The deformation characteristics of rock-like materials with different porosities have been studied experimentally using the specimens of cement mortar and tuff under triaxial compression states. The stress and strain are dealt with respectively by dividing into their isostatic and deviatoric components, and the experimental results are considered from a point of view of continuum mechanics. The results are as follows: (1) The deformation characteristics of the porous rock-like materials depend essentially upon the variation of form of microscopic structure with the stress history. (2) The stress-strain relationships of these materials under the isostatic stress are represented approximately by a bi-linear curve and its bending point may be considered as the yield point for the hydrostatic pressure. (3) In the state of normal compression, the amount of recoverable shearing strain γoct varies in proportion to τoct up to the yield point and its rate of variation does not depend on the mean stress σm. (4) In the state of normal compression, the contractancy arises with the increase of shearing strain under the constant mean stress and its magnitude increases with the mean stress. (5) In the state of over compression, the slope of τoct-γoct curve up to the yield point is steeper than one in the state of normal compression, and the slope does not depend upon the normal stress. (6) In the state of over compression, the contactancy is smaller than one in the normal compression and the dilatancy arises when the mean stress is relatively small. The deformation characteristics of rock-like materials mentioned above may be explained reasonably by assuming the end cap-shaped yield surface in hardening for the yield condition of materials.
The aim of this study is to clarify the fracturing process of granodiorite, known for its unstable behavior in-situ, under various conditions. A stiff, closed-loop servo-controlled testing machine developed recently was used to control the unstable brittle failure and to observe the time dependent fracture under uniaxial compression precisely. The tests of the constant deformation rate, the creep, the relaxation and the cyclic loading were conducted along with the A. E. measurements. The results obtained are summarized as follows: (1) The granodiorite is not stable even in the region below the load vs. axial deformation curve. (2) From the results of the A. E. measurements, the microfracturing is mainly of a tensile crack type which occurs along the axis of the applied load. (3) The pattern of the microfracture accumulation characterizes the inelastic behavior of the rock specimen and the main failure. (4) The dilatancy hysteresis due to the radial strain hysteresis and the absence of A. E. in the unloading cycle seem to show that only the tensile microfacturing contributes to the dilatancy hysteresis. The main feature of these observations is supported by a lack of the shear-related microfracturing except those which might occur just under the platen. From this point of view, to obtain the complete load vs. axial deformation curve, the radial deformation instead of the axial deformation should be selected for the feedback signal of the servo-control system. The complete load vs. axial deformation curve was obtained in this way, and this curve seemed to give the criterion of the failure of granodiorite. It is also concluded that the granodiorite used in these experiments can be classified as the“class-II”type rock defined by Wawersik and Fairhurst.
In this paper, the authors deal with the correlation between the stress-strain diagram and the rate of acoustic emission during the failure process of rocks under uniaxial compression. The rock specimens which contain a rectangular slit were loaded by a stiff testing machine (total stiffness=0.68×106kg/cm) in order to observe the process of fracture crack propagation from the corner of the rectangular slit and the rate of acoustic emission, which was detected with a piezoelectic type transducer (the inherent frequency of vibration=145kHz). The test results show that, if the abrupt and unstable propagation of a fracture crack takes place, the rate of acoustic emission increases abruptly and the applied load drops. On the other hand, the slow and stable propagation of a fracture crack takes place without accompanying any remarkable increase in the rate of acoustic emission. Based on the above test results, the fracture crack is classified into the stable or unstable type, according to the mode of propagation. It is clarified that the unstable propagation of a fracture crack is accompanied with stress drop and depends on the stiffness of the testing system, which suggests that the role of the stiff testing machine is to control the propagation of the unstable type of fracture cracks.
The deformation and fracture of rocks under combined stress states are most fundamental problems in rock mechanics. In previous works, however, the deformation, including the dilatancy, and fracture have been studied under the axi-symmetric stress state (σ1<σ2=σ3 or σ1=σ2>σ3). To discuss the deformation and fracture in actual cases, deformation experiments are needed under more general triaxial stress states (σ1>σ2>σ3). Recent development of a new triaxial compression technique by Mogi (1971) has made it possible to study some deformational properties, such as fracture strength, yield stress, ductility, etc., under general triaxial stress states. In the preceding experiments, the three principal stresses and one of the principal strains were measured. In this paper, the measurements of all components of principal stresses and strains are reported for the first time. The deformational behavior of rocks under general triaxial stress states has been made clear. Particularly, the result shows that dilatancy is highly anisotropic under general triaxial stress states.
