In recent years, the deterioration of social infrastructures, such as highway piers and sewer pipes, has been a significant challenge. Therefore, on-site repair work to extend the life of aging infrastructures has attracted attention. Fiber-reinforced plastics (FRP) are widely used for repairing such structures because they are suitable for on-site molding. To verify the integrity of the repaired infrastructure, a nondestructive inspection of the cured resin is required. As the curing resin is heated from one side of the plate, there is a possibility that the curing process propagates in the thickness direction. In this study, to evaluate the curing distribution in the thickness direction by ultrasonic testing, the effect of an uncured layer on the ultrasonic propagation characteristics is investigated for GFRP-molded plates. Experimental and simulation studies reveal that for a plate with an uncured layer on one surface, quasi-surface waves that propagate mainly in the cured layer are formed. Although it is difficult to detect an uncured layer based on ultrasonic velocity, the attenuation in longitudinal waves can be used to evaluate the uncured layer thickness.

A versatile methodology for fatigue life prediction is indispensable for establishing fatigue strength design methods that guarantee the long-term reliability of carbon-fiber-reinforced plastics (CFRP), In this study, interfacial normal stress (INS) was determined for unidirectional CFRP specimens. The obtained INS was used for predicting the fatigue life of laminated CFRP specimens, assuming that the fatigue life of CFRP is governed by the microstructural stress in the resin. Fatigue tests were conducted on three types of laminated specimens with stacking sequences of [0/30/－30/90]s, [0/45/－45/90]s, and [0/60/－60/90]s. The INS in the resin part of the microstructure was precisely evaluated by two-scale stress analysis, focusing on two spatial scales: microstructure and macrostructure. The sequential fatigue ruptures in the laminae of the specimen have been successfully predicted using INS based on the results of the fatigue test for the pure resin.

Short fiber-reinforced thermoplastics (SFRTPs) have a strength and elastic modulus superior to those of other materials. In injection molding, these thermoplastics continually transition between the solid and liquid phases. The temperatures at which these phase transition behaviors occur are called the crystallization temperature (T_c) and glass transition temperature (T_g). In previous research, we proposed a short beam method for determining the interfacial shear strength (IFSS) of SFRTP injection-molded products. By analysis of the IFSS and weld strength, the solidification temperature (T_solid) could be quantified. However, the relationship between T_solid, T_c, and T_g has not been specified. In this study, T_solid was calculated for three kinds of glass fiber-reinforced thermoplastics in a matrix of polypropylene (PP), polystyrene (PS), and polylactic acid (PLA). Heat flow analysis by differential scanning calorimetry (DSC) was then performed and the heat flow curve in the cooling process was compared with the T_solid value. It was shown that when crystalline plastic was used as the matrix, T_solid equated to T_c, but when amorphous plastic and semi-crystalline plastic were used, T_solid corresponded to T_g.

The mechanical properties of discontinuous carbon fiber-reinforced plastics (CFRTP) were evaluated in this study using a multiscale numerical analysis method. The internal structures of the discontinuous CFRTP were classified into three scales: macro-, meso-, and microscale, based on X-ray CT observation and image analyses. Subsequently, finite element models were prepared while considering the characteristic internal structures in each scale. Using these models, numerical simulations (based on the homogenization method) were performed in sequence to identify the constitutive equations regarding the homogenized materials in the micro- and mesoscales. The numerical simulation (also based on the finite element method) was performed, and the elasto-viscoplastic properties of the discontinuous CFRTP were evaluated. The stress-strain curve obtained from the multiscale numerical analysis method agreed well with the experimental result of the uniaxial tensile test in both elastic and viscoplastic regions. In addition, the strain distributions showed a similar tendency between the numerical and experimental results. These results validated the proposed multiscale numerical analysis method.