Effects of fillet size on bond performance of honeycomb sandwich panels

Honeycomb sandwich structures continually find new applications in many different engineered parts. Much research has gone into improving critical properties that determine the structural integrity of these panels. One of the important characteristics that can have a large effect on the structural integrity is the bond created between the facesheets and the core. A poor bond can lead to delamination and water penetration that can ultimately lead to failure of the structure. The present study investigates this skin to core bond in honeycomb sandwich panels by following two main objectives.;First, a correlation of the strength of the bond with the size and quality of the fillets formed on the honeycomb cell walls was found and discussed. Fillets were measured using a digital microscope, and different dimensions were examined to determine which dimensions correlate best with the bond quality. The bond quality was assessed by comparing G1c values found from a sandwich Single Cantilever Beam (SCB) test. A critical size of fillet was found to exist, wherein the geometry of the fillet no longer becomes a determining factor in the energy release rate associated with disbond between the skin and core. Applications for using this critical size are also discussed.;Second, panels cured with different damming heights and shapes were constructed in order to evaluate the effect on the bond quality. Special emphasis was placed on examining the edge effect found when constructing a panel from which test coupons are obtained. It was determined that damming height and shape has a negligible effect on bond quality, and that all test coupons need to be cut at least 25 mm (1 inch) from the edge in order to avoid edge effects.