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A series of bending fatigue tests were conducted and S-N data were obtained for two groups of 4320 steel samples: (1) carburized, quenched and tempered, (2) carburized, quenched, tempered and shot peened. Shot peening improved the fatigue life and endurance limit. The S-N data exhibited large scatter, especially for carburized samples and at the high cycle life regime. Sample characterization work was performed and scatter bands were established for residual stress distributions, in addition to fracture and fatigue properties for 4320 steel. Moreover, a fatigue life analysis was performed using fracture mechanics and strain life fatigue theories. Scatter in S-N curves was established computationally by using the lower bound and upper bound in materials properties, residual stress and IGO depth in the input data. The results for fatigue life analysis, using either computational fracture mechanics or strain life theory, agreed reasonably well with the test data.

The main objective of this paper is to investigate contact fatigue life models and to evaluate the effect of surface finish on contact fatigue life. The effect of surface finish on contact fatigue life was investigated experimentally using two roller contact fatigue tests. The test samples, i.e. rollers, were carburized, quenched and then tempered. Two different roller surface finishes were evaluated: machined and as heat-treated surface (baseline rough surface) vs. super finished surface (smooth). Because many factors are involved in sliding/rolling contact fatigue, contact fatigue modeling is still in the early development stage. In this work, we will analyze our contact fatigue test results and correlate contact fatigue life with several empirical contact fatigue models, such as the lambda ratio, a new surface texture parameter, and a normalized pitting model which includes Hertzian Stress, sliding, surface roughness and oil film thickness.

Axle primary gearing is normally carburized for high and balanced resistance to contact fatigue, wear, bending fatigue, and impact loading. The focus of this work is on bending fatigue which is a key design consideration of automotive and commercial vehicle axle gearing. Since a carburized component is basically a composite material with steep gradients in carbon content, hardness, tensile strength and microstructure from surface to the middle of the cross section combined with non-linear residual stress, its bending fatigue life prediction is a complex and challenging task. Many factors affect the bending fatigue performance of axle gearing, such as gear design, gear manufacturing, loading history during service, residual stress distribution, steel grade, and heat treatment. In this paper, the general methodology for bending fatigue life prediction of a carburized component is investigated. Carburized steel composites are treated as two homogeneous materials: case and core.