AlN thin films were deposited on the polyimide films by reactive RF magnetron sputtering in Ar and N2 gas environment. Most of the films possess <10·0> or <00·1> 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 <10·0> to <00·1> with increasing gas flow ratio. The intensities of AlN 11·0 and AlN10·3 diffraction were also changed. Residual stress of the films was affected by the deposition conditions. The compressive residual stress increased with increasing N2 gas flow ratio or decreasing process pressure. The surface of the film was observed by using SEM. For the films with strong <00·1> fiber texture, the grain structure observed with scanning electron microscope was equiaxial. On the other hand, elongated grains were observed for the films with weak <00·1> fiber texture. For the films sputtered at lower temperature, many voids were observed between grains. In-situ tensile tests were carried out for each specimen in order to estimate the strength of the films. The maximum stress measured by X-ray method during tensile loading increased with decreasing intensity ratio of 00·2 diffraction. When compared at the same intensity ratio, the strength of the films sputtered at higher temperature was higher than that sputtered at lower temperature.
It is required to evaluate a strain-free reference, α0, to perform accurate stress measurement using neutron diffraction. In this study, accuracy of neutron stress measurement was quantitatively discussed from α0 evaluations on a dissimilar metal butt-weld between a type 304 austenitic stainless steel and an A533B low alloy ferritic steel. A strain-free standard specimen and a sliced specimen with 10 mm thickness taken from the dissimilar metal butt-weld were utilized. In the lattice constant evaluation using the standard specimen, average lattice constant derived from multiple hkl reflections was evaluated as the stress-free reference with cancelling out an intergranular strain. Comparing lattice constant distributions in each reflection with average lattice constant distribution in the standard specimen, αFe211 and γFe311 reflections were judged as a suitable reflection for neutron strain measurement to reduce intergranular strain effects. Residual stress distribution in the sliced specimen evaluated using α0 measured here exhibited higher accuracy than that measured using strain gauges. On the other hand, α0 distributions were evaluated using the sliced specimen under the plane-stress condition. Existence of slight longitudinal residual stresses near the weld center decreased accuracy of the α0 evaluations, which means that it is required to optimize the thickness of the sliced specimen for accurate α0 evaluation under plane strain condition. As a conclusion of this study, it was confirmed that procedures of accurate α0 evaluation, optimization of the measurement condition, and multiple evaluations on the results play an important role to improve accuracy of the residual stress measurement using neutron diffraction.
It is necessary to establish a measurement method that can evaluate accurate stress on the surface. However, the microstructure evolution takes place near the surface due to severe plastic deformation, since structural members have been superpositioned a lot of working processes to complete. As well known, a plane stress can't be assumed on the severe worked surface. Therefore we have been proposed the measurement method that can be measured the in-depth distribution of residual stress components by using high energy X-ray from a synchrotron radiation source. There is the combination of the constant penetration depth method and tri-axial stress analysis. Measurements were performed by diffraction planes for the orientation parameter Γ = 0.25 of which elastic constants are nearly equal to the mechanical one. The stress components obtained must be converted to the stress components in real space by using optimization technique, since it corresponds to the weighted average stress components associated with the attenuation of X-ray in materials. The predicted stress components distribution agrees very well with the corrected one which was measured by the conventional removal method. To verify the availability of the proposed method, thermal aging variation of residual stress components on the severe worked surface under elevated temperature was investigated using specimen superpositioned working processes (i.e., welding, machining, peening). It is clarified that the residual stress components increase with thermal aging, using the diffraction planes in hard elastic constants to the bulk. This result suggests that the thermal stability of residual stress has the dependence of the diffraction plane.
When the elastic-plastic strains vary according to its location in an irradiated area, the stress/strain and the damage measured by X-ray method are different from those obtained from specimens deformed uniformly. In this study, influence of the distribution of the elastic-plastic strains in the irradiated area on diffraction profiles was investigated by using a numerical approach. The change of the full width at half maximum, ΔH, could be approximated as a function of plastic strain. For the distribution of the elastic strain, the value of ΔH increases with increasing range of the elastic strain in spite of absent of plastic strain. When the diffraction profile was approximated by the pseudo-Voigt function, the weight coefficient of Lorentzian decreased with increasing range of the elastic strain. For the crack problem, the distribution of the elastic strain calculated from the diffraction angle agreed well with the average value within the irradiated area. The elastic strain took the maximum at half distance of the irradiated size ahead of the crack-tip. For the large irradiated area, the value of ΔH decreased and took the maximum value behind the crack-tip.
The stress and strain relationships for polycrystalline materials having tetragonal crystal system with  fibre texture, which are required for stress analysis by X-ray diffraction (XRD), were formulated within the framework of the Reuss model of elasticity. In formulating the relationships, two types of orientations of constituent crystallites in the specimen, which contribute to XRD, were shown to exist by taking into account the Laue classes of the crystallites. The formulae representing strains measurable by XRD were found to be different, depending on which the Laue class, 4/mmm or 4/m, the specimen belongs to, even though both Laue classes are in the tetragonal crystal system. The formula obtained for the Laue class 4/mmm shows that the profile observed by XRD is formed in a symmetrical shape so that the strain can be the average value of those for the two different types of crystallites. On the other hand, the formula for the Laue class 4/m shows that the profile is formed in an asymmetrical shape so that the strain can be determined by the magnitudes of the structure factors in the two different types of crystallites. Additionally, it is shown that some of the Debye-Scherrer lines change their widths with the measurement direction. The order of magnitude of such a line broadening is discussed for two materials, BaTiO3 and MgF2.
