The interlaminar fracture toughness is a measure of the ability of material to resist delamination. The experimental determination of theresistance to delamination isery important in aerospace applications. Different kind of specimens and experimental methods are employed to measure the interlaminar fracture toughness of composite materials. The aim of the present research is to gain a better understanding of interlaminar facture of polymer matrix composites in different modes, and to develop mathematical model to predict the critical strain energyreleaserates.Emphasis has been placed on the root rotation at the crack tipwhich was believed to be a critical factor which affects the delaminationfracture toughness, and critical load. A combined experimental and theoretical study has been conducted to determine the role of root rotation on critical load. The goal of predicting the dependence of root rotation on critical strain energy release rate under mode I is achieved. The first part of the present study examines interlaminar fracture toughnes of Double Cantilever Beam (DCB) specimens based on a modified Timoshenko beam model considering the rootrotation.Load, displacement and crack length are prone to measurementerrors. A weighted residual approach is proposed in this thesis to minimize the scatter in measurements. The critical fracture energy has been evaluated based on the proposed approach and hence the influence of error in measurements has been minimized. The critical fracture energy evaluatedbased on the weighted residual approach gives a unique value for theparticular specimen. The present model requires the applied loaddisplacement history and crack extension to generate the crack growthresistance curve (R-curve). From the generated R-curve, the interface failurelod has been estimated for the specimens and the same is found to be in goodgreement with the experimentally recorded value.
