Center notch flexure sandwich geometry for characterizing skin-core adhesion in thin-skinned sandwich structures

The fracture properties of a number of sandwich structures have been investigated using a simple three point bend test geometry. The center notch flexure sandwich (CNFS) test involves loading the lower skin of the sandwich structure and propagating a central crack along the lower skin-core interface. A finite element analysis has been used to determine the crack tip loading conditions as a function of crack length and mechanical properties of the sandwich panel. The analysis agrees well with strain energy release rate data obtained via experiment, and shows the CNFS test to be predominantly mode I. The CNFS technique has been used to characterize the interfacial fracture properties of several foam, balsa, and honeycomb-based sandwich structures similar to those currently used in the marine and aerospace industries. Experimentally-determined values for interfacial fracture energy varied between approximately 170 J/m² for a glass fiber reinforced polyester/linear PVC system and 2750 J/m² for a glass fiber reinforced sandwich system based on a Nomex honeycomb core. The fracture toughness values obtained through the CNFS test agreed well with those values obtained via two different test methods. The J integral has been proposed as a fracture criterion for CNFS specimens that do not behave in a linear elastic manner; initial findings suggest this fracture criterion is applicable to CNFS specimens.