Observation of crack tip strain distribution for brittle materials by means of photoelastic coating technique and non-linear fracture mechanics.

Abstract

Granite, grahite and mortar are typical brittle materials. Recently, for evaluating the strength of these materials, a study based on fracture mechanics has been suggested. In general a microcrack process zone at the rack tip of brittle materials is created, which may determine the fracture process. So using the photoelastic coating method with a PLZT (transparent dielectric ceramics) plate (100-200 μm thickness), the strain field at the crack tip was directly measured. As a result, it was found that the strain field did not show the singularity of γ-1/2, (γ=a distance from crack tip), as the strain field of linear elastic materials. When the strain distributions are normalized by J integral and the tensile stress σult, they were expressed by a form of Δe=Δe0((J/ σult)/γ)m : thus it is found that the strain field within the process zone can be described by the J integral, where Δe and m are material constants, and m is near 1. The strain field at the crack tip of the brittle materials is proportional to approximately γ-1. The extension of the process zone, denoted by ω, is closely related to the J integral. The relationship between ω and the J integral can be expressed as a bilinear line. It is shown that the J integral at the knee point is equal to the critical J integral, value determined by AE technique. Thus the J integral can be an important parameter to evaluate the fracture behavior of brittle materials.