Measures of Stress
In this chapter, we abandon the purely kinematic aspects of the motion of bodies and will focus on the actual forces causing the motion. Two distinct types of forces are recognised in continuum mechanics: body forces are conceived as acting on the particles of a body, and the surface (or contact) forces as arising from the action of one part of a body on an adjacent part across a separating surface. Just as there are many different strain measures, there are several different measures of the surface forces. We will see that the surface forces can be described as a second-order tensor. Thus, the surface forces can always be quantified by a set of nine numbers, and the various different definitions are all equivalent.