This paper describes recent work in which improved nondestructive evaluation (NDE) techniques for impact damage in graphite/epoxy composites have been used in conjunction with two different approaches to damage modelling to improve prediction of structural response under compressive loading.
Impact-induced delaminations occurring at different interfaces within the laminate exhibit different orientations and sizes. A model has been developed to explain this behavior, in which the forces acting on two adjacent nonparallel plies during impact are considered. Regions in which the model predicts positive peel forces have been shown to correlate closely with actual delaminations in a variety of impact-damaged laminates. The model also considers the role of intraply cracking, which provides boundaries to the impact delaminations and plays a part in forming characteristic delamination/cracking features in the damaged laminate.
Following detailed characterization of the impact-damaged zone using ultrasonic C-scanning and sectioning, experimental models have been produced in which polytetrafluoroethylene (PTFE) inclusions simulate the shape and extent of all the delaminations present in the impactdamaged zone. Compression tests on these specimens lead to failure strengths that are comparable to those of impact-damaged specimens and which are considerably lower than those of specimens containing single PTFE inclusions. These results suggest that the delaminations are more significant than ply cracks or fiber failures in determining the compressive behavior of impact-damaged laminates.
The predictive capability resulting from the modelling aspects of this work and the improvements in inspection methods provide considerable potential for accurate assessment of impactdamaged components for subsequent repair.