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Effect of Matrix Stiffness on Wavy Fiber Behavior in Single-Carbon- Fiber-Epoxy Composites
COLD REGIONS RESEARCH AND ENGINEERING LAB HANOVER NH
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This research identifies the mechanisms responsible for lowering the tensile strength of unidirectional polymer matrix composites at low temperatures. Since the stiffness of polymer matrices increases and toughness decreases when the temperature is lowered, the effect of low temperature was simulated by changing the matrix stiffness and toughness. Composite specimens containing a single carbon fiber embedded in an epoxy matrix were cast. The fibers were cast in curved geometries, and the specimens were loaded in tension. The fiber and interfacial failure processes were observed under polarized and unpolarized light through an optical microscope. Increasing the tensile load on the single fiber-epoxy specimens broke the embedded fiber into small fragments, whose lengths were smaller in the regions where the fiber was lying parallel to the loading axis. A significant fibermatrix interfacial debonding, observed near the broken fiber ends in all specimens, was much more pronounced when the fiber was to the loading axis. Transverse tensile stresses at the interface caused this interfacial debonding. Specimens with higher matrix stiffness had long matrix cracks at the broken fiber ends, which were perpendicular to the fiber axis. These matrix cracks tend to propagate perpendicular to the fiber axis, increasing the composites cold sensitivity. The major conclusions are as follows 1 When fibers are wavy, they are not loaded to their full capacity because of premature interfacial debonding started by the interfacial shear stresses and the transverse tensile stresses. The transverse tensile stresses at the interface are not present in the straight fiber specimens. 2 At higher stiffness and lower toughness values, the matrix cracks emanating at the broken fiber ends make the composite weaker.
APPROVED FOR PUBLIC RELEASE