High strain-rate fracture and failure of a high strength low alloy steel in compression

The deformation and fracture of quench-hardened and tempered AISI 4340 steel samples at high strain-rates were investigated. The steel samples were quench-hardened and tempered at 205, 315 or 425°C for 1h before they were subjected to high velocity impact testing. Irrespective of the tempered condition of the martensitic steel samples, failure occurred predominantly by ductile shear fracture along the path of heat waves generated in the material during impact. Adiabatic heating and the associated thermal softening were found to play a more dominant role in the deformation and fragmentation of the samples than the dislocation multiplication effect of plastic deformation. Rather than the anticipated cleavage fracture or shattering of the quench-hardened steel samples into several fragments, the test specimens showed clear evidence of ductile shear and knobby fracture modes for all tempering temperatures. The elongated dimples observed on the fracture surface of the samples tempered at 205°C show a higher aspect ratio than with the other samples. This would indicate that temperature rise along the path of the heat waves and the accompanying stress-drop appears to favor the observed dimpled fracture over the expected cleavage fracture.