A comparative fracture study of a slightly prestrained low alloy steel and a slightly prestrained austenitic stainless steel

Abstract

The effect of slight prestraining on the fracture toughness (as expressed by the J integral) of a low alloy steel and an austenitic stainless steel has been studied. Fracture toughness tests were performed on AISI 4340 steel and AISI 310 steel, prestrained from 0% (annealed) to 15%. The dislocation structure and distribution were examined on annealed and prestrained materials using transmission electron microscopy. While a significant increase (fourfold) in the fracture toughness was found to occur at 2% prestrain followed by a sharp decrease above 2% prestrain for the low alloy steel, there was a continuous decrease in the fracture toughness of the austenitic stainless steel as the prestrain increased from 0% to 15%. This significant increase in the fracture toughness of the low alloy steel is attributed to the availability in the crack tip region of a large number of active Frank-Read sources which emit dislocation loops that blunt the crack as they expand in their slip bands and are pushed into the crack tip. The continuous decrease in the fracture toughness of the austenitic stainless steel is explained by a higher work-hardening rate exhibited by this material, where numerous Lomer-Cottrell locks prevent any effective blunting of the crack tip. This is in contrast with the low alloy steel, in which cross-slip and multiple cross-glide lead to a multiplication of slip lines intersecting the crack tip and, thus, increase the extent of blunting of the crack tip. It is concluded that the mobile dislocation density and the extent of work-hardening rate in the plastic zone are the two most influential parameters controlling the fracture toughness in materials that exhibit significant plasticity prior to fracture.