Fatigue Crack Growth Rate Models for Variable Amplitude Loading: Novel Model Formulation, FE Implementation, and Benchmark Experiment

  • Jensen, Simon Mosbjerg (Aalborg University)
  • Carreras, Laura (University of Girona)
  • Bak, Brian Lau Verndal (Aalborg University)
  • Lequesne, Cedric (Siemens Digital Industries Software)
  • Xiong, Hu (Siemens Digital Industries Software)
  • Lindgaard, Esben (Aalborg University)

In session: - Fatigue

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Delaminations often initiate and propagate in laminated composite structures under variable amplitude (VA) loading conditions during operation. However, the VA loading conditions are far from the idealized constant amplitude loading conditions subjected to test specimens during fatigue characterisation, and state-of-the-art models neglect the effects of VA loading although this is highly non-conservative. VA loading leads to transient fatigue crack growth phenomena following amplitude step changes, i.e. transient phases of significantly increased crack growth rate, which have recently been discovered in two-level block amplitude loading experiments [1]. This conference contribution will present: A novel formulation of crack growth rate models with transient crack growth capabilities [2]; a Simcenter SAMCEF implementation of the new model in a finite element (FE) formulation as an extension to the interlaminar fatigue damage cohesive zone model in [3]; and a new benchmark experiment providing unique high-precision crack length and crack growth rate measurements under multi-level block loading. The transient crack growth rate are well-described by exponential decay functions and relationships between governing model parameters and fatigue load quantities have been derived. The transient exponential decay functions are well-integrated in the FE formulation of fatigue cohesive zone models using the cycle-jump approach, and the implementation is numerically validated. The experimental benchmark example uses a pure moment loaded double cantilever beam specimen subjected to multi-level block loading with real time control of the applied energy release rate. The FE simulated benchmark example using the Simcenter SAMCEF solver shows a significant improvement in the delamination growth prediction in comparison to the crack growth rate models that neglect transient phenomena. The work is part of the UPWARDS EU H2020 project (Grant No. 763990).