Polycrystalline materials are composed of great amount of crystal grains, each of which is joined with neighboring grains. Since the slip system of each crystal grain is randomly distributed over the domain, the materials show mechanical inhomogeneity. This paper discusses the effects of grain size, the resistance of crystal grain, as well as the grain boundary sliding on the deformation behavior of polycrystal. Finite element analyses are carried out with reference to the combined model of crystal plasticity and grain boundary sliding. It is shown that both the size and the flow stress parameters are dominant on the inhomogeneity and that the deformation around the grain boundary is relaxed by the grain boundary sliding.
Type IV creep cracking in the heat affected zone (HAZ) of welded joint has been one of the serious problems in 9 to 12%Cr ferritic steels during long time service. We have already shown that the specimens heated to peak temperature close to Ac3 do not have clear lath martensitic microstructure and that the coarsening of precipitates, M23C6, in these specimens is observed after post weld heat treatment (PWHT). In this study, a coarse grained steel is produced by the higher temperature normalizing heat treatment. The average grain size of coarse grained steel was 250μm and that of the Ac3 simulated HAZ specimens was 8μm, compared to 30μm and 6μm respectively for a normal grain size steel. The creep life, microstructural changes and distribution of precipitates of simulated HAZ specimens for coarse grained steel are investigated and compared with those for the normal grain size steel. The creep rupture time of the simulated HAZ specimens for coarse grained steel is found to be longer than that for the normal grain size steel, because the regions of large precipitates in the coarse grained steel, which are considered to have low creep resistance, are relatively smaller than those for normal grain size steel.
Grain size dependence of creep rupture strength and creep strain rate was experimentally decided for tungsten containing 11% chromium ferritic steels at elevated temperatures. A different forging ratio for the same cast ingot following the same heat treatment extracted the grain size effect of temepered martensitic microstructure on a creep deformation rate and a microstructure evolution during the creep. Fine block grain increased the nucleation site of intergranular ferrite during the creep which was observed through Transmission Electron Microscope for thin foils prepared from ruptured specimens. Grain boundary microstructure recovery was explained by modified “core-mantle-model”, proposed by Terada et al., assuming the mantle width increase during the creep deformation in ferritic microstructure equivalent to a grain boundary slip or a grain boundary diffusion mechanism for austenitic microstructure.
Fatigue tests were conducted at 1273K using smooth bar specimens of the first-generation nickel-base single crystal superalloy CMSX-2. Oxide-layer on the surface of the specimen and the base metal were carefully observed by means of scanning electron microscope after the tests. It was clarified by the observation that crack initiation in high-temperature fatigue of CMSX-2 was mainly brought about by cracking of the oxide-layer which was formed on the surface of the specimen. Comparing the results obtained from the tests with different stress and strain waveforms, cracking of the oxide-layer found to be easily occurred when the following two conditions are satisfied. The first one is sufficient time for the formation of oxide-layer since cracking occurred in thick oxide-layer and the crack continued to grow inside the base metal. The second condition is that there is tensile-going strain after the formation of oxide-layer. The brittle oxide-layer was easily cracked by the following tensile loading. Moreover, there observed change in γ/γ'-structure during the fatigue test and this structural change brought the decrease in cyclic deformation resistance. This might have quickened the oxide-layer cracking particularly in stress-controlled condition.
The purpose of this study is to clarify the effect of CoCrAlY coating on biaxial thermo-mechanical fatigue (TMF) life property of lnconel 738LC. In-phase and out-of phase TMF tests at temperature between 450°C and 850°C were carried out by using a tension/torsion and compression TMF testing machine. In addition to these fundamental TMF tests, a complicated temperature and strain history tests, so called a blade waveform which simulates temperature and strain histories of actual gas turbine blade, were performed. Failure life of coated specimens under the in-phase condition is longer than that of the substrate and the main crack propagated from boundary between the coating and substrate to inside of the substrate. Therefore the coating plays a positive role for the substrate life due to suppression of crack initiation and propagation in the coating. On the other hand, failure life of the coated material under out-of-phase and blade waveform tests is approximately 1/2 of the substrate life indicating negative effect. It was suggested from failure surface observation that crack initiation period in the coating is shorten by the difference of deformation behavior between the substrate and coating, and that crack propagation rate in the substrate of the coated specimen is accelerated by the cracking of the coating compared with the crack propagation rate of substrate itself.
