Improving Damage Tolerance of Integrally Machined Panels

Abstract

Integral structures offer large benefits in terms of manufacturing cost, but suffer often from a lower degree of fail safety compared to built-up structures. The purpose of the study is to illustrate benefits that can be obtained from an improved design of integral structures. AA 2027 test panels with local pad-ups, called crenellations, are tested and modeled. These features decrease the fatigue crack growth under constant amplitude loading. A doubling of the fatigue life is obtained, which corresponds roughly to an improvement of 15% of the stress allowable. Precise prediction of the stress intensity factor variation is not yet obtained. AA 7749 and AA2027 alloy test panels are designed to promote crack turning. This is obtained on AA2027 panels with slanted stringers. AA 7449 panels show delamination failure. These results show that integral structures, with appropriate design optimizations, should be considered even for components demanding a rather high degree of damage tolerance.