In order to study the ground movement due to shallow tunneling, the lowering panel model was analysed by the distinct element method (DEM) as well as by the experiments under the same conditions. The quite similar deformation behavior of granular ground was observed both in the time scale and in the geometry by the two methods. This fact seems to verify the usefulness of the DEM simulation. Based on the results obtained by the DEM analysis and the experiments, the behavior of particles and the pattern of ground movements were discussed. As the characteristic particle behavior, the initial failure zone, roof-arch, ground-arch and sliding plane were clarified in the model. It was found that they were controlled by the contact angle of particles in the particle array. Thus, the different particle arrays cause different types of ground movements, which can be classified in such typical patterns, as V-funnel type, W-funnel type, chimny type and funnel-chimny coexistence type.
Mechanical effects of discontinuity are of paramount importance in estimating deformation and collapse behaviour of geotechnical structures in discontinuous rock mass. Although several numerical methods have been proposed to treat distinct discontinuities of relatively large scale such as fault, there is no powerful method which can take into account the effect of distributed discontinuities of small scale. Kyoya et al.1)2) proposed a damage mechanics theory for discontinuous rock mass which treats the behaviour of rock mass involving distributed discontinuities. In the theory, the effect of distributed discontinuities is characterized by a second order symmetric tensor, called the damage tensor, which is originally proposed by Murakami and Ohno3)4) in their creep damage theory for metallic materials. In this paper, the damage mechanics theory is applied to an underground opening problem in jointed rock mass, and then the numerical results by finite elements are compared with a conventional finite element analysis. It is shown that the damage model presents presumably reasonable results for deformation and failure zone around the cavern. That is, the damage analysis can estimate the anisotropic behaviour of rock mass caused by the distributed discontinuities.
New cracks are formed in a rock around Griffith cracks or Griffith inclusions with various aspect ratios and various mechanical characteristics. Parabolic Griffith-type criteria for fracture can be derived from the analysis of the condition for formation of new cracks from the penny-shaped cracks or inclusions. The parabolic criteria for fracture for the incompressible Griffith inclusions become more flattened as they are less sensitive to compressive stress. The closure of Griffith cracks with various aspect ratios leads to the linear Coulomb criteria for brittle fracture. However, the effect of intermediate principal stress is neglected in the conventional theory of generalized Griffith fracture criteria, although it is against the recent results of general triaxial compression experiments of rocks. The statistical distributions of aspect ratio and orientation of Griffith cracks or Griffith inclusions should be considered in order to explain the effect of intermediate principal stress on the fracturing of rocks. The von Mises criteria and Mogi criteria are derived as the special cases of the statistical generalized Griffith theory.
This paper describes two sets of results of long-term creep tests by bending of rock beams, one for large granite beams (215×12.5×6.8cm) loaded since 1957 and the other for small beams of granite (21×2.5×2.0cm) and gabbro (16×2.0×1.5cm) loaded since 1974. The results during last 27 years for the large beams and 10 years for the small beams are summarized as follows. (1) The creep curve of each specimen showed an undulatory pattern accompanying “turn-backs” in deflection. The most remarkable turn-back gave a reduction of over 10% on the total deflection in the duration of about 650 days. (2) The initial and the delayed deflections due to elasticity changed depending upon specimens. With the small specimens, the delayed deflection lasted for 200 to 300 days, reaching 10 to 45% of the initial deflection. (3) Owing to reconstruction of the laboratory buildings, the large specimens were obliged to move twice after 10 years and 24 years since the start of the experiments. The creep curve for the unloaded beam (the maximum bending stress of 12.8kg/cm2) showed a discontinuous increase in deflection after the first move and a discontinuous decrease after the second move, surprisingly with no change in the creep rate in the general trend. On the other hand, the creep curve for the center-loaded beam (the maximum bending stress of 24.8kg/cm2) was not affected by these two moves. (4) Taking the general trend as the secondary creep, the viscosity of the tested specimens was calculated by neglecting the yield stress. It was 3×1020 to 6×1020 poise for granite and 0.9×1021 to 6×1021 poise for gabbro.
AE and thermal expansion of both stressed and unstressed Oshima granite were investigated. The samples were subjected to slow cyclic temperature change (0.5K/min) between 290K and 370K or between 210K and 290K. A large number of AE events occurred only during the first cycle both heating and cooling. Under unstressed conditions, the thermal expansion coefficient became highly temperature dependent when the occurrence of AE events was observed. When the rock was subjected to thermal cycling under uniaxial stress conditions, the thermal expansion coefficient in the axial (loading) direction became temperature independent, while that in the circumferential direction was unchanged. The reactivation of the AE events was observed during the first heating after the removal of stress. These observations suggest that microcracks are produced by the mismatch of thermal expansion of individual mineral grains and cause the temperature dependency of the thermal expansion coefficient of Oshima granite. The anisotropy of thermal expansion observed when the samples were uniaxially compressed, is explained by the closure of microcracks perpen-dicular to the axial direction.
