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In this study, the stiffener geometry was optimized using curvilinear 3D printing to enhance the buckling resistance. A bio-inspired skin/stiffener composite that mimicked spider-web structures was generated. A dataset was formulated for the regression analysis, covering buckling stresses under distinct feature values. The regression equations, crafted using a deep neural network trained on the dataset, were evaluated. The derived regression equation was subjected to sequential quadratic programming, a mathematical optimization, to determine the optimal value of the explanatory variable. This was aimed at maximizing the buckling stress-to-stiffener volume ratio, which is the objective variable. The optimized arrangement exhibited significantly improved buckling resistance, with approximately 163% higher buckling stress than conventionally designed structures with straight stiffeners of similar weight. [ABSTRACT FROM AUTHOR]

In this study, we evaluated the effects of the fiber orientation and the presence of resin-rich layers on electromagnetic induction testing (EIT) of carbon fiber reinforced thermoplastics (CFRTPs). First, a finite-element analysis was performed to evaluate the distribution and path of the eddy and displacement currents induced in interlaminar conductive and interlaminar non-conductive CFRTP specimens. Second, interlaminar electrically conductive and interlaminar non-electrically conductive specimens of unidirectional CFRTP and cross-ply laminated CFRTP were fabricated using a 3D printer, with control of the number of PA-6 layers and fiber orientation. EIT of the specimens revealed that the presence of resin-rich layers and fiber orientation affected the measured EIT output. The results also indicated that the EIT of CFRTP with resin-rich layers requires penetration of displacement currents between the layers. [ABSTRACT FROM AUTHOR]

For continuous carbon fiber-reinforced plastic (cCFRP), in the case of loading in the longitudinal direction, occurrence of kink band failure causes compression strength to be lower than the tensile strength. Furthermore, 3D-printed cCFRP may exhibit intrinsic compressive behavior depending on the printing orientation. In this study, two types of rectangular specimens were fabricated using the Markforged Mark Two > 3D printer. The first was the in-plane specimen, whose width direction was parallel to the print bed and thickness direction was along the layup direction. The second was the out-of-plane specimen, whose width direction was the layup direction and thickness direction was parallel to the print bed. Compression tests were performed, and the experimental results were compared. As a result, the compressive strengths of both types were similar, and the kink bands always grew in the lay-up direction. For the out-of-plane specimens, nonlinear stress–strain curves were observed. This may be due to the delamination crack growth caused by the low strength in the lay-up direction and the existence of initial defects. This study experimentally investigated the effect of two different printing orientations of 3D printed cCFRP to its strength and failure mode, and this work shows the importance of considering structural deformation due to compression loading in 3D printed cCFRP. [ABSTRACT FROM AUTHOR]

This work proposes a smart eddy current testing (ET) method for identifying crack orientation by introducing an additional electrically anisotropic plate. The key idea of the proposed approach is to insert the anisotropic plate between an eddy current (EC) probe and a test piece for controlling the electromagnetic field. Distribution of the space magnetic field and induced current density was numerically investigated. The simulation results showed that deformation of the magnetic field distribution occurs in the case of inserting an anisotropic plate. Meanwhile, it makes the magnetic field distribution directional and its direction will be changeable and controllable by rotating angle θ of the anisotropic plate. Furthermore, redistribution of the induced current density can be observed in the test piece, which results in two identical peak areas, and they are symmetric with respect to an axis perpendicular to the rotation angle. Subsequently, output voltage of the EC probe was calculated to quantitatively analyze the sensitivity for identifying crack orientation. Findings show that the output voltage from the probe was affected by the angle between the conductivity angle θ of the anisotropic plate and the crack orientation. [ABSTRACT FROM AUTHOR]

The aim of this paper is to investigate the cause of nonlinear behavior mechanism of change in electrical resistance of 3D printed carbon fiber reinforced plastics (CFRP) due to tensile loading. The electrical resistance change method is a nondestructive testing method that helps detect strain and damage. In previous research for electrical resistance change method applied to 3D printed CFRP, the change in electrical resistance is different compared with the one of conventional CFRP. The behavior makes health monitoring difficult, and the phenomenon needs to be clarified. In this research, we focused on the electrode fabrication method and viscoelasticity effect during cyclic tests. Polishing and laser processing are used as surface treatments for electrode fabrication. Consequently, the treatments are found to affect the reverse piezo-resistivity. In addition, the decrease in electrical resistance depends on the test speeds of cyclic test. The viscoelasticity is found to affect the nonlinear behavior. [ABSTRACT FROM AUTHOR]

