Load History Effects on the Fracture Toughness of a Modified 4340 Steel

The effect of load history on fracture toughness was examined for a modified 4340 steel where toughness was evaluated in the elastic-plastic regime by the J_R-curve test method. Four types of load histories were applied: (1) cyclic loading during R-curve development; (2) fatigue precracking at high load levels; (3) bulk prestraining, both monotonic and cyclic; and (4) slow loading during the R-curve test. These load histories were applied in four separate sets of tests. The resulting J_R-curves were compared with R-curves generated for specimens tested in a normal monotonic toughness test. Raising the R-curve was judged as an increase in toughness and lowering it as a decrease. The results showed that (1) the R-curve developed during a cyclic load history has both a monotonic component and a cyclic component, (2) fatigue precracking at high load levels does not affect the R-curve, (3) monotonic bulk prestraining lowers the R-curve whereas cyclic bulk prestraining raises it, and (4) the R-curve may be lowered by decreasing the monotonic loading rate.