Metal behavior laws - Elastoplasticity. Viscoplasticity

This document is dedicated to determining the properties of elastoplastic and viscoplastic materials. Unlike elastic materials, plastic materials retain an irreversible deformation after discharge. Formally, plastic behavior is completely independent of time: it is independent of the rate of deformation, which does not change when the load is kept constant. The deformation of an elastoplastic material comprises a reversible elastic component and a non-elastic plastic component. Metals typically exhibit elastoplastic behavior, at least to a first approximation at not too high temperatures.

Not only is the deformation of a viscoplastic material irreversible, it is also a function of time. It depends on the rate of loading, and changes when loading is held constant. Creep is one of the classic manifestations of viscoplastic behavior. Polymers generally exhibit such behavior, but metals also do, especially at high temperatures. A reversible elastic component of the deformation is very often also present, so that we should strictly speaking speak of elasto-viscoplastic behavior. In most cases, however, it can be neglected in comparison with the viscoplastic component.

The section on viscoplastic properties will focus exclusively on creep and relaxation tests at high temperatures. The influence of strain rate on behavior at lower temperatures will be dealt with in the section devoted to the determination of viscoplastic properties. This arrangement is preferable, even if it means sacrificing strict rigor, since these are the types of test we are most interested in.

For the same reason, the section on determining elastoplastic properties includes brittle materials for which the plastic deformation is very small.