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The mechanical properties of materials are known to be rate- and temperature-dependent. Owing to this, investigations aimed towards the exploration of material behavior (i.e. plasticity, strength, and failure) under thermomechanical extremes has been a subject of sustained interest. The extreme temporal and precise nature of these studies produces special experimental challenges, and as a consequence, knowledge regarding the dynamic response of materials, especially in thermomechanical extremes, is still limited by the deficiency of experimental data. The main objectives of the current study are to 1) develop a reliable experimental scheme for investigating the dynamic inelasticity of metals under thermomechanical extremes. In particular, the focus is on elevated temperature dynamic compressive and shearing resistance of metals at plastic strain rates in excess of one-million/sec and sample temperatures approaching melt. And, 2) to address the need for experimental data on the dynamic response of FCC metals in previously unexplored but important thermomechanical regimes, such as elevated temperatures and plastic strain-rates on the order of 10^5 – 10^9 /s. In order to conduct this research, the single-stage gas-gun facility at CWRU was modified to include a breech-end sabot heater system and a novel fully fiber-optics based normal and transverse motion diagnostics system, which enabled reverse geometry normal and pressure-shear plate impact experiments to be conducted on pure aluminum at elevated temperatures. Additionally, a full characterization of the WC anvil plates was performed. Using these capabilities, elevated temperature normal and combined pressure-shear plate impact experiments were carried out to better understand the high temperature dynamic compressive and shearing resistance of aluminum. These experiments were used to shed light on the temperature-dependence of the shock impedance of aluminum at pressures of around 1.0 – 1.6 GPa, and the temperature-dependence of shear flow stress at levels of strain approaching 50% and strain-rates in the order of 4 – 8 x 10^5 /s. The results showed an overall decrease in the shear flow stress with temperatures in the range of 23 – 593 ̊C, showing that temperature facilitates plastic flow of aluminum when deforming at strain-rates approaching 10^6 /s. Additionally, in an effort to better understand the relaxation behavior of this material at incipient plasticity at ultra-high strain-rates, a series of laser-driven shock compression experiments are carried out on pure aluminum films at temperatures ranging from 23 – 400 ̊C. The results are used to correlate the temperature-dependence of the rate-sensitivity of the Hugoniot elastic limit (HEL) of pure aluminum at strain-rates up to 10^9 /s. In contrast to the previous case (i.e. large plastic strains, and lower strain-rates), the results reveal a monotonic increase in the HEL with temperature for strain-rates in the range of 10^4 – 10^9 /s. This effect is shown to decrease with increasing strain-rate.
Dynamic Behavior of Materials, Volume 1 of the Proceedings of the 2019 SEM Annual Conference & Exposition on Experimental and Applied Mechanics, the first volume of six from the Conference, brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on fundamental and applied aspects of Experimental Mechanics, including papers on: Synchrotron Applications/Advanced Dynamic Imaging Quantitative Visualization of Dynamic Events Novel Experimental Techniques Dynamic Behavior of Geomaterials Dynamic Failure & Fragmentation Dynamic Response of Low Impedance Materials Hybrid Experimental/Computational Studies Shock and Blast Loading Advances in Material Modeling Industrial Applications
High-temperature, pressure-shear plate impact experiments were conducted to investigate the rate-controlling mechanisms of the plastic response of high-purity aluminum at high strain rates (106 s−1) and at temperatures approaching melt. Since the melting temperature of aluminum is pressure dependent, and a typical pressure-shear plate impact experiment subjects the sample to large pressures (2 GPa-7 GPa), a pressure-release type experiment was used to reduce the pressure in order to measure the shearing resistance at temperatures up to 95% of the current melting temperature. The measured shearing resistance was remarkably large (50 MPa at a shear strain of 2.5) for temperatures this near melt. Numerical simulations conducted using a version of the Nemat-Nasser/Isaacs constitutive equation, modified to model the mechanism of geometric softening, appear to capture adequately the hardening/softening behavior observed experimentally.
The results of an experimental study of the material dynamic response of aluminum exposed to a high-fluence, low-energy, pulsed electron beam are presented. The experimental results are compared with calculations based on two equilibrium equation of state (EOS) models for metals in the melt regime. The first model is the simple Mie-Gruneisen (M-G) scheme for solids extrapolated into the liquid phase. The second model (the GRAY EOS) is a three-phase EOS which provides a more detailed and thermodynamically complete description of metals in the melt region. After the onset of melting the M-G theory differs appreciably from experiment, whereas the GRAY EOS provides good agreement. The results indicate that the P-V-T relationship during the melting process can be described by an equilibrium--but thermodynamically consistent--EOS model. (Modified author abstract).
An experimental study was undertaken to investigate the ignition phenomena of 6061 aluminum alloy as a function of oxygen pressure. Cylindrical aluminum alloy specimens were ignited in a pure oxygen environment by a focused cw CO2 laser beam. To study the effect of oxygen pressure on the surface temperature at ignition of 6061 aluminum alloy, the experiments were conducted at oxygen pressures ranging from 0.084 to 2.413 MPa. The temperature history of the entire upper surface of the specimen and of a 0.5 mm diameter spot located initially at the center of the specimen top surface was recorded by using a commercial two-color ratio pyrometer and a fast-response, narrow-band, two-color pyrometer. Mass, brightness, and interior temperatures, for certain experiments were also recorded throughout the experiment. The results show that the surface temperatures at ignition of the alloy obtained from the temperature curves are below the melting temperature of the aluminum oxide and are slightly dependent on oxygen pressure. The data indicate that the ignition mechanism is complex and probably composed of several phenomena acting both separately and in conjunction with each other.