Based on the maximum stress deviator yield criterion and the associate
d flow rule, the quasi-linear differential equation systems of stress
and velocity fields in the plane stress problem of an ideal rigid-plas
tic body are established in this paper. Judgements on the types of the
se differential equation systems are made by using the theory of chara
cteristics. They may be elliptic or hyperbolic, depending on the consi
dered stress state. In the hyperbolic case, equations of two families
of characteristics and relations connecting the stress or velocity com
ponents along characteristics are derived. Three examples are given to
illustrate the application of the aforementioned method of characteri
stics derived. As a conclusion, the effectiveness and advantages of th
is method compared with those based on the von Mises and Tresca yield
criteria are expounded in the last section of this paper.