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The Society of Automotive Engineers Fatigue Design and Evaluation (SAEFDE) Committee has been conducting a long-term program aimed at the development of a predictive capability for fatigue life of SAE 1045 induction-hardened shafts. As a part of a larger-scale investigation provided by the SAEFDE committee, this research provided an analytical model capable of predicting the total fatigue life, both crack initiation and crack propagation, of an induction-hardened shaft under applied bending stress. The analysis procedure incorporated the effects of residual stresses. Total stress intensity factors were calculated and superimposed using applied bending stress intensity factors and residual stress intensity factors along the subsurface elliptical crack front. Fatigue tests were conducted using SAE 1045 induction-hardened shafts to verify the analytical models of subsurface fatigue crack growth. The total fatigue life calculations of subsurface failure showed a factor from 0.6 to 0.8 compared with the experimental results. The analytical model and experimental data confirmed that the majority of the total fatigue life is spent in the crack propagation phase.
The objective of the present paper is to evaluate the fatigue crack growth behavior in press-fitted axles using a fracture mechanics approach and to predict the fatigue strength regarding crack propagation (?w2). The relationship between nominal bending stress (?n) and non-propagating crack length in press-fitted axles is also discussed. Rotating bending fatigue tests were conducted on the induction hardened and quench-tempered axles of 38 and 40 mm in diameter. The equation for ?K was formulated from the result of FEM analyses in which the micro-profile at the contact edge was taken into consideration. The threshold stress intensity factor range ?Kth for small cracks was estimated from the crack size measured after the fatigue tests by using a modified stress ratio effect at fully compressed stress reversals due to high compression residual stress. ?w2 and the relationship between ?n and non-propagating crack length were predicted by using the above mentioned ?K and ?Kth. The predicted ?w2 and non-propagating crack length were in good agreement with the experimental values.
Single tensile overloads were applied to 4340 steel specimens which were heat-treated to give 120 and 220 ksi yield strength levels. The influence of yield strength level on the number of nonsteady state crack growth cycles subsequent to the application of 100 percent overload was noted to be substantial. The number of nonsteady state cycles for the 120 ksi yield strength steel was approximately an order of magnitude greater than that of the higher strength steel. A retardation model was developed using a residual stress intensity factor concept similar to that proposed by Willenborg, et. al. The model was found to predict to within 10 percent the number of nonsteady state crack growth cycles required to move a crack from the pre-overload position to a subsequent position, one overload induced plane stress plastic zone radius ahead of the pre-overload position. The model indicates that the reason for substantial increases in nonsteady state crack growth cycles observed for the low strength steel is due to a corresponding increase in the overload affected zone size.
The importance of delay, retardation in the rate of fatigue crack growth, produced by load interactions in variable amplitude loading, on the accurate prediction of fatigue lives of engineering structures is discussed. The effects of a broad range of loading variables on delay in fatigue crack growth at room temperature are examined for a mill annealed Ti-6Al-4V alloy. The results are used to estimate crack growth behavior under programmed loads.