Makoto Kawagoe

Growth and healing of a surface crack in poly(methyl methacrylate) under case II diffusion of methanol

The behaviour of a pre-crack inserted in a surface swollen layer formed by case II diffusion of methanol in poly(methyl methacrylate) (PMMA) was investigated with reference to the distribution of internal compressive stress induced under constraint by the inner glassy core. The elastic analyses of internal stress were conducted by a finite element method using the data for the mechanical properties and the swelling strains preliminarily obtained for the sheet specimen overall swollen in methanol. The growth of the inserted crack in the surface swollen layer under static tension in methanol at 20°C was almost arrested for a long period at a position where the internal compressive stress in the tensile direction took the maximum magnitude near the boundary between the swollen layer and the glassy core. After being released from the internal compressive stress, the crack rapidly progressed to lead to a general fracture going across the boundary. A relatively long crack inserted in the thinner surface swollen layer completely disappeared in methanol at 40°C during 3 min under no external load. Crack healing, however, was not observed either in methanol at the lower temperature of 20°C or in the entirely swollen specimen, even at 40°C. These results suggest that both high temperature above the glass transition temperature of swollen PMMA and the significant internal compressive stress are required for crack healing.

Effect of water absorption and desorption on the interfacial degradation in a model composite of an aramid fibre and unsaturated polyester evaluated by Raman and FT infra-red microspectroscopy

The interfacial degradation in a model composite of an aramid fibre (Kevlar 49) and unsaturated polyester (UP) followed by absorption and desorption of hot water at 90 °C was monitored by measuring the variations in residual stress in the embedded fibre by means of the micro-Raman technique. The Fourier transform infra-red (FTIR) microspectroscopic analyses were also conducted to study the structure and the quantity of absorbed water causing interfacial degradation. Copyright

Fracture mechanisms of poly(methyl methacrylate) under static torsion in alcohol environments

Static torsion tests of poly(methyl methacrylate) were conducted in several alcohol environments at room temperature. The critical torsional stress for crazing and/or cracking increased with increasing molar volume of alcohol, and was not correlated with the solubility parameter, nor with equilibrium solubility as reported in previous results on n-alkanes. Crazing stresses in alcohols were generally lower than those in alkanes. According to Fourier transform-infrared microscopy of the surface scratch made in several environments, both in ethanol and in 1 -butanol, some absorptions were detected at 3450–3650 cm−1, probably due to the hydroxyl group of these alcohols caught by hydrogen bonding to oxygen of the carbonyl group in the side chain, whereas in other environments (1 -octanol, n-hexane, and air) there were no absorptions in this region of wave number. These results suggest that at a flaw tip strained under stress, penetrating alcohol molecules are chemically adsorbed in a cluster, which breaks a weak bond (e.g. dipole interaction) between the polymer chains and, as a result, facilitates craze formation and breakdown leading to brittle fracture.

Fatigue behaviour of injection-moulded polymer blends of polypropylene and liquid crystalline polyester

Abstract Fatigue processes in injection-moulded blends of polypropylene (PP) and a liquid-crystalline polyester (LCP) were investigated with reference to variations in the inner microstructure and the dynamic viscoelastic properties under cyclic tensile loading. For neat PP, whitening caused by inter-spherulite and/or interlamella microcrazing was observed over a wide range within the specimen, and the storage modulus E ′ and the loss tangent than δ monotonically decreased and increased, respectively, until fracture. For the neat LCP specimen, on the other hand, the molecular orientation progressed all over the specimen during fatigue, and, as a result, E ′ remained almost constant and tan δ clearly decreased, until they were drastically reversed just before fracture. In the blends containing over 40 wt% of the LCP, the influence of LCP was so strongly reflected as to reduce tan δ during fatigue and to extend the fatigue lifetime. In addition, it was observed that the PP and the LCP phases closely attach to each other under loading. Such morphological change in the blended specimens with higher LCP content was also considered to contribute to a decrease in tan δ during fatigue and to the extension of their fatigue lifetimes.

