Imagine that you have two cracked components that are identical to one another except that Component A has a preexisting crack that is twice as long as that found in Component B. Does that mean that the fatigue lifetime of Component A will be 50% that of Component B?
Design Against Fluctuating Loads
Machine elements are subjected to varieties of loads, some components are subjected to static loads, while some machine components are subjected to fluctuating loads, whose load magnitude tends to fluctuate. The components of a machine, when rotating at a high speed, are subjected to a high degree of load, which fluctuates from a high value to a low value. For the machine elements under the action of static loads, static failure theories are applied to know the safe and hazardous working conditions and regions. However, most of the machine elements are subjected to variable or fluctuating stresses, due to the nature of load that fluctuates from high magnitude to low magnitude. Also, the nature of the loads is repetitive. For instance, shafts, bearings, cams and followers, and so on.
Design Against Fluctuating Load
Stress is defined as force per unit area. When there is localization of huge stresses in mechanical components, due to irregularities present in components and sudden changes in cross-section is known as stress concentration. For example, groves, keyways, screw threads, oil holes, splines etc. are irregularities.
Imagine that you have two cracked components that
are identical to one another except that Component A has a
preexisting crack that is twice as long as that found in
Component B. Does that mean that the fatigue lifetime of
Component A will be 50% that of Component B?
Fatigue is a very complex process and it depends upon
1)type and nature of loading
2) size of component
3) stress or strain distribution
4) surface finish and directional properties
5) stress or strain concentrations
6) mean stress or strain
7) environmental effects
8) metallurgical factors and material properties
9) strain rate and frequency effects.
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