A study of the fracture behavior of unidirectional fiber-reinforced composites using Coherent Gradient Sensing (CGS) interferometry

The feasibility of using Coherent Gradient Sensing (CGS) interferometry for studying the fracture behavior of unidirectional fiber-reinforced composit es is investigated in this paper. First, the solution for the deformation f ield surrounding the tip of a crack in an orthotropic material is summarize d. Specifically, the most singular term in the asymptotic expansion is expl icitly presented. Then, the quantities that relate to the CGS measurements are derived in terms of the spatial position, stress intensity factors, and material constants. Based on these results, synthetic CGS fringe patterns are plotted numerically, and the effects of material anisotropy and crack-t ip mixity on the shape of CGS fringe pattern are investigated. in addition, a finite difference interpretation of CGS fringes caused by the finite spa cing of the CGS diffraction gratings is taken into account in the simulatio n. Finally, the initiation fracture toughness and the subsequent resistance curve behavior of a particular unidirectional graphite/epoxy composite are measured using the CGS method. The optically measured stress intensity fac tors compare successfully to values obtained from the load measurements and the available analytical solutions.