A Rheological Study of a Superplastic Sheet Metal Forming Process
Superplastic alloys, first known over half a century ago, [Refs. (l).(2)] have been considered by engineers and metallurgists, with less than perfect justification, to be materials of exceptionally high ductility. Actually, superplasticity is not a matter of ductility — which is defined by the strain at fracture under quasi-static conditions — but is due to the dependence of flow stress on the strain rate, in other words, it is a rheological phenomenon. The basis of superplasticity is neither in the late development of the localized neck in the material when stretched, nor in the large strain at the fracture, but lies in the fact that the neck does not aggravate itself after it is formed, the strain-rate effect on the flow stress preventing the formation of the highly localized necking in the tension test specimens of ordinary ductile metals.
KeywordsFlow Stress Strain Path Localize Neck Bulge Test Meridional Section
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