This paper proposes a 0º tensile strength prediction model for unidirectional composites with a low interfacial strength or low matrix yield stress by investigating carbon fiber reinforced polypropylene. First, a critical cluster dimension in the presence of splitting is formulated using the principles of fracture mechanics. Based on the geometry of fiber packing, a relation between the rate of fiber breakage and the probability distribution of the cluster dimension is then derived. Lastly, the assumption that the cross-sectional area separated by splitting sustains no load is used as a discounting factor to reduce the tensile stress in the unidirectional composites; a stress–strain curve of the composites can then be predicted. The discounting factor is calculated by comparing the probability distribution for the cluster dimensions estimated from the rate of fiber breakage with that of the critical cluster dimension, both of which are functions of fiber stress. When the interfacial toughness related to the critical cluster dimension is estimated from the single-fiber pull-out test and substituted into this model, the predicted tensile strengths are in good agreement with the experimental results.

A unidirectional carbon fiber reinforced plastic (CFRP) was scanned by an X-ray computed tomography (CT) system. Based on the X-ray CT images, a three-dimensional model with random fiber waviness was developed. Each fiber location was identified in a sectional CT image. Subsequently, the relative displacement of fibers between adjacent sectional CT images was obtained with a digital image correlation method. This procedure was repeated to obtain fiber waviness along the axial direction. The constructed three-dimensional fiber model showed random waviness of each fiber in the unidirectional CFRP. Finite element analysis was performed using the three-dimensional model. Simulation results showed bending and twisting deformations coupled with axial contractions during axial compression, which developed due to fiber waviness. A reduction of the axial Young’s modulus due to fiber random waviness was quantitatively evaluated.

Carbon fiber reinforced thermoplastics (CFRTP) using polyamide 6 (PA6) as the matrix resin were examined for the effects of ozone oxidation treatment and water absorption on the mechanical properties. Strength tests were conducted on CFRTP samples subjected to vacuum drying, conditioning, and temperature and humidity cycles. The relationship between the water content and the mechanical strength was clarified from the results. The water content of the samples was measured using the Karl Fischer method. In comparison with the untreated sample, the CFRTP sample subjected to ozone oxidation treatment (for both the CF fabrics and the PA6 films) exhibited higher values of flexural and tensile strengths at all the environmental conditions. This is attributed to the improved interfacial adhesion between the CF and the PA6 in the ozone-oxidized CFRTP sample even after water absorption. However, it was found that the rate of increase due to the ozone oxidation treatment tended to reduce under increased water penetration, because the interfacial adhesion between the CF and the PA6 matrix decreased.

Discontinuities in a particular pattern are introduced into a unidirectional prepreg to improve flowability while maintaining desirable mechanical properties. In the compression molding of this unidirectional discontinuous prepreg, discontinuities in a certain pattern appear, but it is difficult for finite element methods to simulate the appearance of discontinuities because constitutive laws between nodes around discontinuities are unknown. In response, we developed a method to simulate the compression molding of unidirectional discontinuous prepreg based on the Moving Particle Semi-implicit method. Particle methods are preferable to simulate this phenomenon because they do not use mesh and particles can move freely. To represent the unidirectional discontinuous prepreg, the particles were connected as fiber bundles separated by discontinuities and forces worked in/between bundles. The present method can simulate differences in the shape of discontinuities according to the patterns. And the following conclusions could be drawn: (1) The shape of discontinuities depends on the geometric position of discontinuities and the flow perpendicular to fiber direction. (2) The oblique pattern has better geometric adaptability in response to local shape change than the orthogonal pattern. (3) In compression molding, the laminate part near the edge flows farther than the laminate part around the center.