Introduction

This final chapter presents a collection of ideas related to work hardening, as well as some thoughts on modern descriptions of the mechanical response of soils. Recall from Chapter 3 that work hardening materials, in contrast to perfectly plastic materials, may change their response during yielding. These changes are accomplished by altering, in some fashion, the shape or size of the yield surface as plastic flow occurs. Initially the concept of work hardening was introduced to give a better representation of the stress–strain response of metals. The ideas involved are straightforward, although there is a price to pay in terms of increased levels of complexity. We have avoided the topic until now, not because it is unimportant, but because it plays such an important role in the modern theories of soil plasticity that are also considered in this chapter.

Geotechnical engineers have found the general concept of work hardening extremely useful whenever there is a need for response calculations that are more detailed than is possible with perfect plasticity. In particular, the closed yield surfaces described in Chapter 3 would be nearly useless without work hardening. With a closed yield surface there is a possibility of plastic response under increasing isotropic stress, but one cannot arbitrarily limit the amount of stress increase by establishing a fixed yield surface at some arbitrary stress level. We must be able to increase the isotropic stress, to move the stress point out along the space diagonal, without limitation.