3.11 PLASTIC DEFORMATION

Plastic deformation refers to a permanent dimensional change after a mechanical or non-mechanical load has been applied. Elastic, and to some extent, viscoelastic strains are recoverable; plastic or viscoplastic strains are not. As such, plastic deformation equates to instability, and it is clearly to be avoided if dimensional stability is desired. Nonetheless, elastic followed by plastic (elastoplastic) behavior is typical of many materials and must be considered in any discussion of the dimensional stability of composite materials.

First of all, we should use true strain and not engineering strain when plastic deformation is involved. (This was outlined in Section 1.3.3.) True elastic behavior is rare (perhaps a quartz watch spring). That is, if we load any material and then unload it, some permanent deformation will remain. This is described further in Section 3.12-the microyield stress. The application of a load for longer periods leads to both viscoelastic deformation and nonrecoverable creep, described in Chapter 8. The plasticity of a constituent, such as a polymer, will change as it absorbs solutes such as water (see Section 5.3). High-temperature use of ceramic composites may also involve plastic flow of one or more constituents.

Adams [166] has outlined the fundamental models describing plastic behavior of composites. He points out that plasticity is a necessary capability of most matrices, as local stress concentrations can therefore be accommodated before the composite as a whole exhibits nonelastic behavior. Thus, plasticity in composites should be initially addressed through micromechanics models, which can become complex...