| Description |
Sandwich construction consists of using light weight cores with stiff and strong faces, and is widely used to provide high bending stiffness and strength combined with light weight. In detail, the design of sandwich structures is complicated. The bending stiffness depends not only on the stiffness of the faces, but also on the shear and compression deformation of the core. A number of competing failure modes can limit the bending strength, depending on the test conditions, materials, and dimensions considered. All of these factors must be understood in order to design efficient sandwich structures. The present investigation considers the stiffness and strength of sandwich beams that use glass fiber cloth faces made with a wet lay-up of epoxy resin and with rigid polyurethane cores. While other materials such as metals or carbon/epoxy composites are used for face materials, glass cloth is a widely used material in cost-sensitive applications. Expanded polymer foam cores, such as the polyurethane foam used here, are also widely used in cost sensitive applications. Sandwich beams were made with these materials, using two different face thicknesses, three values of foam density, and three values of foam thickness for the cores. These specimens were tested in three-point bend tests to determine the bending stiffness and strength. The stress and strain distributions in the specimens were predicted using a planar elasticity solution that has been shown previously to agree with both detailed finite; element analysis and experimentally measured strain distributions. Predictions were made of the bend stiffness, as well as the bend strength as limited by either failure of the faces or failure of the core in either shear or compression. Comparisons of measured and predicted bending stiffness showed good agreement for most tests. The specimens were found to fail either in the face, or in the core, depending on the combination of face thickness, core thickness, and core density. The predictions of failure in the face material and failure of the core in compression showed reasonable agreement with experiment. The shear failures occurred at about half of the predicted value, and this is believed to occur because of a weak; interface between the core and the face, although this could not be proved definitively. |