As the first step of investigation to avoid alkali-silica reaction, a study was carried out on the characteristics of reactive rocks in Japan, especially andesites of the Tertiary and Quaternary Periods. The test methods used were as follows. (1) Observation under a microscope (2) X-ray diffraction analysis (3) ASTM C 289 Standard Test Method for Potential Reactivity of Aggregates (Chemical Method) It was recognized that the rocks containing a large quantity of reactive silica (such as cristobalite, tridymite, and amorphous silica) were susceptible to alkali-silica reaction. Most of the andesites of the Quaternary were classified as ‘deleterious’. Many cracks and cavities, which occurred during the cooling of the magma, were observed at the inner part of andesite aggregates in deteriorated concrete structures.
In the previous paper, one of the present authors proposed an analytical method useful for designing optimum fibrous laminated sandwich plates which can stand a given axial compressive load. The plate aspect ratio, however, was assumed to be less than unity in the previous study. This paper deals with the case where the aspect ratio of the sandwich plate is greater than unity. The previously proposed optimum design method, the Boundary Slope Method, is generally valid when the aspect ratio is greater than unity, but it is found to be not valid when the aspect ratio approaches to such values as 1.5, 2.5, 3.5, and so on. The reason is that the actual half number of buckling is always different from the assumed half number in such a range of aspect ratio. For instance, when the aspect ratio is near 1.5, the actual half wave number of the laminate which is optimized by assuming that the half wave number is one, becomes two, and the converse is also true. So, another new optimum design method in such a range of aspect ratio is proposed in this paper. The basic idea of the method is that the fiber orientation angles are determined so as that the buckling stress does not change even if the buckling mode changes. This design criterion is found to give the optimum fiber orientation angle of the laminate of which aspect ratio is in the range of critical aspect ratio where the former design method is not valid.
Weld lines, incomplete filling and warp are some of the serious problems in injection moulding of FRTP (Fiber Reinforced Thermoplastics). In this study, it was tried to establish a method of deformation analysis useful to predict the out-of-plane deflections. By considering a state of fiber orientation, the material constants for the analysis were determined from experiments, and the analytical results were compared with the experimental ones for the specimens of simple disk shape. The following conclusions were obtained. (1) The material constants of FRTP greatly depend on the state of fiber orientation, being nearly isotropic in the case of low values of orientation factor, but becoming strongly anisotropic with increasing orientation factor. (2) Warp in a specimen is determined by two factors, the anisotropy of thermal contraction and the stiffness of materials. The mode of warp depends upon the state of fiber orientation. (3) From the comparison of the analytical result with the experimental one, it is found that this deformation analysis satisfactorily predicts the deformation mode and the deflection in the specimen. (4) The above results are expected to bring a considerable improvement in injection moulding technology.
When a helical-wound composite cylinder is subjected to lateral compression load, delamination ocurres in the interlamina. This delamination causes a remarkable decrease in strength. In this paper the behavior of macroscopic failure of helical-wound composite cylinder under impact lateral compression load is observed experimentally and the elemental property is made clear. From the numerical analysis, it is made clear that stress wave propagation induces macroscopic fracture. The following are the main results. (1) The fracture behavior of helical-wound composite cylinder under impact lateral compression load is affected by the deformation velocity and the winding-angle of filament. (2) The impact absorbed energy of helical-wound composite cylinder under impact lateral compression load increases with increasing deformation velocity, and this effect becomes remarkable when the winding-angle was large. (3) The fracture of helical-wound composite cylinder ocurres in fiber boudle and interlamina.In paticular, interlamina fracture causes a remarkable decrease in strength, because the speed of stress wave propagation is affected by the different mechanical impedance of the fiber boudle-layer and the resin-layer.
The signals of acoustic emission (AE) which are emitted in the vicinity of notch were detected during the fracture process on the test specimens of glass fiber reinforced polycarbonate (FRPC) in which the fiber weight contents were changed. The AE cumulative event, the AE cumulative energy and the distribution of AE amplitude were discussed with relation to the fractography in order to study the fracture mechanism of these specimens. It was found that the AE signals of these specimens were the burst emission type, and the AE events increased with an increase in displacement and also in fiber content. The relationship between the AE cumulative event or the AE cumulative energy and the stress intensity factor could be represented by the folded line in log-log representation. It is considered that the folded point represents the initiation point of stable crack in the specimen. The fracture pattern for the specimen of lower fiber content differed from that patterns for the specimen of high fiber content. It seems that this difference is reflected in the distribution of AE amplitude. The AE amplitude for the specimen of low fiber content was sharply distributed in a narrow range, while that for the specimen of high fiber content was widely distributed in a wide range. When the emission source of AE is estimated from the observation of fractured surfaces, it is considered that the low AE amplitude represents the debonding between fiber and resin as well as the pulling-out of fiber, while the high one represents the friction between fibers and the rupture of fiber.
