Cellulose nanofibers (CNFs) are highly crystalline, fibrous materials with a high aspect ratio of long cellulose molecules linked together with strong hydrogen bonds. These long cellulose molecules are incorporated into hemicellulose and lignin, the cell walls of higher plants. The modulus of elasticity of CNF remains constant between −200 °C and +200 °C. However, the linear coefficient of thermal expansion of cellulose fibers was 0.17 ppm/K, which is comparable to that of quartz glass. Further, CNFs have a thermal conductivity in the same order of magnitude as glass. Therefore, they are promising next-generation fiber owing to the excellent mechanical and thermal properties, sustainability, and biodegradability of CNF. Most studies on using CNFs to increase strength have focused on polymer matrix composites, particularly biodegradable polymers. However, it is difficult to increase the strength of materials using CNFs due to the agglomeration of CNFs. Compared to other composite materials, the uniform dispersion of nanosized CNFs is crucial. This article reports that CNFs may behave not as reinforcing fibers but as cross-linking agents to strengthen the polymer when the amount of CNFs is very small in polymer matrix composites, and the addition of CNFs is also effective in strengthening silk yarn and Ti. The mechanical properties of the ceramic green bodies can be increased using CNFs as an auxiliary agent in ceramic slurry. Moreover, the fabrication of composite materials using CNFs is essential for expanding the CNFs applications.
The mechanical properties of fiber-reinforced thermoplastics (FRTP) are greatly affected by the interfacial shear strength, so it is important to evaluate the interfacial properties. The single fiber pull-out test and micro-droplet test are typical methods to evaluate interfacial shear strength. However, these test methods are difficult to evaluate only the effect of the chemical bonding on the interfacial strength because the interface is not only affected by chemical bonding but also by residual stress, which is easily influenced by external factors. In this study, we conducted stud-pull test and tensile shear test using a glass plate coated with aminosilane and a rod-shaped polyamide 66 resin to evaluate the chemical bonding at the interface while the effect of the resin's tightening force on the fibers was small. The stud-pull test and the single fiber pull-out test were also conducted under high temperature condition, to clarify the effect of test temperature on the chemical bonding. In the stud-pull test at 25 °C, the aminosilane treated glass plates had a 7.7 times higher interfacial strength compared to the untreated glass plates. In the single fiber pull-out test, which is greatly influenced by the force with which the resin tightens the fiber, the fiber matrix interfacial shear strength of aminosilane-treated glass fiber and PA66 at 80 °C tended to be lower than that at 25 °C; whereas there was no difference in interfacial strength of aminosilane treated glass plate and Polyamide 66 obtained by the stud-pull test. These results indicated that the chemical bonding which affects the interfacial strength can be evaluated by stud-pull test, and the chemical bonding at the interface between aminosilane treated glass plate and polyamide 66 was less affected by the 80 °C condition.
One of the molding methods for the glass fiber reinforced thermoplastics (GFRTP) is press and injection hybrid molding, in which GFRTP is pre-heated and press-molded into a mold and the thermoplastic is injection-molded on the surface of the GFRTP. Since the press and injection hybrid molded product is easily fractured at the interface between GFRTP and the injection material, several methods have been proposed for improving the interface. From the viewpoint of interfacial bonding between GFRTP and injection thermoplastic, however, the reported methods are composed of the same kind of resin, so it is difficult to have both the high interfacial bonding strength between GFRTP and injection resin, and the high mechanical properties of GFRTP, when polypropylene (PP) is used for injection resin. In this study, we evaluated the mechanical properties of matrix-hybrid GFRTP, which has been developed by stacking the outer layer consisting of PP or PP with maleic anhydride modified PP (M-PP) and the inner layer of PA6. Furthermore, tensile test at the rib root of press and injection hybrid molded product using PP as the injection material was also conducted to clarify the effect of matrix hybridization on the interfacial bonding strength at the rib root. The matrix hybrid GFRTP using M-PP and PA6 showed 2.5 times higher in bending strength as compared to the GFRTP which is only composed of M-PP as the matrix resin. In addition, from tensile tests at the rib root of a press and injection hybrid molded product using PP as an injection resin, the matrix hybrid GFRTP exhibited higher interfacial bonding strength than the single resin GFRTP. These results indicated that the press and injection hybrid molded product with high interfacial bonding strength at the rib root and high mechanical properties can be obtained by using the matrix hybrid GFRTP composed of M-PP, which is the same type as the injection PP on the outer layer and PA6 with GFRTP exhibiting high strength on the inner layer as the shell structure.
