細束X線回折法によるへき開破面近傍の塑性ひずみ測定

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Tensile tests were performed on cracked low-carbon steel plates at low temperatures. The plastic strain distribution near the cleavage fracture surface at the initiation site was measured using the X-ray microbeam diffraction technique. The fracture criterion for cleavage fracture initiation and the quantitative nature of plastic strain distribution are discussed. An attempt was made to correlate the fracture toughness with the microscopic fracture criterion and some mechanical properties. The results obtained are summarized as follows: (1) The equivalent fracture strain ε_f, which was defined as the plastic strain measured on the original fracture surface at the initiation site, was interpreted as the average strain over the region of the structural size ρ_s near the fracture surface.ρ_s was found to be a material constant for the initiation of cleavage fracture.ε_f was correlated to the deviation from the cleavage-to-fibrous transition temperature Td, independently of the specimen geometry and loading mode. (2) The plastic strainε^^-_p concentrated within a very localized region near the fracture surface. The distribution of plastic strain was expressed by the following equation: ε^^-_p=(ρ_s/r)^<1/(1+n)> ε_f where n is the strain-hardening exponent and r is a distance from the fracture surface. This singular behavior of plastic strain was in good agreement with the theoretical analyses of the plastic strain singularity in a hardening material. (3) The critical value of J integral J_C and the plane strain fracture toughness K_<IC> could be expressed in terms of the microscopic quantities by J_C=(2.67+2.67n+215n^2)Fρ_sε_f^<(1+n)> and K_<IC>=2.67(EF(1+n) Fρ_sε_f^<(1+n)>)^<(1/2)>. The plastic strain distribution near the cleavage fracture surface and the fracture surface and the fracture toughness parameters could be derived from the equivalent fracture strain. It was concluded that the equivalent fracture strain is the feasible, practical fracture criterion for cleavage fracture initiation.
社団法人日本材料学会の論文
1974-01-15

細束X線回折法によるへき開破面近傍の塑性ひずみ測定

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