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. Materials properties are evaluated separately with simulated case and simulated core samples. Two fatigue life analysis approaches are applied for bending fatigue life analysis: the strain-life approach and the crack growth fracture mechanics approach. The fundamental materials fatigue properties needed include strain controlled axial fatigue test data, crack growth rates and fracture toughness. Residual stress distributions are characterized and their effect on bending fatigue life is included in both fatigue life analysis approaches. Bending fatigue tests are conducted on carburized U-Notch bar samples, and the test results are compared with the life prediction results. Several important factors in bending fatigue life analysis of carburized components are discussed and included in the life prediction, such as bending stress gradient, mean stress, initial defect size, and residual stress. Finally the pros and cons of the two fatigue life analysis approaches are compared and summarized.