allows the uninterrupted simulation of crack propagation in engineering structures where the component geometries and local loading conditions may develop complex 3D crack configurations. Collocation with radial basis functions (RBFs) is an effective methodology for the interpolation of arbitrary scalar and vector fields defined over scattered datasets. By defining a mesh displacement field over a volumetric domain, the RBF collocation approach may be used to smoothly map a user-defined displacement of elements onto the entire domain, thereby "relaxing" the mesh around the imposed displacements. This mesh deformation leads, in most cases, to significantly improved element quality in comparison to traditional mesh-relaxation approaches such as Laplacian relaxation. In particular, elements that lie close to the source of a large displacement can be expected to exhibit significantly improved characteristics (such as aspect ratio and skew) in comparison to traditional approaches. The application of RBF deformation to fracture-tracking problems introduces many additional complexities that require novel and creative solutions. The most significant of these is the large difference in length scales between the imposed deformations - which are of element scale, and the constraints at the domain boundaries - which are of problem-scale. These differences in length scales make the problem unsuitable for use with compactly supported collocation methods. To retain a computationally efficient interpolation which is scalable to large problem sizes, a new method for RBF collocation has been developed which is based on large numbers of overlapping local collocation systems, using the underlying elemental structure as a framework. By linking together these overlapping local collocation systems a sparse global matrix may be formed, which can be solved to obtain the displacement at each node within the relaxation domain. For crack propagation simulation it is desirable to allow the mesh to move freely within the domain, as defined by the imposed displacements around the crack-tip, and to constrain the motion of surface and edge nodes such that they remain within their pre-existing geometric surfaces. In this work we describe surface-constraint methods which are suitable for use with complex 3D geometries where the mesh relaxation is performed using globally or locally supported RBF collocation systems.

A.R. Maligno [a,b], C. Soutis [c], V.V. Silberschmidt [d]
[a] Zentech International Ltd., London NW11 7RX, UK
[b] Unmanned Vehicle University, Lake Havasu City, AZ 86403, USA
[c] Aerospace Research Institute, University of Manchester, Manchester M1 3NL, UK
[d] Wolfson School of Mechanical Engineering, Loughborough University, Loughborough LE11 3TU, UK

Engineering Fracture Mechanics, Volume 99, February 2013, Pages 62-78

Abstract

In this study, fatigue crack growth behaviour of a cracked thick aluminium-alloy plate repaired with a bonded composite patch is investigated. An adhesively bonded composite-patch repair technique was successfully applied to military-aircraft repair and recently extended to commercial aircrafts. Also this technique has been extended its application from secondary structure repair to the repair of load bearing structures and components. Thus, a thorough understanding of crack growth behaviour of a thick panel repaired with a bonded composite patch is needed. Such behaviour is investigated in this paper using a finite-element code Zencrack in combination with ABAQUS. This approach enables a user to determine the crucial fracture-mechanics parameters such as stress intensity factors. Furthermore, a damage model was implemented for the adhesive layer, in order to monitor the consequence of a partial or total loss of adhesion on crack propagation in the repaired aluminium plate.

Keywords

FEA; Zencrack; Composite patch.

Chris Timbrell, Ramesh Chandwani, Zentech International Ltd.
Duncan MacLachlan, Steve Williams, Rolls-Royce plc, Derby

NAFEMS European Conference: Multiphysics Simulation, Frankfurt, Germany, Oct 16-17 2012

Abstract:
Alloys, especially nickel based ones used in the aerospace industry, are continuously being improved to provide greater strength against component failure and also to increase resistance against crack propagation. This involves altering their composition and, under controlled conditions, modification of precipitate and grain sizes. At high temperatures under both sustained and cyclic loading conditions, these microstructural changes interact synergistically with time dependent mechanisms such as creep, oxidation and corrosion and affect the crack growth rate (CGR). The individual effects of environmental conditions such as oxidation and corrosion and microstructural evolution of grain size at high temperatures, are generally difficult to evaluate. In addition, thermo-mechanical testing of large numbers of specimens under a variety of conditions can be prohibitively costly. Attempts have been made over the last few decades by a number of investigators to conduct standardised tests under controlled environmental conditions and compare them with the results obtained in neutral environments such as vacuum or inert gas [1-4]. It has been found that these environmental effects interact and their combined effect is generally greater than if they were considered separately. In this paper a time dependent crack growth law, COMET (Creep Oxidation Microstructure Environment Temperature), is described which considers the effect of these combined processes using a temperature dependent parameter based on an Arrhenius equation. Using this time dependent law in conjunction with a fatigue crack growth law, a finite element based implementation has been developed to carry out detailed 3D crack propagation analysis and simulation of a cracked component under the effect of thermo-mechanical loading at high temperatures.

Chris Timbrell, Ramesh Chandwani, Angelo Maligno, Zentech International Ltd.
Charly Ma, Consys Company Limited

ISSI 2011 - Structural Integrity in Nuclear Engineering, Heifei, China, Oct 27-30 2011

Abstract:
Many nuclear power plant facilities have been operating for longer than twenty years. Along with ageing of structural materials come other issues, such as creep and corrosion, which can impact upon plant safety and the integrity of the primary circuit. Plant lifetime extension and continued safe and economic operation depends on ageing and lifetime management. To be effective, this requires an understanding of how safety may be maintained as components degrade over extended time periods under operational conditions. Potential structural integrity issues for future generation IV reactors are also paramount to further develop the nuclear industry. This paper describes some aspects of the numerical analysis tool, Zencrack, which can assist in crack growth prediction and fitness for service investigations for a range of nuclear power plant applications.

V. Infantea [1], J.M. Silva [2]
[1] Departamento de Engenharia Mecânica, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal [2] Departamento de Ciências Aeroespaciais, Universidade da Beira Interior Edifício II das Engenharias, 6200-358 Covilhã, Portugal

Engineering Failure Analysis, Volume 18, Issue 2, March 2011, Pages 616–624

Abstract

This paper presents three case studies based on the numerical determination of the stress intensity factor (K) related with different crack front geometries due to fatigue using two commercial finite element analysis codes (ZENCRACK® and ABAQUS®).
Cases 1 and 2 considered a double-U notch specimen used in high temperature fatigue testing. Stress distribution in the vicinity of the U notch has been obtained, as well as numeric solutions of K for crack front geometries with rectilinear, corner and unconventional crack front shapes. In the case of the standard crack geometries the results have been validated with classic solutions existing in literature.
In the third case study, several computational simulations were carried out aiming at determining the K solution for distinct crack front positions, considering the influence of different geometric features of the specimen. Additionally, a stress triaxility parameter was used in order to confirm the stress state condition in the crack propagation region.
The conclusions of this study are encouraging concerning the possibility of using ZENCRACK® as an advantageous computational tool for obtaining K solutions in the case of particular crack front geometries as obtained from fatigue experimental testing.

