Non-destructive inspection and bending performance analysis of sandwich panels manufactured through wet compression molding process

In this work, wet compression molding (WCM) and pressure-controlled WCM (PC-WCM) processes were applied to fabricate sandwich panels. PC-WCM slightly modifies the traditional WCM process by controlling a mold closing gap in real time to avoid a sudden increase of pressure inside a mold cavity, which may cause unwanted defects to a product. Commonly for both processes, liquid-state epoxy resin was poured onto a top skin fabric and the resin permeated into a bottom fabric through a porous foam core. Eight types of sandwich panels were fabricated by combining the following constituents: two skin types (carbon and glass fabric), two core types (PET and PVC foam), and two process types (WCM and PC-WCM). CT-based non-destructive inspection (NDI) confirmed no defects inside all the samples, resulting in non-distinguishable performances between WCM and PC-WCM sandwiches. Instead, as a process signature, it was found that the foam cores were partially impregnated by the resin. The shear moduli of the resin-impregnated foam cores were computed from the three-point and four-point bending test results. The computed moduli of the PET and PVC cores were about 200% and 20% greater than each of the datasheet properties. However, the increased shear modulus of the cores did not improve flexural stiffness of the sandwich structures. They were mainly determined by the skin properties with a marginal contribution from the cores, confirmed through a conventional sandwich theory and analysis of variables.