High-velocity fragmentation and spall fracture of steel AF9628

Abstract

This paper investigates the mechanics of high-velocity fragmentation and spall fracture of steel AF9628. For this purpose, we have conducted an experimental campaign comprising 25 ring expansion tests and 36 planar plate impact experiments utilizing a single-stage light-gas gun, resulting in the largest and most comprehensive investigation to date on the dynamic fracture properties of AF9628. The ring expansion tests involve the axial impact of a conical-nosed cylindrical projectile on a stationary thin-walled tube, over which the specimen is inserted. The cross-section of the cylindrical part of the projectile exceeds the inner diameter of the tube, prompting expansion of the sample as the projectile advances, ultimately leading to the formation of multiple necks and fractures across the circumference of the ring. The experiments were documented using two high-speed cameras to capture time-resolved insights into the specimen’s deformation and fracture mechanisms. The video footage was synchronized with a photonic Doppler velocimetry system to measure the time evolution of the radial speed of the ring, thereby establishing a correlation between the nucleation of necks, the formation of fragments, and the actual strain rate in the specimens, which ranged from $\approx 8000{\text{s}}^{-1}$$\approx 8000{\text{s}}^{-1}$ to $\approx 15000{\text{s}}^{-1}$$\approx 15000{\text{s}}^{-1}$ for the range of impact velocities investigated, spanning from $\approx 240\text{m/s}$$\approx 240\text{m/s}$ to $\approx 370\text{m/s}$$\approx 370\text{m/s}$. The fragments were soft-recovered, weighed, sized, and the fracture surfaces were analyzed utilizing scanning electron microscopy and X-ray tomography. The experimental results demonstrate a general increase in both the number of necks and fragments with expansion velocity. The fractographic investigation and the 3D reconstruction of the fracture surfaces showed a mix of equiaxed dimples indicative of tensile failure and elliptical dimples suggestive of shear failure, with the predominance of each type varying across fractures. The planar plate impact experiments consists of propelling a disc-like projectile towards a stationary disc-like target at velocities ranging from $\approx 380\text{m/s}$$\approx 380\text{m/s}$ to $\approx 780\text{m/s}$$\approx 780\text{m/s}$. The target is twice the thickness of the projectile, positioning the spall plane approximately at the center of the target. A photonic Doppler velocimetry system was utilized to measure the axial velocity of the free surface of the target, providing data on shock pressure, shock velocity, shock width, Hugoniot elastic limit, spall strength, and strain rate within the spall plane, which varied from $\approx 50000{\text{s}}^{-1}$$\approx 50000{\text{s}}^{-1}$ to $\approx 170000{\text{s}}^{-1}$$\approx 170000{\text{s}}^{-1}$ across the tested impact velocity range. The target specimens have been soft-recovered, sized, and analyzed using scanning electron microscopy and X-ray tomography. The investigated impact velocities range from the onset of incipient spalling, characterized by discontinuous cracking and limited void growth, to the formation of a complete fracture spanning a large portion of the central section of the specimen, resulting from extensive cracking and the coalescence of numerous large voids. The X-ray tomography analysis provided three-dimensional reconstructions of the spallation, yielding quantitative data on the evolution of fracture size and volume with impact velocity. The scanning electron microscopy investigation revealed the mechanisms of void growth, coalescence, and intervoid cracking leading to spallation, without revealing a clear influence of the material microstructure on the crack propagation path.