The effect of dynamic strain-ageing on the ductile fracture process in mild steel

Abstract A study has been made of the effect of dynamic strain-ageing on the initiation and propagation of ductile fracture in mild steel. Grain boundary carbide cracking has been found to contribute to void formation in the temperature range 77°K to 573°K. Using scanning electron microscopy it has been shown that this crack density increases with strain. The rate of increase of crack density with strain is greatest at 77°K but deformation under dynamic strain-ageing conditions also leads to an increased rate of crack initiation over that observed at room temperature. These results have been interpreted in terms of the effectiveness of slip lines as stress concentrators increasing at low temperatures and under conditions of dynamic strain-ageing. Examination of fracture surfaces has revealed no change in fracture mode associated with ‘blue brittleness’ although there is some indication that voids link up easily under dynamic strain-ageing conditions. It is suggested that the low ductilities observed in the ‘blue brittle’ region occur because of a change in the work hardening characteristics of the material leading to the onset of necking instability at an early stage in the stress-strain curve.