Introduction

Abstract

Solid materials, when loaded beyond the critical state of stress, undergo permanent irreversible deformations. The deformations are called the plastic deformations, and the process of their growth — the plastic yield process. Imagine a small material element of plastically deformed solid. One can distinguish material fibres passing through the centre of the element. Theory of plasticity is aimed at finding relations between the stresses applied to the element and its plastic deformations. The solid is called a plastically isotropic one, if the relations are the same for all fibres of the element. If the relations depend on the choice of fibres, the solid is a plastically anisotropic one.