Continuous carbon fiber-reinforced T_g-less epoxy resin was produced and evaluated to create a creep-less composite material usable in continuously loaded structural parts. The T_g-less epoxy resin was obtained through the anionic curing of ordinary epoxy resin with an alkaline salt of carboxylic acid. In the viscoelastic behavior of the cured T_g-less epoxy resin, the storage modulus did not drop and the resin remained in a glassy state even at elevated temperatures, such as 300ºC. Additionally, the loss tangent was low at all testing temperature ranges. This indicates that stress relaxation hardly occurred; thus this resin may enable the creation of a creep-less composite material. The brittleness of the cured T_g-less epoxy resin was successfully overcome through modification with nano-rubber particles. Finally, creep tests were conducted using a three-point bending mode for carbon fiber-reinforced composite materials using modified and unmodified T_g-less epoxy resins as the matrices. As a result, in the case of T_g-less epoxy resin containing 16 wt% of nano-rubber particles, the creep strain rate became almost zero after a testing time of over 50 h.

The tensile strength distribution of carbon fibers at short gauge lengths is vital for understanding the tensile strengths of carbon-fiber-reinforced plastics (CFRPs). However, determining the tensile strength distribution at short gauge lengths is challenging, and only a few studies have attempted to do so. This study presents the key factors required to determine the accurate tensile strength distribution, such as, measurement of cross-sectional areas of single fibers, clamp effect in single fiber tensile (SFT) test, residual strain in single fiber composite (SFC) test, and the consistencies of the SFT and SFC tests. With these considerations, the bimodal Weibull distribution model was employed to describe the experimental results. The tensile strength distributions of polyacrylonitrile (PAN)-based carbon fibers at short gauge lengths were narrower than those at long gauge lengths. This study provides significant implications to understand the accurate tensile strength distribution of PAN-based carbon fibers and the tensile strength of resin-impregnated fiber bundles. Furthermore, this study may provide design criteria for the material development of CFRP composites and for enhancing their tensile strengths.

In recent automobile developments, the compatibility between a lighter vehicle body for lower fuel consumption and high clash safety performance has been an essential requirement. It is necessary to increase the energy absorption (EA) efficiency of car body frames, for which the continuous brittle destruction phenomenon of carbon fiber reinforced plastics (CFRP) pipes can be effectively utilized. In this paper, we addressed the effect of curing temperature and thickness on the process-induced residual stress and the destruction behavior under clashing. We measured the residual stress, residual strain, and EA characteristics of CFRP pipes cured at 80ºC, 100ºC, and 130ºC. Fiber Bragg grating (FBG) sensors were embedded into the specimens during the lay-up process to measure the cure strain and residual stress. We used a drop impact test for the evaluation of EA characteristics. The results confirmed that with increasing curing temperature, both, the residual stress in the in-plane direction and the EA amount increased, because of the suppression of fiber breakage in the hoop direction. In contrast, with increasing thickness, the residual stress in the out-of-plane direction increased, while the EA efficiency decreased due to increased interlaminar fracturing. The insight gained is useful for designing appropriate cure cycles for CFRP pipes with high EA efficiency.

A versatile methodology for the fatigue life prediction of unidirectional carbon fiber-reinforced plastic (CFRP) specimens is presented to enable the optimal use of the high specific stiffness and strength of CFRP. We assumed that the fatigue life of CFRP is governed by that of its matrix resin, and employed a microscopic approach to evaluate the resin stress using a micro model, which considered the carbon fibers and resin separately. Fatigue tests were performed on the resin and unidirectional CFRP specimens by making the direction of the reinforcement to deviate 30º, 45º, 60º, and 90º from the loading axis. Fatigue life prediction was successfully performed using the interfacial normal stress (INS), which is the normal stress on a plane perpendicular to the line segment connecting the center axes of two carbon fibers. The inadequate test results for fatigue life prediction by microscopic evaluation of the first principal stress, Tresca stress, or von Mises stress demonstrate the suitability of the proposed methodology.

In this study, we measured the interfacial shear strength between a thermoplastic resin and a carbon fiber by a novel pinhole-type pull-out method. In this method, the specimen had a pinhole formed on a thin metal plate filled with a resin. A fiber was embedded into the resin and pulled out from the pinhole. Seven types of polymers were used as the resin, including PP, Modified PP, PA6, PA12, PC, PMMA and PEEK. In this method, the resin shape is determined by the pinhole geometry and is always cylindrical. This method can be used to measure the frictional force between the fiber and resin after the occurrence of debonding at the interface. As a hypothesis, we defined the total pull-out load as a sum of the frictional and bonding components in the load. Herein, we separated the frictional and bonding forces from the total pulling load. Then, the bonding strength component was compared with the Mises shear yield stress of the resin. As a result, the bonding strength component measured by this method agreed with the Mises shear yield stress values of the resin.