On the fatigue and fracture behavior of necked double shear lugs for aircraft applications

The fatigue and fracture behavior of double shear lugs subjected to axial loading is investigated. The focus is on specific shapes, so-called waisted or necked lugs. These structural components used in aircraft interior are prone to fatigue loads. Three different sizes of necked double shear lugs made of high strength aluminum 2024-T351 and steel 17–4 PH are tested using constant amplitude cyclic loadings with a load ratio R = 0.01. Measurement data is used to identify the number of cycles to crack initiation and final fracture. Fatigue tests show that cracks initiate either at the inside or outside surface of necked lugs. However, no clear dependency on the load amplitude, lug size and material could be found. Numerical simulations using both conventional finite element method (FEM) and extended finite element method (XFEM) are performed to calculate the stress intensity factors (SIFs) for multiple crack lengths of straight and necked double shear lugs. Calculated stress intensity factors for straight lugs fit well to stress intensity factors reported in literature. Stress intensity factor curves of inside and outside cracks of necked lugs plotted with respect to crack length, cross each other, which could have an influence on the fracture behavior observed in fatigue tests.