Estimation of Residual Life and Failure Mechanism of Cracked Aircraft Wing Skin

Abstract

Many failures in aircraft structures are due to fatigue cracks initiating and developing from fastener holes at which there are large stress concentrations. In a typical wing skin, in the zone of riveted joint of rib/skin, the combination of high stress concentration could potentially lead to the appearance of the crack initiation and then crack growth under cyclic loading. Stress Intensity Factor (SIF) solutions are required for the assessment of fracture strength and residual fatigue life for defects in structures. In this context, many research works focused on evaluating the residual life of various cracked aircraft structures but only a few works have been done on light transport aircraft wing skin. The material used for wing skin is AL 2024-T351. A computational model for estimating the residual fatigue life of cracked wing skin is proposed. The complete computation procedure for the crack propagation analysis using low-cycle fatigue material properties is illustrated with the damaged wing skin. Initially, stress concentration effects at the cracked wing skin rivet holes are determined by applying analytical and numerical methods. Further, residual life and the failure mechanism in the cracked rivet holes of the wing skin are estimated. The wing skin with two cracked rivet holes for a pitch of 26 mm was analyzed using MSC NASTRAN/PATRAN for different crack lengths using MVCCI (Modified Virtual Crack Closure Integral) method by which strain energy release rate as well as stress intensity factors are calculated for different crack lengths, and fatigue crack growth life for progressive cracks for different R ratios is computed using a MATLAB program. Comparisons of the stress intensity factors estimated by FE analysis were in good agreement with the analytical solutions. Further, using SIF solutions, the residual life was estimated for the cracks emanating from the two rivet holes using crack growth models. The work also investigates the first failure mechanism out of two competing mechanisms of failure; Failure due to fracture or Failure due to plastic collapse at the net section between two advancing crack tips of the rivet holes of the wing skin. It was observed that the wing skin with crack rivet holes would fail by plastic collapse due to net section yielding. Further, the study can be extended to a multi-axial stress condition.