Cyclic Mixed-Mode Crack Propagation Calculations for Aircraft Engines: An Overview of the last Decades
Guido Dhondt, MTU Aero Engines AG, Germany
Crack propagation calculations play a major role in the life evaluation of aircraft engine components. Nowadays, considering mixed-mode crack extension subject to time-varying low and high cycle fatigue is state of the art. It took, however, a long time to arrive at that point. Based on the theoretical derivations of Inglis and others at the start of the 20th century and thanks to the availability of ever faster computers, numerically evaluated K-factors were obtained in the sixties and seventies. In the eighties first attempts were undertaken to calculate Mode-I cyclic crack propagation in an arbitrary geometry with the Finite Element method. In the next decade mixed-mode propagation was tackled. The first code at MTU used a cutting procedure for hexahedral elements to insert the crack. Collapsed quarter-point elements at the crack tip guaranteed good-quality K-factors. Next, in order to increase the flexibility, a combined hexahedral-tetrahedral approach was favored, creating a tube around the crack tip filled with collapsed hexahedral elements and taking care of the remaining mesh by an automatic tetrahedral mesher. The application field was gradually extended from simple low cycle fatigue calculations for a given load cycle to combined LCF-HCF loading for complex missions with time-varying mode-mixity and temperature. Furthermore, the complex three-dimensional finite element tool was complemented by simple very fast tools for preliminary analyses.