Variable-Amplitude Load Models for Fatigue Damage and Crack Growth

Abstract

Load models for fatigue analysis and testing are tailored to the level of complexity required for the application. Random variable models are developed and applied to analyses in which load sequence effects can be neglected. Conventional narrow-band load peak and range distributions are applied to crack initiation and growth. It is shown that narrow-band load models provide useful, conservative life estimates for general Gaussian loadings. Distributions of significant peaks and ranges for wide-band loadings are developed empirically through simulations with racetrack filtering. An efficient “sequential” simulation technique is introduced for continuous generation of both narrow- and wide-band random loads. Based on a simplified crack closure model, simulations of crack growth suggest that sequence effects are most influential when any or a combination of the following are present: larger ratios of crack opening stress to maximum applied stress, lower values of applied tensile mean stress, smaller values of yield stress and crack growth coefficient. When sequence effects are present, the regularity of the spacing between tensile overloads can be important. In particular, assumption of regularly spaced overloads can be nonconservative.

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