Crack growth behavior in glass fiber reinforced polycarbonate of 30% fiber content in weight (FRPC) was studied under creep conditions at 30, 60 and 90°C from a viewpoint of fracture mechanics, using two types of geometry of test specimens, a single edge notched one (SEN) and a center notched one (CN). It was found that the creep crack growth curve was similar to the creep curve and could be divided into three stages by the crack propagation rate. A creep crack growth model of FRPC was proposed on the mechanism of creep crack growth at each stage and was confirmed by the macroscopic and microscopic observations of the fractured surfaces. Moreover, it was examined which mechanical parameter, the stress intensity factor or the net section stress, controlled the creep crack propagation rate. It was considered that the crack propagation rate can be normalized by the stress intensity factor in the present temperature range.
Several simple and accurate methods of measuring the impact tensile fracture energy of fiber reinforced composites were examined by using a drop-weight type impact tensile testing machine. The tensile fracture energy was found to be expressible by a normal distribution function and, therefore, the Up-and-down method was suitable for measuring it. The results by this method were compared with those obtained by using the Probit analysis and the Impulse method. In the Impulse method, the fracture energy was calculated from the impulse, or the change of the momentum of the weight. The Up-and-down method was found to be a simplest and most effective method because it requires neither a large-size sample nor an additional device for measuring the velocity of a weight or the displacement of a specimen. The impact tensile fracture energy of glass cloth reinforced polyester resin laminates was remarkably large in comparison with the static tensile fracture energy. It was found that a material which shows large energy absorption in the Charpy impact test did not always show a good performance in the impact tensile test. This result suggests that the energy absorbed in the impact tensile test should be considered as an important property of fibrous composites like Charpy impact strength.
The denture base made of dental resin (PMMA) is widely used because it can be produced rather cheaply and molded easily, but this kind of denture base shows some weakness, such as low stiffness and low strength. So, as the upper partial and complete denture bases, we have developed various types of fiber reinforced denture bases using the organic fiber (Kevlar-49) as their reinforcement, and compared their flexural properties with that of the unreinforced denture base. The reinforcing types tried are reinforcement by unidirectional fiber along the arrangement of the anterior teeth, lamination of cloth FRP on the palatal area, and local reinforcement at the remaining tooth (the notch part) using various methods. All these types are considered practical for dental use. By taking the deformation behavior of the denture base during the mastication into consideration, the loading condition for testing was selected so as to give a uniform bending moment in the denture base. The results show that the fractural strength in bending of these reinforced denture bases in bending improved 10-190% over that of the unreinforced denture base, and the stiffness of these reinforced denture bases increased. Although the effectiveness of such reinforcement is shown here only on one kind of reinforced denture base using the organic fiber as reinforcement, it is expected that such fiber reinforced denture bases should become very useful for dental use.
When a material is subjected to repeated loads, mechanical energy is consumed inside of the material by transforming into other types of energy such as heat and sound. As the thermal conductivity of FRP is low, a part of the loss energy is stored inside as heat, and the temperature of FRP increases. Since FRP is a sensitive material to its environmental temperature, it is important to investigate such temperature increase of FRP during fatigue. If the temperature increase of FRP under repeated loads is caused by the heat equivalent to the internal loss energy, it is affected by such factors as kinds of resin and glass cloth, glass content, constitution of laminate, stress amplitude and cyclic frequency. In this paper, the following problems are discussed. (1) The temperature increase of specimens in the static tensile test, (2) the relations between the constitution of laminates or stress amplitude and the temperature increase of specimens in the fatigue test, (3) the temperature distribution in the inside of specimen in the early period of fatigue test, and (4) the effect of the temperature increase of composite laminate on its fatigue behavior. The temperature measurement was made with an infra-red radiation thermometer, which is capable of measuring surface temperature of objects without mounting sensors on their surface.
It is the purpose of this investigation to clarify the relation between volume resistivity and stress luminescence of GFRP (Glass Fiber Reinforced Plastics). The volume resistivity decreases as the moisture content in GFRP increases. The tensile test was used in this experiment and the luminescence was detected by a photo-electron multiplier. The results obtained are as follows: (1) The intensity of luminescence per unit area on the delamination increases as the volume resistivity increases. (2) The appearance of first luminescence is delayed as the volume resistivity decreases. (3) The intensity of the luminescence per unit area decreases as the appearance of first luminescence becomes later. Furthermore, a simulation test of the luminescence was tried to obtain the amount of energy which is transformed to the luminescence. As the result, 5∼30×10-6[J]is transformed to the luminescence.
The lattice distortion and crystallite dimension of ZnO powder ground by a ball mill and a vibration mill have been studied by Stokes-corrected Fourier Analysis of X-ray diffraction pattern, electron spin resonance, heat treatment, etc.. The following results have been obtained. (1) The internal strain and surface strain are about the same throughout the grinding process by a ball mill, but the surface strain is greater than the internal strain by a vibration mill. (2) In the initial stage of grinding, the strain curve shows a minimum, and the intensity curve of ESR C peak crosses the curve of B peak. It seems that a part of the original strain in ZnO crystallites decreases and another sort of strain appears during the grinding process. (3) From the results of the effect of heat treatment, it seems that the A peak of ESR correspond to the broken bonds associated with edge dislocations. (4) The crystallite dimension is greater in the c-direction than in the a-direction, and this tendency is intensified by the grinding process.
