Mixed mode I/II fatigue crack growth under tensile or compressive far-field loading

This study investigates the rate and path of fatigue crack growth under mixed mode I/II loading in the presence of tensile and compressive stresses. Compact tension shear (CTS) specimens made of 42CrMo4 steel are subjected to pure mode I, pure mode II, and mixed mode I/II loadings. Fatigue crack growth rates are determined for a number of KI/KII ratios and compared with each other. Paris' law constants are found out for different modes of loading on the material. Crack growth paths at various loading angles, in the presence of tensile and compressive stresses, are analyzed and the specimens fracture surfaces are examined. In order to determine the stress intensity factors (SIFs) at different modes, finite element simulation of the CTS specimen is adopted. Using the SIFs yielded by simulation, the crack growth angles are obtained by a number of criteria and compared with the experimental results. At mixed mode loading involving compressive stresses, Richard's criterion for the determination of crack growth angles gets improved. As a result, by determining Paris' law constants and the coefficients of modified Richard's criterion, the material behavior in mixed mode I/II fatigue involving compressive stresses is known. The specimens fracture surfaces reveals the wear of the surfaces under compressive loads.