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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.

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.

Modern fatigue analysis techniques, that can provide reliable estimates of the service performance of components and structures, are finding increasing use in vehicle development programs. A major objective of such efforts is the prediction of the field performance of a fleet of vehicles as influenced by the host of design, manufacturing, and performance variables. An approach to this complex problem, based on the incorporation of probability theory in established life prediction methods, is presented. In this way, quantitative estimates of the lifetime distribution of a population are obtained based on anticipated, or specified, variations in component geometry, material processing sequences, and service loading. The application of this approach is demonstrated through a case study of an automotive transmission component.

The incorporation of reliability theory into a fatigue analysis algorithm is studied. This probabilistic approach gives designers the ability to quantify “real world” variations existing in the material properties, geometry, and loading of engineering components. Such information would serve to enhance the speed and accuracy of current design techniques. An automobile wheel assembly is then introduced as an example of the applications of this durability/reliability design package.

A concise method for modeling nonstationary fatigue loading histories is presented. The mininum number of model parameters is achieved by fitting the variations in mean and variance by a truncated Fourier series. An autoregressive moving average (ARMA) model is used to describe the stationary component. Justification of the method is made by comparing fatigue relevant parameters obtained when subjected to the original and reconstructed histories. In spite of a relatively small number of parameters required, the model is shown to give good results that fall within the bounds predicted by the orginal history.

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.

Three computer programs are presented for predicting the fatigue life of notched members undergoing complex load histories. The programs, while differing in terms of required information, employ a common approach involving the determination of the material response at the notch root, an assessment of damage based on closed hysteresis loops, and a linear damage summation to predict life. Agreement between predictions and experimental results generated by the SAE Fatigue Design and Evaluation Committee is reasonably good. The programs have purposely been kept simple to allow for easy modification. Applications and limitations of the three programs are discussed.

Substantial improvements have been made in the accuracy of component fatigue life predictions through the incorporation of cyclic stress-strain concepts in fatigue analysis procedures. This paper describes recent advances in material characterization, complex history analysis and notch analysis, and their application in computer-aided fatigue analysis procedures. A simplified strain-based approach, which offers conceptual and computational advantages, is then described and shown to be useful in many ground vehicle problems. Finally, several applications of these techniques in engineering design practice are presented.

A new section on fatigue properties of metals is currently being written under the guidance of the SAE Fatigue Design and Evaluation Committee. This paper presents a preliminary copy of the proposed section which includes an introduction, a table of monotonic stress-strain properties, a table of cyclic stress-strain and fatigue properties, definitions of properties, and an example application. The main purpose of the paper is to provide engineers with the available data at the earliest possible time. The final version of the report will be published in the SAE Handbook.

Procedures are developed for assessing the influence of various material and processing factors on the fatigue performance of leaf springs. Cyclic material properties, determined from smooth axial specimens of spring steel, are used to determine the level and cyclic stability of residual stresses resulting from mechanical processing as well as the amount of permanent deformation associated with presetting operations. A damage parameter, incorporating material properties, residual stress effects and applied stressing conditions, is used to predict failure location, i.e. surface or subsurface, and lifetime as a function of processing sequence. Predictions are found to be in good agreement with experimental bending results.

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.

An overview of the basic concepts involved in performing a fatigue analysis is presented as an introduction to a series of specialized papers dealing with the development of fatigue information and its use in engineering design. Following a brief account of the fatigue process and failure definition, methods for describing fatigue resistance are discussed. Cumulative damage concepts based on fatigue life curves are then demonstrated.

The fatigue behavior of representative classes of high-strength low-alloy and dual phase steel is reviewed. Cyclic properties describing stress-strain and strain-life relations are used to quantitatively assess material variability as well as processing and environmental effects. Examples of the use of this materials information in design analysis and, in particular, component downgaging are then presented.

Significant progress in materials and structures research leading to improved analytical and experimental capabilities for evaluating the mechanical durability of automotive structures is reviewed. Recent experiences in incorporating modern fatigue analysis methodology in easily used computer-based formats are then presented. This is followed by a general discussion of the application of design aids at various points in the product development cycle with emphasis on future trends and needs.

An overview of the current status and emerging trends in durability-related technologies is presented as an introduction to a series of papers covering applications of durability analysis in design. Problems of information management associated with technology integration are discussed along with the probable impact of new design tools on product development and validation.