Special Issue on Fracture Mechanics. Effect of Strain Rate on Elastic-Plastic Fracture Toughness in the Transition Temperature Region.

Static and dynamic fracture toughness tests at three strain rates were performed on two ASTM A508Cl. 3 steels by using 1T-CT specimens and fatigue pre-cracked instrumented Charpy specimens. The KJC converted from JC of the small specimens indicated a wide scatter. When the strain rate increased, the fracture toughness transition curves shifted to a higher temperature region. An increase in strain rate reduced the scatter of KJC dramatically, especially in the high temperature region, and decreased the lower bound fracture toughness. Fractographic examination of the fractured specimen surfaces indicated that the relationships between KJC and the stable crack growth, Δa0, distance from stable crack front to trigger point, X, or distance from fatigue crack front to trigger point, Δa0+X, were expressible by a single curve, respectively. The scatter of KJC was caused by the varience of Δa0, X, and Δa0+X. With increasing strain rate, Δa0, X, and Δa0+X decreased significantly, leading to a small scatter of KJC. The KJC value at Δa0=0, X=0, and Δa0+X=0 was proposed as the lower bound fracture toughness of a steel and labeled KJCi. The shape of KJCi versus temperature curve was controlled by the critical stretch zone width, SZWc. The Weibull slope m of KJC became larger with increasing strain rate and decreasing temperature. Higher toughness data with larger stable crack extension than 100μm violated the linearity of Weibull plots. In the statistical approach to determine the lower bound fracture toughness in the transition region, much more analytical development is needed. The KJC value with 3% fracture probability coincided with the KJCi value in the low temperature region even in a high strain rate.