The corrosion fatigue (CF) crack growth behavior of a duplex (α and γ phases) stainless steel, SUS329J1, weldment has been investigated in ASTM synthetic seawater (SSW) at the free corrosion potential and the cathodically charged potential of -957mV vs. Ag/AgCl. The tests were conducted with a sinusoidal load at stress ratios of 0.1 and 0.8 with stress cycle frequencies of 0.17Hz and 10Hz. The CF crack growth rate of the weld metal in terms of stress intensity factor range, ΔK, was considerably lower than that of the base metal, since a crack opening load was increased by the compressive residual stress by welding. In air tests, the crack growth rate of the weld metal in terms of the effective stress intensity factor range, ΔKeff, or a modified one, ΔKcont, was larger than that of the base metal, because the acceleration of the growth rate by nonmetallic inclusions in the former is more prevailing than in the latter. At low ΔKcont at the free corrosion potential, the crack growth rate of the weld metal, which was accelerated by the inclusion and hydrogen embrittlement (HE), was higher than that of the base metal, which was accelerated by dissolution. On the other hand, at high ΔKcont at f=0.17Hz of R=0.1 at the free corrosion and the cathodically charged potentials, the growth rate of the weld metal was lower than that of the base metal, since the base metal with lower area ratio of γ phase was more sensitive to HE than that of the weld metal.
The system that spatially measures the distribution of corrosion current density with time by means of the scanning vibrating electrode technique (SVET) has been developed. The SVET system developed has a high resolution of current density, so that the corrosion current at the grain boundary of the bicrystal of Inconel X-750 in 0.1mol/l Na2SO4 solution or that of the corrosion pit of a pure aluminum in water containing 30ppm chloride ion can be measured. This system was applied to make in-situ observations of corrosion damage progression during corrosion fatigue of an offshore structural steel, 3.5% Ni-Cr-Mo-V steel, in synthetic seawater (SSW). Immediately after a specimen was loaded in SSW, localized corrosion was observed along an edge of the exposed area, which gradually spread to the middle part. The localized corrosion area was more localized at higher stress levels than at lower stress levels; the localized corrosion current density generally increased with increasing applied stress. Anodic current was dependent on the distributions of localized corrosion and of small cracks initiated on the surface; the factors controlling the measured current density distribution were discussed.
A quartz crystal microbalance (QCM) technique can detect sensitively a small weight change during minute corrosion of metal thin film from the measurement of change in resonance frequency of quartz crystal. The QCM technique has been applied to monitor the minute corrosion of copper thin film in air or nitrogen gas with different relative humidities containing H2S of 20ppm at 30°C. The weight gain due to sulfide formation as corrosion products on the copper thin film obeyed a linear rate law at the initial stage and subsequently obeyed a prabolic rate law. The linear and parabolic rate constants obtained from the time-variation of weight gain were influenced by the relative humidity and oxygen in air. The corrosion mechanism was discussed from the view-point of electrochemistry. Furthermore, the QCM technique has been applied to investigate the anodic oxidation process of copper thin film in pH 8.4 borate solution. The real electric current, Ireal measured during anodic oxidation at a constant potential in the passive region was compared with the apparent current, Iapp calculated from the frequency change in QCM. It was shown that the dissolution rate of Cu2+ ions through the passive film into solution and the uptake rate of oxygen in the film could be separately evaluated from the comparison between Ireal and Iapp at the high anodic potential where the dissolution of Cu+ ions was negligibly small. These results has revealed that the QCM technique is one of the most powerful tools for the study on minute corrosion of metal thin film.
A method for the detection of porosity in protective coatings and materials was presented. The defects were marked by bubbles rising from the immersed surface under low vacuum. The appearance of bubbles from surface defects was supported by electrical resistance and capacitance changes of the coating. Anodized aluminium, sprayed metal coating, paint film, and plastic molded IC were used for the tests. The variation of resistance, obtained by a moisture meter, showed a good correlation with the effective properties of the coatings.
Several nondestructive methods, for example, measurements of thickness, repulsive hardness, surface roughness, electrical characteristics, ultrasonic test, were tried on the debased test specimens to estimate the degradation of epoxy lining (0.6-1.2mm) on steel plates. It was found that the measurements of capacitance, tanδ, and intensity of reflected ultrasonic on the boundary between steel and epoxy were useful for estimating the degradation of epoxy lining. Especially, the value of tanδ was most useful, and also it was not dependent on thickness of lining.
