In October 2000, the 24 years old Ikata-1 PWR-type nuclear power plant suffered cracking in pipes, where poly vinyl chloride (PVC) tape had been placed and had become baked over time. The existence of residual stress over 100MPa in the pipes, a bit of chlorine and a feather like-pattern on the crack faces suggested the event was one of stress corrosion cracking. Engineers of Shikoku Electric Power Co. groups examined experimentally the production of chloride from PVC tape on a steel, and elucidated the temperature depending production peaks is induced by the properties of FeCl3, the principal ingredient of the chloride. Semi-quantitative expression of the production of soluble chlorine was proposed. In next report, authors will elucidate the stress corrosion cracking in steels with baked PVC tape and the cracking potential in pipes with the fluid of various temperatures and various duration.
The morphology of cracks, the chemical adhesions on the cracks, large residual stresses in pipes, and the location of cracks restricted to pipes of short hot water flow were reported in the previous paper. It was also found that the tempe rature depending peaks in chloride production was caused by the properties of FeCl3. Semi-quantitative expression of the chloride production was proposed. A four points bending stress test was performed on the steel plates with the baked on PVC tape in humid air at 80°C. It is found that the baked PVC tape is the most harmful in 50% humid air, and its harm is same as NaCl in 100% humid air. The highest possible stress in pipes and the highest possible atmospheric temperature surrounding pipes is estimated to be 200MPa and 40°C, respectively. By taking both temperature- and stress-dependences into consideration, the SCC curve relating PVC-tape-induced-contamination for pipes in stagnant was established. Under the assumption of linear damage rule in stress corrosion cracking phenomena, damage to be accumulated in 60 years to pipes with fluids of various temperature and duration were calculated and a damage contour was graphed with a fluid temperature ordinate and a flow duration abscissa. The fluid conditions of major pipes at the Ikata-1 nuclear power plant, which had not received the full inspection, were positioned on so low area on the damage contour that the plant was estimated to be safe for the coming forty years.
Hydrostatic pressure dependence of the mechanical behavior of polymers is studied by three constitutive modeling, in which the yield surface is described by the first and the second invariant of stress and the nonassociated flow rule satisfies the incompressible hypothesis. An internal variable theory of rate-independent plasticity is presented, which incorporates isotropic hardening and kinematic hardening. Both evolution equations of isotropic hardening variable and hydrostatic-pressure-dependent variable are formulated and the previous model is modified. The predicted results of the model are compared with the experimental ones of uniaxial tension and compression obtained by Spitzig and Richmond under high pressure. Another plasticity constitutive equation with isotropic hardening model is derived from assuming a different yield function which also expresses the hydrostatic pressure dependence. The predicted results of the model are compared with the experimental ones of torsion obtained by Silano et. al under high pressure. Finally, plasticity constitutive equation is formulated by the application of kinematic hardening theory to hydrostatic pressure dependence. The predicted results of the model are compared with the experimental ones of reversed torsion under high pressure and compression after tension obtained by Kitagawa et. al.
Finite element models of polycrystalline thin films were constructed based on the Monte Carlo method. The models consisted of columnar aggregates of cubic crystals with fiber texture whose axis was ‹001› direction perpendicular to the film surface. In the Monte Carlo method, the nucleus of a crystal was distributed at positions generated by the random number, and the crystal boundary was formed from the coordinates of the nucleus of crystals by using Voronoi tessellation.The number of grains in a sample volume was changed and fifty models with different orientations were produced for each case. A constant uniaxial displacement was applied to the models to obtain the scatter of elastic properties of thin films under the conditions of plane strain and plane stress. The scatter and mean values of Young's modulus and Poisson' ratio were obtained as functions of the number of the grains within a sample volume. A method is proposed to determine the number of grains for thin films to have macroscopic properties for the cases of various thin films with different degrees of elastic anisotropy.
