Dynamic Fracture Behavior in HSLA Steel

The influence of pre-straining process on dynamic fracture behavior of Cu-strengthened high-strength low-alloy (HSLA) of varied amount of pre-straining has been investigated in the current study. The influence of pre-straining on deformation due to tensile loading and on ductile fracture behavior of the steel has been carried out with the help of instrumented Charpy test and standard fracture test. Study of microstructure, its hardness, and tensile properties has been done to understand and correlate these properties with Charpy and dynamic fracture behavior of the investigated steel. Prior to the machining of the specimen, the stock materials are pre-strained at different levels. All the Charpy V notch (CVN) specimens are fatigue pre-cracked to generate natural crack. The pre-crack length was restricted to 5 mm (a/w = 0.5). Force-time behavior curve and the force-displacement curve is determined by double numerical integration of force-time curve. In the investigated steel, normalized energy increased slightly up to a level of 2% pre-strain and then decreased by increasing pre-strain level greater than 2%. The dynamic fracture toughness first increases up to 2% and then decreases beyond 2% pre-strain indicating the influence of pre-strain on dynamic fracture toughness behavior of steel. The fracture surface of investigated steel has been examined under scanning electron microscopy. This study has been also focused to understand (i) The nature of the stretch zone-a featureless band that forms prior to initiation of ductile fracture and (ii) the micromechanism of slow stable crack growth.