Porosity predictions during co-cure of honeycomb core prepreg sandwich structures

Honeycomb core sandwich structures are co-cured to bond partially cured thermoset prepreg facesheets with an adhesive layer to either side of the honeycomb core under a prescribed pressure and temperature cycle in an autoclave. Defects such as porosity and poorly formed fillets of the adhesive bond-line can compromise the mechanical properties of the sandwich structure. High defect levels during the co-cure are common as the process development cycle is strongly dependent on complex relationships among material properties, part geometry and process parameters. A process model to predict bond-line porosity is formulated based on the understanding gathered about the physics of the co-cure process and the physics of bond-line formation. A three-step approach is adopted to simulate the bond-line porosity. The first step uses the knowledge of the diffusion-induced void growth in the form of a stability map to guide the design of the co-cure cycle. The second step develops the void growth model accounting for the diffusion of volatiles within the adhesive and the prepreg resin. In the final step, the voids that escape into the core from the bond-line are taken into account to calculate the porosity of the bond-line. With the help of two examples, it is shown that in addition to the initial radius of voids and the initial porosity of the bond-line, the interplay between the stability map and the process cycle also play an important role in prediction of the final porosity within the bond-line. This process model should prove useful in reducing or eliminating voids by optimizing the co-cure cycle.