Although yield strength is suitable for criterion for designing components which are subjected to be static loads, for cyclic loading the behavior of a material must be evaluated under dynamic conditions. The fatigue strength or endurance limit of a material is therefore used in the design of parts subjected to repeated alternating stresses over an extended period of time.
The fatigue test determines the stresses which a sample of a material of standard dimensions can safely endure for a given number of cycles. This is accomplished using a specimen having a round cross section, loaded at two points as a rotating simple beam, and supported as its ends. The top surface of such a specimen is always in compression and the bottom surface is always in tension. The maximum stress always occurs at surface , halfway along the length of the specimen , where the cross - section is minimum. For each complete rotation of the specimen, a point in the surface originally at the top center goes alternately from a maximum in compression to a maximum in tension and then back to the same maximum in compression.
Specimens are tested to failure using different loads, and the number of cycles before failure is noted for each load. The results of such tests are plotted as graphs of applied stress against the logarithm of the number of cycles to failure. the curve is known as S-N curve. The usual procedure is to make a number of specimens of the same material and test other one under different stress conditions.
Two different curves of S-N curves occur, one showing a definite levelling of the curve , which indicates that the material has a definite fatigue limit. Commonly, ferrous metals shows distinct fatigue limit, while non ferrous metals do not. The fatigue limit is about 40 to 60 percent of the tensile strength.
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