Thermoplastic composites reinforced by discontinuous and long fibers are suitable for continuous production processes because of their high fluidity during compression molding. In addition, the molded structural part possesses high strength and rigidity as well as a light weight. However, the fibers flowing during the molding process disperse changing their orientation and density, and consequently, the consolidated molded part exhibits local variations in the mechanical properties depending on the state of the fibers inside. When the molded part is segmented into sections of a certain size, the strength characteristics fluctuate during each section. This makes it difficult to design and manage the strength of the molded part. In this study, we used three-dimensional X-ray CT to visualize the state of the fiber reinforcement in long-fiber reinforced thermoplastic composites and quantitatively evaluated fiber volume fraction and orientation degree through image analysis of each segmented compartment. A predictive method was proposed to theoretically calculate the strength of molded parts, including the strength of each compartment, and statistically analyze a series of the strength values. The method was validated by comparing the predicted strength values with the experimental results.

The interlaminar strengths of cross-ply carbon fiber-reinforced plastic (CFRP) laminates were evaluated experimentally by combined out-of-plane tensile and in-plane shear stress tests. Spool-shape CFRP laminate specimens glued to T-shape jigs were prepared, and a tensile load or torsional torque was applied to the specimens using a combined tension-torsion testing machine. Uniaxial out-of-plane tensile and interlaminar shear tests were then carried out to obtain the nominal interlaminar tensile and shear strengths of the cross-ply CFRP laminates. The fracture surfaces of the specimens were thereafter observed using a digital microscope and a scanning electron microscope. Smooth fracture surfaces were observed in the out-of-plane tensile tests, whereas rough fracture surfaces with hackle-like fracture patterns were observed in the interlaminar shear tests. The combined out-of-plane tensile and in-plane shear stress tests were subsequently performed on the specimens for five nominal stress ratios. The critical nominal out-of-plane tensile stress decreased as their nominal in-plane shear stress increased. In contrast to uniaxial tests, combined stress tests showed complex fracture surfaces. The critical nominal stresses obtained could be approximated by an ellipse as a failure criterion for interlaminar areas of cross-ply CFRP laminates.

Short carbon fiber reinforced thermoplastic (SCFRTP) composites are expected to reduce production cost by improving production efficiency and increasing the fuel efficiency of transportation equipment. An SCFRTP material is a composite material fabricated by mixing short carbon fibers of length 0.1–10 mm with thermoplastic resins. It can be easily and rapidly molded by pouring it into molds. However, fiber orientations are randomized owing to the flow of the bulk material during molding. This hinders an accurate evaluation of the mechanical properties of the material, such as strength and rigidity, and a reliable evaluation method remains to be developed. In this study, we aimed to establish a macro-scale strength model that considers the orientation uncertainty of SCFRTP fibers. The modelling approach was verified through micro-scale fracture simulations. High-precision X-ray images were obtained from SCFRTP test specimens to construct mesh models that accurately reproduce the micro-scale structure of the specimens. To examine the influence of orientation uncertainty on material strength, multiple fracture simulations were performed on several subdivided unit-domain models obtained from the original mesh model. The results were statistically investigated. Furthermore, this paper discusses the appropriate modeling techniques for macro-scale fracture simulations.

This study delves into the impact of voids within CFRP materials on dark-field images obtained through an X-ray Talbot-Lau interferometer (TLI). Detecting voids in CFRP holds excellent significance because these voids can significantly influence material properties, such as strength and elastic modulus. TLI presents a promising approach for void detection, given its ability to capture a wide area rapidly and effectively identify voids. However, the relationship between voids and dark-field images has been inadequately assessed. Quantitative assessment of the impact of voids on the dark-field signal is imperative to predict the characteristics of voids in CFRP based on dark-field images. This study introduced unimpregnated areas, representative of voids in CFRP, within the material. Subsequently, we photographed the CFRP using TLI and verified that these unimpregnated areas influenced the dark-field signal. This result was used to thoroughly investigate the relationship between the geometric properties of unimpregnated areas and the extent of their impact on the dark-field signal. Our findings revealed a correlation between the geometric properties of these void-like areas and the magnitude of their effect on the dark-field signal.