Development of internal damage evolution in plates and thin tubular specimens of CFRP laminates under static and dynamic loadings are discussed by means of Acoustic Emission measurements and micrographical observations. The mechanical behavior of three kinds of specimens, i.e. undamaged laminate plates [+45°4/-45°4]s, damaged plates [+45°4/-45°4]s subjected to drop-weight impact and undamaged tubular specimens [±45°]4, under quasi-static and fatigue loadings is observed first. Then the mechanism of the resulting inelastic behavior and the change in the mechanical properties are discussed in relation to the evolution of internal damage. Finally the distribution and the evolution of matrix cracks and delamination in the sliced section of the specimens are measured quantitatively in several stages of fatigue process. The dependence of damage distribution on the loading condition is elucidated. Namely, in the case of the stress ratio R=-0.25, the growth of damage zone involving the main crack is localized, and the main crack forms large delamination. On the other hand, for the stress ratio R=0, small cracks are distributed sparsely, but the main crack is not observed until the final stage of the fatigue process.
In this paper, a design procedure for laminated composite structures based on GA (Genetic Algorithms) with function approximations is proposed. Design variables considered to enhance the rigidity of the laminated composite structure are the number of laminae, the fiber orientations and the stacking sequence. The fiber orientations are chosen from some prepared angles. In the proposed procedure, the rigidity of laminated composites is approximated by means of weighing coefficients and transformed elastic constants at the prepared angles. The proposed approximate function can be thought of response surfaces, because the weighing coefficients are adjusted with pairs of input and output data. The effective approximation is generated using auxiliary information about the principal stress distribution. The approximate function is used as the objective function of GA and can be evaluated without finite element analysis. As a result, the proposed procedure can reduce computational cost remarkably. In this paper, the detail of our developed design procedure and data sampling scheme are described.
The deformation characteristics of metallic materials under the single axial tensile testing are analyzed by an upper bound method based on the theorem of plasticity using R. Hill's orthogonal anisotropic yield criterion. This method was applied to estimate the deformation behavior of a GFRP sheet, combining the r-value as an estimate of plastic workability on orthogonal anisotropic metallic materials. The difference in strength between woven and laminated sheets is considered to be caused by the fiber to fiber friction, and this is calculated on a woven sheet using a unit model. Measuring the orientation angle and the elongation of fiber, the relation between the r-value and fiber orientation was evaluated. It was concluded that these concepts can be effectively applied to the estimation of plastic workability.
In this paper, compression tests in GF/Nylon6 tubes were performed to crarify the mechanism of the initial failure process and the characteristic of the energy absorption. It was found that all the specimens were crushed in a progressive crushing mode regardless of the trigger geometry, but that the specimens with an asymmetric trigger demonstrated to have a better energy absorbing characteristic than that with a symmetric one. The formation mechanism of the debris-wedge was examined through the step-wise morphology analysis. It was also found that changes in the trigger geometries affected the performance of the initial failure process which was characterized by the debris-wedge formation process. As for an effect of testing speed on the specific energy absorbing capability, the GF/Nylon6 tubes had small dependency of the testing speed in the range of our study comparing CF/PPEK tubes. From those results, it could be concluded that the GF/Nylon6 tubes are available to practical use.
Markov process was applied to the chain-of-bundles probability model which is often used for the axial strength analysis of a unidirectional fibrous composite. It was assumed to the process that a group consisting of fiber breakage points, the so-called cluster, evolves in time intermittently subject to two kinds of local load shares around a cluster, and that the composite fractures if the cluster achieves a critical size. Then, the two kinds of cluster evolution processes are governed by simultaneous first-order differential equations. A time-dependent Weibull distribution was used as a lifetime distribution function of the fiber, and the cumulative probability solutions for rupture-time of the critical cluster were analytically obtained. The results showed that the larger clusters reduce the width of distribution and form a master-like distribution curve, similarly to the results predicted from a conventional numerical method, the so-called recursion analysis technique. The proposed Markov process analyses were in a relatively good agreement with the distribution curves predicted by the conventional one. In addition, the effect of ineffective length which is gradually increased in time due to the shear stress relaxation of the matrix in metal matrix composites, was taken into the analysis.
