Evaluation of Prestraining and Dynamic Loading Effects on the Fracture Toughness of Structural Steels by the Local Approach

On the occasion of recent great earthquakes, great concern is focused on the prevention of unstable fracture of steel structures against the seismic loading. This paper employs the local approach for the evaluation of prestraining and dynamic loading effects, experienced during an earthquake, on the fracture toughness of structural steels. The prestraining and dynamic loading lead to a similar result: increasing the yield stress and tensile strength and decreasing the fracture toughness. It is shown, however, that the combined effects of prestraining and dynamic loading is not equivalent to the sum of each individual effect. The analysis using the local approach demonstrates that the critical Weibull stress at brittle fracture initiation is independent of prestraining and dynamic loading. Based on the Weibull stress fracture criterion, the prestraining and dynamic loading effects on the fracture toughness can be predicted from static toughness results of the virgin material. As an engineering application, a simplified method is proposed for the estimation of fracture toughness under the seismic condition. This method uses a reference temperature concept: the dynamic fracture toughness at the service temperature T with prestrain is displaced by the static toughness of the virgin material at a lower temperature $T−ΔTPD,$ where $ΔTPD$ is a temperature shift of the fracture toughness caused by prestraining and dynamic loading. The temperature shift $ΔTPD$ is provided as a function of the flow stress elevation in the seismic condition.