Fracture initiation under pure shear revisited: remarks on the mode II fracture in quasi-brittle materials

Abstract Mode II fracture, as one basic fracture mode, remains an interesting topic in fracture mechanics. However, its realization is a challenge in the laboratory, especially for quasi-brittle materials. In this study, two types of specimens were tested: the angled edge crack semi-circular specimen under three-point bending (SCB) and double edge notched compression (DENC) specimen. Digital image correlation (DIC) was used to measure the displacements surrounding the notch tip, which helps to identify the fracture mode. Based on experimental results, this study (i) confirms that a pure shear stress condition is achieved at the notch tip before the fracture initiation; (ii) shows that the initiated fractures are all mode I/opening fractures, not mode II/sliding fractures. More importantly, detailed experimental analyses demonstrate those mode I fractures are actually under the influence of shear load, but they are not the mixed-mode fractures. Experimental evidence is the observation of non-symmetric pattern of opening displacements. With the presence of a fracture process zone with a finite size, it is interesting to observe that shear load cannot produce the slip during fracture initiation, and only makes orthogonal displacements non-symmetric. The phenomenon represents the challenges to understand fracture initiation under large scale yielding. In this study a parameter η is introduced, i.e., the ratio of opening displacements along two fracture surfaces, to quantitatively evaluate the influence of shear load. Although parameter η does not present actual value of shear load, its value change indicates the change of shear load such that fundamental information about shear influence can be obtained. The study shows two different tendencies of parameter η change during fracture initiation, which indicates the consequences of the SCB and DENC specimens. After the examination of various fracture criteria, DIC comparisons of experimental results reveal an interpretation: (i) for the SCB specimen, pure shear condition cannot be maintained once fracture initiates, which refers to the “weak” condition. This “weak” condition creates a fracture initiation following the criterion of local symmetry, because the parameter η changes from a finite value of larger than 1 to almost 1, suggesting local shear influence is gradually removed; (ii) for the DENC specimen, pure shear is maintained after fracture initiates, which refers to the “strong” condition. For the “strong” condition, parameter η increases from a finite value of larger than 1 to an infinite value, which suggests shear influence increases, such that it does not obey prediction from most current fracture criteria such as MTS criterion. In addition, fracture pattern from the “weak” condition is quite simple; but the “strong” condition results in a structural response so that a complex fracture pattern is created.