Effects of absorbed water on the interfacial fracture between two layers of unsaturated polyester and glass

The effects of absorbed water on the interfacial fracture resistance between two layers of unsaturated polyester (UP) and glass of the bilayer specimen were evaluated by measuring a load for producing the fracture by inserting a razor blade into the interface. The specimens were subjected to the cyclic absorption-desorption and the continuous absorption processes of water. The load to initiate the interfacial fracture was markedly lowered by the early absorption process for short period, and then gradually reduced with increasing cycle or period of water absorption, although it slightly recovered after the first great reduction when the specimens were subjected to the soaking process at lower temperatures. The micro-FTIR (ATR) analyses of the detached surface of the UP resin from the glass plate revealed that the water is accumulated in the resin at the interface in the cluster, showing the concentration to increase with increases both in the temperature of environmental water and in the water-soaking period. The IR analyses also demonstrated the hydrolysis reaction to take place on the detached resin surface of specimen exposed to water at high temperature. Thus the accumulated water at the interface may remain and promote the interfacial degradation even under the drying process by various mechanisms like the hydrolysis reaction in hot water environment.

In-Situ Characterization of SiC–AlN multiphase ceramics

AlN and SiC can react and form a solid solution at temperatures above 1800 °C, a result that may be beneficial for sintering silicon carbide ceramics. The pressureless sintered AlN–SiC multiphase ceramics have reached high density at a temperature of 2100 °C for 1 hr in Ar. Analytical scanning transmission electron microscopy was then used to determine the grain boundary, fracture surface, and the local compositions. Because AlN has a higher solid vaporization pressure than SiC, the vaporization rate of the AlN solid would far exceed that of SiC at a sintering temperature. The vaporizing AlN was deposited on the surface of SiC powder; SiC grains then elongated in a random arrangement. The form of elongated rod crystals of 4H SiC is 5 to 8 μm in length and 1 μm in width. It resulted in the sample fracture section producing pulling-out and a strong tearing-open effect. The bending strength and the fracture toughness of the material obtained are 420 MPa and 4.40 MPa × m1/2, respectively.

On the properties of organic liquids affecting the crazing behaviour in glassy polymers

The properties of organic liquids affecting the environmental stress crazing in glassy poly(methyl methacrylate), polycarbonate and polyvinyl chloride were examined with reference to the diffusivity into the polymers. The isochronal bending tests for the sheet specimens were conducted by a traditional method using the elliptical bending device in environments of five alcohols, four n-alkanes and three good solvents at 30°C. The diffusivity of these liquids exclusive of n-pentane and the solvents was evaluated by measuring the weight gain of specimen under the accelerated soaking at 52°C. The absorption rate of each liquid, given by an initial weight gain per unit time, was shown to be correlated with a new factor consisting of the viscosity and molar volume of environmental liquid and of a ratio of dielectric constant of liquid to that of polymer. The critical crazing strains in alcohols and n-alkanes were also demonstrated to relate to this factor. In good solvents no crazing but dissolution of polymer surface was observed. It thus may be concluded that the crazing is strongly correlated with the diffusivity of environmental liquid, and in addition that the liquid with the above factor of greater value clearly tends to cause the environmental crazing in glassy polymers.

Fatigue failure of poly(methyl methacrylate) in alcohol environments

Fatigue tests of poly(methyl methacrylate) were conducted in environments of methanol, 1-butanol, and 1-octanol at room temperature under cyclic tensile loading at 10 Hz. The fatigue lifetime increased as the molar volume of the environmental alcohol increased, and indicated no correlation with the solubility parameter. The scanning electron micrographs of the fracture surfaces reveal that crazes strongly participate in the fatigue fracture, and suggest that a great increase in the fatigue lifetime in methanol brought about by pre-soaking the specimen in methanol for a period below 100 h, which has been previously reported, is caused by the combined effects both of the case II diffusion of methanol producing the softened matter and the internal compressive stress in the swollen surface layer and of a cooperative work of crazing, shear flow and/or shear cracking taking place under cyclic tensile loading.

High ductility in poly(methyl methacrylate) induced by absorption and desorption of an acetonitrile aqueous solution

Tension tests were conducted in air at room temperature on PMMA sheet specimens which had been previously soaked in a 40 vol % acetonitrile aqueous solution at 20 °C for 24 h and then dried in air at room temperature for 480 h. In contrast with an untreated specimen which fractured at a stress of 84 MPa and a strain of 9 %, shear yielding clearly took place at 42 MPa and the elongational fracture strain increased to about 148 %. No crazes were observed on the specimen surface and as a result the transparency of the PMMA was thoroughly maintained until fracture. Thus this soaking treatment may change PMMA to a completely ductile polymer without a crazing mechanism. The results of the dynamic viscoelastic measurements at 1 Hz show that the glass transition temperature was lowered to about 80 °C (as compared to about 110 °C), and the β relaxation became much sharper with a higher peak value of 20 °C (as compared to a broad curve with a peak at 50 °C). This clear β relaxation at room temperature may contribute to shear yielding and large plastic elongation of the treated PMMA.