Since Griffith (1921), the fracture energy, which is the basic material property of both the linear elastic fracture mechanics (LEFM) and the cohesive crack model (CCM), has been regarded as a material constant. The constancy of the fracture energy has been supported by matching analytical and numerical solutions to extensive fracture test data. However, it so happens that, near the fracture process zone (FPZ) of the previous fracture tests of all types, the crack-parallel normal stress, both in-plane and anti-plane, has been negligible. This prevented detecting the interaction of such stresses with the FPZ, whose width is always finite. To investigate such an interaction, we developed a simple yet effective test of notched beams with three crucial features: (1) plastic supporting pads with near-perfect plastic yielding  first introduce notch-parallel compression; (2) rigid end supports are installed with gaps and engage only after the supporting pads are plastically yielding, which (3) delivers a support system that switches from one statically determinate configuration to another, thus allowing unambiguous interpretation. Thanks to this novel set-up, tests of a quasibrittle material---concrete, at different crack-parallel compression levels, undermine the century-old concept of constancy of  fracture energy, for the case of quasibrittle materials. They show that notch- or crack-parallel compression can double the apparent fracture energy of concrete or reduce it to near zero. This is computationally corroborated by the finite element (FE) crack-band model with microplane damage constitutive law M7, in which, importantly, the FPZ has a finite width and is governed by a tensorial microplane constitutive law rather than a single scalar. The applicability of LEFM, CCM (based on which, e.g., the extended finite element models XFEM and scalar phase-field models were built) thus becomes severely limited. The main conclusion is that the FPZ must be considered to have a finite width and be modeled tensorially, in a way that captures the splitting, interlocking,  dilatancy of slip on planes inclined with respect to the crack path, and buckling of slabs between microcracks. Similar behavior may be expected for shale and other rocks, fiber composites, tough ceramics, sea ice, stiff foams, wood and all other quasibrittle materials, including metals on the micrometer scale.

New Perspective of Fracture Mechanics Inspired by Novel Tests with Crack-Parallel Compression

The Virtual Technical Meeting of the Society of Engineering Science 2020 is the first fully virtual meeting of the SES community to envision and connect during the hiatus created by Covid-19, providing a platform for the exchange of ideas, and to showcase the latest research in the field of engineering science.

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