This paper presents results of a series of reverse geometry normal plate impact experiments designed to investigate the onset of incipient plasticity in commercial purity polycrystalline magnesium (99.9%) under weak uniaxial strain shock compression and elevated temperatures up to melt. Â Strategic modifications made to the existing single-stage gas-gun facility enable dynamic material behavior characterization under extreme conditions, i.e. ultra-high strain-rates (~106/s) and test temperatures up to 1000 Â°C. Â In this custom configuration, thin metal samples (flyer plate) carried by the specially designed heat-resistant sabot are allowed to be heated uniformly across the diameter in a 100 mTorr vacuum prior to impact by a resistance coil heater with axial and rotational degrees of freedom at the breech end of the gun barrel. Â Moreover, a compact fiber-optics-based heterodyne combined normal and transverse displacement interferometer is designed and implemented. Â Like the standard PDV, this diagnostic tool is assembled using commercially available telecommunications hardware and uses a 1550 nm wavelength 2W fiber-coupled laser, an optical probe, and single mode fibers to transport light to and from the target. Â Using this unique approach, normal plate impact experiments are conducted on preheated (room temperature to near melt point of magnesium) 99.9% polycrystalline magnesium using Inconel 718 target plates at impact velocities ranging from 100 m/s to 110 m/s. Â The stress at flyer/target interface, as inferred from the measured normal particle velocity history at the free (rear) surface of the target plate shows progressive weakening with increasing sample temperatures below melt; at higher test temperatures, the rate of softening in stress is observed to weaken and even reverse as the sample temperatures approach the melt point of magnesium samples. Â Scanning electron microscopy is utilized to understand the evolution of sample material microstructure following the impact event.
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Dynamic behavior of metals at elevated temperatures and ultra-high strain-rates
Published: 07 May 2018 by MDPI in The Eighteenth International Conference of Experimental Mechanics session ICEM 2018
Keywords: Normal plate impact; commercial purity polycrystalline magnesium; extreme conditions; incipient plasticity; elevated temperatures; longitudinal impedance.