Cleavage Fracture in Prestrained Mild Steel

Notched bars, subjected to varying amounts of uniform prestrain, have been fractured over a range of low temperatures, to examine the effects of prestrain on cleavage fracture. The parameter studied mainly has been the local tensile stress required to produce cleavage fracture below the notch and it has been. shown that this is increased by prestrain, although the effect of such strain was to increase the transition temperature. Detailed metallographic examination failed to show any features to which the increase in fracture stress could be primarily ascribed and a new interpretation of ‘plastic work’ energy terms is suggested. Delaminations caused by heavy prestrains are related to grain-boundary weakening and to residual stresses.