The paper describes the results of preliminary experiments to examine the effect of time-dependent plasticity on ductile crack growth, resistance to ductile tearing, and instability in laboratory fracture toughness tests. Experiments on a carbon-manganese (C-Mn) steel and two specimens of A533B-1 steel have shown that unstable ductile fractures can be induced at ambient temperature when 20- and 40-mm-thick compact specimens are held at constant loads between the load approximating that for crack initiation and the maximum load attained in a conventional displacement-controlled fracture toughness test; the time to failure after holding at constant load decreases with increasing constant load level and increasing strain rate prior to load-holding. The data suggest that final failure occurs under conditions paralleling “tertiary creep” behavior. Specimens were loaded at typical rates used in “static” fracture toughness testing and at a slower rate where the maximum load condition was reached in approximately 24 h. Crack extension under constant load has been related to J-resistance curves derived from multispecimen displacement-controlled tests.

The J-resistance curve for C-Mn steel specimens is dependent upon strain rate at rates of loading which are conventionally taken to be representative of “static” conditions. This effect was not observed in tests on A533B-1 steel.