X-ray diffraction observation of metal fractures provides us with useful information on the mechanisms and mechanical conditions of fracturing. This method has been developed especially as an engineering tool for fracture analysis. (α+γ) dual phase stainless steel JIS-SUS329J4L is a composite material which consists of ferrite αFe and austenite γFe phases. This material excels in corrosion-resistance in the chlorinated environment, and is used for chemical plant and the oil pipes. In this study, X-ray fractogarphy technique is applied to fatigue fracture surface of dual phase stainless steel. Fatigue crack propagation tests were conducted under stress ratios of 0.5 and 0.7. Residual phase and macro stresses near the fracture surface were measured using X-ray diffraction method to the depth direction. In addition, the distribution of the misfit of plastic strain ΔεP between αFe and γFe phase obtained from micromechanics beneath the fracture surface was calculated. From the distribution of these X-ray parameters, the depth of the maximum plastic zone ωy was determined. Relations between ωy and fracture mechanics parameter were examined. As the result, ΔεP was related with the maximum stress intensity factor Kmax divided by the 0.2% proof stress σ0.2. Moreover, it is suggested that ΔεP is more effective parameter than residual stresses for determination of ωy of dual phase stainless steel.
In order to clarify the microstructural change of ceramic matrix composite (SiCf/SiC) which can influence the strength, chemical composition and crystal structure of ceramic fiber and matrix are examined by means of electric probe micro-analyzer (EPMA) and Raman spectrometer. Oxidation tests are carried out at the temperature of 1473K-1673K for 1000h in air. Little change in chemical composition is observed in ceramic fiber. Excess amount of carbon exists as disordered graphite in ceramic fiber, and it is found that the disorder proceeds as the test time increases according to R value, which is defined as the ratio of two carbon peaks in Raman Spectra. The crystallization of SiC in fiber is rarely observed in extent of the test. As for matrix, besides silicon and carbon, oxygen is contained even before the test in most of the matrix, while there is the other kind of matrix which contains relatively low amount of oxygen and large amount of carbon. It implies that the microstructure of matrix is not uniform. Inside the specimen, the chemical composition of matrix does not change due to the oxidation test. However, after the oxidation test, the content of oxygen increases and that of carbon decreases due to the oxidation at the vicinity of surface. Raman spectra show the crystallization of SiC in matrix after the oxidation test. And even in matrix, disordered graphite exists, and, according to R value, the disorder proceeds as the test temperature and time increase.
It was expected that the performance of a polymer based composite and its interfacial behavior of fiber/matrix could be improved by using organic substance and/or organic-inorganic hybrid. In this paper, the new approach of using organic-inorganic nano-hybrid as a middle layer to improve the interfacial performance of composites was proposed. Several typical reinforcements such as continuous glass fiber, chopped strand glass fiber and glass fabric were treated by epoxy/silica nano-hybrid and a middle layer between reinforcement of glass fibers and the matrix of epoxy resin was introduced. The composites with this nano-hybrid middle layer for reinforcements were fabricated. Then the interfacial behavior with and/or without the hybrid middle layer was investigated and compared by the fragmentation test with simultaneous AE measurements, peeling tests and SEM observation. It was found that the interfacial behavior between glass fiber and epoxy resin could be improved greatly by introducing the middle layer of epoxy/silica nano-hybrid. It was confident that the concept of using organic-inorganic hybrid as a middle layer between reinforcements and matrix would be one of effective approaches for improvement of the interfacial behavior in composites.
Contact strength tests were carried out in soda lime glass plate with a small ceramic sphere. As the ring crack initiation stress changes with increasing the contact angle, the peculiar tensile strength of glass is not applicable as the criterion of initiation strength. Initiation strength including steep stress distribution was discussed from the viewpoint of fracture mechanics. Stress inclination changes with increasing the contact angle. Crack lengths which cause the ring crack initiation are extremely short and a submicron order. The ring crack occurs also even at a position away from the contact circle because stress level is high. Arranging the phenomenon by the stress intensity factor, the ring crack was caused in the fracture toughness value of glass. Therefore, the application of the stress intensity factor is appropriate as the evaluation parameter of the ring crack initiation. Furthermore, a probabilistic evaluation equation can be expressed using stress intensity factor and Weibull modulus in which a contact angle and an effective volume are considered.
