Recent publications by a number of authors have investigated the Durability and Damage Tolerance (DADT) of additive manufactured (AM) components, cold spray repairs, adhesively bonded joints / repairs and composite materials using a modified version of the Hartman-Schijve crack growth law to incorporate short crack effects. This modified Hartman-Schijve law has also been referred to as a variant of the NASGRO equation.

The modified Hartman-Schijve growth laws, both stress intensity factor and energy formulations, are available in Zencrack 9.2-1 as "built-in" crack growth law options. This simplifies the use of these laws which, for earlier releases, required a user subroutine to define the growth law.

Publications in this field include:

• Computing the durability of WAAM 18Ni 250 maraging steel specimens
  Daren Peng, Rhys Jones, Andrew S. M. Ang, Alex Michelson, Victor Champagne, Aaron Birt, Samuel Pinches, Sudip Kundu, Alankar Alankar, Raman R. K. Singh
  Fatigue Fract Eng Mater Struct. 2022; 1-11. DOI: 10.1111/ffe.13828

• Assessment of mechanical and fatigue crack growth properties of wire + arc additively manufactured mild steel components
  Muhammad Shamir, Victor Igwemezie, Saeid Lotfian, Rhys Jones, Huzaifa Asif, Supriyo Ganguly, Ali Mehmanparast
  Fatigue Fract Eng Mater Struct. 2022; 45 (10): 2978-2989. DOI: 10.1111/ffe.13797

• Characterising and representing small crack growth in an additive manufactured titanium alloy
  Muhammad Shamir, Xiang Zhang, Abdul Khadar Syed
  Engineering Fracture Mechanics, 253, (2021), DOI: 10.1016/j.engfracmech.2021.107876

• Computing the Fatigue Life of Cold Spray Repairs to Simulated Corrosion Damage
  Daren Peng, Caixian Tang, Neil Matthews, Rhys Jones, Sudip Kundu, R.K. Singh Raman, Alankar Alankar 4
  Materials 2021, 14, 4451. DOI: 10.3390/ma14164451

• Additively manufactured Ti-6Al-4V replacement parts for military aircraft
  R. Jones, R.K.S. Raman, A.P. Iliopoulos, J.G. Michopoulos, N. Phan, D. Peng
  Int J Fatigue, 124 (2019), pp. 227-235, DOI: 10.1016/j.ijfatigue.2019.02.041

• Representing crack growth in additively manufactured Ti-6Al-4V
  R. Jones, J.G. Michopoulos, A.P. Iliopoulos, R.K. Singh Raman, N. Phan, T. Nguyen
  Int J Fatigue, 116 (2018), pp. 610-622, DOI: 10.1016/j.ijfatigue.2018.07.019

Zencrack changes

The values of the NEGATIVE R parameter on *CRACK GROWTH DATA are changed to enable more general use with K and G-based growth laws. The previous DeltaK and Kmax parameter values are now Delta and Max.

A new threshold option, TYPE=delta sqrt(G)th, is added for G-based fatigue growth laws.

User subroutines for fatigue and time threshold (user_threshold_fatigue and user_threshold_time) are modified to allow G-based or Ct-based return values. The material id is also passed into these subroutines to improve their use in multi-material models.

A new ***INFORMATION table is added to the .rep file to report the internal material id (as used in some user subroutines) and the name of the associated material.

New utility functions utility_material_id_from_name() and utility_material_name_from_id() are added to allow access to material id and name lookups from within user subroutines.

A change is made to the interaction between the crack growth law, threshold and failure values. Now, if the crack growth law is K-based, the threshold and fracture toughness values must be K terms. If the growth law is G-based the threshold and fracture toughness terms must be G terms. The exception is in the use of user subroutines where it is possible, with care, to mix K and G-based data.

Changes are made to cater for the use of fracture toughness as a G term rather than a K term for G-based growth laws. Previously the fracture toughness values were always assumed to be K terms.

As a result of the above changes, the threshold and fracture toughness in the input checking phase are reported in more detail than before as the type of parameter must be consistent with the primary parameter of the growth law.

The various user subroutines that return da/dn or da/dt and the ones that return threshold include a check for the returned value being -ve. But there was previously no check for the returned value being NaN. If this happens it can make the integration scheme continue indefinitely because no growth or cycles/time get added and the integration termination conditions are never satisfied. As well as new NaN checks, the existing -ve value checks are made more consistent.

The tangents and normals at the crack front that are sent to the remeshing process are more consistent with the current crack profile. This can lead to better definition of the ring elements. Previously the tangents and normals were carried over from earlier calculations and could be insufficiently accurate in cases where the crack shape changed significantly from one profile to the next.

For a remeshing embedded crack the tangents and normals at the ends of the crack are now made consistent (i.e. previously the two ends of the crack front definition, while at the same physical position, had tangent and normal definitions that were different - those values are now averaged and applied to the two end node positions).

Zencrack GUI changes

Header and footer comments are enabled on the tabs on the GUI Material screen to allow on-screen hints to be included.

Various threshold and basic material options on the GUI screens are enabled or disabled depending upon the selected growth law and retardation status.

The licensed company name is shown in the GUI status bar and reported to the log file.

The About box includes the company name, license type and information about the license server or nodelocked host being used.

Links are added to the Help drop-down menu for the Zencrack web page and Zencrack license terms.

The import option for splines on the Display Tab now offers a range of file type filters, defaulting to *.spl.

The title bar on several file selection screens has been updated to better clarify why a file is being chosen (e.g. 'selection' of a user subroutine, 'opening' of a mesh file).

The 'Close window on completion' option on the Run Zencrack screen is not currently relevant on Linux as any job that is executed runs in the same terminal window that was used to launch the GUI. That option is now removed from the screen on Linux. Also, that option is no longer shown for the Preview run type on Windows as the preview never launches a separate window.

Remeshing changes

The remeshing process now checks the available disk space before remeshing starts and before tet mesh generation during remeshing. If the available disk space is low the process could fail in a way that does not always make it clear that disk space was the issue. To prevent this, the process will now stop if available disk space is below a specific limit - the default is 0.5GB on the disk partition containing the job files. Parameters WarnFreeGB and ErrorFreeGB in the zencrack_gui.ini file can be used to specify the limits for disk space warnings and to stop remeshing respectively. The warning and error checks can also be disabled via these parameters by setting their values to zero.

A change is made to the control of the tet meshing process. If this process appears to take too long or to hang, there are now controls to terminate the process instead of waiting indefinitely for the process to complete. These controls are hard coded in this version:

• The elapsed time after which a still-running tet meshing process is terminated is 18000 seconds (5 hours).
• The percentage CPU usage limit below which the tet meshing process is deemed to have hung is 0.01%.
• The tet meshing process is terminated if 5 consecutive checks of the process have CPU usage below this CPU limit. Checks are made every 5 seconds.