Analysis of fillet function in wood-based sandwich construction

When a porous honeycomb core is glued to plane facings to make a sandwich construction, glue fillets (concave menisci) are formed around the core cell edges. It is known that glue fillets play an important role in strengthening the bond of the construction, but only few studies on the real function of the fillet have been reported. This thesis investigates the relationships between fillet size and bonding strength in sandwich construction followed by a stress analysis of the fillets. Sandwich panels with various fillet sizes were produced by means of a glue applicator of original design using a modified phenol-resorcinol resin glue, kraft paper honeycomb cores and Douglas fir plywood facings. Tensile strength tests normal to the sandwich specimens of 1 by 1 inch, and flexure tests on the sandwich beams of 3.75 by 12 inches were performed. Fillet rupture sizes and actual fillet dimensions were measured. A highly significant correlation was found between fillet size and bonding strength. Larger fillets provided greater bonding strength. When a sandwich was subjected to tensile load, a vertical shear failure took place at the center of the fillet concave meniscus regardless of fillet size. By assuming the uniformity of fillet shape, the following equation: [symbol omitted]= my + d , was found to express the relationship between the vertical shear stress [symbol omitted] at the fracture point B and the fillet height y at B, where m and d were constants. Too large fillets had tendency to form voids or bubbles within them resulting in lowering strength values. The appearance of fracture in the glueline in flexure test specimens was similar to that in the tensile test. Most of the sandwich specimens with smaller fillets failed in the glueline, while those with larger fillets mostly failed in core shear. This observation also indicated the superiority of larger fillets in bonding of honeycomb-to-plywood. The cause of glueline failure in the flexure test was deemed to result from a complex system of shear, compression and tensile stresses. However, a mathematical expression describing that system of stresses was not found.Forestry, Faculty ofGraduat