In order to investigate the influence of stress change on the fatigue behavior and fatigue life of an aluminum oxide-dispersion-strengthening copper alloy at elevated temperature, rotating bending fatigue tests were carried out under two-step loading at room temperature and 350°C. Both of static strength and fatigue strength decreased at 350°C. However, at the same relative stress σa, /σB, fatigue life was longer at 350°C than at room temperature. Although the cumulative ratios Σ (N/Nf) were nearly unity for both the low to high and the high to low block loadings at room temperature, Miner's rule did not hold at 350°C. These results were related to the stress dependence on the log l-N/Nf relation. That is, the crack length initiated at the same N/Nf was larger in higher stress level at 350°C, whereas there was no stress dependence in the relation at room temperature. The stress dependence on the relation at 350°C was caused by the suppression of crack initiation due to the surface oxidation.
Manganese is one of the important elements to improve mechanical properties and control the surface color of Cu-Zn alloys. However, little is known about the mechanical property and season cracking susceptibility of the Cu-Zn-Mn alloys which contain more than 5mass%Mn. In this paper, we intend to examine the effect of manganese on the fundamental properties of Cu-Zn alloys, and to develop a new Cu-Zn-Mn alloy with much lower susceptibility and good mechanical property. The color properties of a* and b* values of the Cu-Zn-Mn alloys can be controlled by the change of Zn and Mn concentrations, and a Cu-25mass%Zn-10mass%Mn alloy has the same white color as Nickel Silver. The ternary alloy also exhibits the lowest season cracking susceptibility which is nearly the same as that for Cu-15mass%Zn alloy, by suppression of the formation tendency of surface oxide film during exposure in the ammonical environment.
An attempt was made to investigate the crack growth behavior in soda-lime glass under mixed mode loading. Mixed mode crack growth tests under monotonic increasing biaxial loading were carried out by using the newly developed in-situ observation type biaxial bending testing machine. Biaxial stresses were applied by “saddle shape bending” for square plate. Crack extension from the residual stress free indentation-induced precrack were observed by a scanning laser microscope. The mode I and mode II stress intensity factors were calculated by Yoda's method. The major results of this study are summarized as follows: (1) Kinked cracks were initiated from indentation-induced precrack. With a crack growth, crack growth direction gradually approached to the direction perpendicular to the maximum principal stress. (2) When crack growth rate was arranged by mode I stress intensity factor KI, the threshold stress intensity KIth under mixed mode tends to decrease with decreasing in the stress mixing ratio R. (3) The criteria in which the crack starts to grow under mixed mode condition was expressed by similar equation for the brittle fracture criteria. (4) The stress intensity factor Keq, which is obtained by combining KI and KII, is useful to discuss a crack growth behavior under mixed mode condition.
Load-controlled fatigue tests were conducted on smooth tubular specimens of an annealed medium carbon steel under fully-reversed torsion with superimposed static torque or tension. Effect of mean stress on initiation and growth of short crack was studied in the intermediate life regime. Microcracks initiated around the maximum shear stress amplitude planes rather than the planes of maximum shear stress, irrespective of mean stress conditions. The dominant crack grew accompanying coalescence of the shear cracks initiated in the early stage of life and the preferential growth direction was different depending on the type of mean stress. The increase of the crack growth rate and the reduction of the fatigue life with application of mean stress were slight in the present material. The relatively smaller mean stress effect appeared to be resulted from the decrease of the plastic shear strain amplitude with static loading. Therefore, two-stage short crack growth model proposed by Hobson and Brown was modified incorporating the effects of mean stress acting on the crack planes and the hardening/softening of material. The model showed a satisfactory estimation of fatigue lives in torsion under various mean stress conditions.
