A theoretical approach for estimating a probability distribution of damage degree of tunnel concrete liners in Hokkaido area is developed. First, a probability density function theoretically derived from a Poisson model describing random deterioration processes of tunnel concrete liners is compared with a frequency distribution of actual damage degree data. Next, in order to improve the approximation in the range of large damage degree, a concept of double-mode model is newly introduced into the Poisson model based upon a result of analysis on physical structure of temporal growth of damage degree. Finally, it is clari.ed that the double-mode Poisson model can give quite accurate estimation for tail properties associated with the probability distribution of damage degree. Further, it is shown that the proposed double-mode model can well reproduce temporal variation of the damage degree distribution.
It is important to investigate time-dependent deformation and fracturing in rock for the purpose of ensuring the long-term stability of structures in a rock mass. Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. It is known that subcritical crack growth is influenced by not only stress but also surrounding environment. Studies of subcritical crack growth have been widely conducted for silicate rocks such as igneous rocks and sandstones. On the other hand, information of subcritical crack growth in carbonate rocks is not enough. Especially, influence of surrounding environmental conditions on subcritical crack growth in carbonate rocks has not been clarified yet. In this study, we investigated subcritical crack growth in carbonate rocks. Specifically, we investigated the influence of temperature and relative humidity on subcritical crack growth in air and in water. Macedonian marble was used as a rock sample. To measure subcritical crack growth, we used the load relaxation method of the double-torsion test. It was shown that the crack velocity increased when the temperature was higher at a constant relative humidity in air and in water. In addition, the crack velocity increased with increasing the relative humidity at a constant temperature. It is concluded that the temperature and relative humidity influence subcritical crack growth in carbonate rock in air and in water. From the results obtained in this study, it can be suggested that the condition with low temperature and low humidity is desirable for the long-term integrity of a carbonate rock.
The stress intensity factor for the crack under mode III loading in the three dimensional elastic body is analyzed numerically by the finite element method. After obtaining the energy release rates, these energy release rates are converted into stress intensity factors. The method considering the difference of the strain energy between the analysis model and the crack extension model is used for the approach obtained the energy release rate in this study. The ratio of the model width(W), the model thickness(T) and the model length(L) for the basic finite element model are in the ratio 1:1:4. After the analytical accuracy is investigated by three point bending model with the mode I loading, the stress intensity factor for mode III loading is analyzed by the same basic finite element model with the different boundary condition. As a result, the result for mode I loading using the basic model agrees well with the two dimensional analytical solution. However, the result for mode III loading has the some differences for the two dimensional exact solution. Therefore, the numerical analyses of the model with different thicknesses are conducted for mode III loading. As a result of those analyses, it is clear that the numerical results approach the exact solution with the increase of thickness length. That is to say, the model thickness needs about twenty times as long as the model width to obtain the result close to two dimensional exact solution by the three dimensional analysis. However, the model length(L) requires eight times(L=8W) of the model width(W) in connection with the influence of the boundary condition.
The effects of notch-geometries and tensile speed on the fracture strength of jute-fiber-reinforced polypropylene (JF/PP) were investigated. Jute fiber is extracted from the stems of plants and has advantages such as low cost and low energy requirements during manufacture compared to conventional fibers such as glass. In addition, JF/PP can be more easily recycled (by methods including thermal recycling) compared to glass-fiber-reinforced plastics. In the present study, JF/PP plates containing 30 wt. % jute fiber were prepared by injection molding. The notch-root radii were 0.5 mm, 1mm, and 2 mm, while the notch depth ranged from 2 to 5 mm. Tensile tests were performed on the JF/PP plates at cross-head speeds of 103 mm/s, 102 mm/s, 10 mm/s, 1 mm/s, 8.33 × 10-3 mm/s, and 8.33 × 10-5 mm/s, at a temperature of 23 ± 1 °C. In order to gain insight into the process of micro-cracking, the notch-root surface was observed with a microscope. The microscopic observations revealed that micro-cracks were present only near the notch-root immediately before the fracture. Further, during the tensile tests, all the notched specimen were found to exhibit brittle fracture at the maximum load. The maximum elastic stress at fracture was determined using the notch-root radius and the time-to-fracture. However, it was independent of notch-depth. The results obtained can be explained based on the concept of stress-field intensity near the notch-root. Furthermore, the applicability of a fracture criterion based on the stress-field intensity near the notch-root was verified for the case of JF/PP.
