Background information on strain-rate sensitivity (described by the m value) and how it affects local elongation was presented in last month's column. A positive m value causes sheet-metal to strengthen as strain rate increases. In contrast, a negative m value causes a softening as strain rate increases. Lower-carbon steels common to the press shop have a positive m value. In addition, research work on these steels by D. Chatfield and R. Rote of National Steel Research in SAE Paper No. 740177 (1974) showed that n and m values were proportional. As the as-received strength of steel increased, both n and m values decreased. Therefore, most metalforming specialists simply ignore the m value and focus only on the n value. If you only form lower-carbon sheet steel, this approach should not cause any major problems.
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