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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.

Fatigue testing of actual components can produce considerable variation in the data. This seems to be especially true for higher strength case hardened components such as gears. Long term fatigue data on gears has shown there may be up to a 9:1 difference between the high and low cycle samples tested under the same conditions. This paper presents a systematic approach to determining some of the causes of this variation. Hypoid gear sets were dynamometer tested at 6 month intervals to determine the contribution of each cause. This study shows that the dynamometer test set up, the heat treatment of the gears, the cutting of the gear teeth, and the heat of steel used all contribute to the variability. Several metallurgical factors were also examined for a correlation to fatigue life.

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.

Computational fracture mechanics based FATIG3D program was used to simulate contact fatigue life of rough surface contacts in boundary to mixed lubrication regimes. Two-rollers contact fatigue tests were conducted and test results were compared with calculated contact fatigue lives. Calculated contact fatigue life agreed with test results well with the selected set of input data. The effect of several important parameters in the input data on contact fatigue life was evaluated computationally using FATIG3D. These parameters include: oil pressure distribution, crack face friction, direction of friction, friction coefficient, initial crack length, Hertzian stress, and residual stress distributions. The results obtained in this work improved basic understanding and the application of FATIG3D in simulating contact fatigue behavior.

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.

Prediction of fatigue performance of large structures and components is generally done through the use of a fatigue analysis software, FEA stress/strain analysis, load spectra, and materials properties generated from laboratory tests with small specimens. Prior experience and test data has shown that a specimen size effect exists, i.e. the fatigue strength or endurance limit of large members is lower than that of small specimens made of same material. Obviously, the size effect is an important issue in fatigue design of large components. However a precise experimental study of the size effect is very difficult for several reasons. It is difficult to prepare geometrically similar specimens with increased volume which have the same microstructures and residual stress distributions throughout the entire material volume to be tested. Fatigue testing of large samples can also be a problem due to the limitation of load capacity of the test systems available.

To improve fuel economy and possibly reduce product cost, light weight and high power density has been a development goal for commercial vehicle axle components. Light weight materials, such as aluminum alloys and polymer materials, as well as polymer matrix composite materials have been applied in various automotive components. However it is still a huge challenge to apply light weight materials in components which are subject to heavy load and thus have high stresses, such as gears for commercial vehicle axles or transmissions. To understand and illustrate this challenge, in this paper we will report and review the current state of art of carburized gear steels properties and performance.

Fatigue testing of components is used to validate new product designs as well as changes made to existing designs. On new designs it is common to initially test parts at the design stage (design verification or DV) and then again at the production stage (production verification or PV) to make sure the performance has not changed. On changes to existing designs typically the life of the new part (B) is compared to that of the old part (A). When comparing the fatigue life Weibull analysis is normally used to evaluate the data. The expectation is that the B10 or B50 life of the new part or PV parts should be equal to or better than that of the old parts or the DV parts. However, fatigue testing has a great deal of inherent variability in the resulting life. In this paper the variability of numerous carburized and induction hardened components is examined.

Current trends in vehicle development, including both automotive and commercial vehicles, are characterized by short model life cycles, reduced development time, concurrent design and manufacturing development, reduced design changes, and reduced total cost. All of these are driven by customer demand of higher load capacity, reduced weight, extended durability and warranty requirement, better NVH performance and reduced cost. These trends have resulted in increased usage of computational simulation tools in design, manufacturing, and testing, i.e. virtual testing or virtual prototyping. This paper summarizes our work in virtual testing, i.e. fatigue life simulations using computational fracture mechanics for commercial vehicle axle gearing development. First, fatigue life simulation results by using computational fracture mechanics CRACKS software were verified by comparing with gear teeth bending fatigue test data and three point bending fatigue test data.