Influence of stiffness increase on a wavy single fiber composite

摘要

Experiments were conducted by using composite specimens containing a single carbon fiber embedded in an epoxy matrix. The fibers were cast in curved geometries, and the specimens were loaded in tension. Increasing the tensile load on the single fiberepoxy 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 fiber/matrix interfacial debonding, observed near the broken fiber ends in all specimens, was much more pronounced when the fiber was at an angle 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 composite's cold sensitivity. The major conclusions are as follows: 1) When fibers are wavy, they are not loabed 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. These two sources lower the strength of unidirectional composites at low temperatures.