Fracture process of a low carbon low alloy steel relevant to charpy toughness at ductile-brittle fracture transition reg
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INTRODUCTION
THE Charpy impact test has been widely employed for assessing steel properties. The toughness of a steel evaluated by the Charpy test is expressed in terms of the fracture appearance and energy transition temperatures or the absorbed energy at a test temperature. The origin of the absorbed energy is complicated, as it involves not only the energy leading to the brittle crack initiation but the strain energy stored by the overall deformation of a specimen and the energy dissipated during the crack propagation. One of the safety design criteria against the brittle fracture is empirically the minimum absorbed energy at a service temperature at which plane strain fracture is dominating, lu In most failures of steel structures, however, the brittle unstable fracture takes place after a substantial plastic deformation around the notch root and often after a stable crack growth. In these situations, fracture mechanics parameters such as the crack-tip opening displacement (CTOD) f21 or J integral I~l have been adopted, and the crack growth resistance as well as the brittle fracture initiation energy has been evaluated in terms of the J integral.141 These parameters have also been utilized for elastic-plastic fracture mechanics analyses of the stress and strain fields. 15-81In such flows of concepts, although a good correlation was empirically revealed between Charpy V-notch energy (CVNE) and the uppershelf Kzc, 191the significance of Charpy energy across the whole ductile-brittle region must be re-examined. Particular emphasis should be made on various fracture processes involving plastic deformation and ductile crack growth with respect to the toughness levels that vary with test temperatures. T. TANI, formerly Graduate Student, Department of Materials Science and Engineering, Waseda University, is with Kobe Steel Corporation. M. NAGUMO, Professor, is with the Department of Materials Science and Engineering, Waseda University, Shinjuku, 169 Tokyo, Japan. Manuscript submitted January 5, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
Comprehensive studies have been accumulated so far on the effects of microstructures such as grain size, 11~ preferred grain orientation, Ij~l and grain boundary impurity concentrations Ij2,~3J on Charpy characteristics. However, understanding the mechanisms of these microstructural effects has not yet been established in the recent analyses of mechanics at the notch front. Most previous considerations have focused on the triggering of the brittle fracture. The grain size dependence of toughness was ascribed to the crack size in the Griffith equation, I1~ while temper embrittlement was based on the decrease of the cohesive strength at the grain boundary.l~3~ Anisotropy of toughness in a control-rolled steel plate was correlated to the development of {100} texture on the fracture plane to facilitate the cleavage fracture propagation, t~41 In these considerations, the existence of a critical local stress is a presumed criterion of the brittle fracture initiation. It should be
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