Spall and subsequent recompaction of copper under shock loading

M C Hawkins,S A Thomas,S J Fensin,R S Hixson,D R Jones

doi:10.1063/5.0011645

Abstract

Research was undertaken to study spall and subsequent recompaction of oxygen-free high-thermal-conductivity copper using a single-stage large-bore light gas gun capable of planar impacts. Gun experiments were conducted that produced an initial spall in the target with a subsequent recompaction of the spall damage/layer by the use of layered impactors. Symmetric spall experiments at similar conditions were also conducted as a control to the recompaction experiments. Photonic Doppler velocimetry was used to obtain the velocity history of the target's back surface; these velocity histories were analyzed, and the results were compared to numerical simulations. After recovery of selected samples, the microstructure was analyzed using optical imaging microscopy and electron backscatter diffraction. Recompaction waves were clearly observed in the time-resolved data obtained at the back surface of the targets. Two different series of experiments were performed to understand the effect of damage morphology on the stress required for spall recompaction. In the initial series, the peak pressure of the first shock was systematically increased to alter the amount of damage in the material. While in the second series, the peak pressure of the second shock was increased, keeping the magnitude of the first shock relatively constant to understand the effect of peak stress on the recompaction process. Findings clearly show that recompaction results in a perturbed band of the microstructure at the location of the expected spall plane in all cases except where the peak stress of the second shock is lower than the critical value.

Highlights

Experimental studies of the dynamic failure process are numerous in the literature.[1,2,3,4] Much of this prior research used plate impact experiments to produce localized regions of tension
The peak stress can be calculated using σp ρoUs(v pk where σHEL is the Hugoniot elastic limit, ρ0 is the initial velocity, Us is the shock velocity obtained from shock Hugoniots of Al and Cu12 as listed in Table II, υpk is the velocity of the peak obtained from the photonic Doppler velocimetry (PDV) data, and υHEL is the amplitude of the HEL extracted from the PDV plot
These experiments follow the methods used by Becker et al.,[8] and include experiments that were designed and performed to understand the effect of peak stress associated with the first and second shock on the recompaction stress

Summary

Introduction

Experimental studies of the dynamic failure process (spall) are numerous in the literature.[1,2,3,4] Much of this prior research used plate impact experiments to produce localized regions of tension. It has been previously shown that the nature and extent of this damaged region depends on the specific material and on the loading conditions. Examples of loading conditions that can affect the region of damage include: the shape of the shock loading profile (flat top vs triangle), tensile strain rate, peak shock stress level, and others. While most of the work to study damage has focused on studying flat top loading, a few works exist on investigating the damage process that results from a triangular wave,[4–6] a process known to be capable of producing a broader region of tension and subsequent damage. A triangular wave (such as is produced by in-contact loading with high explosives) consists of a shock rise followed by an immediate more gradual release where there is no flat-top

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