Publications

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Gas-gun experiments have probed the compression and release behavior of impact-loaded 304L stainless steel specimens that were machined from additively manufactured (AM) blocks as well as baseline ingot-derived bar stock. The AM technology permits direct fabrication of net-or near-net-shape metal parts. For the present investigation, velocity interferometer (VISAR) diagnostics provided time-resolved measurements of sample response for onedimensional (i.e., uniaxial strain) shock compression to peak stresses ranging from 0.2 to 7.0 GPa. The acquired waveprofile data have been analyzed to determine the comparative Hugoniot Elastic Limit (HEL), Hugoniot equation of state, spall strength, and high-pressure yield strength of the AM and conventional materials. The possible contributions of various factors, such as composition, porosity, microstructure (e.g., grain size and morphology), residual stress, and/or sample axis orientation relative to the additive manufacturing deposition trajectory, are considered to explain differences between the AM and baseline 304L dynamic material results.

Steel grades such as A572 and AISI 4140 are often used for applications where high rate or impact loading may occur. A572 is a hot-rolled carbon steel that is used where a high strength to weight ratio is desired. A grade such as AISI 4140 offers decent corrosion resistance due to higher chromium and molybdenum content and is commonly used in firearm parts, pressurized gas tubes, and structural tubing for roll cages. In these scenarios, the material may undergo high rate loading. Thus, material properties including failure and fracture response at relevant loading rates must be understood so that numerical simulations of impact events accurately capture the deformation and failure/fracture behavior of the involved materials. In this study, the high strain rate tensile response of A572 and 4140 steel are investigated. An increase in yield strength of approximately 28% was observed for 4140 steel when comparing 0.001 s-1 strain rate to 3000 s-1 experiments. A572 showed an increase in yield strength of approximately 52% when the strain rate increased from quasi-static to 2750 s-1. Effects on true stress and strain at failure for the two materials are also discussed.

This memo concerns the transmission of mechanical signals through silicone foam pads in a compression Kolsky bar set-up. The results of numerical simulations for four levels of pad pre-compression and two striker velocities were compared directly to test measurements to assess the delity of the simulations. The nite element model simulated the Kolsky tests in their entirety and used the hyperelastic `hyperfoam' model for the silicone foam pads. Calibration of the hyperfoam model was deduced from quasi-static compression data. It was necessary, however, to augment the material model by adding sti ness proportional damping in order to generate results that resembled the experimental measurements. Based on the results presented here, it is important to account for the dynamic behavior of polymeric foams in numerical simulations that involve high loading rates.

Low carbon, high strength steel alloys such as Vascomax steels are used in a wide variety of extreme environments due to their high strength, high fracture toughness, and stability over a wide range of temperatures. In this study, Vascomax® C250 steel was dynamically characterized in compression using Kolsky compression bar techniques at two strain rates of 1000 and 3000 s-1. A pair of impedance-matched tungsten carbide platens were implemented to protect damage to the bar ends. The tungsten carbide platens were experimentally calibrated as system compliance which was then properly corrected for actual specimen strain measurements. In addition, elastic indentation of the high-strength compression sample into the platens was also evaluated and showed negligible effect on the specimen strain measurements. The Vascomax® C250 steel exhibited strain-rate effects on the compressive stress-strain curves. The dynamic yield strength was approximately 18% higher than quasi-static yield strength obtained from hardness tests. The dynamic true stress-strain curves of the Vascomax® C250 steel in compression were also computed and then compared with the previously obtained true tensile stress-strain curves at the same strain rates. The Vascomax® C250 steel exhibited a reasonable symmetry in dynamic compression and tensile stress-strain response. However, the fracture strains in dynamic compression were smaller than those in dynamic tension probably due to different fracture mechanisms in the different loading modes.

