Durability Assessment Based on Initial Material Quality

New U.S. Air Force durability design philosophies are aimed at minimizing evolution and growth of structural cracks to sizes affecting readiness and life cycle repair costs. Durability design requires attention to material quality and manufacturing quality, since both can be sources of in-service cracking problems. Improvement in the quality of Al-Zn-Cu-Mg, 7050 alloy, thick plate through reductions in microporosity has been correlated with increased smooth-coupon fatigue lives. Consequently, in commercial production, the smooth fatigue test is now employed as a lot release test to warrant initial metal quality. This work is aimed at translating results of smooth fatigue quality-screening tests to aircraft component performance in the context of new durability methodologies. For this purpose, a fracture mechanics analysis is used to convert the smooth coupon lifetimes to an equivalent initial flaw size (EIFS) distribution, which then becomes the starting point for probabilistic crack growth assessment in a component. The EIFS distribution back-extrapolated from life data is shown to be in good agreement with the distribution of crack initiating microflaws determined by more labor-intensive post-test fractography. Consequently, the fracture mechanics back-calculation enables cost-effective conversion of coupon lifetimes, a simple measurement, into a “material characteristic,” initial flaw size population, which can then be utilized analytically for durability design trade studies. A hypothetical component study performed in this manner for materials of varying initial quality is used to demonstrate how material quality differentiated at the coupon level can be translated into a performance advantage under representative in-service loading conditions.