Kumagai and Itô, since August 7, 1957, have been carrying out the measurements of creep, by bending, of two large beams of granite of a size of 215×12.3×6.8cm. The results obtained in 7, 10 and 13 years were published in this journal: Vol. 14 (p. 507), Vol. 17 (p. 925) and Vol. 20 (p. 185), respectively. Those obtained hitherto in nearly 20 years are given in this paper, which shows that the creep rate today is almost the same as that 10 years ago. And we have come to the conclusion that granite can do a plastic flow with a vanishingly small yield stress, i.e. a viscous flow. The viscosity of granite under the normal temperature and pressure is 3∼6×1020 poises as found from the general trend of the creep. Itô and Sasajima, since August, 1974, have been carrying out the similar measurements on three granite and three gabbro beams of smaller sizes, 21×2.5×2.0cm and 16×2.0×1.5cm, respectively. In these experiments, an optical flat was set on the upper polished surface of the beam bent convex upward to produce interference fringes of Na-D monochromatic light. By analysing the fringes, the profile of the upper surface was determined to an accuracy of one-tenth of one wavelength. The routine determination of the profile gave a change in the amount of bending with time. Although only 2.5 years have passed, it has been found that the secondary creep of the granite specimens in the general trend is comparable with that found in the previous experiment. As for the gabbro specimens, such a creep has not yet been observed clearly. During the above two experiments, it has been observed that the creep does not show a steady and monotone progress, but does a repeated“return-back”with an irregular interval of more or less than one year. This strange phenomenon was first noticed after correcting the results of the former experiment for the annual variation of humidity. The latter experiment, being carried out in a constant humidity of neary 100%, shows a similar phenomenon.
Recently, the foundations of structures are becoming larger and larger, but an adequate method of determining the bearing capacity of such foundations has not been established yet. This paper points out that the Terzaghi's equation of the bearing capacity is insufficient in the above cases, and recommends to use the graphical solution of plastic equilibrium which can take account of inhomogeneity of a rock mass. A rock mass of weathered granite was chosen to show the usefulness of the graphical solution of plastic equilibrium. The weathered granites were sampled and tested both in the field and in the laboratory. The triaxial test results indicate that the undrained strength is 4.0t/m2 and the rate of strength increase by consolidation is 0.32. This means that the strength of the rock mass increases with depth, and in the stability analysis this fact must be taken into account. The value of the bearing capacity of a foundation (30m wide, for example) on weathered granite determined by the graphical solution of plastic equilibrium was compared to the results by the Terzaghi's equation and the slip circle method. It is found that the latter two methods give 10-20% higher value than the former and this discrepancy becomes bigger when the foundation becomes wider.
In order to investigate the effect of tensile prestrain on the corrosion fatigue strength, high cycle fatigue tests were performed on aluminium alloy (2017) sheet specimens prestrained by pulling under completely reversed plane bending stress in laboratory air and in salt water (3.0% NaCl). The conclusions obtained are summarized as follows; (1) Corrosion fatigue strength for the prestrained specimens decreased in comparison with that for the as-received specimens, and it decreased monotonously with increasing prestrain regardless of the applied stress amplitude level. (2) Cyclic work hardening and subsequent softening behaviours were observed repeatedly twice for the as-received and 3% prestrained specimens during the corrosion fatigue process. In the specimens prestrained above 3%, the saturation of work hardening took place at about 20% of the fatigue life and thereafter the work softening occured continuously until fracture under subsequent stress cycling. (3) The number of cycles to crack initiation in corrosion fatigue decreased with increasing tensile prestrain. On the other hand, the number of cycles in the crack propagation period was independent of the tensile prestrain. (4) It is suggested that the experimental results obtained in this test can be explained from a consideration that the barriers to the motion of dislocations are easily relaxed by the corrosive dissolution of the specimen surface prestrained during the stress cycling and the plastic deformation of grains easily occurs near the specimen surface.
Stress relaxation and creep tests were carried out to estimate the dislocation velocity-stress exponents (m* and m) for nominally pure KCl single crystals deformed in cube orientation at high temperatures (150°C to 680°C) and at room temperature. The stress and strain rate cycling tests were also applied. It was found that the m-value at high temperatures was about 8 to 10 from the measurements under relatively low applied stress, but it increased sharply when the applied stress exceeded about 0.5kg/mm2. The m*-value obtained by stress relaxation was always lower than the corresponding m-value in accordance with the theory of Nix et al. The stress relaxation at room temperature indicated that the power law approach of Johnston and Gilman (1959) is no more valid at room temperature. The effect of friction between sample and jig faces was also discussed. It is considered that the stress exponents can be material constants if sufficient care is taken for measurement.
