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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.

Data are reported for smooth specimens of SAE 4340, RQC-100, and Man-Ten steels subjected to constant stress amplitude cycling below the conventional fatigue limit. The presence of occasional large strain cycles, or overstrains, in the history resulted in finite lives. Possible causes of this effect are discussed, and it is shown that the deformation response of these steels is altered by overstrain. Inelastic strain-life data for the periodic overstrain and initial overstrain tests fall on the same line in a log-log plot that represents the nonoverstrain tests at shorter lives. The use of inelastic strain as a fatigue damage parameter is discussed.

A cooperative testing program on fatigue crack growth rate was organized by a division of the SAE Fatigue Design and Evaluation Committee. Two structural steels in plate form typical of those used in the ground vehicle industry were tested, with seven laboratories performing fatigue crack growth rate tests. Excellent agreement among laboratories is obtained in control tests at a load ratio of R = 0.1. Test data are reported for additional R values ranging from -1.0 to 0.8, and also for cracking both parallel and transverse to the rolling direction.

Various methods for estimating fatigue life for machine or structural components are compared and discussed. Guidelines are offered to aid in choosing methods or combinations of methods for particular situations, such as different stages of design. The methods discussed include those based on local strain, nominal stress, component test data, and fracture mechanics. It is noted that the exact choice of a method or variation of a method may be an unimportant detail when one considers the uncertainties which often exist as to the actual load history in service or the time dependent effects of a hostile environment. Also, it is important to make maximum use of any component test data or service experience data which are available.

Fatigue lives are predicted by a variety of methods, including stress-life curves, local stresses and strains, fracture mechanics, and service simulation tests. These approaches are reviewed as to their advantages and disadvantages, and also as to their relationships with one another and their place in the design process. Life prediction for variable amplitude service-type load histories is considered in some detail. It is concluded that it is important to apply appropriate cycle counting and to account for interactions among high and low stresses. It is suggested that life predictions which consider both crack initiation and crack growth are often appropriate.