The effect of specimen thickness and temperature on plastic yielding and fracture properties of mild steel

Abstract

The effect of thickness and temperature on deformation and fracture behavior is investigated in notched bend specimens of mild steel in low temperature region using a photo-elastic coating technique. The plastic constraint factor is determined and compared with the theoretical value obtained by slip line field theory. The difference of general yielding properties between surface and interior of specimen is examined by the observation of yield propagation. From these results, a qualitative picture of distribution of σ z (the stress of thickness direction) is deduced. The effect of thickness on the plastic constraint factor and the plastic stress concentration factor R at T GY (the critical temperature at which general yielding and fracture coincide) is discussed and so-called “thick specimen” is defined in cleavage fracture region. Then, the thickness effect on twin formation is studied. It becomes clear that the critical temperature T t above which no twins are formed can be determined by the constant value of the plastic constraint factor. Finally, the relation between the plastic constraint factor, specimen thickness and temperature is embodied by the three dimensional diagram and the condition of plane stress is pointed out.