Observations of Fiber Fracture and Interfacial Debonding Phenomena Using the Fragmentation Test in Single Fiber Composites

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Abstract
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. The experimental work here focused on the fragmentation test. The specimens used were AS4-carbon/Epoxy and E-glass/Epoxy composites. The fragmentation process was continuously monitored with careful attention to debonding at each break and debonding growth at higher strain as a function of applied tensile strain. Photoelastic birefringence patterns in the specimen were observed in more detail than ever to accurately monitor interfacial debonding and damage. The fiber fracture and the debonding length between the fiber and the matrix were measured only by visual observation using the photoelastic patterns rather than the Laser Raman Spectroscopy (LRS) which has usually been used in measuring interfacial debonding length or damage. This visual measurement technique provides the much simpler and more precise method than traditional LRS in measuring the fiber fracture and the interfacial damage in fiber-embedded composites. From the experimental data of fiber breaks and interfacial debond length, the interfacial toughness between the fiber and the matrix was analyzed for each composite system using the energy method. The interfacial debond energy, Γd, of AS4-Carbon/Epoxy was 220 J/m2 and 130 J/m2 for instantaneous debond and whole debond, respectively. For E-glass/Epoxy, Γd was obtained as 105 J/m2 and 98 J/m2 for instantaneous debond and whole debond, respectively.