Determination of fracture toughness from stretch zone width measurement in predeformed AISI type 4340 steel

The fracture toughness parameter JIC was determined as a function of prior cold working in AISI type 4340 steel using two different approaches. One approach is the J resistance curve method. The other approach is from measurements of the stretch zone width which is determined by scanning microscopy on the fractured specimens used in the determination of the J resistance curves. Both techniques agree in showing that, up to a prior deformation corresponding to 2%, there is a marked increase in JIC. At higher values of prior deformation, JIC decreases with the increase in prior work hardening. The results are explained in terms of excessive blunting at the crack tip, which may be due to the availability of excess mobile dislocations at low values of prior strain.