Effect of Fiber Length on the Tensile Strength of Unidirectional Composites

Abstract

This study investigates the effect of fiber length on the tensile strength of discontinuous fiber composites. Unidirectional composites with a staggered structure of chopped strands were prepared by regularly stacking sheets of unidirectionally arrayed chopped strands of a predetermined fiber length. The sheets were prepared by cutting slits into the prepreg at regular intervals along the fiber direction. The final failure is caused by an unstable growth of delamination from the tips of the strands. The conventional linear fracture mechanics model cannot explain the knockdown observed in the tensile strength with decreasing fiber length. Consequently, this paper proposes a novel fracture mechanics model that considers a non-linear stress–strain relation for the composite, which changes with the fiber length. Results confirm that this model can explain the relation between the tensile strength and fiber length very well, without any fitting parameters.