Abstract
The fracture toughness of four kinds of structural steels with relatively high fracture elongation at room temperature was evaluated by the electrical potential method and by other methods in the range between room and liquid nitrogen temperatures. The results obtained are summarized as follows:(1) At room temperature, the (JIc)E value obtained by the electrical potential method is very close to the (JIc)R value obtained by the R-curve method. The (JIc)S value obtained by the SZW (stretched zone width) method is 15% higher than the (JIc)E value.(2) In room-temperature test, the crack initiation detected by the infection of electrical potential curves corresponds to the stage where the formation of type II dimple and the internal necking occur. Prior to the crack initiation, voids are formed in front of the stretched zone.(3) The value of JIc increases with decreasing the testing temperature because of an increase in yield strength, although it decreases drastically when cleavage fracture occurs at the precrack tip.(4) The value of JIc, CTOD (δ) and KIc are related with each other by the following equations.JIc=1.79σy•δ for ductile fractureKIc=0.81√ for brittle fracture
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