Compressive stress is the reverse of tensile stress. Adjacent parts of the material tend to press against each other through a typical stress plane as illustrated in Figure 1(b).

Shear Stress

Shear stress exists when two parts of a material tend to slide across each other in any typical plane of shear upon application of force parallel to that plane as illustrated in Figure 1(c).

Assessment of mechanical properties is made by addressing the three basic stress types. Because tensile and compressive loads produce stresses that act across a plane, in a direction perpendicular (normal) to the plane, tensile and compressive stresses are called normal stresses. The shorthand designations are as follows.

The ability of a material to react to compressive stress or pressure is called compressibility. For example, metals and liquids are incompressible, but gases and vapors are compressible. The shear stress is equal to the force divided by the area of the face parallel to the direction in which the force acts, as shown in Figure 1(c).

Two types of stress can be present simultaneously in one plane, provided that one of the stresses is shear stress. Under certain conditions, different basic stress type combinations may be simultaneously present in the material. An example would be a reactor vessel during operation. The wall has tensile stress at various locations due to the temperature and pressure of the fluid acting on the wall. Compressive stress is applied from the outside at other locations on the wall due to outside pressure, temperature, and constriction of the supports associated with the vessel. In this situation, the tensile and compressive stresses are considered principal stresses. If present, shear stress will act at a 90 angle to the principal stress.

Summary

The important information in this chapter is summarized below.

Stress Summary

Stress is the internal resistance of a material to the distorting effects of an external force or load.

Three types of stress

Tensile stress is the type of stress in which the two sections of material on either side of a stress plane tend to pull apart or elongate.

Compressive stress is the reverse of tensile stress. Adjacent parts of the material tend to press against each other.

Shear stress exists when two parts of a material tend to slide across each other upon application of force parallel to that plane.

Compressibility is the ability of a material to react to compressive stress or pressure.