As the top coating, zirconia with 4 mol% yittria was electron beam-physical vapor deposited (EB-PVD) on the bond coating of CoNiCrAlY. The substrates were rotated during EB-PVD process and the rotation speeds were 5 (R5) and 10 rpm (R10). The thickness of the top coating was 0.12 mm. In order to investigate the change of the internal stress in the top coating by a thermal cycle, the specimen was heated from a room temperature to 1273 K, the internal stress was measured in-situ by the strain scanning method with hard synchrotron X-rays at each temperature step. For the specimen R5, the internal stress increased from about –100 MPa to about 100 MPa with the increase in temperature, then the stress relaxation of the top coating occurred over 1073 K. For the specimen R10, the internal stress did not show a tension in the heating process, it was caused by the separation between columnar structure. In the cooling process, the internal stress decreased, however, the change rate of the internal stress was small as compared with the heating process. This was caused by sinterring of the feather-like structure.
Electron backscatter diffraction (EBSD) in conjunction with scanning electron microscopy was used to assess the distribution of the plastic strain on a microstructural scale (local plastic strain) induced in stainless steel. The local misorientation, which is a misorientation between neighboring pixels, was investigated in order to represent the degree of the local plastic strain. Accuracy of the measurement of the local misorientations was improved by a technique, in which an average of crystal orientation was calculated for several data measured from the same domain. It was shown that the local misorientation was inhomogeneous and localized in the vicinity of grain boundaries. The distribution of the local misorientations followed the log-normal distribution and the mean value correlated well with the macroscopic plastic strain induced in specimens. By using the correlation between the averaged local misorientation and the plastic strain, the distribution of local plastic strain was estimated. This procedure for estimating local plastic strain was applied to measurements of non-uniform plastic stain in notched specimens and its validity was shown by comparing with results of finite element analysis.
Three kinds of copper thin films were fabricated by RF-magnetron sputtering. The target power was selected to be 10 and 150W to change the properties of the films. Thin glass sheet was used as a substrate. For the target power of 150W, the deposition time was selected to be 7 and 40min. The thickness was 0.6μm and 2.9μm, and the grain size measured was 243nm and 450nm, respectively. The grain size of thicker film was larger than that of thinner one. On the other hand, for the target power of 10W, the thickness and grain size were 2.4μm and 54nm, respectively. The grain size depends on the target power, too. The residual stress distribution in the films was measured by X-ray method. Several methods such as the grazing incidence X-ray diffraction method, the constant penetration depth method and the conventional sin2ψ method were adopted. The measured weighted average stress increased with increasing depth. After taking the maximum value at about 0.3μm from the surface, the value decreased with increasing depth. The stress distribution near the surface in the films deposited at 150W was almost identical irrespective of thickness. On the other hand, for the target power of 10W, the stress distribution shifted to compression side. The reason could be explained by the effect of the thermal residual stress. The real stress distribution was estimated by using the optimization technique. The stress took the maximum value at 0.5μm from the surface, and was compressive near the substrate.
TiN thin films were deposited on the polyimide films by reactive RF magnetron sputtering in Ar and N2 gas atmosphere. Most of the films possess strong <111> or <001> fiber texture. The properties such as the fiber texture, residual stress and tensile strength of the film were estimated by using X-ray method. The fiber texture of the film was strongly influenced by the N2 gas flow ratio and changed from <111> to <001> with increasing gas flow ratio. This change of the fiber texture affected on the grain size, residual stress and tensile strength. As <001> orientation become strong, the grain size and compressive residual stress increased. Stress distribution below the surface was also measured by using the constant penetration depth method and the grazing incidence X-ray diffraction (GIXD) method. Compressive residual stress was measured near the substrate and surface of the film. On the other hand, tensile residual stress was observed in the intermediate region. In-situ tensile tests were carried out for each specimen in order to estimate the strength of the films. The maximum X-ray stress measured during tensile loading process increased with decreasing grain size. When compared at the same grain size, <001> oriented films tend to show the larger strength than <111> oriented films.
