New four-point bending based method for impact damage tolerance assessment of sandwich panels

When more and more load-bearing composite structures use sandwich construction, the need for a standardised test method to evaluate its compression-after-impact (CAI) strength becomes pressing. At present, there is no established standard CAI test method for sandwich panels. While a direct adaption of the end-loaded CAI method for monolithic laminates with up-scaled in-plane dimensions shows some promise, the sandwich material cost of such approach for generating CAI data could be prohibitive. This work intends to develop an alternative simple low-cost CAI test method via four-point bending (4PB) for as-received sandwich panels without the need for either panel ends machining or using a specific test jig. An ultimate challenge of developing 4PB CAI test method is to find a balance between two contradictory requirements, namely, securing flexure failure in the compressive skin of sandwich panels and low overall cost. Such balance should encompass a wide range of combinations of intrinsic parameters for sandwich configurations. To this end, a total of 15 different sandwich configurations were constructed with two skin thicknesses, each in both cross ply and quasi-isotropic lay-ups, with three core densities, in both symmetrical and unsymmetrical constructions. In the majority of the configurations, skin laminates were made of carbon/epoxy but E-glass/epoxy was also used. A hole was drilled at the centre of selected specimens to simulate impact damage and a range of diameters was selected to offer a range of hole diameter-to-beam width ratios. These sandwich specimens were tested in 18 different set-ups. For the given support span-to-thickness ratios and loading arm, the combinations of core density and compressive skin thickness were found to be the most significant factor of influencing failure mechanisms. The 4PB CAI method worked well for beams with thin skin thickness and high-density core. It was found that having a weaker tensile skin in unsymmetrical beams proved to be better than a weaker compressive skin in failing the compressive skin due to their substantially reduced flexural rigidity and a lesser demand on the magnitude of the loading arms. On the contrary, beams with a core of 70 kg/m3 density or less and 2 mm thick compressive skin were found to fail in the through-the-thickness shear, along with baseline beams.