Resin transfer molding (RTM) has been attracting much attention to produce FRP products due to lower manufacturing cost than an autoclave molding method and unnecessity of human skills unlike a hand lay-up method. The quality of RTM products is highly dependent on the state of resin impregnation into fiber preform during the mold-filling stage, and it is required to avoid creation of molding defects such as dry spots and voids. Ultrasonic testing can be applied to monitor impregnation and cure of resin in RTM. In this study, an experimental setup was designed to achieve unidirectional resin flow, and ultrasonic monitoring system was proposed. By analyzing the reflected waves from the upper and lower surfaces of the FRP layer in the mold, it was found that our method could evaluate not only residual air voids and resin impregnation into fiber bundles, but also cure degree of resin. Therefore, it was proven that the system could monitor entire RTM molding process.
The deformation behaviours of various CFRP laminates are explained by classical lamination theory based on plate theory. The presence of Bij components in the bending-extension coupling stiffness matrix implies the twist deformation under tensile loading. The specific applications of laminated composites require antisymmetric laminates to achieve design requirement, for example, some form of coupling is necessary to design a turbine blades with pre-twist without using a complex mold. It was well known that the antisymmetric laminates don’t have zero components for Bij(i≠j). In previous studies, the deformation of antisymmetric CFRP laminates have been analysed, on the other hand, the accurate evaluation have not been experimentally measured due to jig friction. Therefore, in order to make it possible to experimentally evaluate the tensile-twist coupling deformation of the antisymmetric laminate, we developed a novel jig to measure twisting under tensile loading. Three types of experiments were conducted by using thrust bearing that reduces twisted friction and radial bearing that reduces friction on the twisted center axis, and then changing the mounting method to the testing machine. In order to evaluate the validity of the proposed method, we compared the Bij value obtained by the classical lamination theory and the twist angle obtained by the FE analyses with the experimental results. As a result, the twist deformation under tensile loading using the proposed jig combined thrust bearing with radial bearing had a strong correlation of 0.96 with the results of the FE analyses.
Friction stir spot welding (FSSW) using thermoplastic resins as an adhesive, which uses the friction induced heat to melt them, has been developed as a method for welding Al alloys and Carbon Fiber Reinforced Plastics (CFRP). Since this method requires a process of inserting thermoplastic resins as an adhesive, efficient method for higher production cycle and higher interfacial strength between CFRP and the adhesive are expected to be developed. In this study, we have developed a proper laminated composite in which PA12 resin is laminated on the outermost layer of CF/Epoxy prepreg for the usage in FSSW. A co-curing laminate in which the PA12 resin in the outermost layer was melted before the curing of the epoxy resin, a co-curing laminate in which the PA12 resin was melted after the curing process, and a laminated composite in which PA12 resin was laminated by press molding as the next process after molding the CFRP were prepared, and their mechanical properties were evaluated by a three-point bending test. Their maximum flexural loads at fracture were the same as CFRP. Furthermore, FSSW of the prepared laminated composites and aluminum alloy was performed and their welded strength was evaluated by a tensile shear test. The shear strength of 12.1 ± 0.6 kN, which was higher than that of FSSW when the thermoplastic resin was inserted between the Al alloy and CFRP, was obtained for the specimen using the laminated composite in which the PA12 resin in the outermost layer was melted before the curing of the epoxy resin in co-curing process.