Fracture mechanics have been successfully applied for damage tolerant design of engineering structures against fatigue and stress corrosion cracking. The stress intensity factor is a key parameter to predict the progress of cracking behavior, and is computed from the shape of cracks and the loading stress distribution. In the present study, using high energy monochromatic X-rays of energy 66.4keV from the synchrotron radiation source, SPring-8, we have developed a system to perform the hybrid measurement of imaging of cracks and strain distribution around cracks. This system was applied to a fatigue crack made in a round bar made of carbon steel with the diameter of 4mm. Computed tomography (CT) of the specimen gave the three-dimensional shape of a thumb-nail crack. Scanning of lattice strain along the loading axis around cracks was conducted under the zero and maximum applied stresses. High tensile strain ahead of the crack was measured at the maximum stress, while the strain on the crack face was low because of crack opening. The full width at half maximum (FWHM) increased near the crack tip under loading, and then decreased after unloading. This recoverable part of FWHM by unloading was caused by the steep distribution of the applied stress in the vicinity of the crack tip. The increase of FWHM due to plastic deformation did not change by unloading.
Observation of the internal crack of steel by a computed tomography (CT) and strain mapping near its tip were investigated using a white X-ray obtained from BL28B2 beam line at SPring-8 in Japan. A low-alloy and high-tensile steel was used as a specimen prepared in the tensile bar with a parallel part of 5mm diameter. A fatigue crack was introduced into the specimen under the pulsating tension load (stress ratio R = 0). CT observation of the crack in the specimen was carried out by using the CCD camera that can detect indirectly the X-ray transmitted through the specimen. To measure the strain using the energy dispersive X-ray diffraction technique, the synchrotron white X-ray beam, which had a height of 100μm and a width of 100μm, was incident on the specimen with the Bragg angle θ of 5 degrees. As a result, the crack in the specimen under the tensile loading could be observed by the CT using the white X-ray. The resolution of CT, which was about 18μm3, was considerably lower than it using a monochromatic X-ray reported previously. The large tensile strain was measured near the crack tip. It was, however, relatively small under the influence of the long slender gauge volume as compared with the crack tip distributed circularly. It was confirmed that the FWHM of diffracted X-ray profile measured near the crack tip was increased due to the steep change in the strain distribution.
Recently, it is very important to grasp the stress states in sub-micro scale area. In this study, components of stresses/strains in a single crystal sapphire are determined by Raman microspectroscopy with sub-micro spatial resolution. The sapphire belongs to a D3d point group with two A1g and five Eg vibration mode Raman lines. First, the relationships between the change of Raman shift and strain components for A1g and Eg modes are theoretically derived based on energy change under mechanical loading. In these equations, 20 unknown parameters are included. Second, these parameters are experimentally determined. In A1g and Eg modes, the relationships between Raman shift and applied strain indicate the linearity. Third, components of strains are determined by Raman microspectroscopy, and stresses are calculated by using Hook's law. Last, stress measurements around the notch root in the single crystal sapphire are conducted. The measured stresses are good agreement with the FEM results. Therefore, the applicability of Raman microspectroscopy to stresses/strains measurement in micro scale area is confirmed.
Recently, the improvement of the high strength of fine grain steels has been investigated actively. Fine grained steels have high yield stress, as expected by the Hall-Petch relationship. Therefore, these materials are considered for use as structural material. Welding is one of the most effective methods for connecting structural components. Nevertheless, the negative influence of tensile residual stresses and coarse grains due to the welding process must be taken into consideration. It has been proved that the shot peening process can effectively overcome these problems. In this study, samples prepared with various mean grain sizes were processed by shot peening. The triaxial residual stress distribution after shot peening was measured by X-ray diffraction. Moreover, the distribution of the hardening effect and nanocrystalline layer near the shot peened surface was observed. In this paper, the relationships between the effects of triaxial residual stress, structure, fatigue and hardness are discussed. As a result, thin nanocrystalline layer was formed on the surface layer of over 90 percent of surface area. Therefore, hardness of the surface, fatigue limit and fatigue life improved. The surface of plastic flow layer became the starting point of the crack initiation. Moreover, compressive residual stress by shot peening processing was confirmed and the depth of the nanocrystalline layer and the plastic flow layer correlated with depth of changing point of the triaxial residual stress distribution.
