Textural vs structural plastic instabilities in sheet metal forming

Negative work-hardening rates are precursors of plastic instabilities. The objective of the work is to determine the origin of the stress decrease in the case of prestrained sheet metals. Samples of low carbon steel and pure aluminium deformed in rolling and reloaded in simple shear exhibit softening for specific shear directions with respect to the rolling direction. Structural and textural evolutions are analysed with transmission electron microscopy and X-ray measurements, respectively. The low carbon steel shows clear evidence of intragranular structural destabilization while the Taylor factor, estimated by self-consistent calculations, remains nearly constant. It is deduced that for steel, the softening has a structural origin. By contrast the aluminium samples present identical structural changes whatever the sign of the work-hardening rate. It is shown that the latter is strongly correlated with Taylor factor evolutions. A textural instability is obtained in this case. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.