The fatigue crack propagation test was carried out in order to clarify the effect of humidity on the characteristics of fatigue crack propagation in spherical graphite cast iron. Spherical graphite cast iron (FCD450) was used as a specimen. Then, two kinds of heat treatments were conducted to the material. One was an austemper treatment (ADI), the other was a quenching and tempering treatment (QT). Moreover, as cast material was added, and with these three materials fatigue crack propagation test was carried out. The experiment conformed to ASTM. Stress ratio R was 0.1, and specimen used was 1CT type, 12.5mm thick. The test was carried out at room temperature and under three or four kinds of humidity, 0, 20, 40, 60, 80%. The relationship between the characteristics of fatigue crack propagation and the crack closure which produced on fracture surface, was investigated. The obtained results were as follows. (1) In the low ΔK region, since the oxide inducement crack closure is generated, the propagation speed lowered, and ΔKth increased. (2) In high ΔK region of QT and ADI, the fatigue crack propagation speed was accelerated by the effect of humidity. (3) ΔKth of As-cast and QT were increased with the rise in humidity. (4) ΔKth of the ADI was highest in the humidity 40% atmosphere. However, it couldn't be clarified that the factor of decreasing ΔKth over the 40%.
To ensure reliable operation and high performance of small-size liquid cooling systems, it is essential to evaluate the liquid-permeation behavior of polymer and to optimize the liquid weight in the system. Accordingly, the present study quantifies the liquid-permeation behavior of an isobutylene-isoprene rubber (IIR) tube and a nickel-plated syndiotactic polystyrene (SPS) plate in antifreeze, namely, a water-propyleneglycol solution. It was found that the IIR tube is permeable to only water in the antifreeze, not to propyleneglycol, because the water vapor pressure is 340 times higher than the propyleneglycol vapor pressure. Moreover, the nickel plated SPS plate has negligible low liquid-permeability, because the liquid barrier produced by the nickel plating is 10 times better than that by the non-plated SPS plate. Accordingly, after long-term operation of the small-size liquid cooling system, the above-mentioned selective water permeation in antifreeze leads to degradation of cooling performance. This is because the viscosity of the antifreeze and pressure losses in the cooling path increase as a result of the increased antifreeze concentration. These liquid-permeations and concentration changes of antifreeze must therefore be taken into consideration when specifying the required liquid weight in the liquid cooling system.
Thermal cycle tests for solder joints of electronic devices are generally performed to investigate their reliability under field service (design) conditions. It is important to clear a relationship between an accelerated test condition and a service condition and to determine a practical accelerated test condition. The relationship between an accelerated test condition and a service condition is explained by use of the solder joint fatigue curve. An accelerated reliability test condition is shown taking account of several factors such as a number of test samples, strain ranges of solder joints under accelerated test temperatures conditions, standard variations of fatigue life of solder joints, reliability and confidence level of devices under service conditions. Moreover, the cases that the standard deviation of fatigue life is known or unknown are investigated. A specific analytical and experimental result for single outline packages is shown.
In order to clarify the fatigue behavior of Cu-6Ni-2Mn-2Sn-2Al alloy which was aged (400°C, 3h) after 80% swaging, fatigue tests of smooth specimens have been carried out. In addition to this, fatigue tests of the alloy with 0.1% Zr have been performed. The plastic replication technique was used for monitoring the crack initiation and propagation behavior. Experimental results showed that the crack initiation life of Zr added alloy is larger than no Zr alloy. The crack initiation sites of the alloys were both the grain boundary and slip bands independent of Zr addition. With regard to a major crack which led to the final fracture of the specimen, however, the initiation sites for Zr added alloy tended to be in the slip bands. For no Zr added alloy, the major cracks tended to initiate from the grain boundary. Thus, it can be concluded that the crack initiation resistance at the grain boundary increases due to the Zr addition. This increased resistance for crack initiation may result from the Zr particles precipitated at the grain boundary by the heating of ageing. On the other hand, a slight effect of Zr addition on crack growth behavior was observed.