The objective of this experimental study is to evaluate the practicality of RC slabs with the UFC permanent forms. Two types of test specimens, i.e. RC slab using ordinary forms and RC slab with UFC permanent forms composite structure, were prepared and subjected to a static load test, running load test, fixed point fatigue load test, and running load fatigue test. As a result, the load-carrying capacity and the number of loading cycles of the RC slabs with the UFC permanent forms were 1.2–1.4 times and 6.3 times larger than those of the RC slabs using ordinary forms, respectively. This is due to the cross-linked steel fibers that have the effect of controlling cracking and forming the bond surfaces with irregularities, which in turn have the effect of improving the bond between RC slabs with UFC permanent forms. It can be demonstrated that the RC slabs with UFC permanent forms have applicability as a composite structure highly resistant to fatigue.
In this paper, fatigue tests and finite element method (FEM) analyses are carried out on spot welded joints of mild steel (270MPa class) and ultra-high strength steel (980MPa class) in order to investigate the influence of strength level of base steels on fatigue strength and fracture morphology of spot welded joints. From the fatigue tests the following results are obtained : (1) The fatigue limit of spot welded joints is almost the same in both steels. (2) Three-dimensional morphology of crack initiation and growth around a nugget is made clear. (3) The fatigue fracture morphology of spot welded joints depends on the load level in the ultra-high strength steel, but not in the mild steel. From the discussion based on the FEM analyses the following results are obtained : (4) The fatigue limit of spot welded joints can be predicted by stress intensity factors around a nugget, fracture criterion from a mixed mode crack and threshold values for fatigue crack growth in base steels. (5) The plastic deformation around a nugget in spot welded joints strongly affects the fatigue fracture morphology.
Carbon fiber reinforced thermoplastics (CFRTP) have a lot of attention for their application to mass-produced-cars. While large amount of process and material data are available for carbon fiber reinforced thermoset composites (CFRP), CFRTP dose not have enough data in manufacturing process and mechanical properties compared with CFRP. Polyamide 6 (PA6) resin is expected to the matrix of CFRTP because of its low cost, good bonding to the carbon fiber, and good moldability. In order to establish design criteria and life assessment of carbon fiber reinforced PA6 resin composite, quantitative assessment of the environmental influence on the fiber/matrix interfacial strength is needed. The single fiber pull-out test was carried out to investigate the influence of water absorption on the interfacial shear strength of PAN-based carbon fiber/polyamide 6 composites. The interfacial shear strength was 38.3MPa for dry specimen, and those of one day and one week wet specimens are 22.9 and 11.3MPa, respectively. Water absorption test was carried out in distilled water at 80°C. The fiber/matrix interfacial shear strength was severely decreased by water absorption. Dry and one day wet specimens were fractured by adhesive failure with interfacial crack. For one week wet specimens, fracture morphology was divided into two groups. For most of one week wet specimens, the crack propagated in the matrix near the fiber/matrix interface. For two of them, the crack propagated in the interface. From this difference of the fracture morphology, process of water absorption to the pull-out specimen is considered as follows. The water absorbed through the interface and degraded the interfacial shear strength. Then, absorbed water diffuse to the resin and the resin was degraded.
High-speed compression molding process of carbon fiber reinforced thermoplastics (CFRTP) by means of electromagnetic induction heating system (IH system) using non-woven stitched multi-axial cloth (NSMC) were proposed in this study. This IH system allows heating of the mold surface instantaneously. There is no need to preheat the mold and the material before placing it in the mold. This system can also reduce production cycle times and the cost of manufacturing composite parts. Specimens were molded by a traditional hot press processing and IH system. While the hot press processing required 50 minutes to raise the temperature of the mold from 50°C to 250°C, IH system required only 70 seconds. Mold cooling of the IH system finished in only about 150 seconds. Comparing with the traditional hot press processing, this IH system can reduce production cycle time drastically. The CF/polyamide 6 specimens molded by IH system had higher bending strength and better impregnation than these molded by the hot press processing. While the void content of specimens molded by the traditional hot press processing were more than 10%, specimens molded by IH system had only about 5% void content. In case of IH system, not only mold but also carbon fiber itself is heated by electromagnetic field. Since this heated carbon fiber accelerates the resin impregnation into the carbon fiber bundle, the void content of the specimens molded by IH system is smaller than that by hot press.