Mixed mode crack paths in terms of plastic stress intensity factors based on conventional and strain gradient plasticity

Abstract

The effect of an initial mode II loading on subsequent mixed-mode crack path 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. An optical microscope and the drop potential method were used to determine the actual position of the crack tip on the curvilinear crack path monitoring and continuous measurements of the crack size were made along the path in the CTS specimen. In accordance with these experimental observations, the main feature of the initial mode II fracture is the crack growth on a curvilinear path. For subsequent mixed-mode crack propagation, the crack front continuously changes the shape and direction with each loading cycle. For a mode II loaded initial crack, the scenario becomes more complex because of the change in the propagation direction when a kinked crack is formed. The experimental data on crack growth for all tested materials are represented in terms of the elastic and new plastic stress intensity factors (SIFs). To this end, both the classical and the conventional mechanism-based strain-gradient plasticity (CMSGP) theories were employed. The material constitutive equation is implemented in a finite element code and the elastic and plastic fracture resistance parameters are calculated as a function of the position along the curvilinear crack path in all tested CTS specimens produced from 34X and P2M steels, 7050 aluminum and Ti-6Al-4V alloys. As a result of the finite element calculations, nonlinear amplitude factor solutions are determined across a wide range of material work hardening conditions, and the role of the Taylor’s intrinsic material length and both the coupled effect of these parameters are established. The potential of fundamental and practical applications of introduced fracture resistance parameters are discussed.