Satoshi Somiya

Strength improvement by densification of C/C composites

Abstract The tensile strength of carbon/carbon composites (C/Cs) was examined as a function of the density in the range above ρ=1.6 g/cm3. Three processing routes of preformed yarn, resin char, and HIP processes were adopted to densify the C/C composites. The density was increased to 1.95 g/cm3 through these routes. The interfacial strength between the fiber and matrix was significantly and unexpectedly varied by repeating the HIP process, though the detailed source mechanisms could not be identified. The tensile strength of the densified C/Cs was shown to be dependent on the processing routes. In particular, opposite tendencies were obtained as a function of ρ, i.e. the monotonic strength increases for the C/Cs with the preformed yarn and resin char treatments, while a monotonic decrease was observed using the HIP process. However, after the tensile strength was re-examined, the ultimate tensile strain was found to be expressed by a monotonic decreasing function of the interfacial strength. This implies that weakening fiber interfaces is a key factor for obtaining C/C composites with the higher tensile strength.

Shear fracture of C/C composites with variable stacking sequence

Abstract Cross-ply laminated carbon–carbon composites with changing ply ratio of 0° and 90° were fractured under shear using several test methods. In these tests, special attention was placed on the understanding of the damage mechanisms. The ±45° off-axis shear test yielded the most precise shear stress–strain relation and was recommended to measure the shear modulus G 12 . On the other hand, for shear strength measurement, the Iosipescu test was superior. The non-linearity in the shear stress–strain curve was found to be solely due to matrix cracking, while the peak shear stress was determined by fibre failure. In order to measure the shear strength by the Iosipescu test, two fracture planes normal and parallel to the loading axes have to be considered. The shear strength increased linearly with fibre volume fraction normal to the corresponding plane. The overall highest shear strength was obtained for the (0/90) n laminate.

Study of the Effect of Aging Progression on Creep Behavior of PPE Composites

In this study, creep behavior of stainless fiber-PPE composites was analyzed in an oil environment at elevated temperatures, and the effects of physical aging on creep behavior were intensively investigated. The results showed that the creep phenomena of metal fiber-PPE composite with pre-aging treatment correlated with the Arrhenius time-temperature reciprocation law and within the aging range; time-aging time superposition also held good. Thus, prediction of short-term creep behavior for any pre-aging time was possible, based on the grand master curve of the creep compliance master curves and the shift factor for aging progression. It was clarified that a pre-aged composite can withstand higher temperatures and longer times. It was also observed that the energy of activation during creep decreased with an increase in pre-aging treatment time.

Creep Behavior of Metal Fiber-PPE Composites and Effect of Test Surroundings

The effect of environment on creep behavior of Poly-Phenylene Ether (PPE) composites with stainless steel fiber was investigated in this research. The results of creep behavior of PPE composites, carried out both in air and oil surroundings at elevated temperatures, show very good agreement with the Arrhenius reciprocation law of time-temperature. It was, however, observed that there was comparatively greater departure from good superposition in the creep compliance curve for oil surroundings in long period creep. The minute changes in activation energy for creep phenomena in different surroundings were observed. The effect of fiber volume fraction on creep behavior was also studied. In addition, a brief investigation of the effect of physical aging was done, with the results clearly showing that smoothness in the creep compliance master curve depends on the degree of physical aging of the matrix resin.

Effect of Crystallinity and fiber volume fraction on Creep Behavior of Glass Fiber Reinforced Polyamide

Thermoplastic Polyamide (PA) is known as Nylon®, and its FRP are one of the engineering plastics. It is very important for them to reveal accurately the visco-elastic behavior. Furthermore, PA is a crystalline polymer, and it is necessary to consider the effect of crystallization on mechanical properties. The purpose of this study was to make clear the effect of crystallization and fiber volume fraction on creep behavior of PA and its composites. The creep test was performed using the materials which adjusted crystallinity on each fiber volume fraction. As a result, these materials conformed to Arrhenius type of time-temperature superposition principle, and these effects on creep behavior had the time retardation effect. To compare the creep behavior on these effects, we made the grand master curves for crystallinity and fiber volume fraction. Obtained two grand master curves were compared each other, the shapes of these curves are similarly, and we can superposed them. It is therefore, the creep behavior including each conditions have the same behavior. Using these curves and shift factors, we can calculate the creep behavior, and be able to estimate the creep behavior with the effect of arbitrary time, temperature, crystallinity and fiber contents.

Degradation phenomena under water environment of cotton yarn reinforced Polylactic-acid

Recently, many green composites have been developed, and utilization of those materials have been is advanced. Because many biodegradable plastics usually show affinity to a human body, in this research, using cotton yarn and Polylactic acid (PLA), which is a typical biodegradable polymer, a green composite was developed as a medical substance. Although the mechanical properties of some biodegradable plastics were far lower than that of an engineering plastic. It is necessary to raise the mechanical properties of a biodegradable plastic to the level which is equal to metal medical material. Furthermore at the practical use, the influence from water absorption is strongly received. The material design of green composites using Cotton yarn/PLA has been done including the effect of water absorption.

Oxidation and Strength Degradation Behaviors of SiC-coated C/C Composite.

In this paper, oxidation behavior of bare and SiC coated C/C composites and its effect on mechanical properties have been studied experimentaly. It was found in oxidation tests that oxidation of an SiC-coated C/C composite occurred by oxygen diffusion through the cracks of SiC coating and thus the weight loss was concentrated just underneath of the coating cracks. Due to this behavior, bending strength degradation of the SiC-coated C/C composite was found to be more serious than that of the bare C/C composite when the comparison was made under the same weight loss level. In particular, it is noted that the bending strength degradation of the SiC-coated C/C composite is more rapid at higher oxidation temperature even in low weight loss condition. On the other hand, interlaminar shear strength, ILSS, degradation was shown to be higher in case of low oxidation temperature than in high oxidation temperature. This tendency suggests that main source of the bending strength degradation of the SiC-coated C/C composite should not be the matrix damages but fiber damages at least under low weight loss level.

Time Dependence of Degradation Phenomena of Plain Woven AFRP in Hot, Wet Environmental Exposure

Plain woven AFRP samples were immsersed in hot water at 80°C and exposed to humid air at 80°C, 80% RH for 300~days. Furthermore, they were tested by a tensile method to investigate time dependence of their degradation. The longer the immersion time, the greater the degradation of tensile strength. But on comparison with samples exposed to hot humid air, those immersed in hot water showed more degradation, although both conditions had the same temperature. Hot water caused the yarn/resin interface to degrade and led to delamination. Warps, which are weakened by absorption, were further damaged by delamination. Damaged warps broke and final fracture occurred at a low level of stress.At an average stress level, AE event rate of samples in hot, wet conditions was lower than that of virgin samples. This was because cracks occurred in the resin-rich area during water/moisture absorption, i.e., fracture occurred before tensile test. The change of fracture by water/moisture absorption in a hot, wet environment could also be revealed by the AE method.