The reduction in fatigue life due to mean stress is considered in fatigue design of nuclear power plant components. However, for stainless steel, it was pointed out that fatigue life did not decrease due to the mean stress. In this study, the effect of the mean stress on fatigue life and fatigue limit was investigated in detail for Type 316 stainless steel. Axial fatigue tests were conducted in ambient air and room temperature. In order to assess the influence of change in strain by applying the mean stress and its effect on fatigue life, the strain range was measured during the fatigue tests. Furthermore, crack closure due to the mean stress was identified by using the digital image correlation (DIC) technique. It was shown that fatigue life and fatigue limit were not reduced by applying the mean stress because the mean stress caused plastic shakedown and that reduced the strain amplitude under the same stress amplitude. It was found that fatigue limit as well as fatigue life correlated better with the strain amplitude than the stress amplitude. Fatigue life was well correlated with effective strain range Δεeff, which corresponded to the crack opening range. Since the mean stress did not reduce the Δεeff, the fatigue life was extended. It was concluded that consideration of the reduction in fatigue life and fatigue limit due to the mean stress is not necessary in the fatigue design of stainless steel components.
Tensile strength and damage behaviors are investigated for non-crimp fabric VaRTM CFRP composites, which are being applied to aircraft structures, containing stress concentrations such as notch and open-hole, and are compared to that of conventional prepreg/autoclave CFRP laminates. Notched strength normalized by unnotched strength of the VaRTM composites has been found to be comparable to the prepreg laminates, though with some variation. Damage aspect has been also observed to be different from the damage mode in the prepreg laminates. Effects of stitches used in the non-crimp fabric on the notched strength and damage behavior are evaluated. Resin rich region and porosities are found to be existing around the stitch sites, and it is considered that damage onset stress is dependent on the positional relation between the stich sites and the notch tip. In addition, the non-uniform ply thickness and laminate condition, and instability of damage modes in the plates caused by the stiches have been found to result in the variation in their notched strength. Furthermore, point stress criterion that has been used to predict the notched strength of conventional prepreg laminates is applied to the VaRTM composites. It has been confirmed that the criterion is valid for the VaRTM composites to predict their notched strength using same characteristic length as of the prepreg laminates without any modification.
In the present paper, residual strains in matrix resin were monitored by embedded FBG sensors during room-temperature cure and post-cure processes by applying various temperature profiles. From the experimental results, it was found that final residual strain became smaller if post-cure process started faster. In order to investigate the strain behavior of resin during all cure processes, a viscoelastic FE analysis of resin in which an FBG sensor was embedded was conducted. The strain behavior calculated by the FE analysis agreed very well with the experimental results quantitatively. The FE analytical results showed that strain behavior of the embedded sensor is governed by thermal strain, cure shrinkage strain and viscoelastic properties of resin. Since cure shrinkage strain and viscoelastic stiffness are functions of degree of cure, it can be said that a relationship between degree of cure and heating profile plays the most significant role in generation of the final residual strain. The present technique is useful to predict residual strain of inserts embedded in resin after molding process.
Hybrid composite materials are the material consists of more than two kinds of components. In hybrid composite materials, we have extended the concept of fiber hybrid and proposed resin hybrid and interphase hybrid. Resin hybrid composite materials mean that some types of matrix resin are used in one composite material. In this study, the concept of resin hybrid composites was applied to braided composites and the fracture mechanism was clarified. At first, the effects of position and the number of fiber bundle impregnated with flexible resin on the fracture mechanism, tensile properties and three point bending impact properties were investigated. Next, finite element analysis considering micro fracture such as fiber fracture, matrix crack inside the fiber bundle and delamination between two fiber bundles was proposed. By combining the experimental results and finite element analysis, the material constants such as Weibull modulus and interfacial shear strength of fiber bundle impregnated with normal and flexible resin were identified. Then, the prediction of stress-strain curve of resin hybrid composites became realized by considering geometry of textile and micro fractures. The fracture mechanism was different by changing position and the number of fiber bundle impregnated with flexible resin. Finally, some kinds of design guides were proposed. It was considered that these techniques are useful for control of the fracture and the optimization of the material design of resin hybrid composites in which the progress of micro fractures became more complex.
Knowledge of subcritical crack growth is essential to ensure the long-term integrity of engineering structures in a rock mass. It has been found that subcritical crack growth in rock is affected by surrounding environmental conditions. In most cases, rock found underground is saturated by water. The underground water can be fresh water or salt water with different electrolyte concentrations. However, the influence of electrolyte concentration on subcritical crack growth has not been clarified. In this study, we have measured subcritical crack growth in Shirahama sandstone in distilled water and in sodium chloride solutions with various concentrations using the Double Torsion method. We show that the crack velocity was the highest in distilled water and the lowest in sodium chloride solution with a concentration of 1.0mol/l. This result indicates the retardation of subcritical crack growth in Shirahama sandstone in sodium chloride solutions of 1.0mol/l. With increasing the electrolyte concentration, the width of the electric double layer on the surface of a solid decreases, which causes the decrease of the repulsive force acting on the crack surface. This leads to the decrease of the crack velocity up to the concentration of 1.0mol/l. On the other hand, when the concentration is higher than 1.0mol/l, the crack velocity increased. This is probably caused by the nucleation of microscopic defects on the boundary between clays and solid mineral grains, such as quartz and feldspar. It can be concluded that electrolyte concentration affects subcritical crack growth by decreasing the width of the electric double layer and compacting clays.