Estimating Void Nucleation Statistics in Laser-Driven Spall
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RESEARCH PAPER
Estimating Void Nucleation Statistics in Laser‑Driven Spall D. D. Mallick1,4 · J. Parker2,4,5 · J. W. Wilkerson3 · K. T. Ramesh4,5 Received: 7 March 2020 / Accepted: 29 May 2020 © This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2020
Abstract We examine the statistical distribution of critical nucleation pressures necessary to dynamically grow voids during the spall failure of an AZ31B magnesium alloy. The approach uses laser-driven micro-flyers to generate spall over times of the order of tens of nanoseconds, allowing us to focus on void nucleation processes rather than void coalescence processes. Our methodology combines quantitative postmortem characterization of void mediated failure with time-resolved interferometry of the failure event, and reveals the dynamics of the failure process. We infer the distribution of the underlying nucleation pressures and explore the associated strain rate dependence of spall strength in these alloys. Keywords Spall failure · Void nucleation and growth · Micro computed tomography · Photon doppler velocimetry · AZ31B Mg alloy · Laser-driven micro-flyer plates
Introduction Spall failure occurs in a material under high-rate loading conditions where stress waves interact to create localized high tensile stresses, activating failure mechanisms such as dynamic void growth [1]. Improving the resistance of a material to the failure mechanisms associated with spall is then relevant to the goal of developing next-generation protection materials that must withstand ballistic or explosive loading. The role of dynamic void growth as a failure mechanism during spall has been a subject of study since Rinehart [2] characterized the ultimate tensile strength of steel, brass, Al, and Cu alloys under explosive loading. In dynamic void growth, the local dynamic hydrostatic tension causes the unstable nucleation and growth of a cavity [3, 4]. These * D. D. Mallick [email protected] 1
US Army CCDC Army Research Laboratory, 321 Colleran Road Aberdeen Proving Ground, Adelphi, MD 21005‑5066, USA
2
US Army CCDC Soldier Center, Natick, MA 01760, USA
3
J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA
4
Hopkins Extreme Materials Institute Johns Hopkins University, 140 Malone Hall, Baltimore, MD 21218, USA
5
Department of Mechanical Engineering, Johns Hopkins University, 223 Latrobe Hall, Baltimore, MD 21218, USA
cavities, or voids, coalesce to degrade the load-bearing capacity of the material. This phenomenon is conventionally studied with gas guns and explosive loading, typically resulting in a single valued “spall strength”. Such loading techniques can achieve strain rates as high as 1 07 s−1 or more, but can make specimen recovery difficult due to the large kinetic energies from loading. Instead, laser-driven approaches have been sparingly employed in recent spall studies to improve experimental throughput and impart less k
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