Almost all rocks are more or less anisotropic and each rock has its own anisotropy in microstructures. The possible effect of anisotropy should be evaluated rather than neglected. To check the microdefects which characterize each rock's anisotropy, we analyzed two or three thin sections oriented parallel to and/or normal to the quarry plane for four kinds of rocks, Ogino tuff, Shimazaki andesite, Ukigane gabbro and Oshima granite. To check physical anisotropy, we used the P-wave transmission method for each rock. Thin section analyses showed that, in the case of Ogino tuff, flat pores oriented parallel to the quasi-horizontal plane and had preferred orientation, while in the case of Shimazaki andesite, plagioclase grains were oriented parallel to the quasi-horizontal plane. Healed cracks preferentially oriented within hornblende and plagioclase grains were typical in Ukigane gabbro. Open cracks and/or healed cracks oriented parallel to the three mutually perpendicular planes, one of which is quasi-horizontal, were found in Oshima granite. The ultrasonic transmission method indicates that P-wave is strongly sensitive to open crack orientation and sensitive to low aspect ratio pore orientation and mineral grain orientation, but insensitive to healed crack orientation. This study suggests the method to be available for qualitative evaluation of rock anisotropy.
A recently developed testing method which takes a linear combination of axial stress and strain as control variable was applied to investigate the post-failure behavior of rock under various confining pressures up to 40MPa. Though the method was only applied to uniaxial compression test in the previous studies, this study showed that it is also applicable to triaxial compression test, and the following results were obtained: Firstly two out of the four rocks showed transition from class II to class I characteristics as the confining pressure is increased. And after the transition, a rapid decline in load bearing capacity was found to become gradually less remarkable as the confining pressure is further increased. For tested rocks except Akiyoshi marble, when the loading rate was raised by 10-fold, the compressive strength increased by nearly same quantity no matter how much the confining pressure was. The compressive strength of Akiyoshi marble did not always increase when the loading rate was increased. Such behavior may be related to the mechanical property of calcite crystal, which is remained to be further investigated.
Sleeve fracturing is very similar to hydraulic fracturing except for the method of loading. Sleeve fracturing is a technique by which radial fractures are induced at any depth without introducing fluid into the rock during the fracturing process, and it is realized by pressurizing a jacketed borehole. Diametral deformation of a borehole is measured to examine the fracturing process of the primary radial fractures, which occur in the direction perpendicular to the minimum in-situ stress in the plane normal to the borehole axis, and the secondary radial fractures which extend perpendicular to the maximum in-situ stress. In this paper, the fracturing process with multi-stage loading is analyzed by the boundary element method and a procedure for determining in-situ rock stresses is presented. Relationships among borehole pressures, length of fractures and in-situ rock stresses on the fracturing process are formulated. It is clarified that the in-situ rock stresses are determined by the length of fractures and the borehole pressures at the stages of the completion of free separation and the re-propagation of fractures, which are evaluated from the curves of pressure-diametral displacement in multi-stage loading test. Furthermore, it is shown that the in-situ rock stresses can be successfully determined by using the presented procedure in practice.
The concept that the scattering of the strength of rocks should be large has been widely accepted in engineering rock mechanics. However, carefully arranged experiments have shown that the coefficient of variation of uniaxial compressive strength for thirty-one specimens of Oshima granite is 1.9%. The effect of environment on the fracture process of granite was then investigated. The compressive strength of the air-dried specimens was higher, by about 20%, than that of the wet specimens. The dilatant strain rate under the wet condition was higher, by a factor of three, than that under the dry condition, at the same stress levels. These facts indicate the important effect of water. A model, based on the assumption that the slow crack growth due to stress corrosion plays an important role in the fracture process of rock under compression, explains quantitatively these effects. Since the different environment may bring about different water content of the specimens, the strength of rocks may scatter within 20%, in a usual room condition.
In order to determine the optimum shape as well as the optimum excavation and support systems for tunnels, a three dimensional back analysis method based on an optimization technique is developed. Equivalent values of mechanical parameters of rocks surrounding the tunnels are back analyzed, which reflect non-homogeniety of the rocks, stress relief condition and structural conditions such as cracks and joints. Application of the method to an extremely squeezing tunnel shows appropriateness of the method through comparison of analytical and field data. To decrease costs and time in three dimensional back analysis, a convenient method is also proposed, by which the maximum and the minimum principal stresses of the ground and the direction of these stresses in the tunnel cross section are directly determined from convergency data.
The Shin-Aimoto Hydro-electric Power Station of 124MW has been completed on Kurobe River by Kansai Electric Power Company in 1985. The construction of the longest headrace tunnel of 5.7km was subject to the critical path in the whole project. The feasibility study was carefully carried out from the rock mechanical point of view. Rocks are gneiss, diorite and limestone which belong to Hida Metamorphic Zone in the central Japan, and the mechanical characteristics of rocks showed remarkable variation with wide range. Rock classification was discussed geostatistically with the combination of CRIEPI's method and Barton's Q-system. In order to get successful result in rapid excavation the TBM and long drill-and-blast methods were applied to the above-mentioned headrace tunnel. Also, the mechanism of “loosening” has been analyzed in comparison between TBM and blasting methods.