Keywords

Stress intensity factor; Finite element analysis; Stress triaxility; Fatigue.

J.M. Silva [1], V. Infante [2], F. Antunes [3], F. Ferreira [1]
[1] Departamento de Ciências Aeroespaciais, Universidade da Beira Interior; 6201-001 Covilhã - Portugal
[2] Departamento de Engenharia Mecânica Instituto Superior Técnico; 1049-001 Lisboa - Portugal
[3] Departamento de Engenharia Mecânica FCTUC; 3030-788 Coimbra - Portugal

International Journal of Structural Integrity, Vol. 1 (4):12, 2010

Abstract

Stress state has a major influence on different phenomena, namely those involving diffusion and plastic deformation (like crack closure and high‐temperature fatigue crack growth, void formation or ductile fracture). The isolation of plane stress and plane strain states is crucial in fundamental studies of material behavior. The isolation of plane stress state is achieved with thin specimens, whilst the isolation of plane strain state is usually done increasing the thickness or introducing lateral grooves. The purpose of this paper is to propose a specimen geometry able to isolate the plane strain state, based on the standard M(T) geometry.
A numerical study was carried out aiming at obtaining a stress triaxiality parameter, h, as a function of different geometrical features of the specimen, such as the notch radius, notch depth and specimen thickness.
Results show that a pure plane strain state is achievable (i.e. 97 percent of specimen thickness has h>0.97) if a specimen with optimized geometrical features is used, which corresponds to a notch radius of 0.5 mm, a notch depth of 1 mm and a total specimen thickness of 12.56 mm.
This type of specimen geometry is a simple and efficient alternative to other common approaches used to obtain pure plain strain conditions for experimental purposes.

J.M. Silva [1], R.A. Cláudio [2], Moura Branco [3], Martins Ferreira [4]
[1] Depart. of Aerospace Sciences, University of Beira Interior, 6201-001 Covilhã, Portugal
[2] Depart. of Mech. Engineering, ESTS/IPS, 2910-761 Setúbal, Portugal
[3] Depart. of Mech. Engineering; IST/UTL, 1049-001 Lisboa, Portugal
[4] Depart. of Mech. Engineering, FCTUC, 3030-788 Coimbra, Portugal

Procedia Engineering, Volume 2, Issue 1, April 2010, Pages 1865-1875, Fatigue 2010

Abstract

Aeroengine components are often affected by high temperature creep-fatigue mechanisms which result in cyclic crack propagation. Some of these components, like turbine and compressor disks, have some geometrical features that act as stress concentration regions with a strong impact in the crack front geometry. Also, recent investigations show that shot peening may have an important influence for crack arresting or even in the delaying of short crack propagation.
This investigation presents some experimental results obtained for two types of specimen geometries of a new generation PM Nibase superalloy. A set of fatigue crack growth curves were obtained from high temperature fatigue testing considering different loading frequencies and two surface conditions of the material: as-machined and shot-peened. The results indicate that the compressive residual stress field due to shot peening contributes to the crack initiation stage, which however diminishes as the initial damage increases beyond a critical dimension. In the case of long crack propagation, it was found that the crack front assumed different geometries during the propagation stage. This fact combined with the high stress gradient region inherent to the specimen’s stress raisers led to the development of a specific stress intensity solution for this non-standard transitory crack front geometry, which was carried out using a finite element analysis. The results from the computational analysis provide a proper stress intensity factor solution that can be used in some experimental particular cases where an alteration of the crack front geometry is expected.

Keywords

Creep-fatigue; Crack propagation; Stress intensity factor; Shot peening; Ni-base superalloys.

A.R. Maligno [a], S. Rajaratnam [b], S.B. Leen [b], E.J. Williams [b]
[a] Loughborough University, Wolson School of Mechanical and Manufacturing Engineering, Loughborough, UK
[b] The University of Nottingham, School of M3, Nottingham, UK

Engineering Fracture Mechanics, September 2009

Abstract

Numerical analyses based on the finite element (FE) method and remeshing techniques have been employed in order to develop a damage tolerance approach to be used for the design of aeroengines shaft components. Preliminary experimental tests have permitted the calculation of fatigue crack growth parameters for the high strength alloy steel adopted in this research. Then, a robust numerical study have been carried out to understand the influence of various factors (such as: crack shape, crack closure) on non-planar crack evolution in solid and hollow shafts under mixed-mode loading. The FE analyses have displayed a satisfactory agreement compared to experimental data on compact specimens (CT) and solid shafts.

Keywords

FEM; Remeshing; Fatigue crack growth.

W. Beres, D. Dudzinski, A. Murzionak
National Research Council Canada, Institute for Aerospace Research Ottawa, Ontario, Canada

12th International Conference on Fracture 2009 (ICF-12), Ottawa, Ontario, Canada, July 12-17, 2009, Volume 4, pp.3265-3273, 2009, Paper 00587

Abstract

This paper describes the destructive inspection analyses of cracks growing from firtree serrations of a turbine disc spun in a spin rig facility. The cracks were initiated by introducing EDM notches in predefined locations of the discs. Finite element based predictions of crack growth from these notches are also reported. ZENCRACK as well as a 2D in-house deterministic fracture mechanics software were used. Both the crack depth and the surface crack length were predicted and correlated with the results obtained during the spin rig testing. The results obtained are used in damage tolerance analyses of critical components of gas turbine engines.

Angelo R Maligno, University of Nottingham
R. Chandwani, C. Timbrell, Zentech International Ltd.

NAFEMS World Congress 2009, Crete, Greece, Jun 16-19 2009

Abstract

Numerical modelling of three dimensional (3D) non-planar fatigue crack growth under mixed mode conditions represent a crucial factor in fracture mechanics in order to assess the residual life of components. This paper focuses on developing a damage tolerance approach that can be used for the design of aeroengine shaft components under the general mixed-mode loading conditions in the presence of stress-raising features. The initial work has validated numerical results against crack growth measurements on uniaxial tensile specimens under Mode-I loading. Then, more realistic loading scenarios have been applied on shafts to investigate the influence of different parameters (e.g. crack orientation) on fatigue crack growth. All of the present work is based on linear elastic fracture mechanics approaches, including the Paris and Walker theories.

