Nucleation threshold stress for the dynamic fracture of a low-alloy NiCr steel

Methods of characterizing the onset of fracture with stress waves are described with emphasis on a technique that allows a determination of the crack nucleation threshold stress, σ no , of material without plastic shock conditioning. With this approach, the tensile stresses in regions of failure were obtained with the aid of the BFRACT 2 computer subroutine based on the nucleation and growth fracture model NAG/FRAG. Computations with this subroutine automatically account for the unloading of stress as voids develop and allows one to determine approximately the tensile stress responsible for failure. Crack densities created with plate impacts were plotted vs. the corresponding computed maximum tensile stresses, and the curve was extrapolated to the no-damage condition, i.e., the stress below which no cracks should develop. This stress level was equated to the nucleation threshold stress. The threshold stress determined for the steel investigated was 1.65 GPa. This value is representative of the material without plastic shock conditioning because of the extrapolation. The nature of the cracking, the effect of shock conditioning on σ n0 , the nucleation rate at low stress levels and the relation of the crack nucleation threshold stress to the Hugoniot elastic limit are described.