Cellular microstructure, or colony, in a single crystal Ni-base superalloy, CMSX-4, that might be formed relating to refurbishment and recoating process has been studied. At first it was shown that this microstructure was frequently originated, when the material experiences a local plastic strain, followed by the re-heat treatments for damage recovery. This phenomenon is not special, but often the case induced by many kinds of processes: e. g. during the in-service period and the fabrication, refurbishment and recoating processes. Once this kind of cellular transformation was formed, the fatigue strength was found to be remarkably reduced. In order to prevent the above problem, two methods were explored: one was a simple preheat treatment that was aimed to release strain energy stored due to the local plastic straining; the second was a new method employing coating technique, based on the new concept that the alloy elements that might be too lacking in base alloys to endure the undesirable effect should be supplemented from the overlay coating layer. It was shown by the fatigue tests that the former was not successful, but the latter might be very effective. The observations of the fatigue crack initiation site, the fracture mode, the crack density in the cellular transformed area, and the crack propagation morphologies near the prior interface, strongly supported the validity of the latter method. The method is expected not only to be a countermeasure against the present problem but also to open a new road as a damage cure coating.
Crack propagation behavior of a perforated plate of normalized and tempered 2 1/4Cr-1Mo steel modeling ligament regions of boiler headers was examined under out-of-phase thermal fatigue at a maximum temperature of 600°C in the air. Inelastic analysis of the perforated plate under the thermal fatigue was carried out, and the nonlinear fracture mechanics parameters such as the J and C* integral were obtained by the line integral method for observed crack distributions. A prediction method for these parameters was proposed, which is applicable to the displacement-controlled condition such as thermal fatigue. In this method, the change of the macroscopic stress-strain relation of the perforated plate with propagating cracks was combined with the reference stress concept under the displacement-controlled condition. The predicted fracture mechanics parameters from this method coincided well with those from the inelastic analysis. The prediction of the crack propagation life on the basis of the proposed method provided a good correspondence with the test results of the perforated plate under thermal fatigue.
In order to clarify the creep-fatigue damage process and to evaluate the creep-fatigue life for boiler 2.25Cr-1Mo header stub welds, a series of creep-fatigue tests were performed on partial mock-up specimens of actual plant under simulated plant loading conditions. Creep voids and micro-cracks occurred along the weld toes at an early stage of life and grew to form many short cracks. These short cracks grew both on the surface and through the wall of the stub tube and later coalesced to form one crack. It was proved that there was a correlation between the maximum crack depth and life ratio and also that there was a correlation between the maximum crack depth and the maximum crack length on the surface. A life prediction method was proposed based on these two correlations.
The performance of repair welds on service-aged 2.25Cr-1Mo header using SMAW with post weld heat treatment was experimentally evaluated. Creep rupture life of the repair weldment was almost the same as that of serviceaged base metal and repair weld was not a weak link. The remaining creep rupture life of aged weldment was improved by repair welding. In creep-fatigue test with strain holding, the fully repair-welded specimen showed almost the same creep-fatigue life as that of new weldment, however, the partially repair-welded specimen showed a shorter creep-fatigue life. FEM analysis revealed that the deformation behavior of repair weldment was quite different from that of new weldment and strain concentrated on softened aged metal.