Nawal K. Prinja, Joseph M. Bushell, AMEC Nuclear UK Ltd.
R. Chandwani, C. Timbrell, Zentech International Ltd.

NAFEMS World Congress 2009, Crete, Greece, Jun 16-19 2009

Abstract

This paper explains how the general purpose FE code Abaqus was combined with Zencrack to predict 3D crack growth as part of an investigation conducted to explain failure of single cylinder bridge roller bearings. Finite Element analyses were conducted to gain an understanding of the stresses caused during operation and explain the possible cause of crack growth resulting in failure. It was important to predict sub-surface crack growth in the rollers induced by repeated rolling as the bridge expands due to daily and seasonal temperature cycles. Models of the bearings were required to represent the contact between the roller and plates, daily movement of the load and the non-linear behaviour of the material. A band of pressure due to contact with the plates traverses back and forth over the roller as it rotates. The ambient temperature changes were such that the roller would experience at least one stress cycle per day of operation. A number of initial crack scenarios are studied, based on crack initiation sites identified during inspections of the failed rollers. Stress intensity factors (SIFs) for all three modes of cracking (KI, KII and KIII) are plotted against angle of roll bearing is misaligned, the roller is subjected to additional twist. The Abaqus and Zencrack analyses showed that in the absence of any twist, the load cycling caused by the rotation of the roller propagates initial end cracks along the roller axis which can lead to the roller splitting in half. Twisting of the roller due to bearing misalignment causes out of plane growth of the crack indicating that a part of the roller can break away.

M. Roy [1], C. Lang [2], I.M. May [2]
[1] QinetiQ Limited, Dunfermline, UK
[2] Heriot-Watt University, Edinburgh, UK

Proceedings of the Institution of Civil Engineers, Structures & Buildings 162, April 2009, Issue SB2, Pages 107-113

Abstract

Composite patches (usually made from carbon-fibre reinforced plastic (CFRP)) are frequently used for localised structural strengthening. They can also be used to repair fatigue cracks in metallic structures in situations where welded repairs are unsuitable: for example, cracks in stress concentrations may re-appear soon after a welded repair is attempted. This application is, however, hampered by the difficulty of predicting the fatigue life extension produced by the repair. This paper presents work which shows that the growth of patched and unpatched fatigue cracks can be predicted using finite-element analysis, allowing the benefits of a potential patch repair to be quantitatively assessed. Simulations of the crack propagation are performed and show a good correlation with test results. The tested repair was successful in significantly reducing the fatigue crack growth rate.

Keywords

Buildings, structure & design, fatigue.

Zhang Weiguo, Mi Weijian, Liu Haiyang
College of Logistics Engineering, Shanghai Maritime University, Shanghai 200135, China

Computer Aided Engineering, Vol. 17 No. 1, March 2008

Abstract

Due to the fact that the analysis of 2D modes can not well reflect the fatigue crack propagation behaviors of container crane, the pipe components of container cranes are studied by 3D crack analysis software Zencrack. The relationship between through crack and partly through crack are analyzed in detail. It can provide references for counteracting fatigue design and crack propagation management of container cranes structure.

Keywords

Container crane; pipe component; through crack; partly through crack; fatigue crack propagation; Zencrack.

R. Chandwani, C. Timbrell

NAFEMS World Congress 2007, Vancouver, Canada, May 22-25 2007

Abstract

Numerical modelling of three dimensional (3D) non-planar fatigue crack growth under mixed mode conditions represent a crucial factor in fracture mechanics in order to assess the residual life of components. This paper focuses on developing a damage tolerance approach that can be used for the design of aeroengine shaft components under the general mixed-mode loading conditions in the presence of stress-raising features. The initial work has validated numerical results against crack growth measurements on uniaxial tensile specimens under Mode-I loading. Then, more realistic loading scenarios have been applied on shafts to investigate the influence of different parameters (e.g. crack orientation) on fatigue crack growth. All of the present work is based on linear elastic fracture mechanics approaches, including the Paris and Walker theories.

Jooho Choi, Gyubeom Lee, Boyoung Lee
School of Aerospace & Mechanical Engineering, Hankuk Aviation University, Korea

The Fourth China-Japan-Korea Joint Symposium on Optimisation of Structural and Mechanical Systems, Kunming, China, Nov 6-9, 2006

Abstract

In the nuclear facilities, fatigue cracks are often observed in pipes during the operation of the Reactor Coolant System (RCS), which originate from the cyclic loading caused by thermal stratification. Though the NDT methods are employed to detect the crack, sufficient number of repeated exercise should be preceded using artificial cracked specimen of a same kind for the reliable on-site detection. The crack of this kind, which has less than 150 m width, can not be made by the conventional machining methods such as EDM, but should be made under thermal cyclic load that is close to that of the RCS. The time for obtaining the wanted crack, however, is prohibitively long, which makes it difficult to be supplied in sufficient number for NDT evaluation. In this work, single edge crack of a rectangular plate is considered as a preliminary study, in which constant tensile as well as repeated thermal loads are applied to let the crack grow. Optimum loading condition is sought that minimizes the time for fabricating the wanted crack size. The crack growth phenomenon is evaluated through the coupling of crack growth simulation software Zencrack with the general purpose analysis code ANSYS.

Keywords

Thermo-Mechanical Fatigue, Crack Growth, Stress Intensity Factor, Artificial Crack.

G. Liu [1], A. Mouftiez [1], J. Lesage [2] and S. Panier [1]
[1] Département Technologie des polymères et composites and Ingénierie mécanique, Ecole des Mines de DOUAI, BP 10838, 59508 Douai, France
[2] Laboratoire de Mécanique de Lille, URA CNRS 1441, U.S.T Lille, IUT A GMP, Villeneuve d'Ascq, France

Surface and Coatings Technology, Volume 201, Issue 5, 25 October 2006, Pages 2086-2091, The 2nd International Meeting on Thermal Spraying - 2nd RIPT

Abstract

The aim of this work is to investigate the fracture behavior of a stationary crack lying along the interface in a coated system during interfacial indentation test. One traditional 3D numerical model and one cohesive element numerical model have been made to study the Vickers indentation test process. A crack-block approach is applied to generate 3D meshes containing the crack front along the interface. The stress intensity factors and the energy release rates (G) on the crack front are computed. The fracture mode mixity is also presented. It is found that the near surface propagation of the crack can be prevented during loading due to compressive stresses. It may occur during unloading due to residual stresses resulting from plastic strain.

