Introduction to Combined Stresses

Chapters 5 to 7 considered only one type of load acting on a machine member, for example, tensile, or compressive, or shear, etc. In actual cases, there may be more than one type of load coming simultaneously. These combinations are described in the subsequent sections.

Bending with Axial Load

A load perpendicular to the axis of a beam causes bending moment, while along the axis causes axial stress. Axial load F can be tensile or compressive. A plus (+) sign for tensile and minus (−) for compressive, is used.

Principal Stresses

In previous chapters we studied loads, which cause direct stress (tensile, compressive) in one direction only, but in actual practice the loads may be acting in different directions, viz., along X-, Y-, and Z-axes, which are mutually perpendicular to each other. Normal stress acts perpendicular to the area under consideration, while shear stress acts in the plane itself. The combined effect of all the loads gives maximum normal stress in one plane and minimum in other plane. These planes with no shear stress are mutually perpendicular to each other and are called principal planes. Direct normal stresses on these planes are called principal stresses. Figure 8.1(a) shows a rectangular element of uniform cross section and unit thickness, which has normal stresses sx and sy, and shear stresses ty and tyx.