Tensile tests under cathodic charging were carried out on sensitized SUS316 steel poly- and single crystals. Maximum susceptibility to hydrogen embrittlement (HE) appeared on the polycrystalline specimen sensitized at 973K, which was characterized by intergranular fracture. On the polycrystalline specimen, the effect of cold-working before sensitized treatment appeared prominently in the HE susceptibility and the morphology of fracture surface. On the single crystal specimen, no effect was recognized in both the sensitized temperature and prior cold-working.
In this study, errors of stress intensity factor KI by stress extrapolation method, displacement extrapolation method and hybrid extrapolation method were investigated on a crack in infinite plate problem. It is found that the errors of stress and strain near crack tip are almost expressed as a simple equation, and we propose the modified distance extrapolation method which assures the high accuracy of KI by finite element analysis. The usefulness of this method is shown by the results of KI calculation on two- and three-dimensional crack problems.
Fatigue crack initiation and propagation in sintered silicon nitride have been investigated with plain specimens. The crack initiation was observed by the in situ observing system equiped with a scanning laser microscope. The crack propagation behavior of the plain specimens was compared with those of the specimens with long cracks and semi-elliptical surface cracks. The main results obtained are as follows; (1) The cracks on the plain specimens can propagate with the low stress intensity factor which is far lower than the threshold stress intensity of long cracks. (2) In some cases, a crack stops propagation or propagates very slowly after crack initiation. In these cases, the crack propagation rate is not determined uniquely by the stress intensity factor, but varies very much. (3) At the final stage of fatigue life, the relationship between stress intensity factor and crack propagation rate of the planin specimen approaches that of the small semi-elliptical surface cracks. This means, for the most part of fatigue life, the power low (da/dN=C·KImaxn) is not applicable.
The fatigue and creep-fatigue crack initiation and growth behaviors of Ni-base single crystal superalloy TMS 12 were investigated at room temperature and 920°C using double edge notched specimens. At room temperature the fatigue incipient crack growth from the notch occurred by failure on the (111) plane, and the crack growth was not dependent on the number of cycles. At 920°C the crack growth behavior did not exhibit anisotropic fracture, which is caused from multiple slips. Under creep-fatigue and creep, rhombic cavities were observed at the crack tip vicinity. Alpha wolfram precipitates were observed at the sites of cavities. Cracks initiated from the interface rafted γ' and γ at alpha wolfram precipitates in (100) plane, which formed square-type cavities.
In order to develop a thermal fatigue life assessment method for high temperature components, out-of-phase thermal fatigue tests were carried out using cylindrical specimens of cobalt-based superalloy HA188 under various temperature ranges. The experiments were conducted in the temperature ranges peaking at 1000°C while starting at 350°C, 760°C and 900°C. A unique testing method was used in which thermal gradients were generated by induction heating for one edge of the specimen and water cooling for the inner and outer surfaces. The results of thermal fatigue test correspond well with the plastic strain range-life data of standard isothermal tests conducted at the peak temperature.
Effect of preliminary surface working on the fatigue strength of type 304 stainless steel in air and pure water both at 288°C was examined. The surface finishings were electropolishing, emery paper polishing, shot peening, shot peening followed by emery paper polishing, and glassbead peening. While the residual stress introduced by shot peening was very highly compressive, the fatigue strength was not so much increased. However, the fatigue strength of the shot peened and then emery paper polished stainless steel was remarkably increased. The fatigue strength of the glassbead steel in air was much more improved compared with the shot peened stainless steel. The fatigue strength of the glassbead peened steel was also increased in high temperature water, but the degree of improvement was not so high compared with that in air.
Flexible coupling plates to transmit the rotating power between traction motor and pinion are often subjected to surface damages such as fretting and/or pitting corrosion which may very rarely lead to fatigue failure. To evaluate the well-founded service life, specimens sampled from the coupling plates used during different service periods have been tested by a Schenck-type fatigue tester. The residual lives dominated by surface defects; fretting and/or pitting corrosion, have been estimated in the lower stress amplitude range than the fatigue limit which occurs in actual loading, using modified Miner's rule. The permitted running distances after surface defects attain morphologically to a stable stage are evaluated to be 5×106km for fretting and 3×106km for pitting corrosion, in comparison with 12×106km in case of no defect initiated, on the assumption that the upper half range in the maximum stress spectrum in service loading is effective to fatigue damage.