Viscous coefficients of CFRP composites that absorb moisture are measured quantitatively and non-destructively by ultrasonic waves. The waves are transmitted through a moisturized unidirectional CFRP laminate which is immersed in water. By measuring the relative amplitudes of transmitted waves at various incident angles, the viscous coefficients are obtained. The moisture absorption is carried out in 60°C hot water for various time period before the specimens become saturated with water. As the moisture content increases, the viscous coefficients increase. Furthermore, these changes are confirmed to be reversible through the measurement after moisture desorption.
As the adhesion of glass fiber to polypropylene is poor, the glass fibers treated by the coupling agent of silane series and sizing agent of urethane series are mixed to the block copolymer of polypropylene (PP) grafted with various contents of maleic anhydride (MAH), and then the glass fiber reinforced polypropylene (FRPP) of the fiber weight content 50% is molded by a direct injection molding machine. The fracture mechanism of the FRPP specimens is investigated by the acoustic emission technique during the tensile test. It is found from the AE frequency analysis that the fiber breaking dominates the damage mode and that the GF/PP interfacial adhesion increases with an increase in MAH content. This phenomenon is discussed based on the power spectrum density at the peak AE frequency and the AE amplitude distribution.
The stress-strain relation and stress relaxation behavior of short-fiber reinforced plastics are affected by coupling agents on fiber surface. In the present study, the shear stress on the interface between fiber and resin in the composites were evaluated from stress relaxation tests and a modified viscoelastic model. In the model, the coefficient of an elastic element is proportional to the ratio of fiber strain to resin strain. The interfacial shear stress in the surface treated fiber composite, which was estimated by the present method, is higher than that in the non-treated fiber composite.
When constructing large underground caverns such as hydraulic power stations and oil storages, it is important to estimate the loosened area around rock caverns caused by excavation. If the loosened area around a rock cavern is known, the most suitable support members and quantities can be determined. The authors have carried out in situ experiment to investigate the crack growth into rock mass by acoustic emission (AE) measurement. In this paper, parameter analysis, locational analysis and moment tensor analysis based on the observed AE signal waves were carried out to make clear the diffusing patterns, locations, orientations and crack types of AE sources. The following conclusions were obtained: (1) Many large amplitude AE signal waves diffused within about ten minutes after cavern blasting, (2) The concentrated areas of tensile cracks and shear cracks around a rock cavern were different, (3) From the distribution of observed AE signal waves, the loosened area was estimated within depth of 2m from the excavated wall.
This paper deals with FLEM-DEM coupled scheme for the analyses of stress and displacement of geotechnical problems. The discontinuous domain in a given problem is modelled as a set of distinct element blocks of DEM. The continuous domain is partitioned by a finite difference grid as a FLEM grid. The FLEM, which is proposed by us, is one of the numerical methods that can be used to analyze large deformation problems. The two methods are coupled together by satisfying the equilibrium conditions of traction and continuity of displacement at interface between the two domains, at each iteration of the solution process. This means that the coupled scheme is well suited for considering joint slips and separation as well as large rotation and displacement of blocks. Verification of the scheme is demonstrated through the analysis of a stacked-drift-type tunnel. The stacked-drift-type tunnel is a new one which is now under investigation as a large cross-section tunnel over 20m in diameter. The numerical results of the contact force between the drifts and force acting to the tunnel as well as the stresses in the surrounding ground are summarized together with the results from a series of centrifugal tests of the tunnel model. It may be concluded that the circumferetial forces acting to the drifts are important to understand the stress-displacement relationship of tunnel.
Fracture permeability is usually estimated by a cubic law that is based on the theory of hydrodynamics for the laminar flow between flat plates. However, the cubic law is too simple to estimate the fracture permeability correctly, because the surface of real fracture is much more complicated and rougher than the surface of flat plate. In this study, therefore, the discrepancy between the cubic law and actual fracture permeability was investigated experimentally and theoretically to develop a correction factor for the cubic law based on real fracture. A fractal model was applied to describe the topography of real fracture surface, because fractal can describe a complicated geometry such as the topography of fracture surface by a simple power law which is determined by fractal dimension and steepness. Consequently, the discrepancy could be written by a the fractal dimension and the steepness, and the following results were obtained. First, the contact condition of the fracture surfaces is most influential on the discrepancy but fractal dimension does not affect much to the discrepancy. Second, steepness is also influential on the discrepancy but it does not affect as strong as the contact condition.
