Aerospace

Zentech has expertise in the aerospace sector in engine, airframe and Space Transportation System (STS) payload applications. Historically the work has usually involved metallic components and the simulation of crack growth in those components. For engine applications, high temperature effects and complex load histories contribute to the difficulty of such simulations.

Although the usage of Additive Manufactured (AM), Cold Spray AM (CSAM) and Cold Spray Repair Technology in the aerospace industry, both defence and civil, is increasing steadily, so far it has remained mainly in low stress regions of an aircraft such as stabilators, fins and other control surfaces. This is to prevent any delamination inherited during manufacturing process from propagating further and affecting the operating safety and integrity of the aircraft. It is clearly stated in the US MIL Standard MIL-STD-1530D (1) and the NASA Standard NASA-HDBK-5010 (2), that it is a mandatory requirement that in order to use additive manufactured components or repairs in high stress areas, Durability And Damage Tolerance (DADT) analyses based on linear elastic fracture mechanics (LEFM) principles should be performed and certified.

It has been shown (3-5) that it is possible to perform Durability And Damage Tolerance (DADT) analysis using a small crack (sub mm) propagation law as postulated by Hartman-Schijve via their modified NASGRO equation. This law, implemented as a standard option in Zencrack, can also be used for assessment of DADT of composite materials, adhesive lap joints and naturally occurring discontinuities due to corrosion.

Zencrack load spectrum capabilities for fatigue crack growth are based on loading described in terms of applied load cycles. The implementation caters for simple constant amplitude loading through variable amplitude spectrum loading to complex thermo-mechanical load cycles in which on-the-fly rainflow counting can optionally be applied. For complex loading history that includes random load types such as may be required in space applications, methods are required to estimate cycle counts over the duration of the random loading sequence. As indicated in NASA Standard NASA-HDBK-5010, VOLUME 1, REVISION A (15 Dec 2023) (6), “an acceptable industry standard for narrow band random vibration processes is to assume that the probability distribution of the stress reaching a certain level during a time period at a certain frequency follows a Rayleigh probability distribution and the total number of cycles is simply the product of the event time duration and the natural frequency.” The random loading is therefore represented by equivalent spectrum loading. Other approaches, for example those documented in NASGRO User Manual, Appendix H (Loading Spectra for Acceptance Vibration Tests), provide alternative methods to estimate an equivalent cycle count at the maximum stress level for various types of vibrations tests, thus allowing an equivalent loading to be included in the Zencrack load spectrum sequence.

Project work includes:

• a range of finite element analysis projects from internal engine components and casings to landing gear
• development of Zencrack software arising from initial consultancy work to develop stand-alone routines for crack simulation
• development of high-temperature thermo-mechanical solutions for combined fatigue and time dependent crack growth simulation
• investigation of the influence of cracks in additive manufactured components and adhesive repairs
• investigation of crack growth from the root of surface pits

Consultancy work in this area involving crack simulation is supported by our proprietary Zencrack software.