Residual stress is generated in coating films due to difference in thermal expansion between film and substrate. Since the residual stress influences the mechanical properties of coated material, it is important to measure the residual stress in the film and the substrate. In this study, 5μm thick TiN and TiC films were deposited on the spring steel substrates by multi-arc (M-A) and dynamic mixing (D-M) PVD methods. In the TiN films deposited by the M-A method, the compressive residual stress of -5.5--4.0GPa was measured, and its absolute value decreased with increasing depositing temperature. Similarly the compressive residual stresses of -2.0GPa and -7.2GPa were measured in the TiN film deposited by the D-M method and TiC film deposited by the M-A method, respectively. In the substrate near the film/substrate interface, small compressive residual stress of -25--10MPa was measured under the as-deposited condition. The residual stress change during the repeated bending stress was measured in the substrate and the film. In the substrate of TiN coated specimen, the residual stress increased in the early stage of repeated bending stress and saturated at the stress value of -110--40MPa. the saturated stress increases with increasing applied stress. On the other hand, the residual stress did not change in the substrate of TiC coated specimen. In addition, the residual stress in the films did not change during the repeated bending stress in all the coated specimens.
In order to obtain fundamental knowledge on the thermal recycling of FRP, the TG and DTA of FRP under air flow were measured. The influences of types of fiber and matrix resin, resin fraction and additive on the thermal degradation and combustion processes were examined for GFRP and CFRP. The following results were obtained. (1) The decomposition and ignition temperatures of GFRP made from UP resin were lower than those of the FRP made from Ep resin. (2) The ignition temperature of FRP was lower than that of matrix resin. (3) Carbon fiber in a CFRP burned at lower temperature than the non-composite carbon fiber itself. (4) The powder form samples of resin and FRP decomposed and burned at lower temperature than the block form samples. (5) The addition of calcium carbonate did not influence the thermal degradation and combustion processes of GFRP.
Laboratory tests have some limitation and problems in estimation of the fiber/matrix interface strength of commercial composites. It is strongly required to clarify the detailed fracture process in composites and to evaluate the properties of interphase in commercial composite materials. New tests based on non-destructive testing (NDT) have been attempted for this purpose, but not enough data have been generated so far by the new methods. So, an experimental assessment was proposed for the failure progression and the strength of the interface in structural composite materials by using acoustic emission (AE) technique in this paper. Samples used are two types, fabricated with fibers differing only in their surface treatment. The mechanical and AE behaviors of E-glass fiber fabric/epoxy laminates were investigated in combination with transmitted light microscopy observation. Failure progress, consisting of several micro-failure mechanisms, contributed to the characteristic AE behavior. They can be discriminated through frequency spectrum analysis of AE signals detected from the laminates. The estimated results were demonstrated as “damage progress curve” to applied loads, being in good agreement with that optically observed through a microscope. The fiber/matrix debonding strength was examined based on a certain AE event rate in the damage progress curve originated by debonding for the two laminate samples.
A new method of X-ray stress measurement was applied to the measurement of residual stress in aluminum thin films sputtered on (100) oriented silicon single-crystal wafers. The film has the  fiber texture perpendicular to the film surface. The strain was measured from Al 222 and 311 diffractions obtained by Cr-Kα radiation. The values of in-plane residual stresses σ11, σ22 and σ12, and out-of-plane normal residual stress, σ33 were determined from the measured strains by using the fundamental formulae derived on the basis of Reuss and Voigt models. The residual stress in the thin films was nearly equi-biaxial tension, and its magnitude decreased with increasing film thickness. The residual stress determined by the curvature method was nearly equal to the value determined by the X-ray method.
In this paper, after the validity of impact tests is examined, impact compressive and tensile tests on three kinds of aluminum alloys are performed under several strain rates, and dynamic stress-strain curves are obtained. It is demonstrated that the JIS2024 shows the strain rate independency, whereas the dynamic properties of other two alloys depend on the strain rate. The JIS7075 shows larger overstress in tension than that in compression. Furthermore, the work-hardening effect in the case of multiple impact tests of aluminum alloys is investigated. It is found that the stress in the plastic region increases every time the impact test is performed excepting pure aluminum.
This paper purposes to improve the focusing effect by taking account of the nonlinear reflection of underwater shock wave. Reflected shock wave by the ellipsoidal reflector does not focus on a geometrical focal point because of the nonlinear reflection. The nonlinearity between incident and reflection angles on the shock wave focusing is evaluated based on the propagation time of reflected shock wave from a rigid flat plate. The peak pressure of the focused shock wave strongly depends on the wave front distribution, especially on the pressure concentration in the radial direction of the reflector. Higher pressure can be obtained by shifting spark gap position toward the second geometrical focal point. The optimal displacement of the spark gap estimated based on the relationship for the nonlinear reflection agrees with the experimental observation. The shape of reflector is modified taking account of the nonlinear effect. The modified reflector demonstrates significant increase of pressure by focused shock wave compared with the standard ellipsoidal reflector.