It is considered generally that the process and style of high voltage low current arc deterioration of electrical insulating materials differ somewhat depending upon the testing condition, resulting in a considerable difference in their arc resistances. In this paper, the effects of the size of electrical insulating material, its spatial configuration, its geometrical form, the electrode arrangement for it and the electrode shape on high voltage low current arc resistance were examined in order to obtain the fundamental data of arc resistance of insulating materials in practical electrical equipments. The test procedure was similar to the ASTM D 495-71 standard method, and in the most experiment the plate type electrodes were contacted closely with a test sample under 50±5g weight, and the arc was stabilized by sharpening the tips of electrodes to 30°. The electrode gap length was 6∼50mm and the max. applied voltage was 50kV. The arc intermittence time was the same as that of the ASTM method. For the variation of arc current due to the differences in electrode shape, electrode gap length and applied voltage, the current control resistance of primary circuit was adjusted in the test circuit so as to maintain the same current as that of ASTM method. Furthermore, the morphological consideration of arc deterioration path and the tests at various testing times in each of the test steps of the ASTM method were carried out to investigate the process of arc deterioration. The results obtained are summarized as follows: (1) The effects of all the variables tested on arc resistance were as large as that of the property of insulating materials. (2) The deterioration path starts to form on the outside, generally a part from the electrode axis, and it deflects easily. The deflection angle found most frequently was 140∼150°. (3) The arc deterioration of insulating materials did not proceed uniformly, and the process of deterioration in the first stage differed from that in the last stage. Direct processes were also observed in various insulating materials.
The problem of the initiation of hysteresis loop of low carbon steels was dealt mainly from the theoretical view point under the condition that the material was subjected to the repeated stress in the range comparable to its long life fatigue. The experimental observations of hysteresis loop were first made on three kinds of low carbon steel with different concentrations of solute atoms. The theoretical calculations were, then, carried out by means of a theoretical model based on the interaction between carbon atoms and dislocations, and the results were compared with the experimental results. The basic concept in establishing the model is that the diffusion of solute carbon atoms is far more rapid through the pipe diffusion process along the bowed out dislocation lines than through the volume diffusion process and the rapid unpinning of dislocation strings takes place, leading to the observable growth of hysteresis loop in each stress cycle even at room temperature.
The effects of tensile speed and temperature on ky of the Petch equation were studied by obtaining the relation between lower yield stress and tensile speed for low carbon steel of various grain sizes. The results obtained are summarized as follows: (1) The Petch slope ky seems to be constant in the low temperature range under a constant tensile speed; it may be explained by the mechanism of dislocations created in the vicinity of the grain boundary. On the other hand, ky decreases with increasing temperature at elevated temperatures; in this range, ky may be a function of unlocking stress and boundary strength. (2) The ky is dependent upon the tensile speed in the low temperature range where it is independent of temperature. It may be explained from the fact that the friction stress increases with tensile speed. On the other hand, the tensile speed dependence of ky disappears at elevated temperatures. It may be explained from that ky contains both the term of unlocking stress which is affected by the tensile speed and that of boundary strength independent of tensile speed.
The effect of grain size on the fatigue crack propagation was studied on pure aluminium in fully reversed plane bending. The crystallographic deformation in the vicinity of fatigue crack was observed by means of the X-ray microbeam technique and optical microscopy. The propagation rate of fatigue crack was found to increase with increasing grain size. The crack propagation was observed to be occasionally retarded by the grain boundary. Since the crack meets the grain boundaries more frequently in the specimen of smaller grain sizes, the crack propagation rate is smaller than that in the specimen of larger grain sizes. This interprets the grain size dependence of crack propagation. On the other hand, the propagation rate of fatigue crack could be expressed by a power function of the excess dislocation density in subboundary (Db)max or the subgrain size t in the grain at the crack tip with a positive or negative exponent. By analysing the development of substructure and the propagation of crack, the crack propagation in one grain seems to be essentially ruled by the substructure.
