FULL-FIELD DAMAGE ASSESSMENT OF NOTCHED CFRP LAMINATES

The work presented in this thesis constitutes the first dedicated application of surface full-field experimental techniques to the comprehensive damage assessment of open-hole compression (OHC) in composite laminates, under both static and fatigue loading. The relevance of the work comes from OHC being one of the two main tests used in industry to measure damage tolerance of composite material systems.
The main motivation for the work is the existence of a gap in the published literature pertaining to the location of the occurrence of different damage events during the life of notched composite structures. Additionally, the effect of toughening laminates by interleaving of particles, intended to improve the damage tolerance, was studied. As such, the main goal was to demonstrate the viability of using full-field non-contact experimental techniques to study the evolution of damage in notched carbon fibre reinforced polymer laminates. The specific techniques used were thermoelastic stress analysis (TSA) and digital image correlation (DIC).
It was found that a characteristic damage sequence is independent of the material system and that final failure of the laminate is controlled by the development of crush zones at the east and west sides of the hole. These crush zones result from the collapse of kink bands whose development is in turn controlled by matrix cracking early in the life of the laminate. Hence, by characterizing the sequence of damage events and their occurrence in notched coupons, the design allowables of actual composite structures can be better approximated. Pertaining to the effect of particle interleaving, statistical analysis of life data demonstrated that it could not be concluded that this kind of toughening improves the OHC fatigue life of the laminates tested.
The work presented in thesis thereby demonstrates that TSA and DIC can be applied to the study of damage in composite laminates and, thus, represents a significant step towards an improved understanding of damage morphology and evolution in heterogeneous materials.