Spall fracture of two high strength armor steels

This study examines the dynamic fracture behavior and spall strength of a high hardness armor (HHA) steel and an improved rolled homogenous armor (IRHA) steel. Flyer plate impact tests were conducted at three different velocities, which provided peak stresses of 3.75 GPa causing no damage, 4.72 GPa that generated incipient spall damage, and 8.47 GPa which resulted in full spall, respectively. Free surface velocities were measured by Photon Doppler Velocimetry (PDV), and the damage examination was conducted by conventional optical microscopy and scanning electron microscopy (SEM). Results show that HHA specimens exhibited higher spall strength and Hugoniot elastic limit (HEL) than IRHA specimens at the same peak compressive stresses. Post-mortem examinations revealed that the HHA steel showed brittle fracture indicated by cleavage seen on the fracture surface and crack propagation through the thickness. In contrast, a more ductile fracture indicative of void growth and coalescence mechanisms was observed throughout the fracture surface of IRHA.