Carbon fiber is a brittle material and its tensile strength is strongly influenced by defects, which are often assessed by the shape parameter of the Weibull distribution. Therefore, the shorter the gauge length is, the higher the tensile strength is. The usual tensile test is conducted with a gauge length of 25 mm and the tensile strength at short gauge is derived using Weibull distribution. However, it is reported that the shape parameter changes when the gauge length is less than 1mm. The objective of this work is to attempt to evaluate the short gauge tensile strength of carbon fiber in tests under optical microscope observation. The shorter the gauge length was, the more test results of fiber separation from adhesive or fiber breakage in adhesive were observed. It is found that the test of short gauge of 0.2 mm requires careful specimen preparation to avoid shear stress due to misalignment of fiber and loading axes. Weibull shape parameters of the tensile strengths at the gauges from 50mm to 1mm were almost the same as the usual tensile test. However, the values for the 0.5 mm and 0.2 mm gauges were higher than those for the longer gauges.
In this study, torsional vibration tests of open-hole Sitka spruce samples were performed, and the influence of the diameter and position of the hole on the value of the nominal shear modulus were examined. The nominal shear modulus value decreased as the diameter of the hole increased and the hole position approached the mid-length of the sample. Nevertheless, such decrements were gentler than the nominal elastic moduli obtained from the longitudinal and flexural vibration tests obtained in a previous publication. A formula for deriving the nominal shear modulus of open-hole sample was proposed by modifying the equation obtained in a previous study, and the accuracy was improved, although the numbers of parameters contained in the equation increased.
This study discusses the etching of a poly(methylvinylsiloxane) (PVMS) silicone rubber sheet (VMQ) using a nanosecond-pulse laser operating at a wavelength of 355 nm under room temperature in air. We obtained a VMQ sheet with etchings of ~60 µm in depth and a surface roughness of 9.5 µm (Sa) through spread-dot patterns of focused laser irradiation. An increase in binding energy and composition of Si2p and O1s peaks, along with an increase in oxygen atom ratio, were observed using X-ray photoelectron spectroscopy (XPS) analysis. The surface oxidation of the etched VMQ sheet was similar to cases of laser irradiation on poly(dimethylsiloxane) based silicone rubber. The etched VMQ surface also showed high contact angle of 156° for water and 107° for propylene carbonate (PC), and both the angles were higher than those of untreated VMQ sheet. Moreover, ethanol showed greater wettability on the etched VMQ surface when compared with the untreated sample. Differences in liquid repellence between surfaces reflect the similarity of chemical structure of PVMS.
It has been reported that the strength decreases due to the waiting time until casting (Issue 1) and the strength homogeneity at the upper / middle / lower part due to material separation (Issue 2). For Issue 1, there is a possibility that the strength decrease due to the waiting time can be suppressed by improving the casting efficiency. For Issue 2, there is a possibility that the mud and filling material are not sufficiently mixed, or that it is partially diluted by spring water. For example, if it is possible to give the property of anti-washout-ability with muddy water, it may be possible to suppress the uneven distribution of mud and the partial decrease in strength due to spring water by placing filling material from the bottom of the hole and replacing it with muddy water. From the above, it is possible that the above issues can be solved if the material separation characteristics are sufficient, the casting efficiency can be improved, and the anti-washout-ability water or water can be imparted. We studied the compressive strength, bleeding, and anti-washout-ability of cement milk in muddy water to which a "surfactant-type liquid thickener" different from conventional fillers was added. It was possible to suppress the occurrence of bleeding by adding the thickener even in very thin cement milk with a W/P=400% or more. However, unlike bentonite, it does not have the effect of improving strength, so it was found that it is necessary to reduce W/P in order to obtain the specified strength. Cement milk that was added the thickener did not change its rheology and the self-fluidity was not disappeared for as long as 2 hours. On the other hand, the yield value of cement bentonite was increased after 2 hours so that the self-fluidity was disappeared. Cement milk containing the thickener did not disperse into muddy water, suggesting that even if the filling material is placed in muddy water, it may prevent unintentional mixing with muddy water. As a future effort, it is necessary to confirm its usefulness as a filling material by repeating further analytical tests to various applications.