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.

Carbon fiber tape reinforced thermoplastics (CTT) have attractive features such as high formability and mechanical performance; however, CTT is known to have a lower strength and higher standard deviation of mechanical properties than continuous fiber composites for the same fiber volume fraction. In this study, a simulation model was developed to evaluate the tensile modulus and strength of CTT. Geometric parameters which were ignored in most of the previous studies, such as the resin-rich area and the variation in the number of UD tape layers, were considered. FEM (Finite Element Method) and the shortest path theory were adopted to evaluate the mechanical properties. The simulation results for the tensile modulus and strength agreed well with the experimental results obtained in previous studies. In the sensitivity analysis, the effect of the tape thickness was well expressed, while that of the tape length was too sensitive to specimen tensile strength in comparison with the experimental results. In addition, the standard deviation of the simulation results was higher than that of experimental results, indicating that further improvement in the simulation model is required for future practical applications.

This study proposed and validated a new method to evaluate the mode II interlaminar fracture toughness both at the initial delamination onset and during the crack growth stage by using a doubly end-notched tension (DENT) specimen. The specimen configuration idealized the growth of two mode II cracks in a double-lap joint. Shear-lag analysis indicated that the energy release rate of the DENT specimen is independent of the crack length and that the crack growth is stable. The initial mode II fracture toughness obtained by quasi-static tensile tests of the proposed specimens largely agreed with that obtained by the end notched flexure (ENF) tests. This trend was also valid in the other CFRP unidirectional laminates with different Young’s moduli. Furthermore, the present study demonstrated that the crack growth resistance curve (R-curve) of mode II delamination could be obtained by unloading-reloading tests of the proposed specimens.