円柱の据込み加工における自由表面延性の解析的評価

概要

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Cold upsetting of a circular cylinder is often used to evaluate the cold forgeability of forming materials. The index of the forgeability is the critical strain where a crack occurs on the expanding free surface of the upsetting cylinder. Occurrence of cracking is associated with the multiplicity of solution from a view point of the mechanics of plasiticity. In general, it would be believed that the multiplicity corresponds to plastic instability. But plastic instability does not necessarily cover all over the multiplicity, but load instability must be taken into consideration. To the evaluation of cracking the criterion of load instability is applied which appears after plastic instability in upsetting. The mode of cracking must be consistent with deformation required on elasto-plastic boundary. Hence, only two kinds of mode are permitted. The strain rate at load instability can determine which mode of the two is realized on cracking, by comparing the energy consumption from load instability to the contraction of plastic region into a plane region. Loading path in nominal stress rate space is used which makes it easy to specify plastic and load instabilities. Strain data developing on the expanding surface is determined with reasonable accuracy using commercial software MARC. It is shown that the criterion of load instability is useful for the prediction of the surface cracking.
1999-09-25