Compression Strength Prediction of Impact-Damaged Composite Sandwich Panels

The objective of this work was to develop a technique for predicting the residual compression strength of sandwich panels containing impact damage in one facesheet. The technique was tailored to predict the residual compression strength of specimens that exhibit a kink-band propagation failure mode, where kink bands are formed at the peak-strain locations adjacent to the impact damage region. Under continued compression loading, the kink bands propagate in a stable manner perpendicular to the applied load. When a critical kink-band length is reached, growth becomes unstable corresponding to panel failure. The prediction method involved an analysis that modeled the impact-damaged sandwich specimen as a single facesheet with an open hole. The method was undertaken in two stages, each representing stages of failure that were observed in testing. For stable kink-band growth, the average stress criterion was used to calculate the far-field stress corresponding to a range of virtual kink-band lengths. Unstable kink-band growth was assumed to take place when the strain energy stored at the kink-band tip reached the fracture toughness of the facesheet. Fracture tests were conducted on sandwich specimens to measure the facesheet fracture toughness. Linear elastic fracture mechanics (LEFM) was then used to calculate the far-field stress required for unstable kink-band growth. Stress values corresponding to stable and unstable kink-band growth were plotted as functions of kink–band length. The intersection of the curves was assumed to correspond to panel failure, yielding a prediction of residual compression strength. The method was calibrated against residual compression strength values of impact-damaged sandwich panels available in the literature. Initial comparison showed good agreement between experiment and analysis.