The high cycle fatigue strengths at the toe of the cruciform welded joints of SM41A and HT60 were investigated experimentally under tensile and bending fatigue loading. The effects of welding process (toe treatment), mean stress and welding residual stress on fatigue strength were studied. The main results obtained are as follows. (1) The fatigue limit of SM41A welded joint under tensile loading was increased up to 64% by the toe treatment of CO2 soft plasma welding. Similarly, in the case of HT60 welded joint the fatigue limits under bending loading were increased to 77%, 43% and 25% by the toe treatment of CO2 soft plasma welding, TIG welding and grinding, respectively. These improvements can be attributed to the change of the toe part geometry; lowering the elastic stress concentration factor at the toe. (2) The relation between fatigue limit of welded joint and mean stress can be expressed by the straight line intercepted at the true rupture stress on the axis of mean stress in fatigue limit diagram. (3) The fatigue limit of welded joint was decreased by tensile welding residual stress. This decrease can be well estimated by regarding the welding residual stress as the static stress.
Al-4wt% Cu alloys were solidified unidirectionally to obtain the columnar structure, and the ultrasonic velocity and the attenuation were measured by using longitudinal waves with a frequency of 5MHz, to investigate the relation between the cast structure and the ultrasonic parameters. The attenuation increased with the diameter of columnar grain, but largely scattered among different ingots. The attenuation also increased with the dendrite primary arm spacing, and a linear relation was obtained between them. Since the attenuation varied with the arm spacing after homogenization, the attenuation was found to be controlled by small pores in the ingot; the attenuation linearly increased with the fraction of porosity area and the average porosity size. The attenuation of ultrasonic waves propagated parallel to the columnar grains was three times larger than that of ultrasonic waves propagated perpendicular to the columnar grains. The ultrasonic velocity slightly decreased with distance from the ingot bottom. However, the velocity had no dependence on the diameter of columnar grain, the solute segregation or the porosity parameters.
Fracture toughness of sintered silicon nitride was evaluated at room temperature, 1000 and 1200°C using the technique of single edge precracked beam method or bridge indentation method. It was observed that the geometry of precrack developed from five Vicker's indents satisfied the requirements of the standard test method for plane strain fracture toughness of metallic materials (ASTM E 399-83). The scatter of the fracture toughness obtained by this testing method was small compared with that of tensile strength for the same material in the temperature range from room temperature to 1200°C, and the Weibull shape parameter was in the range from 21.09 to 60.72. The mean value of fracture toughness obtained at 1000°C was nearly equal to that obtained at room temperature, and the value decreased when the temperature exceeded 1000°C. Furthermore, a slow crack growth was observed at the precrack front on the fracture surfaces at 1000 and 1200°C.
The bending test and fracture toughness test with acoustic emission (AE) measurement were carried out on calcium-silicate/paper composites laminated with various spacings. The bending strength of the divider section specimen became twice as large as that of the calcium-silicate mortar (plain), while the bending strength of the arrester section specimen was 60% of that of the plain. Influence of the laminated specing on bending strength was hardly observed. The strength, however, was increased by producing the composite in which calcium-silicate penetrates in the fiber-structure space because of large adhesion between fiber and calcium silicate. On the fracture toughness test, J integral at maximum load, Jpmax, of the composites was approximately 30 times as large as that of the plain. The laminated spacing did not influence J integral at the on-set of calcium-silicate layer fracture nor J integral at the on-set of paper layer fracture. From the result of J-R curves determined by unloading compliance, on the other hand, the crack growth resistance increased with decreasing laminated spacing. A good correlation between the AE rating parameter, Tac=ΣEAE/B/ΔJ, and the crack growth resistance, ΔJ/Δa, was obtained. It is considered that most of the AE events having high energy were caused by the fiber pull-out and/or failure in paper layer. The high-energy AE relative frequency, RN, is an efficient parameter to predict the effect of reinforcement.
A study has been made of the critical consideration on the possibility of the tolerant defect at the endurance limit of plain carbon steel specimen. There exists the critical size of pre-crack and micropit which would not reduce the level of original endurance limit of the fine homogeneous spherodized cementite structure, and these two sizes coincide well with each other in the range less than 100μm. This suggests the existence of tolerant defect which could be estimated by fatigue experiments on pre-crack and micropit specimens. Furthermore, disagreement of the above critical sizes of the fatigue pre-crack artificial pit specimens becomes distinct with increasing degradation of homogeneity and coarsening of microstructure.