Dr J. Hou
Air Vehicles Division, DSTO, Australia

MSC.Software VPD Conference 2006, Sydney, Australia, July 25-26, 2006

Abstract:
Presentation Title: 3D Crack Growth Modelling in Gas Turbine Engines
DSTO has been continuously enhancing its capability to provide through-life support to the ADF in aircraft engine component life management. One of the major requirements is an enhancement in computational 3D crack growth modelling and analysis. During service, gas turbine engine components experience both cyclic and sustained thermal and mechanical loadings which may result in fatigue and creep damage. As the damage sustained is highly dependent on the variations in both operating stresses and temperatures, finite element analyses (FEA) of the components can be very useful. This presentation outlines the critical issues involved in 3D crack growth and evaluates the results of a 3D crack growth capability in the ZENCRACK software with an emphasis on its validity and applicability to engine components. The primary issues to be dealt with in practice for 3D crack modelling are outlined, and the methodology and techniques implemented in ZENCRACK are described and discussed. Practical applications of ZENCRACK and individual evaluations for particular problems are presented. The various limitations and uncertainties encountered in the practical applications are identified. In particular, it is found that ZENCRACK is a useful tool for the calculation of stress intensity factors. Conclusions and recommendations are made for more accurate 3D crack growth modelling.

A.L. Hutson [1], M. Huelsman [1], D. Buchanan [1], R. John [2], S. Haering [2]
[1] University of Dayton Research Institute, Ohio, USA.
[2] AFRL/MLLMN, Wright Patterson Air Force Base, Ohio, USA.

FATIGUE 2006, 9th International Fatigue Congress, May 14-19 2006, Atlanta, Georgia, USA

R. Chandwani, C. Timbrell, M. Wiehahn

International Conference On Pressure Vessels And Piping, Chennai, India, Feb 7-9 2006

Abstract:
Much of the process plant designed in the early 1970s, in response to increased demand for power, is now 'ageing' and necessitates an on-going 'fitness for service' assessment. This is necessary to satisfy licensing authorities requirements for continuous operations. Fitness for service assessment involves many issues including evaluation of structural flaws, material degradation, damage due to creep, fatigue and plasticity and the overall effect on the residual life of components. Codes of practice such as API 579, BS 7910 and R6 give detailed assessment procedures and may involve very detailed and time consuming finite element analyses. The authors have developed an FEA tool, Zencrack, to accurately evaluate single or multiple cracks in any structure (e.g. pressure vessels, piping, etc.). Further, 3D non-planar crack growth under general fatigue or time dependent loading is possible. The software can help to increase efficiency in performing sensitivity studies, thus allowing more accurate assessment of the residual life of the plant than would otherwise be possible in a given timeframe.

R. Chandwani, C. Timbrell, M. Wiehahn

International Seminar on "Fatigue, Reliability & Performance Considerations In Design", Indian Institute of Science, Bangalore, India, Jul 14-15 2005

Abstract:
The use of fracture mechanics techniques in the assessment of performance and reliability of structures is on the increase and the prediction of crack propagation of an existing or postulated flaw in a structure plays an important part. A software simulation tool, which uses FEA, has been developed to quantitatively predict the propagation of 3D non-planar cracks through structures. This tool allows calculation of stress intensity factors and energy release rates and can be applied in a variety of commercially important applications. These include design of laboratory experiments, testing and developing advanced materials, assessing the effects of surface treatments and the study of component repairs.

Two page article

Aerospace Engineering, July 2005

T.Iijima
Hitachi Ltd., Japan

2005 ASME Pressure Vessels and Piping Division Conference, Denver, Colorado, USA, July 17-21, 2005, paper PVP2005-71259

M.R. Roy, J.D.G. Sumpter, QinetiQ, Rosyth Business Park, Dunfermline
C. Timbrell, M. Wiehahn, Zentech International Ltd.

Abaqus Users' Conference, Stockholm, Sweden, May 18-20 2005

Abstract:
Stiffened plate structures such as ships and bridges often develop local stress concentrations which are a mixture of membrane tension and out of plane bending. Any cracks which initiate due to fatigue soon grow through the plating thickness. Modern steels are resistant to brittle fracture, but if cracks are left un-repaired, they may spread rapidly by fatigue. The rate of increase in fatigue crack length can be quantified using the elastic stress intensity factor, K.
Routines for the determination of K are available in ABAQUS, but they have limitations for performing extensive parametric studies on fatigue crack growth. Multiple fatigue crack tip positions must be analysed, requiring frequent re-meshing; and the crack direction and through thickness profile must be anticipated in advance. Zentech have developed the ABAQUS compatible software Zencrack specifically to deal with these problems.
This paper describes the problems of applying existing ABAQUS J integral methods to model cracks in structures under out-of-plane bending. Shell elements are shown to give path dependent and unreliable J values. Solid elements can be used, but the crack shape under fatigue cannot be easily predicted. Zencrack provides re-meshing techniques which allows the crack shape and speed of fatigue crack propagation to be efficiently predicted.
It is shown that Zencrack analysis overcomes ambiguities over how to take account of variations in K along the crack front in fatigue crack growth predictions. A set of results are presented which suggest that an out-of-plane bending stress is very much less detrimental for the fatigue life of a through thickness crack than a membrane stress of the same magnitude.

F.Leblanc [1], J.Roger [1], A.Zimmermann [1], D.Coutellier [2]
[1] Robert Bosch GmbH, Corporate Research and Development, Stuttgart, Germany
[2] Université de Valenciennes et du Hainaut Cambrésis, France

Journées Nationales sur les Composites #14, UTC, Compiègne, France, March 22-24, 2005

Abstract:
Within the fracture mechanics frameworks, adhesion can be considered in terms of an interfacial toughness. The interfacial fracture energy Gc depends on the mixed mode angle (the relative proportion of tensile and shear forces at a given distance ahead of the crack tip). Such an approach has been shown to efficiently predict the behaviour of multi-layered structures or components containing different materials and to provide a usefull simulation technique for the development of industrial products. In order to determine the interfacial toughness, lots of sample geometries exist (i.e. the Asymmetric Double Cantilever Beam Single Leg Bending, End Notched Flexure, Symmetrical Center Cracked Beam, Brazil Nut Sandwich). Besides, some authors quote the influence of surface pre-treatment on the delamination (chemical as well as physical interactions) or manufacturing processes and underline the influence on the interfacial toughness. The numerous test methods enable the choice of different loading conditions and at the same time allow to reach a wide range of mixed mode angles. In this paper, we present a method to determine the interfacial toughness curves, based on the correlation between experimental techniques and numerical simulations. Experiments were conducted for typical polymer/polymer interfaces, similar to those encountered in electronic packaging applications, and a commercial finite element code was used in combination with a meshing tool to compute the mixed mode angle and the related energy release rate.

