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Fiber reinforced thermoplastics (FRTP) attracts attention as a lightweight material for mass-produced automobiles due to advantages such as excellent formability and recyclability in addition to high specific strength and specific rigidity. Many factors influence the improvement of the mechanical properties of FRTP. We have revealed that the tensile strength of injection-molded glass fiber/polyamide66 (GF/PA) improves as the reinforcement fiber diameter decreases in the wide strain rate range in previous researches. In addition, it was suggested that by using a strength prediction model of discontinuous fiber reinforcement, if the strength can be obtained for one diameter, the strength can be predicted for other diameters. In this study, we investigated the mechanism of the strength enhancement of the GF/PA associated with thinning of the reinforcement fibers and increase of the strain rate. To ascertain the interface properties, single-fiber pull-out tests were conducted on the GF/PA and it was confirmed that the interfacial shear strength (IFSS) of the GF/PA had strain rate dependency. The strength prediction of the GF/PA with the strain rate dependency of IFSS taken into account showed values closer to the experimental values than the values obtained from the prediction with an assumption that IFSS is constant regardless of the strain rate. Furthermore, investigating the factor of the increase in strength due to the thinning of the fibers suggested that the increase of the fiber strength is the dominant factor.

Applications of fiber reinforced plastics have been expanding due to improvement of not only fuel efficiency but also the motion performance of some recent vehicles. Especially, the demand for injection-molded fiber reinforced thermoplastics is expected to increase because of their superior moldability, productivity and recyclability. In this study, the influence the fiber diameter has on the impact tensile properties of long glass-fiber reinforced polyamide (GF/PA) is investigated using the split Hopkinson pressure bar method. Prior to the tensile tests, an investigation of the fiber-orientation distribution was conducted in order to cut out specimens with the same fiber orientation angle from the injection molded plate. Two types of specimens, referred to as specimens with high- and low orientation angle, were manufactured using glass fibers with average diameters of 13, 17, 23 μm. In the tensile test, the GF/PA with smallest fiber diameter showed the highest tensile strength and the most significant strain rate dependency on the strength. These effects were more significant for the specimens with high orientation angle. From SEM observations on the fracture surface and an average fiber length measurement, it was observed that the interfacial fracture and the fiber breakage were dominant failure modes under the considered tensile loading conditions. It was suggested that decreasing the stress acting on the fiber/matrix interface by reducing the fiber diameter affected the improvement of the GF/PA strength. Using the modified linear rule of mixtures, the tensile strength was predicted. The predictions showed good agreement with experimental results. Therefore, it is believed that the decrease of critical fiber length is the reason that the impact tensile properties are higher for the samples with smaller fiber diameter.