Browsing by Author "Gallo, Federico Guido"
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Item Effect of electro-mechanical loading in metallic conductors(2010-12) Gallo, Federico Guido; Ravi-Chandar, K.; Mear, Mark E.; Satapathy, Sikhanda S.; Liechti, Kenneth M.; Landis, Chad M.The development of high powered electro-magnetic devices has generated interest in the effect of combined electromagnetic and mechanical loading of such structures. Materials used in high-current applications – aluminum alloys and copper – 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, these materials are also subjected to large mechanical forces. In previous work reported in the literature, ejection of material from the vicinity of preexisting defects such as cracks, notches or discontinuities have been observed resulting from short-duration high-intensity current pulses; after a series of pulses, permanent deformation and weakening of intact material has also been reported. But a lack of complete understanding of the effects of short duration current pulses hinders the assessment of the reliability of such conductors in high energy applications. Therefore, an investigation was undertaken to examine the behavior of electromagnetically and mechanically loaded conductors. This work investigates the effects of short-duration, high-current-density pulses in combination with viii mechanical loading. The aim is to develop a theoretical model to describe the resulting mechanical response. The model is to provide a characterization of the possible effects of thermally-induced plastic strains on metals loaded beyond or just below their yield strength or below the critical stress intensity factor. In the experiments reported here, two types of specimens, undamaged and damaged, were subjected to combined electromechanical loads. Undamaged specimens were used to observe thermally-induced plastic strains - strains not caused by an increase in mechanical loading, but rather resulting from the reduction of yield strength and post-yield stiffness due to the increase in temperature. The experiments were conducted such that it would be possible to develop a model that would conclusively account for the observed material behavior. The second sets of specimens were weakened a priori by the introduction of a crack in order to study the influence of such crack-like defects on the electrical and mechanical fields, and to produce a safe design envelope with respect to the loading conditions. Failure was found to occur due to melting triggered by joule heating; a quantitative criterion based on current concentration and heat accumulation near the crack tip has been developed based on these experimental results.Item Mechanical properties of metallic materials subjected to current pulses(2007-12) Gallo, Federico Guido; Ravi-Chandar, K.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.