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Technical Paper

The Limitations of Fatigue Testing

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.
Technical Paper

Modified Locati Method in Fatigue Testing

Locati method is suitable in preliminary fatigue tests and production quality control. It is efficient since it uses just one test sample. The method requires that the slope of the S-N curve be known a priori, however. In this paper, a modified Locati method is presented that virtually eliminated this requirement. The method produces a point on the S-N plane that is independent of the slope of the S-N curve. The test design strategy to control the fatigue life of such a point is provided. The presented method has been successfully applied to preliminary fatigue tests of several welded components of ground vehicles.
Technical Paper

Investigation of S-N Test Data Scatter of Carburized 4320 Steel

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.
Technical Paper

Evaluating Designed Gap Closure Effect on Joint Preload

The method presented utilizes the analytical design model and matrix algebra to determine the load required to close a design gap. By performing a linear analysis in MSC® Nastran, relative displacements at fastener contact points due to unit loading are collected. The stiffness equation, F = K x is used in matrix form to determine gap closure force. The effect of this force on preload should be considered in torque specifications. The method prevents overestimation of preload if closure force is not considered and underestimation if only the stiffness equation is used independently.
Technical Paper

Development of an Accelerated Testing Methodology of Rotary Oil Seals for Off-Highway Vehicles

This paper will describe the development of an accelerated testing methodology for an off-highway vehicle rotary oil seal system. There are two typical field failure mechanisms associated with off-highway input pinion shaft oil seals: 1) excessive abrasive wear of soft seal lip and hard shaft surface due to abrasive environment; 2) excessive heat and degradation of the seal lip due to lack of lubricity and wear of the shaft surface run against this seal. The accelerated testing of the rotary oil seal consisted of a combination of the following factors; shaft run-out, eccentricity, testing temperature, rotation and reciprocal motion of the seal lip relative to the shaft surface. The combination of these factors especially reciprocal motion reproduces the same failure mechanism, i.e. shaft wear grooves and oil seal lip wear observed on the field usage samples with 6,300 hours service in only 350 hours of accelerated testing.
Technical Paper

Development of a Maintenance Free Self-Lubricating Ball Joint

Vehicular suspension ball joints can be categorized in the family of tribological systems which can reduce useful service or working capacity through malfunction or breakdown. Detailed metallurgical analysis of the friction and wear mechanisms on typical ball joint bearing surfaces point to a Teflon-based woven fabric, self-lubricating liner as the best bearing material for the joint. Laboratory functional testing was conducted on modern, 4-axis test equipment simulating the applicable loading and motion conditions typically encountered in use. The self-lubricated bearing liner woven with Teflon thread demonstrated higher sustained load capacity, less rotating friction, excellent torque retention qualities and extended life in comparison to existing components utilizing greased metal-on-metal and/or “plastic” bearing materials.
Technical Paper

Contact Fatigue Tests and Contact Fatigue Life Analysis

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.
Technical Paper

Bending Fatigue Life Analysis of Carburized Components Using Strain Life and Fracture Mechanics Approaches

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.
Technical Paper

Atmospheric Pressure Microwave Plasma P/M Sintering of Cam Lobes

A new proprietary Atmospheric Pressure Microwave Plasma Technology, developed for various materials processing applications, has been applied to P/M sintering of cam lobes. The aims were a) to compare the new processing route with conventional process for the same alloy composition and b) to check the possibility of successful sintering at higher temperatures so that different higher temperature P/M alloys may be used. P/M green cam lobes were used, and sintering runs were carried out initially at temperatures comparable to that currently used in the conventional processes; this was followed by runs at higher temperatures that are not very practical in the conventional processing route due to equipment component constraints. Properties such as density and hardness were measured for the sintered samples, together with corresponding microstructural analysis.
Technical Paper

A Study of the Effect of Multiple Braze Furnace Exposures on 304L Stainless Steel Copper-Brazed Assemblies

The effect of multiple braze furnace exposures has been questioned by many because the rework of brazed parts is a common practice in manufacturing. However, there are process controls that limit the number of exposures for an assembly due to known issues with multiple exposures. A common concern deals with the effect of multiple braze furnace exposures on the structural integrity of the base material of the components. Another concern regards the effect of multiple exposures on the structural integrity of the braze joint itself. This paper details experimental results of a physical study to investigate these questions. The material forms used are seam-welded tube and a thin-wall stamped component, both made from 304L stainless steel. The copper paste used in the study has an industry designation of ANSI/AWS A5.8 - BCu-1a.