Fundamental study was carried out in order to make use of coal ash as a ground material in large quantities. A series of examination were done by using three kinds of coal ash produced from boilers of which the combustion system differs each other. The properties of coal ash and the characteristics of solidification and stabilization in the solidified coal ash by steam treatment were examined. The main results obtained were as follows: (1) A solid state reaction takes place between coal ash and CaO at the boiler temperature, generating a large amount of hydraulic substances. The morphology of each particle and the content of amorphous materials in coal ash differ depending upon chemical composition, temperature and pressure in the production atmosphere. (2) As coal ash, particles becomes denser or amorphous materials become larger in quantity, the compressive strength of the solidified coal ash becomes higher. (3) The solidified coal ash satisfies a safty standard in leaching by treating it with a saturation steam of proper condition.
This paper is concerned with the elaborate modeling of ductile spall damage due to void nucleation and growth in viscoplastic materials by means of continuum damage mechanics. The evolution equation of a scalar damage variable is developed by taking account of nucleation and growth of spherical voids in a unit cell; it is expressed in terms of the rates of the void nucleation and its radius growth. The resulting damage evolution equation and elastic-viscoplastic constitutive equations of the damaged material are incorporated into the commercial hydrocode MANJUSRI-3D, and the axisymmetric two-dimensional analyses of the plate-impact tests are performed. The results of the numerical analyses are compared with those of the one-dimensional analyses, axisymmetric analysis without explicit modeling of void growth, and the corresponding experiments.
The impact bending behaviors of beam with open-cross section were investigated. The cantilever bending and three-point bending experiments were performed by a falling weight for aluminum specimens. The plastic deformation in the case of cantilever bending was larger than that of three-point bending. But, the elastic dynamic bending strain measured in the case of cantilever bending was smaller than that of three-point bending when the peak of the first strain pulse in the strain history is compared with each other. Thus, these behaviors are very complicated and quite different from the static one. That is, the elastic bending strain of the first strain pulse is reduced by the interference between the leading waves and the following reflected waves with the reversed phase which are reflected at the free end. These impact bending behaviors in the case of open-cross section beam are similar to those of the hollow section beam.
This paper examines the analytical structure for simulating the localization in soil material. The three components of stress rate associated with the shear plane are related with the equilibrium condition on the plane. Other components can be assumed to be continuous, based on Hill's verification with respect to the continuity of stress on the plane where discontinuous velocity arises. This assumption makes possible application of the conventional constitutive equations to examine the structure. Consequently, all components of stress rate become continuous across the shear plane. Then, the constitutive relation between strain rate difference and stress rate difference must be linear with a constant coefficient independent upon their components. Furthermore, under such circumstances, all components of the discontinuous velocity gradient vector vanish. Hence, it means that they must be continuous under rigorous consideration, and their multiplicity does not appear.
In view of the situation that aging of boiler components proceeds and some “Type IV” cracking incidents are reported recently, the develoment of an accurate remaining life evaluation method for boiler weldment parts is an important subject for the utilities. In this study, simulated materials with coarse and fine grain microstructures in a weldment heat affected zone were produced, and their high temperature strength properties were evaluated. The strength property of the weld joint was also discussed based on the experimental and analytical results. It was found that the creep rupture strength of the fine grain material was as low as 1/2 of the base metal and as 1/5 of the coarse grain material, and the weld joint ruptured at the fine grain region in a shorter period than the base metal as the applied stress level decreased. Many grainboundary creep cavities were also observed in the fine grain region. A creep deformation analysis by FEM indicated that the creep strain accumulation at the fine grain region in the weld joint preceded against other regions, suggesting that the rupture of weld joint occurred at the fine grain region in the experiment.
Fretting fatigue tests were carried out on SUS304 austenitic stainless steel by plain bending. The specimens were set with Al2O3 ceramics or SUS304 contact pads. Corrosion fretting fatigue tests were also carried out by dripping them in salt water. From the test results, when the specimen was set with SUS304 pads, the fatigue limit decreased slightly in salt water as compared with that in air. However, with Al2O3 pads, the fatigue limit decreased significantly in salt water. This is because the specimen was abraded by Al2O3 pad and then the wear debris were washed away by salt water. The distinctive feature of failure in salt water was that the failure occurred near the center of fretting area as a result of the slipping region moving inward the contact area.
Fatigue strength of single crystal of a nickel-base superalloy (PWA1480) was studied. The stress-controlled fatigue tests were performed at 700°C and 5Hz using triangular waveforms. In this study, two kinds of testing procedures were adopted. One is the conventional zero-tension fatigue test (R=0). Another is the procedure in which the maximum stress was held at 1000MPa and the minimum stress was varied from zero to 1000MPa (R=0-1) at 24 and 700°C. The fatigue tests at higher mean stress levels caused creep-induced damage, which resulted in the inversion of the S-N curves at 700°C. The results of fatigue tests at 700°C indicated that the fracture mechanism changed depending on mean stress and also on stress range.
