Abstract
Experimental investigations of the effect of pre-straining on the crack initiation and propagation in fatigue have been carried out on mild steel and stainless steel. X-ray analysis of the material has clearly revealed that pre-straining of the material introduces surface compressive residual stress. With increasing degree of pre-strain the crack nucleation period is delayed and the rate of crack propagation is decreased. The maximum stress intensity factor K max is related to the rate of crack propagation by a relation da/dN=CK max q , where the value of the exponent q is approximately 2.5. The constant C decreases with increasing pre-strain. Thus it has been observed that the increase in fatigue life is contributed by the delay in crack nucleation and a decrease in the rate of crack propagation. The increase in the endurance limit appears to bear a linear relation with the degree of pre-strain.
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