The bonding property of interfaces between matrix polymers and fibers directly affects mechanical properties of injection molded parts of short-fiber reinforced thermoplastics. The bonding property is an important factor for predicting accurate deformation of the parts. Hence, we have evaluated micromechanics models proposed by Nairn et al. and Hashin, which can consider a degree of the bonding by defining a spring constant on the interface. In this paper, the Nairn–Hashin (NH) models are examined using injection molded square plates of short glass fiber reinforced polypropylene. The spring constants and the tensors of fiber orientation are first identified by measuring the elastic moduli for small specimens cut out from several locations and directions in the molded plates. Stiffness matrices are then calculated from the spring constants and the tensors with the NH models. By using the stiffness matrices, finite element analyses are finally performed for bending specimens cut out from the molded plates. The values for the simulated bending loads of the specimens almost agreed with the measured ones.

Fiber waviness is one of the process-induced defects that causes significant reduction in compressive strength of carbon fiber composites. In this study, eddy current techniques for noncontact visualization of waviness distribution are newly developed. In-plane waviness in a carbon fiber composite is visualized by visualizing the path of the eddy current flowing along the carbon fiber through magnetic field measurements. Finite element analyses show that the shape of in-plane waviness can be visualized in the distribution of the magnetic field from the eddy current. However, it is found that in-plane waviness with a smaller angle than the actual misalignment angle is visualized as the distance between tested material surface and measurement plane increases. To avoid the underestimation of the waviness angle in non-contact measurements, a magnetic imaging method to reconstruct the surface magnetic field is proposed. The magnetic field on the sample surface is reconstructed using the magnetic field data obtained in the area away from the surface. Experiments were performed for a cross-ply laminate with an artificially induced in-plane waviness. The distribution of in-plane waviness can be successfully visualized from the measured magnetic field. The angle of waviness can be measured with an error of 1° using the proposed magnetic imaging method for non-contact measurements.

A new 3D printer equipped novel nozzle structure for continuous carbon fiber reinforced thermoplastics (c-CFRTP) was developed and the suitable printing conditions were studied. c-CFRTP filament and additional matrix resin were supplied independently using each extruder, which is useful for variety printing and precise form control in 3D printing. To measure the mechanical properties, specimens for tensile strength testing were fabricated by using c-CFRTP filament (V_f: 50%) without additional matrix resin. The experimental results indicate that the tensile strength and Young’s modulus were 700 MPa and 53 GPa, respectively. The re-crystallization effect through annealing after 3D printing yielded no drastic improvement. The mechanical properties were considerably improved by a hot press process after 3D printing. The tensile strength and Young’s modulus increased to 1,400 MPa and 90 GPa, respectively. These results suggest that one of the useful applications of c-CFRTP 3D printing technology is preforming of small parts in industrial products.

We have developed a model for predicting the tensile strength of discontinuous carbon fiber reinforced polypropylene composites with an open circular hole. To investigate the fracture mechanisms, composite specimens with different open hole diameters were tested in tension, and their fracture morphology was subsequently observed using a scanning electron microscope. Based on fracture surface observations, a Duva–Curtin–Wadley model was chosen as the fiber breakage criterion. This was incorporated into an equivalent inclusion model combined with the Mori–Tanaka theory to predict the tensile strength of the composites. The equivalent material properties together with the Weibull fracture criterion acquired from the tensile-loading experiments on the composites were implemented in a meso-scale finite element simulation package. It was shown that the predicted trend of decreasing tensile strength with increasing open hole diameter was reasonably consistent with the experimentally-obtained data. The discrepancy between the experimental tensile strength and the model predictions was less than 9%, indicating the validity of the simulation model developed in this research.

Ultrasonic waves are applied to detection of uncured regions in glass-fiber reinforced plastic (GFRP) plates. Polymer-based composites play an increasingly important role in the field of civil engineering because of their high specific stiffness, strength, and excellent corrosion resistance. For example, GFRP pipes are used to rehabilitate aged sewerage pipes. Uncured GFRP tubes are inserted in the aged pipes, and are then inflated and cured with steam blowing to create close-fit pipes. In this cured-in-place pipe (CIPP) method, an uncured region may be left in the GFRP pipe, so the constructed pipe must be inspected to ensure the integrity of the reinforcement. In this study, for detection of such an uncured region in GFRP structures, the effect of resin cure on ultrasonic propagation characteristics is investigated experimentally. Through-thickness and in-plane ultra-sonic measurements are conducted, and their sensitivities to resin cure are compared. In addition to wave attenuation and velocity, several AU parameters are evaluated from the power spectrum of a broadband signal. It is found that the A2 parameter, which represents the centroid of the power spectrum, has a good correlation with the mechanical properties of GFRP plates, and is expected to improve the detectability of uncured regions.