Ken Kurashiki

Intralaminar fracture mechanism in unidirectional CFRP composites: Part I: Intralaminar toughness and AE characteristics

Abstract Three types of representative carbon fiber reinforced unidirectional composite materials were used and their intralaminar fracture behavior was investigated using the double-cantilever beam specimen with a simultaneous acoustic emission measurement. The intralaminar fracture toughness was evaluated by both the compliance method and energy area method. As a result, it was found that the intralaminar fracture toughness without bridging fibers had a constant value during crack propagation but it increased greatly when bridging fibers were present. The effect of bridging fibers on the intralaminar fracture toughness was estimated quantitatively by cutting the bridging fibers. Distinct differences in load–displacement curves, compliance, crack propagating behavior and acoustic emission signal characteristics between these three types of unidirectional composite materials were observed. It was also found that bridging fiber failure generated relatively large power spectra and contributed to the peak frequencies of 600–700 kHz in the spectrum analysis of acoustic emission (AE) signals. This suggested that the bridging fibers were also an important source of AE signals. Furthermore, a linear relationship between crack length and normalized cumulative AE event count rate was obtained.

Conductivity stability of carbon nanofiber/unsaturated polyester nanocomposites

Carbon nanofiber (CNF)/unsaturated polyester resin (UPR) was prepared by a solvent evaporation method, and the temperature dependency of electrical conductivity was investigated. The CNF/UPR composites had quite a low percolation threshold due to CNF having a larger aspect ratio and being well dispersed in the UPR matrix. The positive temperature coefficient (PTC) was found in the CNF/UPR composites and it showed stronger effect around the percolation threshold. The electrical resistance of the CNF/UPR composites decreased and had lower temperature dependency with increasing numbers of thermal cycles.

Intralaminar fracture mechanism in unidirectional CFRP composites — part II: analysis

Abstract Three kinds of representative carbon fiber reinforced unidirectional composite materials are used, and their intralaminar fracture behavior is investigated by using the double-cantilever beam (DCB) specimen with a simultaneous acoustic emission measuring. In Part I, the experimental results on the crack propagation, the bridging fibers, the intralaminar fracture toughness acoustic emission characteristics and microscope observations were obtained. Here, we use a bridging fiber model to analyze the debonding force acting on a bridging fiber and try to estimate the number of bridging fibers during the crack propagating process. At the same time, the intralaminar fracture toughness is calculated by both the adhesive force model and the finite element analysis. As a result, it is found that the intralaminar fracture toughness without the bridging fibers will have a constant value during the crack propagation, but it increases greatly when bridging fibers exist. It is clear that the bridging fibers play an important role in the intralaminar fracture toughness. The debonding forces acting on the bridging fibers and the number of bridging fibers are obtained. Furthermore, the quantitative estimation of the increment of the intralaminar fracture toughness contributed by bridging fibers is made according to the adhesive force model and it is comparable with the results obtained by the finite element analysis.

Development of Insulation Sheet Materials and Their Sound Characterization

The research and development in soundproof materials for preventing noise have attracted great attention due to their social impact. Noise insulation materials are especially important in the field of soundproofing. Since the insulation ability of most materials follows a mass rule, the heavy weight materials like concrete, lead and steel board are mainly used in the current noise insulation materials. To overcome some weak points in these materials, fiber reinforced composite materials with lightweight and other high performance characteristics are now being used. In this paper, innovative insulation sheet materials with carbon and/or glass fabrics and nano-silica hybrid PU resin are developed. The parameters related to sound performance, such as materials and fabric texture in base fabric, hybrid method of resin, size of silica particle and so on, are investigated. At the same time, the wave analysis code (PZFlex) is used to simulate some of experimental results. As a result, it is found that both bundle density and fabric texture in the base fabrics play an important role on the soundproof performance. Compared with the effect of base fabrics, the transmission loss in sheet materials increased more than 10 dB even though the thickness of the sample was only about 0.7 mm. The results show different values of transmission loss factor when the diameters of silica particles in coating materials changed. It is understood that the effect of the soundproof performance is different due to the change of hybrid method and the size of silica particles. Fillers occupying appropriate positions and with optimum size may achieve a better effect in soundproof performance. The effect of the particle content on the soundproof performance is confirmed, but there is a limit for the addition of the fillers. The optimization of silica content for the improvement of the sound insulation effect is important. It is observed that nano-particles will have better effect on the high soundproof performance. The sound insulation effect has been understood through a comparison between the experimental and analytical results. It is confirmed that the time-domain finite wave analysis (PZFlex) is effective for the prediction and design of soundproof performance materials. Both experimental and analytical results indicate that the developed materials have advantages in lightweight, flexibility, other mechanical properties and excellent soundproof performance.