Mechanical tests were conducted on four kinds of stainless clad steels to establish test methods for determining crack growth resistance of bimaterial interface. In tension tests, smooth specimens and shallow notched specimens were employed. In these tests, all of the smooth specimens were broken in carbon steel, not along the bimaterial interface. On the other hand, most of the shallow notched specimens were broken along the interface, when the notch root was located at the interface. Therefore, the shallow notched specimens were suitable for estimating the strength of the interface in tension tests. For fracture toughness tests, chevron notched specimens are recommended, since pre-fatigue cracks were susceptible to initiate and grow in carbon steel for conventional straight notched specimens. In fatigue crack growth tests, side-grooved and non-side-grooved specimens were employed. Although the side-grooves were machined so that the minimum cross-sectional plane of the specimens coincided with the plane of the bimaterial interface, cracks did not always propagate along the interface. Therefore, the side-grooves were judged not to be effective for cracks to propagate along the bimaterial interface. Both in fracture toughness tests and fatigue tests, the crack growth resistance along bimaterial interface was much lower than the resistance of matrix steels. In all of the mechanical tests conducted, the crack growth resistance along the interface was higher for the normalized material than that for the as-rolled material. The nickel foil inserted between carbon steel and stainless steel improved the growth resistance of interfacial cracks.
The acoustic emission (AE) signals associated with the cleavage fracture process were measured during fracture toughness tests of Sugi (Cryptomeria japonica) and Red lauan (Shorea negrosensis) woods. In addition, the effects of liquid ammonia treatment on the fracture toughness and the AE parameters such as event count, amplitude distribution and spectrum were examined. The following results were obtained; The AE total count, NT under the tests up to the maximum load in Sugi was larger than that of Red lauan wood. For both woods, NT of radial direction (crack surface: TL plane) was larger than that of tangential direction (RL plane). After the liquid ammonia treatment, AE events were decreased considerably, and NT in Sugi and Red lauan were reduced to one fifth and half, respectively. From the AE amplitude distribution analysis, the difference in distribution profile between RL- and TL-cracking was observed in untreated Sugi, but not observed in treated Sugi, nor in treated and untreated Red lauan. The characteristics of AE spectra in TL and RL planes were examined, but any good result could not be obtained. The fracture toughness, GIC of TL and RL planes in Red lauan were approximately 0.42 and 0.25N/cm, respectively. After treating with liquid ammonia, GIC was increased with the treatment time. The GIC of TL plane had a constant value (1.43N/cm) after 8 hours of the treatment, and the GIC of RL plane took a maximum value (0.97N/cm) at 15 hours.
Combined axial-torsional testings were conducted using tubular specimens of stainless steel (SUS304), on which alumina was coated by the detonation method. In this investigation, two kinds of specimens with different thickness of ceramic coating were prepared, and the loading modes tested were three types of tension, torsion and combined loading. The fracture behaviors were observed by a plastic replication or a direct photography. In each case, the failure of ceramic coating was found to progress through three stages, which were identified as the initiation of small cracks almost vertical to the maximum principal stress direction, the extention of cracking with debonding around the cracking, and the falling-off. The stress states in the ceramic-coated specimens under three loading conditions were analyzed by an elastic-plastic finite element method. The failure modes of ceramic coating were correlated with the maximum principal stress and the equivalent stress of the Mises type, which were calculated using the analytical results in the ceramic coating. The results indicated that the start or transition of each failure mode was explained by using the equivalent stress rather than the principal stress.
In order to study the optimizing for reliability-design ceramic components, a unified estimation method based on 3 parameter Weibull statistical analysis was developed by analyzing the fracture strength data obtained from various fracture tests on sintered SiC. The estimated results based on this method were compared with those estimated based on 2 parameter Weibull analysis about fitness for ceramic fracture strength properties. Furthermore special features for application of the unified estimation method to reliability-design were studied by applying it to the design of ceramic components for a gas turbine. The results obtained were as follows: 1) The fracture strength of sintered-SiC can be estimated by the unified estimation method based on 3-parameter Weibull statistical analysis considering the lower limit strength. 2) As for the normalized strength as a material constant obtained by the unified estimation method, the 3-parameter based estimation method gives a higher value than the 2-parameter based one. 3) The unified estimation method based on 3 parameters is realistic for study of allowable stress against extremely low fracture probability in reliability-design.