The effect of microstructure on fatigue crack growth behavior under constant stress amplitude was investigated using CT specimens of two kinds of Ti-6Al-4V alloy. Both of them were under the same annealing condition and consisted of α+β phases, but their microstructures showed obvious distinction because of the different reduction of plate in the β and α+β rolling processes. One of them was composed of spherical α phase surrounded by thin β phase (material A) and the other possessed longitudinally stretched α and β phases (material B). For both materials, rough crack surfaces were formed in the low Kmax region and roughness-induced crack closure was added to plasticity-induced one while relatively flat crack surfaces were built in the high Kmax region where plasticity-induced crack closure was dominant. However, the crack surface morphologies of material A and B were quite distinct in the low Kmax region due to their microstructures, which had a pronounced effect on crack growth and crack closure behavior. The effect of crack surface morphology was qualitatively evaluated by the optical observation of crack surface profiles and quantitatively by the measurement of the effective stress intensity factor ΔKeff.
The effect of prestrain on the fatigue crack initiation and propagation behavior of an aluminum alloy (2024-T6) reinforced with 20 volume percent of SiC particles was investigated. Cracks were mainly nucleated in the matrix by prestrain of about 0.32% strain. The X-ray diffraction method was used to measure the change of residual stress in the SiC particles and in the matrix before and after prestrain. The residual stress changed from -134 to -107MPa in the SiC phase and from 0.6 to -12MPa in the matrix phase. Fatigue tests were conducted using a specially designed servo-hydraulic fatigue testing machine in a scanning electron microscope. The initiation and propagation behavior of small fatigue cracks in a smooth specimen was observed. Fatigue cracks were mainly nucleated from the matrix cracking and propagated in the matrix near the particles. When compared at the same maximum stress intensity factor, Kmax, the crack propagation rate, dc/dN, of the prestrained material was higher than that of the non-prestrained material because of the damage near the interface.
The first experiment for obtaining the static transverse shear modulus and failure strain of CFRP Laminates is presented in this paper. In order to obtain these values, the difference in vertical displacement of two points on the neutral axis of a short beam specimen is measured by a newly devised electro-optical extensometer. The transverse isotropy is first confirmed by our proposed method. Furthermore, the experimental results of transverse shear modulus and failure strain are compared with those based on a classical lamination theory and a three dimensional FEM code, respectively.
Stress transfer perpendicular to the axis of high performance polyethylene fibers in a fiber reinforced epoxy resin composite was investigated by X-ray diffraction. The intensity of stress σf in the embedded fiber could be measured by monitoring the strain of the crystalline regions of polyethylene by X-ray diffraction. The stress σf increased linearly with the applied stress σo as well as the hydrophilicity of fiber surface. Further the increase of σo made the stress σf reach a plateau. The fracture behavior was variable depending on the surface treatment of fiber. The “X-Ray Diffraction Method” is shown to be a powerful tool for detecting the stress on the incorporated fiber in the composite under load in situ and non-destructively.
The optical method of caustics is a useful technique for determining the stress intensity factor K. The method has been mainly applied to isotropic materials. For anisotropic materials, however, the method has not been much studied. In this paper, the method of caustics is examined for a highly orthotropic unidirectionally reinforced carbonepoxy composite under mode I loading by using a coating technique, and the results are compared with those of finite element analysis. As a result, it was found possible to apply the method of caustics to orthotropic CFRP laminate material.
The analysis of acoustic emission (AE) signals is presented by using the wavelet transform, which is a representative method of the time-frequency analysis, and its properties are discussed. Five analyzing wavelets are used in order to obtain the best suitable one for AE signals. It is shown that Gabor wavelet can make a good decomposition of AE signals, and gives the peaks of frequency that are in good agreement with the results by Fourier analysis. The properly spaced intervals for sampling AE signals are obtained. Furthermore, the analysis using Gabor wavelet provides much time information, such as microfracture time, relation of matrix cracking and fiber break at a fiber breakage, etc., which are impossible to be evaluated by Fourier analysis.
In order to assure the maintainability and reliability of structural components, it is important to examine the structural components for damage and to estimate their remaining life. In recent years, attempts to apply statistics of extremes to the estimation of maximum crack length in a structural component have been made. In such estimation, it is necessary and important that the sample area is made as small as possible to restrain the labor for taking small cracks and measuring crack lengths and that the estimate of maximum crack length satisfies the needed accuracy of estimation. However, a guide for determining the sample area S (the ratio of the sample area to the whole area) and the number of division m that satisfy these two conditions has not been obtained. In the present paper, as a part of the study to obtain this guide, the relationship of √V(Xmax)/σ to log T is examined by conducting Monte Carlo simulation for the case when crack length follow a Weibull distribution (shape parameter α=0.8, 2.0), where √V(Xmax)/σ is the root mean squared error of the estimated value, σ is the variance of the double exponential distribution which the largest crack length in each elemental area follows, and T(=m/S) is the return period. As a result, it is shown that √V(Xmax)/σ by Monte Carlo simulation is not equal to √V(Xmax)/σ (the result of a previous paper) by the theoreti cal analysis quantitatively and that the cause for this difference is not σ but √V(Xmax)/σ. It is also shown that √V(Xmax)/σ by Monte Carlo simulation for the case when individual crack length follows a weibull distribution is not equal to √V(Xmax)/σ (the result of a previous paper) by Monte Carlo simulation for the case when individual crack lengths follow an exponential distribution quantitatively.