Fatigue strength assessment of laser stake welds in web-core steel sandwich panels

Abstract

Industries nowadays demand lightweight and space-saving solutions such as sandwich panels that offer high strength-to-weight ratios. In general, sandwich panels consist of homogeneous core material that carries a shear force and face plates that carry a bending moment. The web-core sandwich panel has an inhomogeneous core as it consists of unidirectional web plates that are periodically distributed and connected to the face plates by laser stake-welded T-joints. The joint has two crack-like notches on each side of the weld. When the panel bends, the out-of-plane shear deformation opposite to the web direction causes local bending of the joined plates in the vicinity of the weld. This leads to tension at one notch tip and compression at the other. The fatigue crack initiates at the tensile tip and propagates through the weld under cyclic loading until the plates are separated and the sandwich effect is lost. The thesis investigates the fatigue resistance of laser stake-welded T-joints in the web-core panels under out-of-plane loading, i.e. lateral loading. The bending of the joined plates in panels can cause contact between the surfaces of the compressive notch. It is determined that the contact causes an increase of the stiffness of the T-joint in the case of significant face plate deflection since it results in a large rotation of the joint. The increase in the stiffness of the joint needs to be considered in the assessment of the global response of the panel, which defines the displacements that are imposed as the loads in the local strength assessment of the joint. If panel contains a low-stiffness filling material inside the voids, the contribution of the material increases the shear stiffness of the panel and reduces the deformation of the joint. Thus, under the same external loading, the filling prolongs the fatigue life of the panel. The evaluation of the J-integral at the tips of the notches, modelled using statistically mean notch depth values, gave agreement on the fatigue strengths between all the series that were investigated at five million load cycles. The agreement is obtained regardless of plate thicknesses, the type of loading or whether the filling material is present. Good agreement with other steel joints is also demonstrated at five million load cycles. However, the slope of the fatigue resistance curve varies, depending mainly on the load type, i.e. tension or bending. The research determined that the slope depends on the distribution of the maximum principal stress in the vicinity of the notch tip. When the stress is characterized by a dimensionless gradient, evaluated in the stress direction, the slope-gradient relation appears linear. The gradient shows sensitivity towards the loading type and the plate thickness effect.