Strength of composites depends on the strength of fibers and interface. The fibers are subjected to shear stress due to the adhesive force of interface. Therefore, to obtain a strong composite, one needs to have strong interface. The interface strength is especially important in the case of short-fiber composites. But, as the matrix is a polymeric material, the strength of composites decreases with decreasing adhesive force of interface at high-temperatures. There are two ways to obtain the interface of high strength; one is chemical adhesion and the other is anchor effect. The former depends on temperature. On the other hand, when the surface of fiber is irregularly roughened, the strength of interface increases due to the anchor effect. But the fiber strength decreases by stress concentration with increasing surface roughness.
In the present study, to improve the strength of short-fiber composites, a new composite was developed with fibers having expanded-ends. Before the experiments, the suitable shape and size of fiber-ends were calculated by using a two-dimensional elasto-plastic FEM. The results of calculation showed that the end shape was spherical with the diameter five times bigger than that of a fiber. Practically, the ends of fibers were formed into disk shape and they were inserted between the laminas of GFRP. The tensile strength of the new composite increased by about 30% at the transition temperature of the resin.