The purpose of this study is to propose an optimal material design for vinyl ester resin used in unidirectional carbon fiber reinforced plastics (UD-CFRP) under cryogenic conditions, with consideration of fatigue loading behavior. Changes in static strength and fatigue life were evaluated using UD-CFRP specimens with both of unmodified and modified matrices. Apparent interfacial shear strengths between the carbon fiber and matrix were assessed via microdroplet tests. Furthermore, the relationship between crack growth rate and the range of energy release rates in UD-CFRP was investigated through fatigue crack growth tests. The results revealed that the static tensile strength of UD-CFRP specimens improved under both of room temperature and cryogenic conditions when a modified matrix was used. Fatigue life under cryogenic conditions was enhanced by approximately 25.9 times with matrix modification using toughening agents. Interfacial shear strength between the carbon fiber and matrix also improved. The crack growth rate of UD-CFRP under fatigue loading revealed that both of the threshold and critical energy release rate ranges were improved through matrix modification. The influence of thermal stress at the carbon fiber/resin interface on the energy release rate associated with crack propagation was analyzed using finite element method (FEM) simulations. The consistency between experimental and analytical trends for both unmodified and modified matrices confirmed the reliability and versatility of the FEM model developed in this study. Moreover, it was found that the improvement in the coefficient of thermal expansion of the matrix due to toughening agent modification was a key factor contributing to the enhanced fatigue life of UD-CFRP.

In recent years, biodegradable composites have attracted attention as alternatives to metallic materials for bone fixation devices, which are prone to corrosion, fatigue, and stress shielding. In this study, a hybrid approach combining interfacial treatment and tensile drawing was investigated to enhance the mechanical properties of tricalcium phosphate (TCP)/poly(lactic acid) (PLA) composites. The surface of TCP, a bioactive ceramic, was modified with L-lactic acid or stearic acid to improve interfacial adhesion with PLA. Composite specimens of interfacial treated TCP and PLA were prepared by kneading and compression molding. Among the two agents, L-lactic acid did not improve tensile strength and fracture strain of TCP/PLA composite, whereas stearic acid enhanced both properties. At a stearic acid amount of 6 phc (per hundred ceramics), the tensile strength and fracture strain reached the maximum values of approximately 1.5 times and 1.6 times those of the untreated specimen, respectively. Infrared spectroscopy revealed no peak shifts indicative of hydrogen bonding between the modified TCP and PLA. In contrast, fracture surface observations showed a fibrillar structure between TCP and PLA only at 6 phc, suggesting bond formation. Subsequently, the interfacial treated TCP/PLA composite specimens were tensile drawn to induce molecular orientation. The hybrid approach yielded superior mechanical properties compared to either treatment alone, demonstrating a synergistic effect of interfacial treatment and drawing.

Banana pseudostems are an underutilized biomass resource, and their fibers (BPSFs: banana pseudostem fibers) have been reported to exhibit lower tensile strength than other natural fibers. This is often attributed to the fact that tensile strength is typically evaluated based on the apparent cross-sectional area, which includes internal lumens. In this study, X-ray micro-CT was employed to determine the effective cross-sectional area excluding lumens, enabling evaluation of the intrinsic tensile strength of BPSFs. The results showed that the tensile strength calculated using the effective cross-sectional area reached 620 ± 330 MPa, indicating that BPSFs possess a strength level comparable to that of conventional natural fibers. Furthermore, to enhance interfacial adhesion required for application as reinforcement in composite materials, an alkaline surface treatment was applied. A 15-min alkaline treatment resulted in the maximum tensile strength of the fibers, suggesting that the removal of non-cellulosic surface components contributed to strength improvement. In addition, when polypropylene containing 4 wt% maleic anhydride-grafted polypropylene (MAPP) was used as the matrix, the interfacial shear strength exhibited high values after short-duration alkaline treatment.