Q. Liu, P. Hamel, W. Hu, P.K. Sharp, A. Lahousse and G. Clark
Air Vehicles Division, DSTO Platforms Sciences Laboratory, Australia

Internal document approved for public release in March 2005

R. Chandwani, M. Wiehahn, C. Timbrell

UK Ansys Conference, Stratford-upon-Avon, England, Nov 15-16 2004

Abstract:
This paper will address methods of performing truly three-dimensional fracture mechanics analyses in ANSYS.
Generally available fracture mechanics techniques and their implementation and use with ANSYS for 3D analysis will be briefly discussed. Techniques include the crack opening displacement (COD) method for LEFM, crack tip opening displacement (CTOD) method for EPFM, and the J-Integral method.
A software implementation using the COD method in conjunction with ANSYS will be presented. This implementation addresses generation of cracked 3D meshes and crack growth prediction. Examples will demonstrate large-scale crack growth under generalised mixed-mode loading and the development of complex 3D crack surfaces.

S. Berdouk, R.J. Grant, P. Meynier, J. Smart
North East Wales Institute, Wrexham, Wales

The First International Conference On Computational Mechanics (CM'04), Belgrade, Serbia and Montenegro, November 15-17, 2004

Abstract:
Previously, Grant & Smart [1,2] have reported on both experimental and numerical results for pin-loaded tubes. In the experimental work, tubes were loaded by means of a central shank and transferred to the tube by means of a transverse pin. (It should be noted that this work originated because this resembles a part of an aircraft structure.) The crack growth rate and trajectory were noted and some experiments were finished early to obtain the crack profile which was found not to lie in a radial direction. In the finite element modelling, at various 'radial lengths', the crack was given a twist and a tilt and the crack profile was estimated. It was found that the numerical and experimental results correlated with each other. The same experimental set up is now been replicated using a boundary element programme, Beasy, and the finite element programme, Zencrack. In both programs, an initial crack is given and the program automatically calculates the crack growth. The objective of the research is to compare the crack growth trajectory and the profile of the crack with the previous results and to compare critically the different methods. This work is currently in progress and the results from the different procedures will be compared and contrasted.

[1] Grant RJ & Smart J (1999) Crack growth in pin-loaded tubes. Part 1: numerical analysis, J Strain Analysis, 34, 253-269
[2] Grant RJ & Smart J (1999) Crack growth in pin-loaded tubes. Part 2: comparison of experimental data with numerical results, J Strain Analysis, 34, 271-284

B.C. Pinheiro, I.P. Pasqualino
Programa de Engenharia Oceânica, COPPE/UFRJ, Rio de Janeiro, Brazil

20º Congresso Nacional de Transportes Marítimos, Construção Naval e Offshore, Rio de Janeiro, Brazil, November 8-12, 2004

C. Timbrell, R. Chandwani, G. Cook

Journée Scientifique 2004: Les méthodes de dimensionnement en fatigue, Centre de Compétences Matériaux & Conception (CCM&C), Fribourg, Switzerland, Oct 27 2004

Abstract:
This paper discusses the issues involved in numerical crack growth prediction for general 3D cracks and describes the state of the art methods that are available to practising engineers. This is a wide ranging subject in which no single theoretical method is appropriate for all cases. Different approaches are adopted, for example, for crack propagation under static load, sustained load, fatigue load and impact load. The historical pedigree of the various approaches dictates the extent to which commercial software can provide practical solutions on a day-to-day basis. A brief overview is given of the relevant fracture mechanics parameters and their use in crack growth prediction under various load conditions.
The difficulties imposed by real-life problems are further compounded by the complex 3D geometries that are involved. These complexities may arise from general component shape such as turbine disk-to-blade connections or from individual geometric discontinuities such as chamfers or stiffeners. Further complications may be introduced from a variety of sources including residual stress effects, propagation along dissimilar material interfaces and propagation in non-metallic materials or metals which are non-homogenous, large grained or anisotropic.
A number of numerical approaches are discussed and the advantages and disadvantages of each are noted. Difficulties associated with growth of a general 3D crack front are considered in general and with respect to each method.
Of the various load types that may cause crack propagation, fatigue is the most advanced in terms of useable prediction capabilities. The current state of the art for fatigue crack propagation allows for growth of multiple non-planar defects through a 3D structure under general mixed mode loading. Stress ratio and load interaction effects may be included within the analysis. The most general integration schemes allow for proprietary crack growth models, including stress ratio and temperature dependency. In addition, the effect of a static load component such as residual stress may be included within the analysis. Sustained load damage (e.g. Creep) may be combined with instantaneous damage due to rainflow counted fatigue cycles.

R. Brockman [1], R. John [2]
[1] University of Dayton Research Institute, Dayton, OH, USA
[2] Air Force Research Laboratory, Metals, Ceramics, and NDE, Wright Patterson AFB, OH, USA

Materials Science & Technology 2004 Conference & Exhibition, New Orleans, Louisiana, USA, September 26-29, 2004

Abstract:
Recent studies have shown that analytical predictions of crack growth in rotating components can be used in conjunction with displacement measurement techniques to identify critical levels of fatigue damage. However, investigations of this type traditionally have focused on the detection of damage at known flaw locations. This presentation will address the related problem of assessing damage associated with flaws at unknown locations, through the combined use of analytical models and measured vibration signatures. Because the measured data are insufficient to identify a unique solution for the location and severity of fatigue cracks, the analytical procedure must be able to bound the extent of damage occurring at life-limiting locations. The issue of analyzing successive measurements to improve estimates of worst-case damage and crack locations will also be discussed.

M. Wiehahn, C. Timbrell, R. Chandwani

FENET Technology Workshops - Durability and Life Extension, Palma, Majorca, Mar 25-26 2004

C. Timbrell, R. Chandwani

FENET Technology Workshops - Durability and Life Extension, Palma, Majorca, Mar 25-26 2004

C. Timbrell, R. Chandwani, M. Wiehahn, G. Cook

FENET Technology Workshops - Durability and Life Extension, Noordwijk, The Netherlands, Oct 9-10 2003

C. Timbrell, M. Wiehahn & G. Cook, Zentech International Ltd., Camberley, Surrey, UK
A.H. Muhr, TARRC/Rubber Consultants, Hertford, UK

Proceedings of the Third European Conference On Constitutive Models For Rubber, London, England, Sep 15-17 2003

Abstract:
Software for simulating crack propagation ("Zencrack") has been developed which handles adaptation of the finite element mesh in the region of the crack tip as the crack propagates, and calls up a finite element solver as a subroutine to calculate the strain energy release rate at each increment in crack length. The objective of this paper is to demonstrate its applicability to rubber components.
First, the reliability of Zencrack, with ABAQUS as the solver, for calculations of strain energy release rate for an edge crack in a strip of rubber in simple extension is investigated. The results are in reasonable agreement with previous analyses and experimental work from the literature. 3D analyses show that the crack front would be ex-pected to develop curvature during propagation. The application of Zencrack to failure of "O" rings by internal cracking is also presented to show how the approach could be applied to a rubber component.

