Investigation of the Fatigue Life Scatter for AA7075-T6 Using Crystal Plasticity Finite Element Method in the High to Very High Cycle Fatigue Regime

Fatigue indicator parameters (FIPs) serve as the measurement of the driving force for fatigue crack initiation (FCI). The extreme value distributions of FIPs for fatigue life scatter (i.e., fatigue crack initiated from surface or subsurface) in the high cycle fatigue (HCF) to very high cycle fatigue (VHCF) regime are computed of AA7075-T6. The FIPs are averaged in the volume of sub-band in the scale of sub-grain proposed based on the concept of slip system of AA7075-T6 where sub-bands are extracted from statistical volume elements (SVEs) using open-source called Dream-3D. SVEs are built via the experimental crystalline microstructure. The material response variables required in the computation of FIPs are obtained by embedding crystal plasticity theory to the crystal plasticity finite element model via open-source software PRISMS-Fatigue. To meet statistical requirements, 50 instantiations of SVEs are modeled for both traction-free and fully periodic boundary conditions, respectively, corresponding to the surface and subsurface cases under different strain amplitudes (i.e., 0.362%, 0.312%, 0.289%, and 0.254%). It is found that the FIPs are higher under high stress levels indicating that fatigue crack tends to initiate at high stress levels. In addition, the low driving force can be found in the simulations with fully periodic boundary conditions throughout the SVEs, whereas the higher FIPs under traction-free boundary conditions tend to form close to the free surface under cyclic loading conditions. Furthermore, the highest FIPs have a propensity to scatter along with the depth of SVEs with the traction-free surface under strain amplitude of 0.254%, which means the fatigue life in the VHCF regime has a higher scatter. The interactions between the high symmetry of FCC crystalline structure and free surface combining the loading amplitudes have a compound effect on the FIPs, especially at low stress amplitudes, which also has a greater impact on the ultra-high-cycle fatigue life. To this end, the VHCF and HCF experiments are performed under an ultrasonic fatigue experimental system with a frequency of 20 kHz with different stress levels corresponding to the strain amplitude in the simulations. Besides, the morphology fracture surface of presentative samples was observed via the scanning electron microscope (SEM). The comparison between the simulation and the experimental results indicates that the EVD of FIPs is efficient in preciously predicting the FCI and well explain the fatigue life scatter of the AA7075-T6 in the HCF to VHCF regime.