In order to get fundamental understanding to establish the refurbishment technology for advanced gas turbines, the nucleation of cellular colonies in a single crystal Ni-base superalloy, CMSX-4, was studied, supposing the case in which the material was subjected to damage associated with local plastic deformation, followed by the re-heat treatments. During the study special attention was paid to get basic understandings on it. The experimental evidences indicated that the cellular colonies were originated and developed from the damaged region, being influenced by the following many factors: the strain field by the local plastic deformation, the re-heat treatment temperature, and the microsegregation in the material. It were also shown that the cellular colonies was actually produced inside the material subjected to fatigue and thermto-mechanical fatigue damages. Furthermore, the effect of the local cellular colonies on the high temperature fatigue strength was experimentally quantified.
A series of deformation tests was conducted on Ni-based superalloy IN738LC under isothermal and anisothermal conditions between 450°C and 950°C. Under the isothermal conditions, the material showed no rate/time-dependency below 700°C, while there was distinct rate/time-dependency above 800°C. Anomalous inelastic behavior was observed under the anisothermal conditions; with an increase of cycles, stress at higher temperatures became smaller in absolute value, and stress at lower temperatures became larger. Based on the experimental results, the previously proposed viscoplastic constitutive model for IN738LC at 850°C was extended to the anisothermal conditions. In the constitutive model, evolution of formally incorporated variable Y was assumed to be active under the higher temperatures and negligible under the lower temperatures. The extended constitutive model was applied to the anisothermal cyclic loading as well as monotonic tension, stress relaxation, creep and cyclic loading under the isothermal conditions. It was demonstrated that the present constitutive model was successful in describing the inelastic behavior of the material adequately, including the “anomalous inelastic behavior” observed under the anisothermal conditions.
In order to examine the applicability of the effective-stress-based analysis method of perforated plates which was proposed by the authors, inelastic behavior and creep-fatigue life of perforated cylinder subjected to cyclic thermal stress are predicted. Firstly, basic creep behavior of perforated cylinder under axial tension at uniform temperature is examined by three-dimensional FEM creep analysis, and the results show that macroscopic and local creep behavior of perforated cylinder is predictable using the proposed method on perforated plates. Secondly, thermal fatigue test of perforated cylinder is carried out, and the elastic-plastic-creep behavior of perforated cylinder is numerically analyzed by modeling the perforated cylinder to the equivalent solid cylinder based on the effective stress concept. Predicted local stress-strain behavior around circular holes is used for creep-fatigue life prediction based on the linear damage rule. Comparison of numerical results and experimentally observed damage clarifies that the difference of damage mode at inner and outer surfaces of perforated cylinder can be successfully reproduced.
On an increasing demand to reduce CO2 emissions and save energy, high strength ferritic heat resisting steels of 9-12%Cr have been developed with a view to elevating metal temperatures of thermal power plants. Type IV creep cracking in heat affected zone (HAZ) of welded joints is a serious problem for these high strengthened steels. The present paper investigates the microstructures and creep properties of welded joints for W strengthened 11Cr-0.4Mo-2W-CuVNb steel. Most of the welded joint specimens were ruptured at HAZ and creep lives decreased than that of base metals. The comparison of creep properties of simulated HAZ specimens showed that fine grains produced by heating around Ac3 temperature was responsible for such degradation of creep strength. The growth of precipitates on grain boundaries and recovery of dislocation structures during creep was faster for fine-grained HAZ. The electron beam (EB) welded joints indicated two times longer creep life than gas tungsten arc (GTA) welded joints, however the brittle Type IV fracture was occurred even in the EB welded joints for long-term creep test. The FEM creep analysis by using the creep data of simulated HAZ specimens could explain the experimental results for creep properties of welded joints.
In the remaining life assessment of high-temperature components such as boilers after long term service, the practical evaluation of crack growth lifetime using nonlinear fracture mechanics is needed. In this paper, creep-fatigue crack growth behavior under displacement controlled conditions is examined using CT specimen of 21/4Cr-1Mo steel which is the typical high temperature structural steel. The J-integral estimation method and the crack growth prediction method using fully plastic solution are also examined. Creep-fatigue crack growth under displacement hold can be separated into fatigue crack growth in the cyclic portion and creep crack growth in the holding portion. Those mechanisms are competitive to each other, and the crack growth behavior is determined by dominant one of fatigue and creep. The J-integral approach using fully plastic solution is introduced for predicting low cycle fatigue crack growth under the complicated loading condition and creep-fatigue crack growth with the relaxation. The propriety of this solution is verified by comparing the prediction with experiments by the slope-line-control method.
