Five Important Basics of Fatigue Stress Design Calculation

What is cyclic loading?

If the applied load (force) on a body changes cyclically (i.e., periodically with time) then the kind of

loading is called cyclic loading.

For example, when the point load applied at the center of a rotating shaft supported by bearings at both the ends, the acted loads periodically changes between compressive and tensile, the fibers at the top experiences compression and the fibers at bottom experiences tension. With the continuous rotation, the top fiber come at bottom and vice versa, so the shaft in total experiences a cyclic loading.


What is fatigue strength?

The stress, at which a body under cyclic loading fails after certain numbers of load cycles, is called the fatigue strength of the body for that number of cycle.

For example, in case of the above rotating shaft loading if the shaft fails at 50 Ksi after 10^4 cycles the fatigue strength of the shaft is said to be 50 Ksi at 10^4 cycles.


High Cycle vs. Low Cycle Fatigue

It is already explained that for describing the fatigue strength, apart from the stress value the number of cycles is also require. If this number of cycle is unto 10^3 then the fatigue situation is called as low cycle fatigue. If the number of cycle is more than 10^3 to 10^6 then it is called high cycle fatigue.


What is a S-N curve?

The S-N diagram (also called as Wohler curve) is the plot of the fatigue strength of a test specimen against the number of load cycles. A typical S-N curve of aluminum looks like below:

Fig.1: A Typical S-N Diagram of Aluminum


Image source: Wiki

The fatigue strength values are plotted in linear scale along the Y axis and the number of the loading cycles are plotted along the X axis in logarithmic scale of the S-N curve. After the S-N diagram for a particular specimen is available, you can find the fatigue strength for any particular number of loading cycles.


What is endurance limit?

The endurance strength or limit is the fatigue strength of a component corresponding to the 10^6 loading cycles practically ( infinite number of loading cycles theoretically). In other words, if the developed stress in the component is lesser than the endurance strength, then theoretically, the component should withstand infinite number of loading cycles.


In the next article, I’ll discuss an example problem of fatigue stress design calculation, and you’ll be in a better position to appreciate the basic concepts of S-N diagram, endurance strength, high and low cycle fatigue and cyclic loading.




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