The hydrogen desorption characteristic of Gr.122 steel welded joint was investigated to examine the applicability of hydrogen as tracer for creep damage evaluation. The thermal desorption analysis (TDA) was applied to the interrupted creep specimens with various degrees of damage. The thin plate-type specimen parallel to the loading direction was taken from the outer surface, the center of thickness and the inner surface of the welded joint. Hydrogen charging into the creep specimens was conducted by means of cathodic electrolysis. Next, the hydrogen-charged samples were subjected to the TDA for measuring the hydrogen evolution curve. The experimental results revealed that the peak height and the amount of desorbed hydrogen, CH, of the heat affected zone (HAZ) increased significantly with creep, and this increase was more pronounced at the outer surface, although that of the weld metal showed no significant change. The increase in CH with creep seemed to be attributable to the defect formation (the geometric damage) rather than the microstructural changes, because the hydrogen desorption characteristic was not significantly influenced by the mere thermal aging. In addition, there was a good correlation between the small punch (SP) creep test result and the CH, and this result indicated that the present hydrogen thermal desorption analysis could be a strong tool for detecting and evaluating the creep damage accumulated in the welded joint.
In high Cr ferritic steels, premature failure at the welded joint has recently been a worldwide issue. This failure occurs at fine-grained heat-affected zone (FGHAZ) and it is known as “Type-IV creep damage”. In this study, the modified small punch (SP) creep test with a rectangular-type specimen (10×6×0.5 mm) was applied to the interrupted uniaxial creep specimens of welded joint of KA-SUS410J3 for investigating the change in local creep property due to the uniaxial creep. The rectangular-type specimens consisting of weld metal, heat affected zone and base metal were removed at the different distances from the outer surface, and all the specimens were subjected to the SP creep test at 690 °C and 200 N. Experimental results revealed that the rupture times measured on the interrupted uniaxial specimens were shorter than that on the as-received, although the fracture occurred along the FGHAZ on all the specimens. This decrease in rupture strength was dependent on the distance from the outer surface, and it was much more pronounced just below the outer surface for the uniaxial specimen interrupted at 93% of the rupture life. Additionally, the distribution of the decrease in SP creep rupture strength was qualitatively in good agreement with the degree of damage and/or degradation such as creep cavity and softening. Consequently, the modified SP creep test was expected to be a strong tool for detecting and evaluating the creep damage accumulated in the welded joint.
Thermal sprayed MCrAlY alloy coatings are applied to protect the substrate from oxidation and corrosion under the high temperature environment. Though there are various MCrAlY alloys, CoNiCrAlY is generally used. And, CoNiCrAlY alloy are coated by various thermal spray processes, LPS, HVOF, and APS. However, few studies have been conducted on influences of thermal spray processes on high temperature oxidation property of CoNiCrAlY alloy coatings. In this study, high temperature exposure test was conducted on CoNiCrAlY alloy coatings applied by LPS, HVOF and APS. Influences of thermal spray processes on high temperature oxidation property of CoNiCrAlY coating is reported based on the growth behavior of the oxide layer formed on CoNiCrAlY alloy coating surface.
In this study, the effect of machined surface layer on stress measurement by means of instrumented indentation technique and X-ray diffraction method was comparatively investigated through the use of three weld specimens of low-carbon austenitic stainless steel with different machined surface layers; as-cutout, mechanically-polished and electrolytically-polished specimens. Tensile and compressive stresses exist respectively in the machined surface layer of as-cutout and mechanically-polished specimens. Meanwhile, no stress and no machined surface layer exist in electrolytically-polished specimen. Tungsten Inert Gas (TIG) bead-on-plate welding was performed under the same welding heat input condition to introduce the residual stress into these three specimens. Against these three specimens, firstly the X-ray diffraction method was applied, then the instrumented indentation technique was applied and finally the stress relief technique was applied to measure the distributions of residual stress after welding. Based on a comparison between these stress measurement results, the instrumented indentation technique was in good agreement with the stress relief technique rather than the X-ray diffraction method, even though both machined surface layer and penetration of indenter had almost the same depths. That is why it can be considered that the instrumented indentation technique measures the residual stress in deeper areas of material surface than penetration depth of indenter. A distinction between the instrumented indentation technique and X-ray diffraction method in surface stress measurement was thus clarified from a viewpoint of measurement depth.
Ultrasonic shot peening was applied to type 304 austenitic stainless steel and four-point rotating bending fatigue tests were performed at room temperature (R.T.) and 573K up to 108 cycles. The fatigue strengths at 573K were much lower than those at R.T. irrelevant to shot peening process. The shot-peened specimens exhibited higher fatigue strengths than untreated ones at both R.T. and 573K, indicating that shot peening had beneficial effect even at the elevated temperature of 573K. Clear fatigue limits were recognized in both shot-peened and untreated specimens at R.T. On the contrary, at 573K, fatigue failure occurred at high cycle fatigue (HCF) region (N > 2×106 cycles) in the shot-peened specimens, while not in the untreated ones. Fatigue crack initiated at the specimen surface due to cyclic slip deformation in the untreated specimens at R.T. and 573K and in the shot-peened ones at R.T. However the shot-peened specimens showed sub-surface crack initiation with a typical fish-eye pattern in the HCF failures at 573K. Inclusions were not recognized at the center of the fish-eye. The transition of crack initiation mechanism at 573K of shot-peened specimens could be attributed to the combined effects of the surface hardening by shot peening and the softening of material at the elevated temperature.