Compressive failure mechanisms in composite sandwich structures

The objective of this work was to investigate the compressive failure response of composite sandwich structures of four different configurations designed to produce different failure modes. Specimens were tested with no damage, with 0.25-inch-diameter open holes, and with three levels of impact damage. The facesheets consisted of two, three, or four layers of carbon/epoxy plain-weave fabric. Nomex and aluminum honeycomb were used as core materials, Strain gages were used to monitor the in-plane strain distribution around the damaged area. Out of-plane displacements were recorded using shadow moire interferometry to monitor initial signs of facesheet instability. After compression testing, representative specimens were sectioned to document the internal damage. The influence of damage state was documented for each specimen configuration. For Nomex-core specimens, the effects of facesheet thickness on strength and strains were investigated. The effect of core material was studied for the four-layer facesheet specimens. Strain distributions around the damage and the load carried by the damaged region were examined as a function of applied stress level. Depending on core material and damage type, different responses were identified. Two distinct compressive failure mechanisms, inward and outward buckling of the impacted facesheet, were identified that were functions of facesheet thickness and damage type, The experimental observations of these failure mechanisms were related to current damage tolerance analysis methodologies. Wrinkling analysis appeared to model the correct undamaged failure mechanisms but needs to be improved to more accurately predict the influence of facesheet thickness and core materials on strength. For open holes, the point stress/strain and average stress/strain criteria may not be appropriate as they are not sensitive to facesheet thickness or core stiffness. Modeling impact damage as an initial facesheet indentation on a damaged core appeared to be a suitable analysis for the inward-type buckling failure mechanism.