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Several two-dimensional finite element analyses of (02/908/02) glass/epoxy and graphite/epoxy composite laminates were performed to investigate some of the characteristics of damage development due to an impact load. A cross section through the thickness of the laminate with fixed ends, and carrying a transverse load in the center, was analyzed. Inclined matrix cracks, such as those produced by a low-velocity impact, were modeled in the 90° ply group. The introduction of the matrix cracks caused large interlaminar tensile and shear stresses in the vicinity of both crack tips in the 0/90 and 90/0 interfaces. The large interlaminar stresses at the ends of the matrix cracks indicate that matrix cracking may give rise to delamination. The ratio of Mode I to total strain energy release rate, GI/Gtotal, at the beginning of delamination, calculated at the two (top and bottom) matrix crack tips was 60 and 28%, respectively, in the glass/epoxy laminate. The corresponding ratio was 97 and 77% in the graphite/epoxy laminate. Thus, a significant Mode I component of strain energy release rate may be present at the delamination initiation due to an impact load. The value of strain energy release rate at either crack tip increased due to an increase in the delamination length at the other crack tip and may give rise to an unstable delamination growth under constant load.
Research scientist, Analytical Services and Materials, Inc., Hampton, VA
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