The present paper is concerned with the fracture toughness of a chopped strand reinforced polyester in water environment. Fracture toughness tests were performed by using compact tension specimens of the composite in distilled water. Acoustic emission signals were detected during the fracture toughness tests. The acoustic emission activity for the specimen in water was extremely lower than that in room condition. After the tests the scanning electron microscope observation was made to explain the difference between the fracture behavior in water and that in room condition. When the fracture toughness Kρ, AE was defined as the critical stress intensity factor corresponding to the onset of the abrupt increase of acoustic emission energy, it was found that the fracture toughness decreased with immersion time and the rate of increase became almost zero at about 60 days of immersion time.
The importance of hybrid composites is related to (1) the wider range of spectrum on materials properties, (2) the good balance between strength, rigidity and toughness of composites, and (3) the possibility of obtaining a more complicated multi-load-path structure. The present paper is concerned with a statistical characterization of tensile failure behavior of hybrid composites with unidirectional carbon/glass hybrid fiber reinforced composites as an example based on the new failure process simulation model which was previously proposed by the authors. The effects of fraction and dispersion pattern of carbon fibers on tensile failure characteristics of hybrid composites are systematically analyzed and the results are statistically examined. As results, it is verified that the hybrid effect in a broader sense exists: the composition effect is apparently larger than that estimated by the rule of mixture as far as strength, elongation and absorped energy of hybrid composites are concerned. It is furthermore shown that the hybrid effect in a narrower sense, which is defined as the net hybrid effect, that is, the initial failure strain enhancement from which the statistical size effect is subtracted, is also statistically valid: the relation between the hybrid effect and the carbon fiber fraction or the dispersion of hybrid composites is clarified. Thus these results give a fundamental guideline for the materials design of hybrid composites.
The bending fatigue behavior of adhesive bonded joints of FRP was investigated under relatively low cycle repeated stress, and the effect of type of over-laid plate in adhesive bonded part and the effect of atmospheric temperature on fatigue strength were examined. The specimens used were orthophthalic acid unsaturated polyester reinforced with glass mat or glass mat combined with roving cloth. The mother plate and the over-laid plate of the specimens were made of the same material. The fatigue testing machine was developed originally by the authors and the bending method of 4 point supporting system was adopted. And the bending moment was applied by an unbalanced rotating weight. The stress cycle was 400cycles/min constant and the testing air temperatures were 23°C, 50°C and 80°C. The following results were obtained from the experiments. (1) The observed fatigue strength of the adhesive bonded joint was increased more as the over-laid plate was strengthened, if the bending stress imposed on the adhesive surface area was adopted as the stress amplitude. (2) The fatigue strength of the both-side adhesive bonded specimen was twice that of the plain specimen, and the increase of fatigue strength for the one-side adhesive bonded specimen was between 1.3 and 1.6 times. (3) The fatigue strength was considerably decreased at above 50°C. (4) Dynamic creep properties were much influenced by the atmospheric temperature. (5) The formula proposed previously by the present authors for the estimation of fatigue life in the tensile fatigue test, which is given as Eq. (1), was found to be applicable also for the bending fatigue test.
This paper deals with the creep properties of CF/GF hybrid laminates. Hybrid laminates used in this experiment were symmetrical hybrid FRP laminates of 7 selected thickness ratio including CFRP and GFRP. The reinforcement used in CFRP was plain woven carbon-fibrous cloth. The conditions of the 3 points bending creep tests were 1 level of maximum bending stress at 5kgf/mm2, 4 levels of environmental temperature at 20°C, 50°C, 80°C and 110°C, 4 levels of off-axis angles at 0°, 15°, 30° and 45°, and 7 levels of thickness ratio [thickness of (sureface=CFRP)/Thickness of (total=CFRP+GFRP)] at 1, 0.5, 0.43, 0.33, 0.2, 0.09 and 0, respectively. The measured creep relaxation properties of these laminates were discussed on the basis of calculating formula of stiffness of sandwich panel and off-axis analysis of bi-directional reinforced theory. From the discussion, it is concluded that some of the selected hybrid laminates can be used as the advanced structural FRP materials for machine tools.