STATISTICS OF FATIGUE CRACK GROWTH IN FASTENER HOLES

Structural components in aircraft are often required to be operative beyond their original design service objectives (DSO). Vital issues for aging infrastructures are estimation and prediction, with confidence, of residual life, reliability, and availability, given the service history. Uncertainty increasingly is considered to be a major factor. Demanding high reliability exacerbates the role of uncertainty. One aspect of life cycle management for aging aircraft was investigated by replicating laboratory specimens subjected to fatigue loading that is typical for military aircraft wing skins. Samples were fabricated from 7075-T6 plate aluminum alloy similar to that used for wing panels. A total of 15 specimens were tested. Tests were terminated when the fatigue life expended (FLE) reached a prescribed value of 100%, 150%, or 200% of the component DSO. Then, microscopy was used to quantify the size of fatigue cracks within high stress regions of simulated fastener holes in laboratory specimens. Cumulative distribution functions (cdfs) for geometrical properties of cracks and constituent particles in the alloy were characterized statistically as input for residual life estimations and for life cycle management analyses. Insights into crack initiation and growth are also provided.