This study investigates the mechanical properties of a recycled carbon-fiber reinforced thermoplastic (rCFRTP) composite that combines discontinuous recycled carbon fibers (rCF) with a twisted-yarn structure. The material was produced by mixing rCF recovered from production scrap with polyamide 6 (PA6) filaments using a commingling process, followed by carding, alignment, and yarn twisting. The resulting rCF/PA6 strands were hot-pressed to form test plates. Uniaxial tensile tests, static four-point bending tests, and finite-element (FE) analyses were conducted to clarify the influence of the twisted-yarn structure on bending properties. In tension, although fiber bundle pull-out failure was observed in some areas, it can be seen that a continuous fracture surface was formed. The rCF/PA6 laminates achieved an average tensile strength of 397 MPa and a tensile modulus of 52 GPa, which exceed reported values for injection-molded rCFRTPs. Under bending, where tensile and compressive stresses occur simultaneously, the influence of fiber-bundle pull-out was minor and the material showed markedly higher properties: an average bending strength of 400 MPa and a bending modulus of 46 GPa. In-situ microscopic observation during four-point bending revealed that initial cracking originated on the compressive side and propagated along fiber bundles. The progressive change in load-bearing layers during crack growth was identified as the mechanism responsible for the observed high toughness. FE analysis incorporating a discontinuous fiber rule of mixtures and twisted-yarn model reproduced the initial bending stiffness of the experiments, confirming that the combined model can predict the elastic property of discontinuous, twisted-yarn composites.

Honeycomb sandwich panels offer excellent stiffness-to-weight ratios, making them common in aerospace, automotive, and renewable energy structures, yet they are prone to face sheet buckling and core crushing under bending. This study explores using recycled carbon fiber non-woven tissue (rCF NWT) as an interlayer in glass fiber reinforced polymer face sheets to enhance performance and promote fiber reuse. Long beam flexural tests and linear-elastic FEM analysis showed that rCF NWT increased flexural rigidity, maximum load, and energy absorption without adding significant thickness and weight. Strain measurements revealed lower tension- and compression-side strains and suppression of tension-side softening beyond ~0.2% strain. The interlayer delayed failure and suppressed core buckling but shifted the dominant damage mode from global buckling and core crushing to interfacial delamination within the face sheets with face sheet–core separation. These results show that rCF NWT improves the flexural performance and damage tolerance of honeycomb panels while enabling carbon fiber recycling.

This study examines the thermal conductivity and electrical resistivity of alumina/epoxy resin composites with silica layered multi-walled carbon nanotubes (silica@MWNT). The silica@MWNT was synthesized by using a sol-gel method, and the alumina/epoxy resin composites with silica@MWNT were fabricated by a planetary centrifugal mixer. Using the manufactured composites, thermal conductivity and electrical resistivity were measured. As a result of these tests, the thermal conductivity of the 1.8wt% silica@MWNT/alumina/epoxy resin composites was lower than that of the composites with 1.8wt% MWNT, but the thermal conductivity of the 5.4wt% silica@MWNT/alumina/epoxy resin composites was higher than that of the composites with 5.4wt% MWNT. This suggests that the synergistic effect of adding silica@MWNT with alumina improves as the content of silica@MWNT increases. The electrical resistivity of the 1.8wt% silica @ MWNT/epoxy resin composites was higher than that of the 1.8wt% MWNT/epoxy resin composites and was comparable to that of the neat epoxy resin. The electrical resistivity of the 1.8wt% silica@ MWNT/alumina/epoxy resin composites was nearly the same as that of the alumina/epoxy resin composites and the 1.8wt% MWNT/alumina/epoxy resin composites. However, the electrical resistivity of the 5.4wt% silica@ MWNT/alumina/epoxy resin composites showed a higher value than that of the 5.4wt% MWNT/alumina/epoxy resin composites, thereby demonstrating enhanced electrical insulating properties.

