Observation of Crack Tip Strain Distribution for Brittle Materials by Means of Photoelastic Coating Technique and Non-Linear Fracture Mechanics

Granite, granite 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 crack 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 σ_ull they were expressed by a form of Δɛ=Δɛ₀ ((J/ σ_ull/r)^m; thus it is found that the strain field within the process zone can be described by the J integral, where Δɛ 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.