Ductile fracture criteria would be a very useful engineering tool in predicting failure of metals in practical deformation processes. In the present study, a ductile fracture criterion for a cracked plate of n-th power strain hardening material has been investigated theoretically and experimentally under plastic uniaxial tension. The relation between the critical strain far from a crack at fracture and a crack length was derived theoretically based on the energy conservation law and by analytical estimation of the strain energy for a cracked plate. The experiment on 0.80%C steel was conducted to compare with theory. Also, the strain and stress distributions near a crack tip were discussed. The following conclusions were obtained: (1) The total strain energy I for a cracked plate is expressed as the sum of I∞ without a crack and Ic due to a crack, where Ic is represented by the product of I∞, square of a crack length C2 and constant α, that is, Ic=α·C2·I∞. (2) The relation between the critical strain ε∞ and a crack length is given by ε∞·C1/1+n=const. (3) The experimental results are in good agreement with theory. (4) The strain distribution ε near a crack tip is predicted as follows, ε∝ε∞(C/r)1/1+n (ε∞: strain far from a crack, r: radial coordinate)
Mechanical degradation of polystyrene (PS) has been studied kinetically by a vibration ball mill under atmospheric pressure at room temperature. The degree of degradation was determined by measuring the number-average degree of polymerization at various stages of degradation. The extent of degradation was represented by the rate constant of degradation (kS) and the final average chain length (P∞). The degradation of the mixture of polycarbonate (PC) and PS was also carried out under the same condition. kS of PS in the presence of PC was increased with the amount of PC, while kS of PC was decreased with the amount of PS. These results are used to discuss the mechanical degradation of polymer in a vibration ball mill.
In order to investigate the effect of understressing on fatigue strength of low carbon steel, the fatigue tests have been carried out under rotating bending and torsion, using the specimens of Al-killed and quenched rimmed steels having different strain aging abilities. Based on the comparison of the experimental results for both materials under both loading conditions, the fatigue behaviours, particularly the effects of strain aging were discussed. The main conclusions obtained are summarized as follows: (1) Under both loading conditions, the understressing effect appeared clearly in quenched rimmed steel but not in Al-killed steel. (2) The understressing effect did not depend on the understress levels. (3) The specimen of quenched rimmed steel aged at room temperature showed an increase in fatigue strength and hardness compared with the unaged specimen. (4) The coaxing effects was revealed slightly in Al-killed steel. In quenched rimmed steel, it was remarkable under rotating bending but appeared under torsion only when the coaxing stress increment was small. (5) The coaxing effect was related to the existence of crack and was most remarkable at the crack length of about 70μ. (6) It seemed that the main causes of understressing and coaxing effect for quenched rimmed steel were precipitation hardening and strain aging.
Recently, it becomes difficult to find economical construction sites for dams because numerous dams have been already constructed in the past for flood control, water resources and power generation. Therefore, it is urgent to develop new materials or construction methods suitable for the embankment of relatively low dams on weak bedrock or soil foundation. This report describes such a new material which we have developed. The main points are as follows: (1) The new embankment material is made by mixing decomposed granite in place with portland cement, but the process requires some elaborate management in quality and construction works. (2) The physical properties of the decomposed granite is greatly improved by the addition of the cement of about 5 weight % of the dry decomposed granite. The reaction products between the cement and clay minerals in the decomposed granite have been confirmed by chemical analysis to be hydrates. This material is proven stable enough for the embankment use from its chemical and physical properties. (3) This material makes the volume of dam banking less and the quality control during construction easier, leading to the economical embankment with improved quality.
For the theoretical estimation of macroscopic elastic moduli of particulate-reinforced-plastic, there are three important factors to be considered; (1) the shape of filler-particles, (2) the orientation distribution of fillers, and (3) the interaction among fillers. In industry, various shapes of fillers are used for the reinforcement of plastics. Except for spherical fillers, it is impossible to neglect the orientation distribution of fillers. The filler-particles can be added up to such an amount that the effects of fillers appear remarkably, so that the interaction among the fillers cannot be neglected in general. We can evaluate the effect of the shape of filler by applying Eshelby's theory, if the shape of filler can be approximated to ellipsoid. However, the factors (2) and (3) have never been analyzed in a satisfactory manner. In this paper, a self-consistent method utilizing Eshelby's theory is proposed. It is shown that by using this method the effects of orientation distribution as well as the interaction among fillers can be evaluated in terms of distribution functions.
An analytical method proposed in the preceding paper, has been applied to calculate the effective elastic moduli of composites in which fillers are oriented uniaxially, plane-randomly or space-randomly. The shapes of fillers are considered to be fibres or disks with various aspect ratios. (In them spheres are included as one of the special cases.) The main results found on the Young's moduli of Glass-Epoxy systems are as follows: (1) The degree of reinforcement with fibres or disks increases logarithmically with the aspect ratio of these fillers. The“critical aspect ratio”for disks is larger than that for fibres, and generally that ratio for plastic-composites is larger than that for metallic-composites. (2) In the finite range of aspect ratio, fibres are more effective than disks to increase the Young's moduli in the reinforcing direction, when these fillers are oriented uniaxially. Continuous fibres are almost equally effective as disks with infinite aspect ratio. (These are usually called FRP and layered composite.) (3) When fillers are oriented space-randomly, in the range of aspect ratio more than 10, disks are more effective than fibres on Young's moduli and Poisson's ratio of composites. (4) The usefulness of approximate solutions in which interactions among fillers are neglected (the case that the elastic moduli of composites are expressed in linear functions of volume fraction of fillers; 1+δv), should be examined for each shape and for each orientation distribution of fillers.