Fatigue fracture of 2024-T3 aluminum plates under in-plane symmetric and out-of-plane antisymmetric mixed-mode deformations

Motivated by the engineering problem of a crack located in a pressurized aircraft fuselage near a lap splice joint, fatigue crack growth in thin 2024-T3 aluminum plates under mixed-mode in-plane symmetric and out-of-plane antisymmetric deformations is studied theoretically, numerically and experimentally. A small scale yielding approach assuming no crack closure and straight, perpendicular crack fronts through the thickness is adopted that models the crack-tip fields using a simple superposition of plane stress elasticity and Kirchhoff plate theory. Experiments are performed using a double-edge-notch specimen configuration loaded in cyclic tension and torsion from which fatigue crack growth rates are measured. This combination of in-phase cyclic loading generates both a standard {dollar}Ksb{lcub}I{rcub}{dollar} in-plane opening mode and a {dollar}ksb2{dollar} twisting/transverse shearing out-of-plane mode that comes from the Kirchhoff plate theory. Geometrically nonlinear plate finite element analyses are performed to compute the {dollar}Ksb{lcub}I{rcub}{dollar} and {dollar}ksb2{dollar} stress intensity factors for the test specimen as a function of applied axial tension load, applied torque and crack length. Fatigue crack growth rate results indicate that in the absence of crack surface roughness-induced crack closure the out-of-plane antisymmetric {dollar}ksb2{dollar} mode may drive crack growth instead of the generally assumed {dollar}Ksb{lcub}I{rcub}{dollar} mode. However, roughness-induced crack closure, caused by a complex process of crack surface contact, friction and wear, is prevalent in both R = 0.7 and R = 0 {dollar}sim{dollar} 0.1 loading ratio tests, where R is the ratio of the minimum to maximum load in the cycle (tension and torque). This closure mechanism appears to be a mitigating factor in the mixed-mode fatigue crack growth scenario in that it shields the crack tip thus reducing the actual {dollar}Delta Ksb{lcub}I{rcub}{dollar} and {dollar}Delta ksb2{dollar} experienced at the crack tip. A number of compelling results regarding mixed-mode crack growth rates, transitioning of the crack surface through the thickness to either a slant or a "V-shaped" orientation and implications of these results for fatigue cracks in aircraft fuselages are presented.