Fatigue crack propagation tests were conducted using thin-walled tubular specimens of pure copper under combined in-phase axial-torsional loadings. In large scale yielding situations, the crack growth behavior under biaxial stresses was investigated based on the elastic-plastic fracture mechanics. The crack morphology was found to be a bent-type in the combined loadings and a branch-type in the torsion case. Cracks were found to propagate in such direction that the Mode II component of the stress intensity factor was minimized. By using a devise to detect the opening and sliding displacements at the center of crack, the deformation behavior of bent-or branch-cracked specimens was also examined to evaluate the crack opening point. It was noted that tails were observed near both tips of the hysteresis loop in the case of torsion. The correspondence between these tails and the crack closure was discussed in the relation with the opening and closing behaviors of ideally elastic cracks. The effective stress intensity factor ΔKIeff of Mode I was calculated by taking account of experimental observations mentioned above. When ΔKIeff was employed to correlate the growth rate, good correlation was not seen because of gross plasticity. A tentative procedure to estimate the J-integral range ΔJ was proposed for the bent crack. The growth rate of bent crack was correlated very well with ΔJ estimated by the proposed procedure.
In this study, the fatigue test to obtain the L-N characteristics was first performed on the spot welded joints in cross tension type. Then the fatigue life distribution was experimentally examined by assigning a number of specimens at two different levels of applied load. Moreover, SEM observations were made on the fracture surfaces by paying attention to the crack initiation site and the crack propagation path. The main results obtained on this type of welded joints are summarized as follows; (1) The both distributions of the static strength and the fatigue life were well represented by the three-parameter Weibull distribution. (2) The fatigue fracture was classified into several types depending on the combination of crack initiation sites and crack initiation lives at respective sites. (3) The fatigue fracture was caused by the propagation of the second crack which was stimulated by the first crack. (4) A correlation was found between the fatigue life and the crack reflected angle, and this was qualitatively explained by the variation of stress intensity factor in the mixed mode.
Two kinds of polyvinyl chloride specimens (Hi-PVC and PVC) with or without blending a reformer for impact resistance were prepared in order to compare their impact resistance properties. The effects of temperature and notch root radius on the specific absorbed energy, the impact strength and the fractographs were investigated by using the instrumented Charpy impact test apparatus. It was found that the impact resistance property of the PVC specimen depended on the temperature as well as on the notch root radius. It was also found that the Hi-PVC specimen showed a tendency similar to PVC at temperatures below 0°C, but improved its impact resistance property at temperatures above 20°C due to the reformer.
Recently, advanced ceramics with superior strength and corrosion resistance have been developed. They have a possibility for various engineering applications as structural materials used in high temperature water, such as steam turbine systems or water cooled reactors. Fundamental corrosion behaviors of 14 kinds of sintered polycrystalline ceramic were examined in oxygen-free (less than 20ppb dissolved oxygen) and oxygenated (about 32ppm dissolved oxygen) pure water at 290°C using a refleshed-type autoclave system. Change in weight and surface morphology detected by X-ray diffraction and scanning electron microscopy on the samples corroded for 72 hours were discussed. Silicon carbide showed the best corrosion resistance among the present ceramics. The corrosion resistance of alumina strongly depended on its purity. 99.99% purity alumina showed high corrosion resistance. For pressure-less sintered and hot-pressed silicon nitride and sialon ceramics, the preferential dissolution of crystalline phase occurred at the surface of the sample and the grain boundary phases remained near the surface. Aluminum nitride formed a thick boehmite layer in the tests. Partially stabilized zirconia, stabilized by 3mol% yttria, flaked away by accompanying a phase transformation from tetragonal to monoclinic. Many of the non-oxide ceramics, particularly silicon carbide, showed more accelerated dissolution rates in oxygenated water than in oxygen-free water.
Thermal expansion coefficients of unidirectional carbon fiber reinforced composites were investigated theoretically. Explicit equations for the longitudinal and transverse thermal expansion coefficients of the composite were derived, employing the extended equivalent inclusion method. The anisotropic property of the carbon fiber as well as the elastic interaction among the fibers were taken into account. The formulae obtained are valid for the whole range of fiber volume fraction; 0≤Vf≤1, and they are simple in the form and easy to use. The availability of these formulae was confirmed by the comparison of the predicted values with experimental data on three types of carbon fiber composites. The formulae derived here are applicable not only to carbon fiber/epoxy but also to carbon fiber/metal composites. In the case of carbon fiber/rubber composite, in which the rigidity of rubber is infinitesimal compared to that of carbon fiber, these formulae can be reduced to more simple equations.
In order to observe the crack initiation and propagation behaviors, an original fatigue testing machine was designed and made, and it was combined to SEM in commercial type. By using this installation, fatigue crack initiation and propagation behaviors were directly observed on aluminum sheet specimens in attention to the long life region. The main results obtained are summarized as follows; Some pores are formed at the intersections of slip bands in certain grains on the specimen surface, and these pores are connected to each other passing through the slip bands. Microscopic fatigue cracks formed in this mechanism are found in many surface grains, but the propagation rates of these microcracks have a large scatter. By adjoining some of these microcracks, a dominant crack appears somewhere on the specimen surface, and this crack grows gradually in company with the coalescense to further microcracks along the crack propagation path. The number of such predominant cracks in a specimen is only a few in the long life region of Nf>2×106. Another finding is that small fatigue cracks having the length such as L<0.1mm clearly open in full time of each cycle in pulsating tension.