G. Cook, C. Timbrell, M. Wiehahn

Abaqus UK Users Group Conference, Warrington, England, Nov 12-13 2002

Abstract:
Many analysts are keen to investigate cracks in components under static or cyclic external loading. In the latter case fatigue crack growth prediction is also of importance. It is known that surface treatment effects such as shot peening have a beneficial effect on component life due to the compressive residual stresses introduced in the vicinity of the surface. Such treatments are used in maintenance programmes to extend the life of components in service. The effect on crack growth rate can be dramatic with significant changes in the crack growth profiles and increase in fatigue life to failure. The approach discussed in this paper applies fracture mechanics techniques to establish crack growth rates based on detectable defect sizes above the crack initiation stage established by non-destructive inspection. Thereby, extended fatigue life can be predicted, extended inspection periods calculated and the retirement of some components may be avoided.
This paper demonstrates how the Abaqus DLOAD user subroutine can be used to include the effect of residual stresses from shot peening in the analysis of crack growth in 3D finite element models. The inclusion of these effects may be so beneficial that a crack will change from a "growing" state without residual stress to a "non-growing" i.e. below threshold state, if residual stress is included.
Initially the effect of residual stresses are presented using a linear fracture mechanics approach for a single edge notched specimen and a corner crack specimen. Subsequently, the non-linear effects of contact at the crack surfaces are modelled and the effects on the cyclic energy release rates are presented.

I.P. Pasqualino [1], I.A. Valeriano [1], T.M.J. Alves [2]
[1] Ocean Engineering Department, COPPE, UFRJ, Rio de Janeiro, Brazil
[2] PETROBRAS/CENPES, Rio de Janeiro, Brazil

The Twelfth International Offshore and Polar Engineering Conference, Kitakyushu, Japan, May 26-31, 2002

A.M. Gilmartin, M.B. Henderson*, T.J.W. Ward, B. Vermeulen
QinetiQ, Cody Technology Park, Farnborough, Hampshire GU14 0LX, United Kingdom
*ALSTOM Power Tech. Centre, Whetstone, Leicester LE8 6LH, United Kingdom

The 2002 Abaqus User Conference, Newport, Rhode Island, USA, May 28-31, 2002

Abstract:
Gas turbine engine combustor manufacturers face the challenge of achieving improved efficiency against the increasingly stringent regulations concerning NOx emissions. Revolutionary changes in design styles will be required with the increased use of high stress concentration features such as effusion cooling holes. Lifetime prediction for new combustor designs will require an estimation of life to first crack and an estimation of the crack propagation life. This paper studies crack propagation in featured low cycle fatigue specimens intended to simulate cooling holes within a combustor liner wall. Based on extensive fatigue crack growth testing of corner notch specimens, linear elastic fracture mechanics crack growth laws have been postulated for the g¢ strengthened, nickel-based superalloy C263 at 300 and 800°C. These laws have been used to predict the growth rates for cracks growing within featured specimens and the results compared with data obtained from experimental testing of specimens with 90° holes. Direct comparison with growth rates obtained from load-controlled fatigue testing of the specimens at 300 and 800°C has shown the growth laws to provide good predictions for the behaviour of the material in the vicinity of the feature. The meshing of the test specimens with elliptical and straight through cracks was performed with the aid of ZENCRACK software, which provides singular elements to a conventional mesh. The 3-D analysis was performed using ABAQUS Standard.

B.J. Wicks, R.A. Antoniou, S.L. Slater and J. Hou
Aeronautical and Maritime Research Laboratory, Defence Science and Technology Organisation, Department of Defence, Box 4331 GPO, Melbourne 3001, Australia

Presented at RTO AVT Symposium on "Ageing Mechanisms and Control: Part B - Monitoring and Management of Gas Turbine Fleets for Extended Life and Reduced Costs", Manchester, UK, 8-11 October 2001, and published in RTO-MP-079(I)

G. Cook, C. Timbrell, B. Browning

Symposium on "Turbine Engine and Airframe Sustainment", AeroMat 2001 - 12th Advanced Aerospace Materials & Processes Conference & Exhibition, Long Beach, CA, U.S.A., June 11-14 2001

Abstract:
No formal papers were required or proceedings issued for the Aeromat 2001 conference.

J. Hou, M. Goldstraw, S. Maan, M. Knop

Airframes and Engines Division, DSTO Aeronautical and Maritime Research Laboratory, Victoria, Australia: Internal document approved for public release in May 2001

B. Browning, G. Cook, C. Timbrell

Abaqus Users' Conference, Maastricht, The Netherlands, May 30 - June 1 2001

Abstract:
Crack growth prediction techniques have been available for 3D models under mixed mode loading for a number of years. Previous techniques were limited to crack growth within a small region of a model. A new method is presented for automatically predicting large scale crack growth in a 3D finite element model by allowing the crack front to move through the model. Some simple examples are presented in addition to comparison of analysis results against published experimental spin test data for a cracked turbine engine compressor disc. This spin test analysis includes high levels of plasticity and crack closure effects.

C.Timbrell, G.Cook, R.Chandwani

1st Northern European Technology Conference, Manchester, England, June 7-8 2000

Abstract:
This paper describes software which is interfaced to MSC.Marc for analysis of arbitrary 3D cracks. The software, Zencrack, can be used in three ways:

1. For generating 3D finite element meshes containing multiple crack fronts from a finite element model of an uncracked component.
2. For determining the distribution of the maximum energy release rates and stress intensity factors along crack fronts.
3. For automatically calculating fatigue or sustained load crack growth in a general 3D body under arbitrary loading.

Items 2 and 3 use the results from the LORENZI option of a MSC.Marc analysis. The raw data from this option is processed to calculate energy release rates along the crack front. There are many complex issues to be addressed in 3D crack growth prediction. This paper presents the key concepts of the software implementation and the interface to MSC.Marc.

