42 C. Rudolf et al. taper features on the back-sides to provide a more gradual load-transition from gage section to grip, which performed well with specimen failures consistently occurring in the gage section rather than in or at the grips. This new design also provided better electrical and thermal contact between the grips and specimen compared to holder 001; Fig. 7.6 shows the new design produces a ~10 ◦C decrease in maximum temperature and lower gradients at a current density of 30 A/mm2 on copper specimens. Additional schematic drawings of the tab holders can be found elsewhere [5]. The wire materials were supplied in wound-coil forms with significant residual stresses. To avoid adding significant plastic deformations during manually straightening, a stress-relief process was developed. Fixtures of identical materials (i.e., Cu bar for Cu wire, etc.) were machined with channels for straightening four lengths of wire (~1150 mm total) at a time. As shown in Fig. 7.7, each bar has four half-circle channels (1 mm diameter) for holding the wires straight when sandwiched between two bars. The assembly was bolted together to provide a clamping force and then oven heated in ambient air atmosphere at the temperatures and times listed in Table 7.1. The process produced straight wires, and subsequent metallurgical examinations showed that the stress-relief process had no effect on the “as-received” microstructures (grain sizes). Fig. 7.6 Comparative plots of temperature across copper wire samples at a current density of 30 A/mm2 for (a) holder 001 and (b) holder 002 Fig. 7.7 Stress-relief fixtures for straightening the wire samples: (a) inside view of copper, (b) inside view of iron, and (c) assembled view of titanium Table 7.1 Stress-relief parameters and grain sizesa Material Temperature (◦C) Time (hours) Grain size (μm) Copper (110) 190 2 3.3 Iron 425 2 5 and 10 bi-modal Titanium (CP Grade 2) 500 2 9.0 aGrain sizes of the as-received and stress-relieved specimens were identical
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