Distinctive features of crack growth rate for assumed pure mode II conditions

•Effect of initial mode II loading on mixed-mode fatigue crack growth was investigated.•Initial mode II has acceleration and retardation effects according to material properties.•New dimensionless parameter was introduced to quantify cyclic fracture resistance. The effect of an initial mode II loading on subsequent mixed-mode fatigue crack growth was investigated through experiments and computations for 34X and P2M steels, 7050 aluminum, and Ti-6Al-4V alloys in a compact tension shear (CTS) specimen. The experimental data on crack growth rate for all tested materials are represented in terms of elastic and plastic stress intensity factors (SIF) and total strain energy density (SED). To this end, the elastic–plastic fracture resistance parameters are calculated by finite element computation as a function of the position along the curvilinear crack path. Careful observation of both the crack path and the crack growth rate behavior revealed that the initial mode II loading has two contrasting effects on the subsequent mixed-mode crack growth, which depends on the elastic–plastic material properties. For each material, a comparison of the experimental results on the crack growth rate for pure mode 1 and mixed-mode fracture for specimens of the same geometry is presented. On this basis, a new dimensionless cyclic fracture resistance parameter in terms of elastic–plastic SIFs and total SED is proposed to establish the effect of mixed-mode loadings on the crack growth rate. Crack growth rate acceleration or retardation was observed because the initial mode II loading with respect to the crack growth rate under pure mode I was attributed to the plastic properties of the tested ductile steels, aluminum, and titanium alloys. This study shows that the mode I crack growth can be considered a predominant factor for evaluating the effect of an initial mode II loading in CTS specimens for subsequent mixed-mode cyclic fractures of different structural materials.