R.Chandwani, I.Larsen

Proceedings of the "Workshop on Subsea Pipelines", COPPE/UFRJ-Federal University of Brasil, Rio de Janeiro, Brasil, December 8-9 1997

Abstract:
Flexible pipes, composed of bonded, unbonded or high strength steel or other alloy materials, used as high pressure dynamic risers have increased multifold in the past 20 years or so. Since 1978, when flexible risers were first specified and installed in the Enchova field, offshore Brazil, as part of a floating production system, developments in the design of particularly unbonded and steel risers have made the exploitation of oil/gas resources in deep waters with hostile environments economically viable. Also, in deep waters a compliant topside structure is more economical than a fixed platform. Various designs of compliant topside structural systems such as converted tankers, semi-submersibles with single or multi-point mooring systems with different types of turret and DP systems, tension leg platforms, deep draft platforms, spar platforms, etc., with varying specifications are currently available which comply with the wave motions rather than resist them. Flexible risers are critical components of such floating production systems because they provide the means of transferring the condensate from the seabed to the topside structure and also to exporting the processed fluids from the structure.
In this workshop lecture, some aspects of the design of dynamic flexible risers, unbonded, steel or titanium, are discussed.

C.Timbrell, G.Cook

Seminar on "Inelastic Finite Element Analysis", Institute of Mechanical Engineers, London, October 14 1997

Abstract:
This paper gives an overview of techniques that have been used by Zentech for analysis of cracks in 3D geometries in a number of commercial applications. These techniques are incorporated into the Zentech program (Ref. 1) which is interfaced to Abaqus (Ref. 2) and Marc (Ref. 3). Simple demonstrative examples and some consultancy applications are included. Some of the difficulties encountered in this type of analysis are noted and future areas of interest are outlined.

P.Chakrabarti, Zentech Inc.; R.Chandwani, I.Larsen, Zentech International

OMAE'96 Conference, Florence, Italy, June 16-20 1996

Abstract:
The design of riser and mooring systems for a floating production system usually assume uncoupled behaviour of the two systems, and each system is analysed for the effects of the vessel response given as input. In certain situations the presence of the risers can affect natural periods, damping, and slow drift response of the vessel thereby influencing the system design. This paper discusses a method for analysing the coupled riser/mooring system using Zentech's suite of programs with emphasis on the effect of the risers on the total system behaviour.

R.M. Andrews, A.P. Dyer
Structural Integrity Department, TWI, Abington Hall, Abington, Cambridge, U.K.

Presented at the 10th U.K. Abaqus User Group Conference, Abington, U.K., September 21, 1995

Abstract:
TWI's Structural Integrity Department uses ABAQUS to support work in fast fracture and fatigue. Two widely differing examples are presented in this paper. A study of ductile tearing in thin sheet aluminium alloys has required the reproduction of extensive crack advance to evaluate candidate fracture parameters. Welded tubular joints in three dimensions have been modelled with solid elements including the weld. Weld toe cracks have been included in these models and both linear elastic analyses (to generate stress intensity factors) and full non-linear analyses have been carried out. Results from ABAQUS analyses are presented and compared with experimental results. Some remarks on the future development of fracture mechanics facilities in ABAQUS conclude the paper.

N.Constable

Cognos North American User Conference, Ottowa, Canada, June 28-30 1995

Abstract:
This paper is about the iterative development methods used by Zentech during a two year system development project for Statoil, a major Norwegian oil company. I called the paper Rapid application Development in PowerHouse after reading an article in a UK computer paper which seemed to have just discovered the techniques which we, and I suspect a good many other PowerHouse developers, have been using for a number of years.
I shall be discussing an extension of the prototyping techniques which I am sure your Cognos salesman cited as one of the benefits of PowerHouse. Strictly prototyping suggests the building of a working model from which design and construction decisions can be made. This model is then discarded and the real business of development undertaken. In practise it is a waste to throw away perfectly good code, so people started to use something called iterative development. That is a cycle of design, construction, user review, change implementation and further reviews. At Zentech we applied those techniques in a larger than usual project, allowing us to meet tight deadlines, while overcoming problems caused by a loose original user specification.
Firstly I will tell you something about the people and companies involved. Then I will give you some project background, explain how the project developed, what changes we had to make to our working practises to meet the challenges presented, and what our final solutions were. I will touch briefly on some of the technical issues raised and how they were resolved. Finally I will try to draw some conclusions from our experiences.

C.Timbrell, P.W.Claydon, G.Cook

Abaqus Users' Conference, Rhode Island, U.S.A., June 1-3 1994

Abstract:
A practical and well proven method is presented for generation of 3D meshes for modelling cracks, including fully automatic techniques for 3D (non-planar) fatigue crack growth prediction. The techniques, used successfully in various industries for a number of years, have recently been applied for the first time with ABAQUS. Results are presented for a number of cases, including comparisons against theoretical and experimental data.

R.Chandwani, D.Mahmood, I.Larsen, Zentech International; J.Neffgen, JMN & CEN Consultants, Scotland

Offshore and Arctic Operations - 16th Annual Energy Sources Technology Conference, Houston, February 1993

G.Cook, P.W.Claydon

MARINFLEX 92 Conference on Flexible Pipes, Umbilicals, Marine Cables, London, November 1992

Abstract:
Flexible risers have been traditionally used to transport fluids at high pressure under dynamic loading conditions. Typically within the offshore industry they are used in a catenary type configuration to connect say a floating production system to the sea bed, or, either a fixed structure or another floating vessel. The risers themselves are designed to have a high axial stiffness yet be flexurally compliant to withstand the static and dynamic offshore loading. This loading arises from direction induced motion of the connected vessel.
A riser configuration is designed to withstand extreme wave and current loading conditions, typically the 100 year conditions, within the allowable integrity limits of the riser. Once the system configuration has been established, an additional and equally important requirement for full qualification is the prediction of its design life under operational loading conditions. Due to dynamic loading and the different layer structures forming the riser cross-section its design life is governed by combined wear and fatigue. The evaluation of stresses, slip and wear within unbonded flexible risers under these conditions have been addressed by a number of papers [1-6]. It is the intention of this paper not to reiterate these evaluation techniques but to present an engineering approach using them for predicting the design life of flexible risers.
A basic chart showing the input into the design life prediction is given in figure 1. From the frequency of occurrence diagram defining the environmental wave data the configuration experiences, all possible damaging load cases are analysed using a 3D flexible riser package, in this case Zenriser. The corresponding time history results are stored for subsequent life prediction using a post-processor referred to as ZENLIFE. In addition to the general equivalent cross-sectional riser properties available from the riser analyses the processor must also access the pipe geometrical description, material properties and the criteria against which the design life prediction is to be made. The processor can then evaluate stresses, slip and wear during each load case from which the design life for the riser can be assessed. This is expanded in greater detail throughout the paper.
The procedure outlined in this paper has been used on a number of cases ranging from existing risers already in use to the qualification of future riser designs. However, at present confidentiality prevents the disclosure of any of these results. Instead a demonstration case is presented - the design life assessment of a lazy 'S' configuration. For simplicity all wave loading was applied in a single direction with the riser configuration in its 'far' position. In practice a spread of wave directions consistent with environmental conditions would have to be applied in addition to all vessel offsets and rotations consistent with this wave loading.

