Tests and Interpretation of Small Fatigue Crack Growth in Metallic Rotorcraft Structures with Emphasis on the Statistical Characteristics

摘要

This report presents the results of an experimental program on the growth of small, multisite fatigue cracks on smooth surfaces in 7075-T7351 aluminum. It was shown that the resulting population of measured cracks was bimodal. That is, it consisted of primary cracks that can grow until failure by fracture in each specimen, and secondary cracks that ultimately arrest. An experimental procedure for separating the two distributions was developed and used to obtain confidence intervals for crack length versus loading cycles. A microstructurally based transition point between small and long crack behavior was also developed and used. Topics for additional research that need to be further developed and applied to the bimodal model of cluster small crack growth are presented.