Mechanical properties of metallic materials subjected to current pulses
Gallo, Federico Guido
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Materials used in high-current applications -- Al6061-T6 and Cu 102 -- are subjected to heat pulses of short duration (in the range of a few hundred microseconds to a few milliseconds); immediately following or along with such heat pulses, the specimens are subjected to large mechanical forces. The heat pulses are obtained by discharging a current (of ~ 10⁹ A/mm² density) through the specimens. Specimens of Cu 102 and Al 6061-T6 were loaded into the plastic range and then subjected to a short duration current discharge from a capacitor bank. The resulting heating causes both a thermal expansion and unloading of the specimen; upon cooling, the stress could not recover to the pre-discharge levels. Many experiments of this type have been performed in order to understand the cause of this stress drop. Accurate measurements of the strain and stress evolution with time were obtained using digital image correlation methods and a load cell; current measurements were obtained with a Rogowski probe. Finally, the measurements were interpreted with a viscoplastic model in order to determine the reasons for the load drop observed experimentally. The comparison of the experimental results with this viscoplastic model generates material constants that do not appear to be compatible with calibration in quasi-static load conditions; alternate methods to calibrate such constants have been implemented validating the previous results, but raising further issues. Specifically, a test was performed at constant loading conditions monitoring the strain in order to reduce the number of parameters to calibrate. It appears that other effects that have been ignored in the present modeling or other viscoplastic models may have to be considered in order to fully understand the effect of the short duration current pulse and generate appropriate models. The second part of the work relates to the effect of multiple pulses on the same specimen. In this part, the Cu 102 and Al 6061-T6 specimens were strained into the plastic range and then subjected to 50 or 100 current pulses of different magnitude, with the cross-head of the loading machine kept fixed. After each pulse, the specimen was allowed to cool to room temperature by waiting for 1 min. A drop in load that decreased with each subsequent current pulse was observed in both materials, with the load leveling off to a constant value after a few tens of current pulses. After 100 pulses, the stressstrain curve of the specimen was obtained once again and found a permanent drop in yield stress compared to the virgin material. Finally, a series of multiple discharges at fixed load and current density was performed showing an unstable behavior in the plastic strain accumulation.