Tensile properties and fatigue strength under rotating bending were investigated on two aluminum-lithium alloys, 2090 and 8090, and the results were compared with conventional aluminum alloys, 2024-T4 and 7075-T6511. The proof stress and tensile strength of Al-Li alloys were approximately the same as those of 2024 alloy, but were lower than those of 7075 alloy. On the other hand, the elongation and reduction of area were extremely small compared to conventional Al alloys. The fatigue strength of Al-Li alloys was superior to that of conventional Al alloys in the region of long fatigue life (Nf>105). It is suggested that the excellent fatigue strength of Al-Li alloys is mainly due to their high resistance to crack initiation.
Fatigue crack growth behaviors such as the crack growth rate and crack closure were studied using medium carbon steel (S45C) and two phase stainless steel (SUS329J1) under various combinations of the negative stress ratio, R and the maximum stress relative to the yield strength, σmax/σy. The main results obtained are as follows; (1) In the range of intermediate fatigue crack growth rate, the crack growth rate, da/dN increased with a decrease of R and an increase of σmax/σy for both the carbon steel and the two phase stainless steel. In this case, it becomes clear quantitavely that the compresion stress in the load cycle relative to the yield strength, Sc/σy is a very important factor which dominates the crack opening level and consequently da/dN in the both materials. (2) In the range of low fatigue crack growth rate, however, the compression stress in the cycle, Sc caused da/dN to decrease for the carbon steel but to increase slightly for the two phase stainless steel. This is closely related to the fact that the oxide deposite forms easily on the fracture surfaces of the carbon steel but does not form on the stainless steel.
The fatigue test data of adhesive bonded butt, lap and tube joints under the same adhesive condition have been collected. The fatigue lives of these joints depend on the shape and size of adhesive joints, i.e. the lap length, adherend thickness and so on, because the stress distribution of adhesive layer varies with such shape parameters of adhesive joints. Hence, in this study the collected S-N data were tentatively standardized by the maximum stress in the adhesive layer obtained by the finite element method. Then the standardized fatigue strength was compared with that of butt joints. The results indicate that the fatigue strength of lap and tube joints can be estimated based on that of the butt joints of the same adhesive thickness. Furthermore, the statistical analysis was conducted in order to evaluate reliability of these estimated values.
A new life prediction method is proposed for a cast Ni base superalloy, René80, under fatigue -creep interaction (FCI) conditions based on its surface crack propagation behavior. In the previous paper, we proposed that the frequency modified total strain energy parameter was useful for evaluating the failure life (Nf) and crack initiation life (Nc) of a high strength superalloy under FCI conditions. The tensile strain energy parameter was obtained from the total tensile strain energy (ΔWTT) and the loading time (τTT) under tensile stress. In this paper, not only the failure and crack initiation lives but also surface crack propagation behavior are found to be evaluated by this parameter under FCI conditions. The surface crack propagation rate (da/dN) is a function of the parameter and surface crack half length (a); da/dN=B'(ΔWTTτnTT)l·awhere B', l, n are material constants. A new life prediction method taking account of surface crack propagation behavior and initiation life can be defined; N2a=A(ΔWTTτnTT)m+B(ΔWTTτnTT)-l(log(2a)-log(2a0))If the grain size (2a0) and the number of cycles [Nc2a0=A(ΔWTTτnTT)m] needed for the surface crack length to grow to 2a0 are known, the number of cycles (N2a) in a certain crack length (2a) can be determined from the above equation.
In order to clarify the influence of moisture in air on fatigue crack propagation characteristics of high strength steels, fatigue tests were conducted in laboratory air and dry air using CT specimens. The following results were obtained: (1) In case of low stress ratio (R=0.1), the retardation-effect by moisture in air was recognized in low ΔK region (stage 2a) of the specimen whose tensile strength was below about 2000MPa. This behavior was explained due to the oxide-induced crack closure promoted by moisture in air. (2) In case of high stress ratio (R=0.8), the acceleration-effect by moisture in air was recognized in the medium ΔK region (stage 2b) of the specimen whose tensile strength was higher than 1000MPa. This acceleration-effect might be caused by the hydrogen embrittlement which occurred near the specimen surface, and became more remarkable when the test frequency was low.
Environmental cracking tests for deep-drawn austenitic stainless steel cases were carried out in standing water in a manhole and under accelerated conditions, to test for hydrogen embrittlement. The main results are as follows. (1) 8.6% and 9.1% Ni austenitic stainless steel cases with a sacrificial zinc electrode became cracked after setting for several months in standing water in a manhole. The electrochemical condition was -790mV at the lowest electrode potential, and a current density of 50mA/m2. (2) 8.6%, 9.1% and 10% Ni austenitic stainless steel cases became cracked under accelerated conditions, to test for hydrogen embrittlement. The electrochemical conditions were the same for those cases with a sacrificial zinc electrode in sea water. However, 12% Ni austenitic stainless steel case did not become cracked under these conditions. (3) Susceptibility to hydrogen embrittlement depended on the ratio of residual stress to tensile strength, and on martensite content. Thus, the cracking probability of deep-drawn cases in this environment could be estimated. (4) Stable austenitic stainless steel cases did not became cracked after more than 7 years in these manholes.
