Publications

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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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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. As a result, the C250 alloy was more ductile than the C300 alloy under dynamic loading.

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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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A current Kolsky tension bar has been implemented with pre-tension-load capability to investigate the effect of preload on the high-rate response in tension of materials and structures. In this study, fully threaded brass studs have been experimentally investigated in terms of pre-tension-load effect on the tensile stress-strain response at the same high strain rate. The preload is not observed to significantly influence the plastic flow stress. The failure responses are quite different, however, when different pre-tension loads are applied. © The Society for Experimental Mechanics, Inc. 2013.

Various loading and measuring configurations have been developed in Hopkinson bar fracture toughness experimental techniques. It is well known that several fundamental issues, such as force equilibrium, pulse shaping, stress-wave propagation, etc., must be evaluated in order to obtain a reliable measurement. In our previous work of characterizing Mode II dynamic fracture toughness of a woven composite, highly sensitive polyvinylidene fluoride (PVDF) force transducers were employed to check the forces on the front wedge and back spans in a SHPB ENF experiment. The results show that proper pulse shaping is necessary so the specimen can achieve stress equilibrium before the crack starts to propagate. This study addresses the issue that stress wave propagates through the non-uniform section, which is between the incident and transmission bars including the specimen, loading wedge, and supporting fixture. The transmitted signals are compared with PVDF measurements, and also with numerical simulations of stress waves propagate through supporting fixture and down to the transmission bar. © The Society for Experimental Mechanics, Inc. 2013.

This paper describes the development of infra-red imaging methods to visualize and monitor damage evolution in metallic alloys. Imaging is performed in-situ during tensile and notched tensile experiments at the microstructural grain level. Specimen preparation and imaging techniques are described. The results are anticipated to guide and improve alloy-specific damage evolution constitutive models to enable improved deformation and failure predictions. © The Society for Experimental Mechanics, Inc. 2013.

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