Effect of the type of prior straining on the fracture toughness of AISI 4340 steel

The effect of prestraining on the JIc value for AISI 4340 steel was examined. Prestraining was achieved either by compression (cold rolling) or in tension. Compact tension specimens machined from prestrained material were used to determine JIc by the J resistance curve method, stretch zone measurements were performed to confirm the JIc values obtained and microhardness tests were carried out to examine the strain distribution. For both types of prestraining, JIc increases significantly for values of prestraining up to 2%. Differences in JIc at 1%, 2% and 5% prior deformation were observed, depending on the type of prestraining. These differences are attributed to inhomogeneous deformation and Lüder's band formation. The results confirm that crack tip blunting due to mobile dislocation mechanisms is responsible for the significant increase in toughness at low values of prestraining.