In this study, ultrasonic testing equipment is developed to evaluate 3D-printer filaments, particularly short-fiber reinforced reproduction filaments. Recently, 3D printers based on fused deposition modeling (FDM) using thermoplastics have become popular because of the simplicity and relatively low cost. In addition, filaments containing carbon or glass fibers have also been developed to improve the mechanical properties of products. In 3D printers using reproduced filaments, the quality of the filament affects the reliability of the products, particularly in the case of fiber-reinforced filaments. In the equipment developed for this study, ultrasonic waves that passed through the target filament along the longitudinal direction were measured, and the physical properties of the filament were predicted based on wave propagation velocity. To improve the accuracy of the velocity measurement, the Akaike information criterion (AIC) was introduced to detect the wave arrival time of signals. In addition, a finite difference time domain simulation was conducted to confirm the validity of the ultrasonic inspection. It was determined that change in velocity in ultrasonic waves can be used as an index of filament degradation.

3D printing technology has been receiving increased attention as a new molding method for carbon fiber reinforced thermoplastics (CFRTPs). This study investigates the effect of process parameters on the mechanical properties of 3D printed CFRTPs with in situ resin impregnated fiber yarn. The CFRTP specimen was manufactured through fused deposition molding using carbon fiber yarn consisting of 3000 carbon fiber monofilaments and poly(lactic acid) filaments. The mechanical properties of the CFRTP specimen were evaluated through tensile testing. A mathematical model was developed to describe the relationship between the process parameters and the tensile strength and modulus of the specimen. Based on the results, the high molding temperature print speed of about 100 mm/min and low filament extrusion speed were established as the optimum process parameters for the actual manufacturing of the CFRTPs.

Continuous carbon fiber composites can be printed with 3D printers. Many studies detailing elucidations of the mechanical properties of such 3D printed composites have been published, all of which employed a conventional tensile specimen configuration with surface resin layers. In the present study, 0º, 90º, ±45º, and lay-up direction type specimens were newly designed for 3D printed composites without surface layers. Using the 3D printer, both conventional and newly designed specimens with serpentine folded fiber bundles were fabricated and investigated experimentally. The lay-up direction specimen was fabricated using 800 layers. The specimens without the serpentine folded fiber bundles were experimentally shown to be adequate for tensile tests. The lay-up direction specimen had the lowest strength and stiffness, which seems to be related to its surface roughness.

Owing to the superior mechanical properties and moldability of carbon fiber reinforced thermoplastics (CFRTP), they have been increasingly adopted in various applications. Injection molding is a typical method for fabricating CFRTP, however, this method requires a mold, and hence it is difficult to use this method to fabricate complex geometries. Meanwhile, 3D printing is suitable for fabricating complex geometries and for optimizing the structure. Recent advancements in 3D printing technology have enabled printing of composite materials that have arbitrarily curved continuous reinforcements. Previous studies have revealed that a fiber bundle becomes twisted in a curved section, and this might affect the mechanical properties of 3D printed CFRTP. In the present study, the mechanism of twisting a fiber bundle in a 3D printed CFRTP is investigated. In addition, fiber fracture in a curved section is evaluated by measuring the electrical resistance of a fiber bundle. As a result, it has become clear that the twisting of a fiber bundle is caused by the flattening of the fiber bundle, the adhesive force on the print bed, and tensile load from the print nozzle. Moreover, the electrical resistance measurements verified the fracture of fibers in a curved section.

Continuum damage mechanics (CDM)-based finite element analysis was performed to predict the mechanical behavior of a 3D-printed curvilinear continuous carbon fiber reinforced thermoplastic (c-CFRTP). Elasto-plastic properties of the 3D-printed c-CFRTP, including its damage initiation, evolution, and propagation, were identified by conducting monotonic and cyclic tensile tests on three specimens (i.e., [(±45)₂]_S, [(0/90)₂]_S, and [(±67.5)₂]_S). Then, an S-shaped curvilinear c-CFRTP was 3D-printed and tested under monotonic tensile loading conditions. The non-linear mechanical behavior of the S-shaped curvilinear c-CFRTP due to tensile loading was well predicted by means of finite element analysis using the constructed CDM-based material model.

Three-dimensional (3D) printing is a promising technology that can produce complex structures without the requirements of expensive tools and molds. Additives are usually incorporated into the plastic materials used in 3D printing to increase their strength and rigidity. Particularly, carbon fiber-reinforced plastic (CFRP) has shown great promise as a 3D printing material. However, the strength of CFRP after printing remains unknown, although it is well-known that its strength is affected by plastic melting during printing. Herein, we analyzed the fracture behavior of CFRP specimens before and after bending to different curvature radii. From the experimental results, a fracture criterion describing the material behavior by considering the tensile and compressive loads was developed. The fracture mechanism remained the same for CFRP specimens with different curvature radii. This study deepened our understanding of the mechanical properties of CFRP materials used in 3D printing.