Publications

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Kolsky compression bar experiments were conducted to characterize the shock mitigation response of a polymethylene diisocyanate (PMDI) based rigid polyurethane foam, abbreviated as PMDI foam in this study. The Kolsky bar experimental data was analyzed in the frequency domain with respect to impact energy dissipation and acceleration attenuation to perform a shock mitigation assessment on the foam material. The PMDI foam material exhibits excellent performance in both energy dissipation and accele-ration attenuation, particularly for the impact frequency content over 1.5 kHz. This frequency (1.5 kHz) was observed to be independent of specimen thickness and impact speed, which may re-present the characteristic shock mitigation frequency of the PMDI foam material under investigation. The shock mitigation characteristics of the PMDI foam material were insignificantly influenced by the specimen thickness. However, impact speed did have some effect.

A Hopkinson bar was employed to conduct transverse impact testing of twisted Kevlar KM2 fiber yarns at the same impact speed. The speed of Euler transverse wave generated by the impact was measured utilizing a high speed digital camera. The study included fiber yarns twisted by different amounts. The Euler transverse wave speed was observed to increase with increasing amount of twist of the fiber yarn, within the range of this investigation. The higher transverse wave speeds in the more twisted fiber yarns indicate better ballistic performance in soft body armors for personal protection.

Vascomax® maraging C250 and C300 alloys were dynamically characterized in tension with Kolsky tension bar techniques. Compared with conventional Kolsky tension bar experiments, a pair of lock nuts was used to minimize the pseudo stress peak and a laser system was applied to directly measure the specimen displacement. Dynamic engineering stress–strain curves of the C250 and C300 alloys were obtained in tension at 1000 and 3000 s−1. The dynamic yield strengths for both alloys were similar, but significantly higher than those obtained from quasi-static indentation tests. Both alloys exhibited insignificant strain-rate effect on dynamic yield strength. The C300 alloy showed approximately 10 % higher in yield strength than the C250 alloy at the same strain rates. Necking was observed in both alloys right after yield. The Bridgman correction was applied to calculate the true stress and strain at failure for both alloys. The true failure stress showed a modest strain rate effect for both alloys but no significant difference between the two alloys at the same strain rate. The C250 alloy was more ductile than the C300 alloy under dynamic loading.

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A modified laser occlusive extensometer technique was developed to measure the specimen strain with reasonably high and tunable resolutions in Kolsky tension bar experiments. This new technique used a novel concept where a laser beam was split into two independent sections to track the displacement histories of the incident and transmission bar ends, respectively, with independent resolutions. This technique ensures highly precise small strain measurements without sacrificing the capacity for large strain measurement. The uncertainty caused by rigid body motion of the specimen during dynamic strain measurement, which is induced by slight variation of laser intensity along the gage length, was also minimized in this technique. The validation test on Vascomax® maraging C250 alloy demonstrated that the new technique was capable of making both small and large strain measurements in Kolsky tension bar experiments, which is also applicable to Kolsky compression bar experiments.

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Threaded joints are used in a wide range of industries and are relied upon in maintaining component assembly and structural integrity of mechanical systems. The threads may undergo specific preparation before assembly in applications. In order to ensure a tight seal the threads may be wrapped with PTFE tape or to prevent loosening over time an adhesive (thread locker) may be used. When a threaded joint is subjected to impact loading, the energy is transmitted through the joint to its neighbors while part of it is dissipated within the joint. In order to study the effect of the surface preparation to the threads, steel and aluminum joints were tested with no surface preparation, application of PTFE tape, and with the use of a thread locker (Loctite 262). The tests were conducted using a Kolsky tension bar and a frequency based analysis was used to characterize the energy dissipation of the various thread preparations on both steel/steel and steel/aluminum threaded joints. © The Society for Experimental Mechanics, Inc. 2015.

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Iridium alloys are known to have superior strength and ductility at elevated temperatures, making them useful as structural materials for certain high-temperature applications. However, experimental data on their high-strain -rate performance are needed for understanding high-speed impacts in severe environments. Kolsky bars (also called split Hopkinson bars) have been extensively employed for high-strain -rate characterization of materials at room temperature, but it has been challenging to adapt them for the measurement of dynamic properties at high temperatures. In this study, we analyzed the difficulties encountered in high-temperature Kolsky bar testing of thin iridium alloy specimens in compression. Appropriate modifications were then made to the current high-temperature Kolsky bar technique to obtain reliable compressive stress–strain response of an iridium alloy at high-strain rates (300–10 000 s^-1) and temperatures (750 and 1030 °C). Finally, the compressive stress–strain response of the iridium alloy showed significant sensitivity to both strain rate and temperature.

Iridium alloys have superior strength and ductility at elevated temperatures, making them useful as structural materials for certain high-temperature applications. However, experimental data on their high-temperature high-strain-rate performance are needed for understanding high-speed impacts in severe elevated-temperature environments. Kolsky bars (also called split Hopkinson bars) have been extensively employed for high-strain-rate characterization of materials at room temperature, but it has been challenging to adapt them for the measurement of dynamic properties at high temperatures. Current high-temperature Kolsky compression bar techniques are not capable of obtaining satisfactory high-temperature high-strain-rate stress-strain response of thin iridium specimens investigated in this study. We analyzed the difficulties encountered in high-temperature Kolsky compression bar testing of thin iridium alloy specimens. Appropriate modifications were made to the current high-temperature Kolsky compression bar technique to obtain reliable compressive stress-strain response of an iridium alloy at high strain rates (300 – 10000 s^-1) and temperatures (750°C and 1030°C). Uncertainties in such high-temperature high-strain-rate experiments on thin iridium specimens were also analyzed. The compressive stress-strain response of the iridium alloy showed significant sensitivity to strain rate and temperature.

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Dynamic tensile experimental techniques of high-strength alloys using a Kolsky tension bar implemented with pulse shaping and advanced analytical and diagnostic techniques have been developed. The issues that include minimizing abnormal stress peak, determining strain in specimen gage section, evaluating uniform deformation, as well as developing pulse shaping for constant strain rate and stress equilibrium have been addressed in this study to ensure valid experimental conditions and obtainment of reliable high-rate tensile stress-strain response of alloys with a Kolsky tension bar. The techniques were applied to characterize the tensile stress-strain response of a 4330-V steel at two high strain rates. Comparing these high-rate results with quasi-static data, the strain rate effect on the tensile stress-strain response of the 4330-V steel was determined. The 4330-V steel exhibits slight work-hardening behavior in tension and the tensile flow stress is significantly sensitive to strain rate. © 2013 Society for Experimental Mechanics.

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