3D printers using fused filament fabrication (FFF) with thermoplastic filaments have been employed for rapid prototyping applications. The present research focuses on 3D-printed short carbon fiber/PA-6 composites. Specimens with four types of build-up orientations (0°, 90°, ±45°, and layup) were fabricated using two types of 3D printer: Mark Two, produced by Markforged, and X-Plus produced by Qidi Tech. Tensile tests were performed on these specimens. X-ray CT and SEM observations were conducted to investigate the defects and gaps between the print paths. The effect of the bed temperature and extrusion rate were experimentally investigated. For the ±45° specimens fabricated using Mark Two, a large gap between the print paths was confirmed. An increase in the extrusion rate of 5% (1.05) and a minor increase in the nozzle temperature of 3.6% (from 275 °C to 285 °C) yielded a smaller gap between the print paths, 20 % higher tensile strength of the ±45° specimen, 10 % higher tensile strength of the 0° specimen, and 4% higher strength of the lay-up direction specimen with X-Plus. The bed temperature had a minor effect on tensile strength. [ABSTRACT FROM AUTHOR]

The 3D printing of carbon fiber reinforced thermoplastics (CFRTPs) is crucial in many fields such as aerospace engineering and electrical motorbikes. Structural health monitoring of 3D printed CFRTPs is important for its practical application. CFRTPs are monitored using the electrical resistance change method, but this method has not been applied to 3D printed CFRTPs. In this study, we used the electrical resistance change method to monitor CFRTPs. The electrical properties were measured in terms of fiber orientation and electrode distance. In addition, a cyclic test was performed to reveal the change in the electrical resistance. Our results reveal that the volume resistance along the 90° orientation has a larger standard deviation than those for the other specimens. The change in resistance exhibited reverse piezo-resistivity at the low stress range in the cyclic tests. The reverse piezo-resistivity occurred due to fiber waviness, which is generated during the 3D printing process. [ABSTRACT FROM AUTHOR]

The recent technology enables us to fabricate carbon fiber reinforced plastics which have continuous reinforcement fibers using a 3D printer. Arbitrarily curved reinforcement fibers are able to be configured using this technology, and it had been revealed that a fiber bundle is folded in a printing process of a curved section in the previous research. However, this mechanism has not been clarified. The present study, therefore, experimental investigations were conducted to clarify the folding of a fiber bundle in a printing process of a curved section. In addition, the electrical resistance was measured to clarify the effect of folding to fiber breakage. As a result, we concluded that folding of a fiber bundle attribute to positional relation of two forces; adhesive force to the print bed and tensile force from the print nozzle, and change of configuration of fiber bundle in the printing process. This mechanism is verified using the finite element analysis. In addition, fiber breakage was also revealed measuring the electrical resistance of a printed fiber bundle, and this presumably affects the mechanical properties of curved sections. [ABSTRACT FROM AUTHOR]

Continuous carbon fiber composites can be printed using 3D printers. Many studies detailing 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, 0°, 90°, ±45°, and lay-up direction type specimens have been designed for 3D printed composites without surface layers. Both conventional and newly designed specimens with serpentine folded fiber bundles were fabricated and investigated experimentally using a 3D printer. 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 appears to be related to its surface roughness. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) printing is an attractive technology to produce complex structures without the need for expensive tools and molds. Additives are usually incorporated with the plastic materials used in 3D printing to increase their strength and rigidity. In particular, carbon fiber-reinforced plastic (CFRP) shows promise as a material for use in 3D printing. However, the strength of CFRP after printing is still unclear, although it is known that its strength is affected by the plastic melting during printing. In this study, we analyzed the fracture behavior of CFRP specimens before and after bending to different curvature radii. From the experimental results, a fracture criterion that described the behavior of the materials by considering tensile and compressive loads was developed. The fracture mechanism was the same for CFRP specimens with different curvature radii. These results increase our understanding of the mechanical properties of CFRP materials used in 3D printing. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) printing of carbon fibre reinforced plastic (CFRP) components shows promise for building light-weight strong components for industry. Many structures require multiple components to be joined to form the completed parts. However, tightening of metal bolts used to join CFRP can damage the composite and the bonding strength of adhesive bonding is variable. Here, we propose a new joint system, based on all-3D printed parts. Snap-in joints and Ajax-pins induce a mechanical joining force at the interface between parts, together with a bearing force. This joint system offers comparable strength to that of adhesive and metal bolt joints, while adding negligible weight to the parts. Furthermore, 3D printing of the surface structures, holes and pins enables unprecedented control over the joint design, enabling strength optimization through the thickness and in-place directions as required. The Snap-in joint system shows great potential for the fabrication of strong light-weight 3D printed structures. [ABSTRACT FROM AUTHOR]

