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Experimental investigations indicate that the damage tolerance of graphite/epoxy systems, as measured by the laminate's residual compression strength after low-velocity impact, is directly proportional to the neat resins strain to failure. Results also indicate that laminate damage tolerance is significantly increased when thin, discrete layers of tough, ductile resin are “interlayered” or placed between the graphite/epoxy lamina. The physical mechanics which lead to this improved impact resistance are examined in this investigation. Interlayered and noninterlayered 36-ply pseudoisotropic [(±45/0/90/0/90)2/±45/0/90/±45]s laminates were subjected to low-velocity, drop-weight impacts. Incident impact energy levels ranged from 1110 to 8890 N · m/m of laminate thickness (that is 250 to 2000 in. · lb/in. (Note: Original measurements were in English, which appear in parentheses in this paper.) The resulting impact-induced damage was characterized by a combination of destructive and non-destructive test methods, including ultrasonic C-scan, light microscopy, and scanning electron microscopy. The differences in the amounts and in the distributions of impact-induced damage in the interlayered and noninterlayered laminates will be discussed in detail. The ultrasonic C-scan data indicate that interlayering significantly reduced the size of internal delamination at all impact energy levels. Microscopic investigation of sectioned, polished impact specimens indicates that the pattern of damage development was also altered through interleafing. Although transverse cracks developed in the interlayered laminates at intermediate impact levels, they do not form delaminations at the ply interfaces as they do in the uninterleafed laminates. Damage at these levels is limited, for the most part, to a system of transverse cracks. While interlayered laminates do delaminate at the high-impact energy levels, the number and size of the delaminations are reduced.
impact damage, interleaf, delamination, transverse cracks, hackles, resin toughness, damage patterns, Mode II strain energy release rate, residual compression strength
Senior research engineer, American Cyanamid Co., Stamford, CT