The present paper investigates the applicability of Genetic Algorithms (GA) to accelerated determination of the material parameters required in order to evaluate creep-fatigue life and remaining life of Mod. 9Cr-1Mo steel. The GA program, which was written in Visual C++, was developed in order to estimate the values of 23 material parameters required to describe the partitioned inelastic strain range versus life equations and the creep-fatigue damage growth model based on the strain range partitioning concept. Three types of test, constant-strain amplitude creep-fatigue tests, constant-amplitude creep-fatigue crack growth tests and variable-strain waveform creep-fatigue tests, were needed in order to determine the above-described parameters experimentally. The GA analysis was able to determine the parameters without using the creep-fatigue crack growth test data. The effects of the number of data used in the GA analysis for the estimation accuracy of the material parameters and the creep-fatigue life were evaluated. The obtained results suggest that the number of tests required for GA determination of material parameters is half of the number of creep-fatigue tests (175) that must be conducted in order to determine the material parameters experimentally.
In the past the microstructural observation was mostly applied to understand the materials behavior qualitatively in the R & D of new materials and life prediction for the fast breeder reactor components. However, the correlation between the changes in properties and microstructures must be clarified quantitatively to ensure the structural integrity. Particularly we are interested in the method to correlate the long-term properties with microstructural changes at high temperatures. The current research is to quantify the changes in microstructure of the weld metal for the welded structure of the reactor vessel. In this research we have conducted creep testing of a 16Cr-8Ni-2Mo weld metals at 823, 873 and 923K up to 76, 320h. Based on the change in area fraction of the precipitates, the evaluation of creep-rupture properties with time and temperature was attempted. Correlation between ferrite content and the area fraction of the precipitates was not recognized. It was possible to correlate linearly the amount of the precipitates to log (creep exposure time) with the Larson-Miller parameter.
The experiment was conducted on the compact normal and shear (CNS) specimens made of homogeneous and dissimilar materials subjected to mixed-mode loading. Many Young's fringes patterns around the crack tip were taken and analyzed by the image-processing system developed in my laboratory. The displacement obtained by speckle photography is not as smooth as that obtained by the finite element analysis (FEA). Therefore, the displacement data were smoothed by 2-D FFT filtering and least squares method. The intelligent hybrid method proposed by Nishioka et al. was applied to the stress-strain analysis. Consequently, the stress and strain near the crack tip can be evaluated with high accuracy by the present stress-analyzing system. Then, the stress-intensity factor was evaluated by the virtual crack extension method (VCEM) and displacement extrapolation. The accuracy of stress-intensity factor at the free surface was discussed from both viewpoint of experiment and 3-D FEA.
Chloride ingress into concrete with high strength lightweight aggregate made of fly ash (HFA aggregate) as the coarse aggregate was evaluated by a steady state migration test and chloride ponding test. It was found that the migration coefficient obtained from the migration test could represent appropriate ranking on chloride ingress into the concrete with different W/Cs. The effect of HFA aggregate on the chloride ingress was negligible and a similar migration coefficient to that of normal weight aggregate was given. In addition, an observation using EPMA showed no chloride ion present in the HFA aggregate itself after the migration test. Chloride ponding test also exhibited same ranking providing increased apparent chloride diffusion coefficients with increased W/C. Migration coefficients might be related quantitatively to the apparent diffusion coefficient with chloride binding isotherm being properly considered.
Silicone gel is usually applied to electrical automotive devices to protect them from corrosion. However, under a vibration environment, the silicone gel vibrates bonding wires in the devices, thus, to evaluate the reliability of the devices, the vibration analysis of the gel/wire structure is indispensable. In this study, we clarify the relation between the fatigue life of gel-protected bonding wires and the geometry of the gel and bonding wires experimentally. It was founded that the diameter of wires and the thickness of the gel have a significant influence on fatigue life. Then, we developed a method, based on a vibration analysis model that takes into account the visco-elasticity of a gel, for predicting the fatigue life of the wires. It was confirmed that the predicted fatigue life showed good agreement with the measured fatigue life. Finally, we developed a design tool for easily calculating the fatigue life of the wires. This tool estimates the strain range by using a response surface, i. e., a neural network. As Bayesian regularization was executed in learning of unknown parameters in the neural network, we could make the response surface and ensure good generalization ability.