The Effect of Periodic Overloads on Biaxial Fatigue of Normalized SAE 1045 Steel

摘要

During the past decade it has been observed that periodically applied overloads of yield stress magnitude can significantly reduce or eliminate crack closure under uniaxial or Mode I loading. This paper reports the results of a series of biaxial in-phase tension-torsion experiments that were performed to evaluate the effects of overloads on the fatigue life of smooth tubes constructed of normalized SAE 1045 steel. Five strain ratios were investigated, including uniaxial (λ =ε_(xy)/ε_(xx)= 0), pure torsion (λ=∞), and three intermediate ratios (λ= 3/4, 3/2, and 3). Periodically applied overloads of yield stress magnitude allowed cracks to grow under crack face interference-free conditions. Strain-life curves were developed by computationally removing the overload cycle damage from test results and calculating equivalent fatigue lives. A factor of two reduction in the fatigue limit was found at all ratios when these results were compared with constant-amplitude results. Cracking behavior was observed and it was noted that for strain ratios greater than one, cracks initiated along the rolling direction (longitudinally); otherwise, the cracks initiated on maximum shear planes. This observation was used to help explain the similarity in fatigue life results for all strain ratios for both constant-amplitude and overload data. Parameter-life curves were developed using the equivalent fatigue life data and several common multiaxial damage parameters, and the damage parameters were evaluated. It was found that the simple maximum shear strain criterion together with uniaxial overload data provided a good estimate of the fatigue behavior for all strain ratios.