A simulation model of mixing process presented in the previous paper was based on the assumption that the mixing process is affected by exchanging the positions of high resistance particles with those of low resistance ones in the whole system. In this report, we proposed a modified new simulation model which differs from the previous one in a point that the new one has an expanding mixing zone. By means of this model, the actual mixing process of a stationary mixer with impeller blades fixed on the vertical rotational axis was characterized. The results obtained were summarized as follows; (1) Validity of the simulation method for the actual mixing process was confirmed by comparing parameters (σ, R) related to the actual mixing process with those of modified model. (2) The arrangement of particles and the mixing zone were determined on the basis of the results obtained from the photographic observation during an actual mixing process. The standard deviation of the total inspection, σTL was found to be affected by the number of samples, N and the sample size, Sn. The optimum values of N and Sn were 16 and 25, respectively. (3) The value of the sampling fraction, ζ was found to be affected by the sampling conditions. In the spot sampling, when the correlation coefficient between σSL (: Standard deviation in the spot sampling) and Rs (: Total electric resistance calculated by applying the approximation method) was greater than 0.8, the value of ζ became minimum, 20 percent. (4) The mechanism of actual mixing process seemed to be accounted by the hybridization of two processes, one is the process of expansion of mixing zone and the other is the process of random exchange of position of particles in the mixing zone.
In order to study the influence of transition elements present in grain boundary on the stress corrosion of zirconia, yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) containing transition metal oxides such as CoO, FeO and NiO were chosen and annealed in air or soaked in water. The relations of the amount of monoclinic ZrO2 formed by annealing or soaking against the annealing or soaking time were obtained in each sample. The results were summarized in the following. (1) The doped transition elements were present primarily in grain boundaries of specimens. The amount of monoclinic ZrO2 formed by annealing or water-soaking decreased with an addition of the transition elements. (2) The transformation rate was proportional to the concentration of tetragonal ZrO2 on the surface. This indicated that the transformation induced by annealing or water-soaking was caused by the stress corrosion by water. (3) In the early stage of soaking in water the transformation rate of species containing transition elements greatly changed itself, which could be explained by the stress corrosion mechanism.
The most important recent activities in the electrical measurement of the earth structure and its interpretation techniques include the use of large continuous data, high speed computer simulation with effective visualization, and setting of underground solid electrode arrays such as cross-hole (hole-to-hole) or hole-to-surface (surface-to-hole) electrode configurations. These developments including the computerized tomography technique would provide more powerful exploration tools for detecting deep or small mineral deposits, energy and groundwater reservoirs and fracture zones, and also for monitoring hazardous waste disposal environments, groundwater contamination and various changes of the earth structure given by the natual and artificial energies in the shallow and deep underground spaces. In this paper a brief review of the solid electrode array technique is given first, which should be effective in order to extract the response (signal) due to the deep or small targets. Then, the sensitivity distributions of two different subsurface solid electrode configurations are presented to illustrate the effect of electrode element array on the characteristics of response (signal) due to the target inhomogeneities. Furthermore, helpful inversion elements in the resistivity interpretation and evaluation procedures are shown by taking the examples of the cross-hole and hole-to-surface computerized section constructions for the purpose of improving the electrical solid array system.
The stresses generated as a result of difference in thermal expansion coefficient between matrix and fiber have been calculated on the heating and cooling processes of the γ-Al2O3 continuous fiber reinforced Al composite with 50 vol% fibers. A method used for calculating the thermal stresses was based on Garmong's one which had been discussed by Tyson. In the present study, on the process of heating the composite from low temperatures the Bauschinger effect of the matrix due to the plastic deformation received during the previous thermal history was incorporated into the constitutive equation for the Al matrix. As for the matrix creep at high temperatures, the Dorn formulation for steady-state creep of polycrystalline Al was used on the assumption that the creep stress contains a threshold stress below which the matrix creep can not take place. The analysis of the matrix stress relaxation at high temperatures indicated that a considerable degree of the threshold stress must be taken into account. The matrix thermal stress-temperature hysteresis curves were observed by an X-ray diffraction technique during the thermal cycling processes of the composite. The thermal stresses calculated on the basis of the present model were in good agreement with the experimental results.