Effect of fiber strength on tensile fracture of unidirectional long carbon fiber-reinforced epoxy matrix composites

Carbon fiber-reinforced epoxy matrix composites, with a tensile fracture stress of either 3.5 or 5.5GPa (designated as 3.5 and 5.5GPa CF/RE composites, respectively), were studied to determine the effect of fiber strength on the tensile fracture behavior. The smooth and notched tensile specimens were loaded in the direction parallel to the fibers. The 5.5GPa CF/RE composite exhibited a higher smooth tensile fracture stress but a significantly decreased notch strength ratio compared with the 3.5GPa CF/RE composite. For the smooth tensile specimens of the 3.5GPa CF/RE composite, a zigzag fracture approximately perpendicular to the loading direction occurred, and the fracture process involved a brittle fracture, or pull out, of the fibers; whereas, for the 5.5GPa CF/RE composite, the fracture was approximately parallel to the loading direction, and fiber-matrix interfacial fracture was observed. For the notch tensile tests, fracture occurred at the fiber-matrix interfaces independent of the composite type. The results are described and discussed.