In order to predict the weathering process of rock foundation and rock materials for engineering purpose, fundamental studies have been carrying out to investigate the factor of weathering, changes of some properties and rock deterioration mechanism by weathering. The outdoor exposure tests on Cretaceous Granitic Rock (Kitakyusyu granite) were carried out in this study. The columnar specimens used in this experiment are composed of a series of rocks that include fresh rocks to weathered rocks. Their weathering properties were clarifed by the outdoor exposure, and the regression equations for the deterioration process were obtained as a function of outdoor exposure time. Using these equations, one can predict the weathering process from the initial physical and mechanical indices of rocks.
The purpose of consolidation grouting for dam foundation is to improve the permeability and mechanical properties of rock masses. For permeability, the grouting effect is usually confirmed by the permeability test at check holes. But for the change of mechanical properties, it has been difficult of confirmation. So, the mechanical improvement of foundation by grouting, like the changes of elasticity and strength, has not been considered in dam foundation design. In this study, an in situ experiment has been made in order to examine the grouting effect on mechanical properties of rock masses. The testing yard where the experiment was performed was 10m square, and 13 boreholes for grouting, divided into four (4) stages, were laid out. These boreholes had the length of 7m each, and the injection material was cement mixed with water. The experiments mainly consisted of elastic wave prediction and borehole expansion tests. As the results, the followings were obtained. (1) The change of distribution of elastic wave velocity by grouting represents the uniformity of rock masses. (2) Grouting can improve the deformability of rock masses. (3) For more deformable rock masses before grouting, the more mechanical improvement is achieved. (4) The reological model can be used to explain the grouting effect. By this model, the experiment results of poor rock. masses, which have lower mechanical properties, can be well expressed.
The effect of Ti in low carbon high strength steels on the microstructures and mechanical properties of laser welds was investigated. The addition of Ti content up to 0.06mass% scarcely increased the hardness of weld metal. However, Ti addition inhibited the decrease in hardness by heat treatments at 773-1073K because of TiC precipitation. Using Johnson-Mehl equation, the decrease in hardness of Ti free steel after the heat treatments can be approximately estimated as a function of temperature and time. Further, in the steels with Ti addition, the effect of TiC precipitation, on hardness being evaluated, the decrease in hardness after the heat treatments can be also estimated. In tensile test, the elongations of laser welded specimens were lower than those of the base metals in as-welded condition. They increased to the same level as the base metal by a heat treatment at 973K.
Low-carbon/medium nitrogen 316 stainless steel called 316FR is a principal candidate for the high-temperature structural materials of a demonstration fast reactor plant. Because creep-fatigue damage is a dominant failure mechanism of the high-temperature materials subjected to thermal cycles, it is important to establish a reliable creep-fatigue life prediction method for this steel. Long-term creep tests and strain-controlled creep-fatigue tests have been conducted at various conditions for two different heats of the steel. In the constant load creep tests, both materials showed similar creep rupture strength but different ductility. The material with lower ductility exhibited shorter life under creep-fatigue loading conditions and correlation of creep-fatigue life with rupture ductility, rather than rupture strength, was made clear. Two kinds of creep-fatigue life prediction methods, i.e. time fraction rule and ductility exhaustion method were applied to predict the creep-fatigue life. Accurate description of stress relaxation behavior was achieved by an addition of “viscous” strain to conventional creep strain and only the latter of which was assumed to contribute to creep damage in the application of ductility exhaustion method. The current version of the ductility exhaustion method was found to have very good accuracy in creep-fatigue life prediction, while the time fraction rule overpredicted creep-fatigue life as large as a factor of 30. To make a reliable estimation of the creep damage in actual components, use of ductility exhaustion method is strongly recommended.