Strength of composite sandwich panels containing debonds

A method for determining the critical debond size between the facesheet and the core in composite sandwich panels under in-plane compression is described. The approach uses fracture mechanics together with a buckling criterion for a debonded faceskin. The technique yields predictions for the critical in-plane compressive load for debond propagation as a function of core-to-faceskin debond size, faceskin thickness, lay-up, composite material properties, and honeycomb properties and geometries. A computer program, developed in this work, calculates the critical buckling load and facesheet deformed shape by solving an eigenvalue problem. An experimental study was conducted to determine the onset of delamination buckling in composite sandwich panels containing such flaws. Sandwich panel specimens of graphite/epoxy faceskins and aluminum honeycomb core were constructed with embedded delaminations and with varying faceskin thicknesses and core sizes. Four-point bending tests were conducted such that the faceskin containing the debond was under in-plane compression. The predicted critical fracture loads, computed using the proposed theoretical models which were solved using a numerical computational scheme, closely followed the experimental measurements.