P.W.Claydon

Engineering Fracture Mechanics, Volume 42, Issue 6, August 1992, Pages 961-969

P.W.Claydon, G.Cook, P.A.Brown, R.Chandwani

Marine Structures Journal, Volume 5, Issue 5, 1992, Pages 399-429

D.Hoffman, HMC Offshore Corp.; N.M.Ismail, R.Nielsen, Wellstream Corp.; R.Chandwani, Zentech International Ltd.

Offshore Technology Conference, Houston, May 6-9 1991, Paper Number OTC-6724-MS

G.Cook, C.Timbrell, P.W.Claydon

STRUCENG & FEMCAD Conference, Grenoble, France, 17-18 October 1990

Abstract:
A method is presented for automatically generating meshes containing 3D crack fronts. Further, an adaptive meshing scheme is presented for automatically updating meshes during full 3D fatigue crack growth prediction.
Cracks are introduced into a valid mesh of the intact component by a mapping scheme which replaces standard 20 noded brick elements with 'superelements'. In the context of this paper, the term 'superelement' refers to a set of 20 noded brick elements which models a quarter circular or through crack front using collapsed 'quarter point node' brick elements. Replacement of elements in a mesh allows introduction of one or more distinct crack fronts, with more than one superelement permitted on each distinct crack front. A special mapping controls the crack size in each superelement and maintains original isoparametric surfaces thus creating a valid mesh for the cracked component. Following incremental fatigue crack growth prediction, all superelements are re-mapped to obtain a mesh with the new crack position. The mapping scheme allows non-planar crack growth in 3D geometries.
The procedure has been found to be flexible, efficient and a great time saver in 3D fatigue crack growth analyses. Good agreement with experimental fatigue crack growth data has been demonstrated using this meshing scheme.

G.Cook, P.W.Claydon, C.Timbrell

STRUCENG & FEMCAD Conference, Grenoble, France, 17-18 October 1990

Abstract:
A method is presented for predicting full 3D fatigue crack growth of arbitrary 3D crack fronts under mixed I, II and III mode loading. It uses a generalized 3D virtual crack extension (VCE) method to evaluate the distribution of the local maximum energy release rate, Gmax, along the crack front. A modified Paris equation, relating the local Gmax to the local crack growth rate, is then used to evaluate the magnitude of the crack growth. The direction of local crack growth is taken as the direction of the local Gmax.Good agreement with experimental data is produced for fatigue growth against the number of cycles for an elliptical crack under mode I cyclic loading. Although no experimental results are available, the prediction of full 3D crack growth for a crack front under mixed mode loading is also demonstrated.

P.A.Brown, R.Chandwani, I.Larsen, Zentech Consultants; E.Dasambiagio, Petrobras/Cenpes, Rio de Janeiro, Brazil

SPE Latin American Petroleum Engineering Conference (LAPEC), Rio de Janeiro, Oct. 1990, Paper Number SPE-21191-MS

R.Nielsen, R.S.Colquhoun, A.McCone, Wellstream Corp.; J.A.Witz, Univ. College London; R.Chandwani, Zentech, London

Proceedings of The First ISOPE European Offshore Mechanics Symposium, EUROMS-90, Norway, Aug. 1990, Paper Number ISOPE-E-90-054

P.A.Brown, A.Soltanahmadi, R.Chandwani, I.Larsen

The Institute of Marine Engineers, London, Oct. 1989

Abstract:
During the last seven years, Zentech Consultants have been involved in a number of projects that may be broadly classified within the heading 'Analysis and Design of Flexible Riser Systems'. The basis for their involvement has been the development of an advanced analytical tool for detailed design and hydrodynamic response of these systems. This paper summarises some of the experience gained by Zentech over the last seven year and presents an investigation into optimisation techniques used during design of single and multiple flexible riser systems. An introduction to this subject is given which addresses the reasons behind performing advanced analysis within the design procedure. The purpose and definition of flexible riser systems and the characteristics of flexible pipe are also presented, followed by a discussion of the methodology commonly used within design. The analytical tools required during design are identified and guidelines are given for validation of these software packages. A series of case studies showing typical system analysis are then presented with emphasis on parameters used for system optimisation.

P.A.Brown, A.Soltanahmadi, R.Chandwani

CIVIL-COMP 89, Civil & Structural Engineering Conference, City University, London, Sept. 1989

P.A.Brown, A.Soltanahmadi, R.Chandwani

International Seminar on Flexible Risers, University College, London, January 1989

This paper was originally presented at a meeting on ‘Flexible risers’, held on 9 January 1989 at University College, London, UK and later published in:

Engineering Structures, Volume 11, Issue 4, October 1989, Pages 234-241

M.W.Spiteri, R.Chandwani

Proceedings of Seminar on Subsea Emergency Shutdown Systems, London, 1989

E.Rasten, Seanor Eng., Oslo; E.Lorentzen, Zentech Consultants; L.Bilstad, Statoil, Stavanger

Proceedings of SUBSEA '88, International Conf., London, December 1988

R.Chandwani, D.Mahmood, Zentech Consultants; J.Colloff & D.Daintith, British Nuclear Fuels, Risley

U.K. Proceedings of Conference held at University Of Bristol, March 1988

P.A.Brown, R.Chandwani, I.Larsen

Proceedings of International Conference on Offshore Structures, City University, London, Sept. 1987

Abstract:
An extensive series of 3-D numerical simulations is required in order to design a dynamic flexible riser system. This paper presents an efficient method of solving the corresponding non-linear equation system using a lumped mass finite difference scheme and gives details of the software package developed, namely FLEX-RISER. The verification of this numerical model is discussed and a set of example analyses are presented showing some applications of the package. These examples describe the key modelling points for this type of analysis and show the type of results necessary for the design of such systems. It is concluded that an efficient analytical tool is essential for the comprehensive dynamic design of a flexible riser system.

R.Chandwani, N.D.Nathan

Journal Of Prestressed Concrete Institute, Vol 16 Issue 3 Pages 10-18, May-June 1971