We evaluated the strain rate dependence of the tensile stress-strain relationship of bisphenol A polycarbonate using the relaxation modulus obtained by dynamic viscoelasticity measurement. The temperature dependence of the storage modulus, loss modulus, and loss factor was measured at multiple frequencies using dynamic viscoelasticity testing, and a shift factor for converting temperature and time was obtained, which was then converted into the relaxation modulus. The storage modulus, loss modulus, and loss factor could be back-calculated from the obtained relaxation modulus, demonstrating that polycarbonate is thermorheologically simple. Tensile tests of polycarbonate were performed at low and high strain rates using a universal testing machine and the split Hopkinson bar method.The stress-strain relationship estimated from the relaxation modulus was compared with the tensile test results, and it was shown that the strain rate dependence of the stress-strain relationship can be estimated from the relaxation modulus at small strains.

Dynamic crack propagation in brittle solids is numerically analyzed using a two-dimensional finite element method based on the phase-field model of fracture to investigate the limiting crack velocity. Straight crack propagation without branching or kinking is achieved by modeling an elastically homogeneous and isotropic medium that contains a narrow rectangular region with a relatively low critical energy release rate. Mode I and mode II crack propagation are studied for two types of strain energy degradation functions in the phase-field model: quadratic and cubic forms. For both degradation functions, the mode I crack accelerates toward the Rayleigh wave velocity, which is the theoretically predicted limiting velocity according to linear elastic fracture mechanics. In contrast, the two degradation functions are found to yield different propagation characteristics for the mode II crack. With the quadratic degradation function, the crack velocity increases smoothly beyond the shear wave velocity. In the case of the cubic degradation function, however, the crack accelerates discontinuously from below the Rayleigh wave velocity to above the shear wave velocity due to the nucleation of a secondary crack ahead of the main crack tip, followed by coalescence of the secondary and main cracks. This behavior is consistent with theoretical predictions based on linear elastic fracture mechanics as well as previous molecular dynamics and finite difference simulations.

A laser welded butt joint was fabricated using 1180MPa recycled steel plate and a tensile test was performed. Two types of fatigue tests using different loading methods were also performed. The tensile properties of this test material were 1218 MPa tensile strength and 23.8% elongation, which were almost the same strength and ductility as the base material. There were no abnormalities in the welded parts, and the joints were properly joined. The 10⁷ cycles fatigue limit of the base material (as pickled) was 450 MPa for both axial and bending loads at a stress ratio R=-1. In addition, at 10⁷ cycles fatigue limit of the bending fatigue test at R=0 was 340 MPa, which overlapped with the modified Goodman line on the fatigue limit diagram. The fatigue strength of the laser welded butt joint was constant under axial and bending loads in the low life range of R=-1, but varied in the high life range. The fatigue fracture surface under bending load was present only on one side and was caused by a defect located approximately 100 μm away from the surface. For R=0, the fatigue properties were in good agreement under axial and bending loads. The fatigue limit was 250 MPa for both R=-1 and R=0 conditions. When the laser input side was set to the maximum stress side and the stress ratio was set to R = 0 on the plane bending fatigue testing machine, the fatigue limit was dramatically improved to 340 MPa, and the same fatigue strength as that of the base material after pickling was obtained.

The quality of Long-Term Supply Type Liquefied Stabilized Soil for Removal and Backfilling of Existing Piles at several permanent plants in the Tokyo metropolitan area was investigated over a period of approximately one year. The inspection was carried out at the time of arrive to the sites, and the results of a quality inspection carried out at the time of shipment were received separately from the plants and compared with the results of the on-site inspection. Results showed significant variability in strength between construction sites and plants test results, with field measurements showing a high coefficient of variation compared to plant data. Many samples did not meet Tokyo Metropolitan Government specifications, particularly in cases where bleeding rates exceeded 1%. While quality variability was minimal between vehicles produced at the same plant, significant differences in quality were observed between plants. XRF analysis was used to compare the design mix with the actual mix. This study proposes quality standards and recommends that existing pile removal backfills should adopt Grade A (semi-structural) specifications, including enhanced traceability and standardized testing procedures.