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A simple test specimen configuration is proposed to evaluate the bond strength between a skin and secondarily bonded or co-cured stringer or frame when the dominant loading in the skin is flexure along the edge of the frame. The specimen consists of a flange bonded onto a skin, loaded either in a three- or four-point bending test fixture. Two configurations for the flange termination were examined, either square-ended or with a 20° taper. Observations show that failure initiates at the tip of the frame flange, either in the adhesive or in the topmost skin ply. Loads at which failure initiates were recorded, as well as the load at which a delamination propagates along the skin/flange interface. Detailed finite element models were built to examine the stress distribution in that critical area. Two failure modes were considered, failure in the adhesive material due to maximum tensile stress and formation of transverse cracks in the skin topmost ply due to maximum transverse principal tensile stress. When applying the experimentally measured failure initiation loads onto the finite element models, results show that the calculated transverse stress in the top skin ply is very close to the transverse strength of a unidirectional tape ply for all the configurations tested. Results also show that the stress levels in the adhesive are almost identical for all the configurations tested.
composite materials, testing, design, bond strength, skin/flange interface, secondary bonding, co-curing
Senior technical specialist, Boeing Defense & Space Group, Philadelphia, PA
Senior research scientist, U.S. Army Research Laboratory, Vehicle Structures Directorate, NASA Langley Research Center, Hampton, VA