New approach to evaluation of fiber/matrix interface

A double cleavage drilled compression (DCDC) specimen has been analyzed by the boundary element method (BEM). At the same time, the DCDC specimen of FRP composites was proposed and used to evaluate the interfacial behavior and fracture energy in CFRP model composites. As a result, it was confirmed that the DCDC specimen had a mode I stress distribution for the hole offsetting displacement of b equals 0, which was of a symmetrical hole, while it had the mixed- mode stress distribution for b does not equal 0. The approach of calculating the interfacial fracture energy was established by using both of experimental results and analysis. Furthermore, the bridging fiber DCDC specimen was proposed and the effect of bridging fibers on the loading phase was made clear. It was shown that the DCDC test was a useful method to quantitatively evaluate the interfacial behavior of fiber/matrix in composites.

Interfacial Behavior of Fiber/Matrix Based on of Middle Layer of Organic-Inorganic Nano Hybrid

It was expected that the performance of a polymer based composite and its interfacial behavior of fiber/matrix could be improved by using organic substance and/or organic-inorganic hybrid. In this paper, the new approach of using organic-inorganic nano-hybrid as a middle layer to improve the interfacial performance of composites was proposed. Several typical reinforcements such as continuous glass fiber, chopped strand glass fiber and glass fabric were treated by epoxy/silica nano-hybrid and a middle layer between reinforcement of glass fibers and the matrix of epoxy resin was introduced. The composites with this nano-hybrid middle layer for reinforcements were fabricated. Then the interfacial behavior with and/or without the hybrid middle layer was investigated and compared by the fragmentation test with simultaneous AE measurements, peeling tests and SEM observation. It was found that the interfacial behavior between glass fiber and epoxy resin could be improved greatly by introducing the middle layer of epoxy/silica nano-hybrid. It was confident that the concept of using organic-inorganic hybrid as a middle layer between reinforcements and matrix would be one of effective approaches for improvement of the interfacial behavior in composites.

Effects of Matrix Resin and Fiber Content on Mechanical and Fracture Properties of Continuous-Fiber-Mat Reinforced CP-Resin Composites.

The purpose of the present work is to reveal the behavior of two kinds of different matrix resins in composite materials through the experimental results of mechanical and fracture properties and observation of macro and micro fracture surfaces. The composite specimens are made of ductile or brittle resin and continuous glass fiber mats with various fiber contents. The elastic modulus and the tensile strength of these two resins are nearly of the same values, while the elongation is different.The results obtained are as follows:(1) The tensile strength of the ductile matrix resin composites is higher by 20-30% than that of the brittle matrix resin composites. The values of both composites with fiber content 20wt%, however, are lower than those of the matrix resins. These results can be understood based on the observations of macro fracture processes and fracture surfaces.(2) The fracture toughness Kin at crack growth initiation increases with increasing fiber content for both composites. The values of Kin for the ductile matrix resin composites with fiber contents 0wt% and 20wt% are higher than those of the brittle ones. For the composites with fiber contents 40wt% and 60wt%, the values of Kin for both composites scatter randomly.