The present study deals with electrical resistance changes in woven-fabric CFRP during loading. Four kinds of plain weave woven-fabric CFRP laminated specimens are prepared and subjected to cyclic tensile loading that does not cause any damages, and the electrical resistance changes of the specimens are measured experimentally by the four-probe method. As a result, the present study shows that the electrical resistance of a specimen comprised of six ±45° plies decreases remarkably with increasing number of loading cycles. The decrease is caused by shear plastic deformation of ±45° plies. The thickness shrinkage caused by shear plastic deformation increases the number of fiber contacts, and this decreases the interlaminar contact resistance between the plies. For a single ±45° ply, the same electrical resistance decrease caused by the shear plastic deformation is observed, and the magnitude of the decrease is smaller than that of the six-ply laminate tested. This is because the effect of interlaminar contact resistance decrease does not exist for a single ±45° ply. For the six 0°/90° plies, the present study shows that electrical resistance in the through-thickness direction is decreased by out-of-plane plastic deformation of carbon fiber and misalignment of the plies. [ABSTRACT FROM PUBLISHER]

Carbon-fiber-reinforced polymer (CFRP) composites are used in aerospace and automobile structures. For many CFRP structures, mechanical metallic fasteners are adopted. In internal structures such as a wing box, damage to CFRP structures around the fastener holes is difficult to see. A simple method of finding damage around fastener holes is thus required for CFRP structures. This study applies self-sensing time-domain reflectometry to detect bearing failure around fastener holes of CFRP structures. In a previous paper, the effect of fasteners was minimized when the micro-strip line (MSL) was placed apart from the fasteners. This study numerically and experimentally investigates the peeling of glass-fiber-reinforced polymer (GFRP) insulator. A bearing failure results in embossing at the edge of a fastener hole. The embossing produces a wedge that causes the peeling of GFRP, generating a gap between the GFRP and CFRP. The gap strongly affects the characteristic impedance of the MSL. [ABSTRACT FROM AUTHOR]

This paper presents analytical solutions to eddy current distribution in carbon fiber-reinforced plastics induced by line current. Derived solutions show that eddy current distribution in unidirectional carbon fiber composite is dependent only on electrical conductivity in the drive current direction when the drive current is directed in the fiber direction or the transverse direction. Moreover, according to the derived analytical solution, skin depth of eddy current depends not only on frequency of drive current but on lift-off and width of the drive current unlike general expression of skin depth. Finally, we discuss improvement of sensitivity of eddy current conductivity sensor using analytical solutions. It is found that analytical solutions can clearly divide effects of magnetic field from drive current and that from eddy current. Sensitivity of eddy current sensor can be improved by 10 times placing pickup coil at region where the effect of magnetic field from eddy current is large. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer (CFRP) composites have low inter-lamina strength. One of the monitoring technologies is a self-sensing method that uses the electrical resistance change of a CFRP structure for detecting damage. The electric current distribution is vital information for the self-sensing method when optimizing the arrangement of probes to measure electric potential changes. We have developed a new orthotropic electric potential function analysis approach using affine transformation for unidirectional CFRP. In this study, the orthotropic electric potential function analysis method is improved to calculate the electric current of a thin cross-ply CFRP. Two types of stacking sequences for the beam-type cross-ply laminates were calculated to confirm the effectiveness of the improved method. The electrical voltage changes caused by multiple delamination cracks of a cross-ply laminate are new outcomes of this study. The analytical results were compared with computed results using the finite difference method. Consequently, the new equivalent electric conductance method proved to be effective for calculations of the electric current density of a cross-ply CFRP laminate. Furthermore, the new method for calculating the electric potential difference changes caused by multiple delamination cracks using orthotropic distributed doublet analysis, with the equivalent electric conductance, has also proved to be effective. [ABSTRACT FROM AUTHOR]