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With the renewed current interest in high strength steel and aluminum automotive body and chassis components it has again become important to properly assess the effects of corrosion on the fatigue behavior of structures. The present work summarizes the past work on fatigue and corrosion and presents new results on current automotive materials. The Neuber plasticity correction method, used throughout fatigue software of the ground vehicle industry to account for localized plastic behavior during fatigue, was found to give a very simple and useful technique for the computation of fatigue life of materials in corrosion environments. Data is offered for many common automotive structural materials and a method is given to adapt finite element calculations to compute corrosion fatigue life.

A main requirement for a satisfactory function and service life of a forged powder metal connecting rod is the fatigue strength. Fatigue strength mainly depends on design, material, microstructure, and surface condition. Much work has been accomplished to optimize these factors, but still a variety of surface defects such as localized porosity, roughness, oxide penetration, decarburization, etc., can be developed during manufacturing. These surface defects impact the fatigue strength in various ways. The impact of the decarburized layer depth on the fatigue life of a forged powder metal connecting rod is the focus of this work. Several connecting rods were submitted to a Weibull test at the same loading pattern. After the fatigue tests, the connecting rods were divided into groups with different decarburized layer depths. Both Maximum Likelihood Estimates (MLE) and Rank Regression (RR) techniques were used to analyze test results from all the groups obtained.

A procedure for the computation of fatigue life of shafts subjected to variable amplitude independent bending and torsion loading is described. The elastic superposition technique, followed by a Neuber plasticity correction, also allows for the initial and possibly cyclic plasticity dependent residual stress states caused by induction hardening or other surface altering processing effects. The present study documents the formation and alteration of residual stresses caused by initial induction hardening and followed by a straightening process. Sample calculations are presented for two critical finite elements of a prototype shaft, and lives are predicted using a traditional equivalent axial stress and by a new procedure that searches for and computes lives at critical angles.

The fatigue overload behavior of single overlap 5754 aluminum spot welds has been investigated. As a baseline, constant amplitude tension-tension tests with an R=0.1 (=Pmin/Pmax) were conducted. These tests were compared both with several different series of high but variable mean constant maximum load tests, and with periodic overload tests. The high mean load tests, tested with maximum loads of 3560N, 2670N, 1780N, and 1330N all showed a significant reduction in the fatigue limit which ranged from less than ½ to almost 1/3 of the baseline fatigue curve. Further, the fatigue limit reduction from the periodic overload tests was below 1/3 of the constant amplitude baseline tests. The results of these tests indicate that mean loads and variable amplitude loading can both have a significant deleterious impact on fatigue life.

Increased use of powder-forged connecting rods in the automotive industry prompted an investigation into the suitability of powders from different suppliers for this application. Specifications developed by North American users call for ultra clean powders to enhance machinability and fatigue life. Powders from four manufacturers were each blended with graphite and lubricant, then pressed, sintered and forged to full density. Metallographic samples were prepared and evaluated for inclusion content. In addition, the powders were mixed to the composition of connecting rods, (C - 0.5%, Cu - 2% and MnS - 0.3%), and were similarly pressed, sintered and forged. Test bars were machined from the forged discs. Uniaxial fatigue tests were performed in the tension-compression mode and strain-life curves were developed. It was determined that all powders examined were very clean and were comparable in their inclusion content.

Shot peening is a commonly used surface treatment process used to improve the fatigue life of aircraft, automotive and other highly stressed structural components. This improvement is attributed to the formation of compressive residual stress on the surface layer of the material by the impingement of spherical media (shot). The compressive residual stress usually decreases the tensile stress created in the component by “in service” external forces and therefore increases the fatigue strength of the part. To quantify the improvement resulting from shot peening, the fatigue behavior of powder-forged connecting rods and laboratory test bars from the base material (2% copper steel), both in the stress-free (unpeened) and surface treated (shot peened) condition were compared. The fatigue data were correlated with the residual stress generated at the surface. The stress magnitude and depth were determined using x-ray diffraction analysis.

The cyclic behavior of single overlap aluminum joints joined through a number of different methods has been investigated using Alcan 5754-O, an alloy that potentially could be used in structural applications. Overlap shear tests of spot welded, clinched and riveted joints are compared on the basis of their fatigue performance. The fatigue response of the spot welded joint was the baseline to which the other fasteners were compared. Test results showed an improvement of approximately 25% for both the mechanical clinch joints and aluminum rivets in fatigue strength at 106 cycles. The most significant improvement in fatigue strength of 100% was found for the self piercing rivets at 106 cycles. The failure behavior of the various joining methods is discussed as well as the surface appearance.

A computer-based technique was used to produce fatigue life predictions for a variety of steels using service histories recorded from several automotive components. The materials considered, both hot and cold-rolled sheets, ranged in yield strength from 200 MPa (30 ksi) to 580 MPa (84 ksi) and included most of the steels currently under consideration for material substitution to achieve vehicle weight reduction. The results of these computations were used to produce estimates of the weight savings potential, in fatigue limited designs, of the substitution of higher strength materials for conventional hot or cold-rolled low carbon steel; the indicated weight savings were from 16 to 50% depending on the nature of the loading of the component. This wide variation in savings is an indication of the importance of design in the engineering for minimum weight. Differences in the fatigue notch sensitivities of the materials were found to be an important factor in the predicted fatigue lives.

Fatigue characteristics of representative cold-rolled, high strength steels, in gages ranging from 0.072 in. (1.83 mm) to 0.055 in. (1.39 mm), were determined in fully-reversed, axial strain cycling at amplitudes up to 0.01. Alloys were selected from three families of high strength steels: recovery annealed steels, conventional microalloyed steels - nitrogenized steel and rephosphorized steel, and dual phase steel. Cold rolled low-carbon steel provided a comparative baseline. Cyclic stress-strain curves are presented to indicate the degree of cyclic stability achievable by various strengthening mechanisms while relative fatigue resistance is determined from strain-life curves. The implications of these behavioral trends to component down gaging are discussed.

Fatigue properties of sheet steels are examined beginning with a brief overview of the more common strengthening mechanisms used in the manufacturing and processing of sheet products. Cyclic and monotonic flow properties are reviewed with a particular emphasis on processing variables. Strength ductility tradeoffs for sheet steels are discussed and several alloy steels are presented in terms of a Neuber-life cure. The fatigue of cast iron is approached as an internally flawed material. Fatigue life predictions are made by comparing the response of similar structure and composition cast steel to that of cast iron and then applying a Neuber analysis to the results. Fatigue results are given for both non-heat-treatable and heat-treatable aluminum alloys. Finally, the role of residual stresses induced by surface treatments is discussed.

A round-robin low cycle fatigue test program was conducted by the SAEFDE Committee using A356-T6 cast aluminum alloy. Three different microstructures representative of three solidification rates were sought, but only two significantly different secondary dendrite arm spacings, DAS, resulted. The smaller DAS had slightly greater monotonic yieid and ultimate strengths, greater permanent deformation at fracture and better low cycle fatigue resistance. Under strain-controlled axial low cycle fatigue conditions. A356-T6 was observed to cyclicaiiy strain harden and hysteresis loops were skewed toward the compressive stress from about 1 to 10 percent. Fatigue failures usually initiated at surface or near surface porosity. About 25 percent of the 173 test specimens that were considered valid failed between the strain gage knife edges and the specimen fillet radius.