Longitudinal Stress:

While the deforming force is applied along the length of the body (Figure), the stress produced is called longitudinal stress.

Normal (or Compression) Stress

Suppose the deforming forces are applied uniformly over the entire surface of the body normally (Figure 19(a)) which produce change in volume of the body without changing its shape (Figure 19(b)). The restoring force acting per unit area normal to the surface of the body is called normal or compression stress. For instance, while we apply hydrostatic pressure uniformly over the entire surface of the body than the normal stress is produced.

(a)                                                       (b)

Figure: (a) Deforming Forces Acting Normally on a Body; and (b) The Deforming Force Cause Change in its Volume

You may wonder: What is the difference between pressure and normal stress? Pressure is defined as the external (deforming) force per unit area normal to the surface. While, normal stress is defined as the internal restoring force developed per unit area normal to the surface.