| Fiber-reinforced polymer composites are being studied for many applications because of their potential for outstanding mechanical properties. To be used in new applications, composites have to be more fully understood. The fiber, the matrix, and the fiber/matrix interface are the building blocks that need study. This research has concentrated on the interface where debonds, or damage need to be studied more thoroughly. Several experimental methods are used for researching interfacial properties of polymer composites. The fragmentation test, the microbond test, the pull-out test, the microindentation test, and the push-out test have all been used for interfacial analysis of composites, but all these tests involve complex interfacial stress states. The experimental work here focused on the fragmentation test. The specimens used were AS4-carbon/Epoxy and E-glass/Epoxy composites. In fragmentation specimens, a single fiber is aligned axially in the cavity of a dog-bone shape silicone mold, and then an epoxy matrix is cured around the fiber. The resulting tensile specimen is loaded in tension. The tensile loads on the fiber cause the fiber to fracture within the matrix. The fiber fracture process continues until the fiber breaks into smaller fragments. Eventually the interfacial forces no longer induce fracture in the fiber. In this research, the fragmentation process was continuously monitored with careful attention to debonding at each break, debonding growth at higher strain, and matrix cracks, all as a function of applied strain. Photoelastic birefringence patterns in the specimen were observed in detail to accurately monitor interfacial debonding and damage.; The observations of debonding were interpreted using fracture mechanics methods. The application of fracture mechanics to composites is not simple, because composites are anisotropic and heterogeneous with the fiber and matrix having different properties such as disparate thermal expansion coefficients. In the past, simple elastic-plastic stress analysis methods were used to analyze fragmentation test results in terms of an interfacial shear strength, τ. The analysis in this research revised an energy method for analyzing interfacial debonding by including residual stress and friction effects in the analysis. This new analysis can be used with detailed debond observations to deduce interfacial toughness properties. |
