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Time dependent mechanical breakdown of materials has been a long-standing concern for structural components, and it is especially exacerbated by randomness in material properties, loading conditions, and other sources of uncertainty. A generalized phenomenological cumulative distribution function for lifetime, which is consistent with fundamentals of reliability theory, is presented. The form of the cdf includes a kernel that incorporates functions for time dependent loading, mechanical breakdown, and statistical behavior. The formalism is applicable for a variety of loading conditions including creep rupture, rate dependent tensile loading, and fatigue. Analysis of failure under stress histories such as variable stress amplitude or step-stress loadings is manageable as well. The model is a generalization of the classical accelerated life model in which covariates are included to account for physical attributes that directly influence component lifetime.

Estimation and prediction of residual life and reliability are serious concerns in life cycle management for aging structures. Laboratory testing replicating fatigue loading for a typical military aircraft wing skin was undertaken. Specimens were tested until their fatigue life expended reached 100% of the component fatigue life. Then, scanning electron microscopy was used to quantify the size and location of fatigue cracks within the high stress regions of simulated fastener holes. Distributions for crack size, nearest neighbor distances, and spatial location were characterized statistically in order to estimate residual life and to provide input for life cycle management. Insights into crack initiation and growth are also provided.