Engineering Design
DESIGN FOR STRENGTH-VARIABLE LOADING
INTRODUCTION
The analysis and design of parts subjected to static loading are considered in the earlier chapter. In this chapter we will be considering how parts or component fail when subjected to variable (nonstatic) loading and how to design them.
Variable stresses
In a number of machine elements, conditions frequently arise, in which the stresses vary or they fluctuate between levels, even though the magnitude, direction and point of application of load do not change. For example, in a rotating shaft subjected to the action of constant force, inducing bending loads, a particular fiber on the surface undergoes both tensile and compressive stresses of the same magnitude for each revolution of the shaft. If the shaft rotate let us say 1750 rpm, the fiber is stressed in tension and compression successively 1750 times each minute. These and other kinds of such loading occurring in machine members produce stresses, which are called repeated, alternating, or fluctuating stresses.
Fatigue failure
Often machine members subjected to such repeated stressing are found to have failed even when the actual maximum stresses were below the ultimate strength of the material, and quite frequently at stress values even below the yield strength. The most distinguishing characteristics is that the failure had occurred only after the stresses have been repeated a very large number of times. Hence the failure is called fatigue failure
A fatigue failure begins with a small crack, the initial crack may be so minute and can not be detected. The crack usually develops at a point of discontinuity in the material, such as a change in cross section, a keyway or a hole. Once a crack is initiated, the stress concentration effect become greater and the crack progresses more rapidly. As the stressed area decreases in size, the stress increase in magnitude until, finally, the remaining area is unable to sustain the load and the component fails suddenly. A fatigue failure, therefore, is characterized by two distinct regions. The first of these is due to progressive development of the crack, while the second is due to the sudden fracture. The zone of sudden fracture is very similar in appearance to the fracture of a brittle material, such as cast iron, that has failed in tension.
A fatigue failure almost always begins at a local discontinuity such as a notch, crack, or other area of stress concentration. When the stress at the discontinuity exceeds the elastic limit, plastic stain occurs. For fatigue fracture to occur, there must exist cyclic plastic strains. Thus we shall need to understand the behaviour of materials subjected to cyclic plastic deformation.
