FRACTURE MECHANISMS OF THE FIBER MATRIX INTERFACIAL BOND IN FIBER-REINFORCED POLYMER COMPOSITES

The fracture behavior of the fiber/matrix interfacial bond is an important governing factor of both composite fracture behavior and its environmental resistance. Methods have been developed to investigate the ultimate and dynamic fatigue fracture behavior of the interfacial bond in both a dry and a saline environment at 37-degrees-C. A single fiber embedded within a thermoplastic microdroplet pull-out test was used as the sample model for this work. Ultimate bond strength testing was conducted on a CGS mechanical test machine while fatigue testing was conducted on a custom-built miniature closed-loop mechanical test machine designed to apply accurately controlled dynamic loads to test samples within the range 0.000-0.200 N. The fracture behavior of the interfacial bonds formed between polysulfone thermoplastic and both carbon fiber and polyaramid fiber were investigated using these methods. Results show that the ultimate and fatigue strength of the carbon fiber/polysulfone interface is significantly greater than the polyaramid fiber/polysulfone interface in both the dry and saline environments. The ultimate and fatigue strengths of the interfaces of both sample types were significantly lower in the saline environment compared to the dry condition. Evaluation of interfacial bond fracture surfaces by SEM indicated that interfacial bond fracture occurred by a combination of both interfacial adhesive and matrix cohesive failure. Results indicate that matrix cohesive failure occurs as a function of both the absolute magnitude of the applied interfacial shear stress and its value relative to the ultimate strength of the interface.