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Delamination is the most commonly observed damage mode in laminated composite materials. A major source of delamination damage is from low-velocity impact. In thin composite laminates under point loads, matrix cracks develop first in the plies, and delaminations then grow from these cracks at the ply interfaces. The purpose of this study was to quantify the combined effects of bending and transverse shear loads on delamination initiation from matrix cracks in composite laminates. Graphite-epoxy laminates with 90-deg plies on the outside were tested and analyzed to provide a two-dimensional simulation of the back-surface damage observed during low-velocity impact. Three plate-bending problems were considered: four-point bending, three-point bending, and an end-clamped center-loaded plate. Under bending, a matrix crack will form on the tension side of the laminate, through the outer 90-deg plies and parallel to the fibers. Delaminations will then grow in the interface between the cracked 90-deg ply and the next adjacent ply. Laminate plate theory was used to derive simple equations relating the total strain energy release rate G, associated with the delamination growth from a 90-deg ply crack, to the applied bending load and laminate stiffness properties. Three different lay-ups, [904/0/±45]s, [904/03]s, and [903/0/±45]s, were tested and results compared. Test results verified that in all cases the delamination formed at the interface between the cracked 90-deg ply and the next adjacent ply. Calculated values for total Gc from the analysis showed good agreement for the three lay-ups and the three different test configurations. Using an assumed value for GIc from a previous study, the analysis was able to predict the delamination onset load for the cases considered. The result indicates that the opening mode component (Mode I) for delamination growth may be much larger than the component due to interlaminar shear (Mode II) from a matrix crack.
composites, delamination, matrix crack, strain energy release rate, bending, graphite/epoxy
Research engineer, Aerostructures Directorate, U.S. Army Research and Technology Laboratories (AVSCOM), NASA Langley Research Center, Hampton, VA
Graduate student, Texas A&M University, College Station, TX