Brittle fracture initiation associated with the strain localization in a heat-affected zone of a low carbon steel
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I.
INTRODUCTION
INITIATION of brittle fracture is the critical stage that determines the fracture toughness of a steel. The criteria for the initiation have been discussed by many authors. Maximum local stress ahead of the notch or crack tip has often been employed as the trigger of an incipient crack.[1,2,3] When the plastic zone extends, the maximum tensile stress has been realized at the elastic-plastic boundary, which moves more distant from the precrack as the applied stress increases.[4,5] It has been shown that the fracture toughness scales with the distance from the precrack front to the brittle fracture initiation site in the brittle temperature region.[6,7,8] However, in the transition region, the plastic zone tends to diffuse and maximum local stress does not increase after the general yield.[5] In the ductile-brittle transition region, where plastic work preceding brittle fracture initiation is substantial, geometrical correlations between the precrack and the initiation sites are not uniquely determined. Ritchie et al. proposed the concept of characteristic distance to take into account microstructural origins of incipient crack formation and showed that fracture toughness, KIC, scales with the square root of the characteristic distance.[8] Lin et al. explained the inconsistency in the grain size dependence of transition temperatures between Charpy and fracture mechanics tests from the statistical distributions of second phases in the plastic zone.[9] In the ductile-brittle transition region, where plastic work preceding to the brittle fracture initiation is substantial, geometrical correlations between the precrack and the initiation sites are not uniquely determined, being controlled by both stress and strain fields.[10] One of the present authors KEN’ICHI YOKOYAMA, Graduate Student, and MICHIHIKO NAGUMO, Professor, are with the Department of Materials Science and Engineering, Waseda University, Tokyo 169, Japan. Manuscript submitted April 1, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
showed in low carbon steels that brittle fracture initiation sites shift to the maximum strain site from the maximum stress site.[11] A critical strain model was also proposed for the ductile fracture at the upper shelf region.[12] In the case of inhomogeneous microstructures with constituents having different mechanical characteristics, constraint of plastic deformation associated with the microstructures gives rise to locally varying stress and strain distributions. The locations of the brittle fracture initiation sites thus should distribute in a complicated way,[11,13] and identification of the initiation sites with respect to microstructures is expected to elucidate the role of plastic deformation on the brittle fracture initiation. Toughness of steel components is normally the lowest at weldments, the microstructures of which are complex and inhomogeneous. In low carbon steels, the microstructure of the heat affected zone (HAZ) of weldments consists primarily of bainitic ferrite, a7B , partially mixed with
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