AN EXPERIMENTALLY VALIDATED MODEL FOR LOCAL PLASTICITY EFFECTS ON FATIGUE CRACK GROWTH IN AIRCRAFT STRUCTURES

摘要

Localized plasticity can commonly occur at stress concentrations in aircraft structures. This is especially true for high performance fighter aircraft, where the requirements for maximum load carrying capability with minimum weight often result in structural elements that are highly stressed. When fatigue cracks form and grow at notches or other stress concentrations in such elements, the local plasticity can significantly affect the cracking behavior. A new engineering methodology has been developed to perform damage tolerance analysis for aircraft structural integrity programs where local elastic-plastic behavior is significant. The methodology comprises several essential elements, including a analytical cyclic shakedown model that calculates the local stress relaxation and redistribution that occurs when local elastic stresses exceed yield, new weight function stress intensity factor solutions for cracks at holes with arbitrary stress gradients, and a strip yield crack closure model modified to address any crack geometry and any applied or residual stress field with a rigorous K-analogy formulation. A critical set of spectrum crack growth experiments was performed with open-hole coupons of aluminum alloy 2124-T851, including tension-dominated, fully-reversed, and compression-dominated spectra scaled to produce either substantial or limited plasticity at the edge of the hole. Conventional crack growth analysis methods were found to be generally unsuccessful at predicting the test lives, but the modified NASGRO strip yield methodology was generally successful without arbitrary calibration.