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The objective of this work was to investigate the fatigue damage mechanisms and to identify the influence of skin stacking sequence in carbon epoxy composite bonded skin/stringer constructions. A simple four-point-bending test fixture originally designed for previously performed monotonic tests was used to evaluate the fatigue debonding mechanisms between the skin and the bonded frame when the dominant loading in the skin is flexure along the edge of the frame. The specimens consisted of a tapered flange, representing the stringer, bonded onto a skin. Based on the results of previous monotonic tests, two different skin layups in combination with one flange layup were investigated. The tests were performed at load levels corresponding to 40, 50, 60, 70, and 80% of the monotonic fracture loads. Microscopic investigations of the specimen edges were used to document the onset of matrix cracking and delamination and subsequent fatigue delamination growth. Typical damage patterns for both specimen configurations were identified. The observations showed that failure initiated near the tip of the flange in the form of matrix cracks at one of two locations, one in the skin and one in the flange. The location of the 90° flange and skin plies relative to the bondline was identified as the dominant layup feature that controlled the location and onset of matrix cracking and subsequent delamination. The fatigue delamination growth experiments yielded matrix cracking and delamination onset as a function of fatigue cycles as well as delamination length as a function of the number of cycles.
composite materials, fatigue testing, design, bond strength, skin/flange interface, secondary bonding
National Research Council resident research associate, NASA Langley Research Center, Hampton, VA
Senior scientist, U.S. Army Research Laboratory, Vehicle Technology Center, NASA Langley Research Center, Hampton, VA
Senior tech specialist, Boeing Defense & Space Group, Philadelphia, PA