Localized damage response of composite sandwich plates

The objective of this article is to derive closed-form solutions for the deformation and fracture responses of a composite sandwich plate subjected to static indentation of a hemispherical-nose indenter. The composite sandwich is modeled as an infinite, orthotropic, elastic plate resting on a rigid-plastic foundation. The facesheet deflection is several times the laminate thickness so that bending moments may be neglected and only membrane forces are considered in the facesheet. The rigid-plastic foundation force is given by the honeycomb crushing resistance. The deformation of the facesheet is found by using the principle of minimum potential energy. The elastic strain energy resulting from the membrane forces in the facesheet, the plastic work dissipated in crushing the honeycomb, and the external work are evaluated using an appropriate shape function for the facesheet deflection. The relations between the indentation load and the transverse deflection and length of deformation are obtained by minimization of the total potential energy. Minimization of the total potential energy has to be done numerically because of an implicit expression for the contact radius between the hemispherical-nose indenter and the facesheet of the honeycomb. An approximate solution for the load–indentation response is derived by assuming an average value of the contact radius. For the particular composite sandwich plates and indenters considered, the difference between the numerical and approximate solutions is about 3%. Furthermore, the approximate predictions are within 15% of the experimental results. Conservative estimates of the failure loads which cause cracking of the facesheet are predicted using the Maximum Stress and Tsai–Hill Criteria. The equations derived by the above failure criteria yield important design considerations for composite sandwich plates. It was observed that the failure load increases with the square of the ply thickness and indenter radius and is inversely proportional to the crushing resistance of honeycomb.