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Silicone foams have been used in a variety of applications from gaskets to cushioning pads over a wide range of environments. Particularly, silicone foams are used as a shock mitigation material for shock and vibration applications. Understanding the shock mitigation response, particularly in the frequency domain, is critical for optimal designs to protect internal devices and components more effectively and efficiently. The silicone foams may be subjected to pre-strains during the assembly process which may consequently influence the frequency response with respect to shock mitigation performance. A Kolsky compression bar was modified with pre-compression capabilities to characterize the shock mitigation response of silicone foam in the frequency domain to determine the effect of pre-strain. A silicone sample was also intentionally subjected to repeated pre-strain and dynamic loadings to explore the effect of repeated loading on the frequency response of shock mitigation.

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In this research, a new type of highly stretchable strain sensor was developed to measure large strains. The sensor was based on the piezo-resistive response of carbon nanotube (CNT)/polydimethylsiloxane (PDMS) composite thin films. The piezo-resistive response of CNT composite gives accurate strain measurement with high frequency response, while the ultra-soft PDMS matrix provides high flexibility and ductility for large strain measurement. Experimental results show that the CNT/PDMS sensor measures large strains (up to 8 %) with an excellent linearity and a fast frequency response. The new miniature strain sensor also exhibits much higher sensitivities than the conventional foil strain gages, as its gauge factor is 500 times of that of the conventional foil strain gages.

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Additive manufacturing (AM) technology has been developed to fabricate metal components that include complex prototype fabrication, small lot production, precision repair or feature addition, and tooling. However, the mechanical response of the AM materials is a concern to meet requirements for specific applications. Differences between AM materials as compared to wrought materials might be expected, due to possible differences in porosity (voids), grain size, and residual stress levels. When the AM materials are designed for impact applications, the dynamic mechanical properties in both compression and tension need to be fully characterized and understood for reliable designs. In this study, a 304L stainless steel was manufactured with AM technology. For comparison purposes, both the AM and wrought 304L stainless steels were dynamically characterized in compression Kolsky bar techniques. They dynamic compressive stress-strain curves were obtained and the strain rate effects were determined for both the AM and wrought 304L stainless steels. A comprehensive comparison of dynamic compressive response between the AM and wrought 304L stainless steels was performed. SAND2015-0993 C.

Conventional Kolsky tension bar techniques were modified to characterize an iridium alloy in tension at elevated strain rates and temperatures. The specimen was heated to elevated temperatures with an induction coil heater before dynamic loading; whereas, a cooling system was applied to keep the bars at room temperature during heating. A preload system was developed to generate a small pretension load in the bar system during heating in order to compensate for the effect of thermal expansion generated in the high-temperature tensile specimen. A laser system was applied to directly measure the displacements at both ends of the tensile specimen in order to calculate the strain in the specimen. A pair of high-sensitivity semiconductor strain gages was used to measure the weak transmitted force due to the low flow stress in the thin specimen at elevated temperatures. The dynamic high-temperature tensile stress–strain curves of a DOP-26 iridium alloy were experimentally obtained at two different strain rates (~1000 and 3000 s−1) and temperatures (~750 and 1030 °C). The effects of strain rate and temperature on the tensile stress–strain response of the iridium alloy were determined. The iridium alloy exhibited high ductility in stress–strain response that strongly depended on strain-rate and temperature.

Iridium alloys have been utilized as structural materials for certain high-temperature applications, due to their superior strength and ductility at elevated temperatures. The mechanical properties, including failure response at high strain rates and elevated temperatures of the iridium alloys need to be characterized to better understand high-speed impacts at elevated temperatures. A DOP-26 iridium alloy has been dynamically characterized in compression at elevated temperatures with high-temperature Kolsky compression bar techniques. However, the dynamic high-temperature compression tests were not able to provide sufficient dynamic high-temperature failure information of the iridium alloy. In this study, we modified current room-temperature Kolsky tension bar techniques for obtaining dynamic tensile stress-strain curves of the DOP-26 iridium alloy at two different strain rates (~1000 and ~3000 s-1) and temperatures (~750°C and ~1030°C). The effects of strain rate and temperature on the tensile stress-strain response of the iridium alloy were determined. The DOP-26 iridium alloy exhibited high ductility in stress-strain response that strongly depended on both strain rate and temperature.