In the preceding paper, we investigated theoretically the influences of filler shape, orientation distribution and interaction among fillers on the elastic reinforcement, assuming that the shape of filler was ellipsoidal. In this paper, these theoretical results were examined experimentally, using model specimens. The specimens used were films or plates made of epoxy-resin and various fillers oriented uniaxially or at random. The results obtained are as follows: (1) As for the fibre reinforced epoxy-resin in which fibres are oriented uniaxially, the Young's modulus in the reinforcing direction is lower than that predicted theoretically. This is due to the facts that a small inclination of fibre axis from the reinforcing direction causes a substantial drop of Young's modulus, and that in practice the perfect orientation of fibres is difficult to be obtained. (2) As for the specimens in which short glass fibres or flakes are filled at random, the orientation distribution of fillers is restricted by the thickness of specimen. And it seems that short fibres are oriented plane-randomly in the film specimens and space-randomly in the plate specimens. On the other hand, flakes seem to be oriented nearly uniaxially in the film specimens, but in the plate specimens the orientation of flakes seems to vary from space random to uniaxial as the dimension of flakes increases. (3) When fillers are spherical, the 1st-order approximation of theoretical solution is useful for the range of lower than 10vol% of fillers, and the 2nd-order approximation is useful for the range of lower than 20vol%, with practical accuracy.
Recently, fiber reinforced plastics (FRP) are being widely used as the elements of sandwich constructions. Such sandwich constructions are superior in improving flexural rigidity as well as making laminated structures. However, few fundamental studies have been carried out on the sandwich constructions composed of plastic materials, and their mechanical and functional properties remain unclear. The present paper examines the mechanical properties of an unsymmetrically laminated sandwich construction which is composed of the polyurethane form (PUF) as the core material and aluminium (Al) and glass mat FRP (GRP) as the facing materials so as to enhance hybrid effectiveness. Particularly, the basic problems associated with flexural properties of the sandwich construction GRP/PUF/Al, such as the behaviors of flexural fracture, and the test methods of laminated construction are examined. Two types of flexural tests are carried out by using three and four point loading systems and the results are treated theoretically as well as experimentally to clarify the characteristics of the sandwich construction in which the core material has comparatively lower elastic modulus than the facing materials.
Heat contents of a series of mixed alkali glasses with the composition (1-x)R2O·xR'2O·2SiO2 (R, R'=Li, Na, K; x=0, 0.3, 0.5, 0.7, 1.0) were measured in the temperature range 25° to 900°C with a continuous type high-temperature calorimeter. The data obtained showed various temperature dependencies of the mixed alkali effect of these glasses on their heat capacities; i.e., a negative deviation from linearity for all pairs of alkali species in the temperature range below about 250°C and a positive deviation above this temperature. The temperature dependencies of these heat capacities in the temperature range below the glass transition, Tg, were discussed in terms of Debye temperature (θD), which leads to the conclusion that the deviation of θD from linearity was correlated only to the change in Young's modulus. Therefore, the present results do not contradict with the mixed alkali effect on Young's modulus reported by Jagdt, which indicates the similar reversal at about 250°C. The mixed alkali effect on heat capacity above Tg was discussed in relation to the activation energy of viscosity, which showed a negative deviation from linearity in the mixed alkali silicate glasses.
The purpose of this study was to investigate the effect of the individual addition of sucrose (S) or sorbitol (B) to tobacco laminae upon three characteristics of shreds (fragility, filling capacity and equilibrium moisture (content), and then compare the effects of “S” and “B” with that of glycerol (G) studied in our preceding paper. The experiment was carried out under the same condition as the previous experiment with “G”. The important results obtained were as follow: (1) The increase of the addition rate to tobacco laminae or of the additive content in shreds tended to increase the equilibrium moisture content, to decrease the filling capacity and to decrease the fragility of shreds for all additives of “S”, “B” and “G”. (2) The effecct of the addition of “S”, “B” or “G” was large for fragility, medium for filling capacity and small for equilibrium moisture content of shreds. (3) The effect of the addition of “G” on the three characteristics was stronger than that of “B”, and that of “B” was stronger than that of “S”. This order of additives in their effectiveness corresponded well to the order of strength of hygroscopicity of these additives. (4) The additive content in the shorter part of tobacco shreds was larger than that in the longer part.