Effect of specimen geometry and friction in the high strain rate compression tests
Pun, Lalit (2019)
2019
Materiaalitekniikan DI-ohjelma - Degree Programme in Materials Science and Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2019-11-13
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-201911085836
https://urn.fi/URN:NBN:fi:tuni-201911085836
Tiivistelmä
The Split Hopkinson pressure bar (SHPB) or Kolsky bar is widely used to characterize the mechanical behavior of materials at different strain rates and different temperatures. The proper characterization of mechanical behavior of different materials at different strain rates and temperatures requires minimization of potential errors that can occur due to different factors such as friction, improper selection of specimen geometry and size. In this study, the effect of lubrication, specimen geometry, and size on the dynamic behavior of aluminum alloy has been investigated experimentally and numerically.
Dynamic compressive tests were performed at different strain rates and at room temperature using the specimens made of aluminum alloy 7050-T351, with diameters of 5mm, 6mm, 8mm, 10mm and 12mm and different length to diameter ratios of 0.6, 1 and 1.4. The tests were performed using cylindrical specimens, but a few experiments were also carried out using the cuboid specimens to check the compatibility of the used specimens.
The shorter specimen of smaller diameter (5mm) was found to be sensitive to lubrication, but larger specimens did not show larger deviations in the results. The strain rates and calculated temperature rise in the specimens due to adiabatic heating decreased with the lubrication applied between the specimen and pressure bars. Also, the suitable specimen diameter and aspect ratios were identified for high strain rate compression tests. The aluminum alloy 7050-T351 showed the increased strain rate sensitivity above strain rates of 1000 s-1. Moreover, the cuboid specimens should be discarded in the compression tests as they provide unreliable results.
The experimental results were fitted to the Johnson-Cook constitutive material model. The obtained parameters were then used in the numerical simulation where a replica of the test device used in the lab was constructed using a 2D-axisymmetric model in the y-axis. The effects of friction were also studied simulating the reflected and transmitted stress-waves using coefficients of friction ranging from 0-0.5. The results showed that the reflected stress-waves decrease slightly with the increase in friction, but the transmitted stress-waves increase significantly. The simulated yield stress also increases with increment in coefficient values of friction.
Dynamic compressive tests were performed at different strain rates and at room temperature using the specimens made of aluminum alloy 7050-T351, with diameters of 5mm, 6mm, 8mm, 10mm and 12mm and different length to diameter ratios of 0.6, 1 and 1.4. The tests were performed using cylindrical specimens, but a few experiments were also carried out using the cuboid specimens to check the compatibility of the used specimens.
The shorter specimen of smaller diameter (5mm) was found to be sensitive to lubrication, but larger specimens did not show larger deviations in the results. The strain rates and calculated temperature rise in the specimens due to adiabatic heating decreased with the lubrication applied between the specimen and pressure bars. Also, the suitable specimen diameter and aspect ratios were identified for high strain rate compression tests. The aluminum alloy 7050-T351 showed the increased strain rate sensitivity above strain rates of 1000 s-1. Moreover, the cuboid specimens should be discarded in the compression tests as they provide unreliable results.
The experimental results were fitted to the Johnson-Cook constitutive material model. The obtained parameters were then used in the numerical simulation where a replica of the test device used in the lab was constructed using a 2D-axisymmetric model in the y-axis. The effects of friction were also studied simulating the reflected and transmitted stress-waves using coefficients of friction ranging from 0-0.5. The results showed that the reflected stress-waves decrease slightly with the increase in friction, but the transmitted stress-waves increase significantly. The simulated yield stress also increases with increment in coefficient values of friction.