Prediction of the post-impact compressive strength of CFRP laminated composites

This paper analyses the results of several selected experimental studies concerned with impact and post-impact compression damage and failure, of continuous carbon-fibre/epoxy composites, in order to predict their residual strength. Drop-weight (low-velocity) impact causes three principal types of damage in laminated polymer composites, namely matrix cracking, delaminations and fibre breakage, which together can seriously degrade the laminate monotonic compressive strength. The distribution of impact damage through the thickness, determined from sectioning studies, was roughly cylindrical in shape; ultrasonic C-scan images and X-ray shadow radiographs also indicated that the shape of the overall damage is approximately circular. Under uniaxial compression loading, local buckling grows laterally (like a crack) from the impact site as the applied load is increased. These delaminated regions continued to propagate, first in short discrete increments and then rapidly at failure load. The damage pattern is very similar to that observed in laminated plates with open holes under compression loading. Because of this resemblance, a cohesive zone model, developed initially to predict notched compressive strength, was applied to estimate compression-after-impact (CAI) strength. From independently measured laminate parameters of plain compressive strength and fracture toughness, the model successfully predicts the residual strength of various carbon-fibre systems and lay-ups; in most cases the error was less than 10%. © 1996 Published by Elsevier Science Limited.