Fatigue tests were conducted at 1273K using smooth bar specimens of a nickel-based single crystal superalloy CMSX-10, and small cracks were observed on the surface of the specimens by means of microscope. It was clarified by the observation that fatigue fracture of CMSX-10 takes place as follows; (1) Oxide-layer was formed on the surface of the specimen. (2) A lot of small cracks of the size of about 100μm were initiated perpendicular to the stress axis on the surface of the oxide-layer. (3) Some of them grew and penetrated the oxide-layer to reach the base metal. Processes (1) to (3) were completed at the very early stage of fatigue life. (4) Growth of the cracks that reached the base metal brought about the final fracture. As the formation of oxide-layer on the surface of the specimen was considered to be a trigger of crack initiation, a fatigue test was also conducted with repeated removal of the oxide-layer by emery paper and diamond paste. In this test, no crack appeared on the surface of the specimen even at 20000 cycles when a lot of cracks were observed in the normal fatigue tests. This proves that removal of the oxide-layer retards the initiation of small cracks and extends the fatigue life.
This paper describes tension-torsion multiaxial creep-fatigue for CMSX-2 nickel base single crystal superalloy at 1173K. Strain controlled tension-torsion low cycle fatigue tests with hold-times were carried out using hollow cylinder specimens, of which the axis agrees with  crystallographic direction. Hold-times in torsion tests were more detrimental to lifetimes than those in tension tests. Mises' equivalent strain range parameter gave a large scatter for the correlation of creep-fatigue lives. The relationship between creep-fatigue lifetime and elastic strain range had strong dependence on a principal strain ratio but that between the lifetime and plastic and creep strains had almost no dependence. A linear damage rule gave a proper estimate in tension tests, but substantially overestimated lifetimes in torsion tests. A new simple lifetime prediction method, taking account of anisotropy of the elastic modulus, was proposed. The method predicted the experimental lifetimes within a factor of 3 scatter band. Cracks in tension tests propagated in a principal strain direction but these in torsion tests in a maximum shear direction. In torsion tests, cracks were only observed in soft-zones predicted by numerical analyses.
Dissimilar diffusion boning between an oxide dispersion strengthened (ODS) Ni-base alloy, MA758, and conventional casting Ni-base superalloy, CM-247LC, was studied by combining and controlling some bonding parameters: bonding temperature, bonding pressure, and the post weld heat treatment. The optimum bonding condition was filtered and refined on the basis of the microstructure and thickness of the diffusion layer, the hardness distribution across the welded interface, and the high temperature tensile properties of the joints. The experimental evidences indicated, the bonding temperature and the post weld heat treatment played an essential role. For example, the longer the post weld heat treatment time, the better the high temperature tensile properties were. The fatigue strength of the joints thus optimized was also evaluated at high temperature in air, and was compared with that of the base material, MA758. The fatigue strength of the joints optimized was slightly lower compared with that of the base material, MA758, however, it was high enough for engineering applications.
In the present paper, the effect of two-step aging on fatigue strength in beta Ti-22V-4Al alloy is studied on the basis of observation of crack initiation and small crack growth and fractographic analysis of fracture surfaces. Rotating bending fatigue tests have been conducted at room temperature in laboratory air using smooth specimens prepared with materials subjected to different two-step aging treatments (STDA), and the obtained results were compared with the data of material solution treated and aged at a single temperature (STA) and as-solution treated material (ST). The fatigue strength of STDA materials aged at the first-step temperatures close to the second-step temperature was slightly lower than, or nearly the same as, that of STA material. Particularly, the STDA material aged at high temperature followed by low temperature showed no subsurface crack initiation which was seen in STA material in the long life regime around or more than 107 cycles, thus the fatigue strength was superior to that of STA material in that region. Fatigue cracks were initiated at beta grain boundaries in STDA materials aged at low temperature followed by high temperature, which was the same behaviour as in STA material, while were generated within beta grains in STDA materials aged at high temperature followed by low temperature, but small crack growth was not affected by the aging condition. Based on the above experimental results, the role of alpha precipitate distribution in fatigue behaviour was discussed and it was indicated that bi-modal microstructure composed of coarse and fine alpha precipitates was one of the recommended microstructures of the present alloy.
In this paper, a statistical method for the evaluation of the data related to the super-long life fatigue region is proposed. According to a research paper published by the “Research Group on the Statistical Aspects of Material Strength”, failures are classified into two modes. That is, the failures due to cracks originated on the surface and those due to crack resulting from internal material defects. Therefore, to evaluate the fatigue properties in this region, the experimental data should be separated according to these two modes. To decide the optimum separation between these two modes, we make a first separation based on the number of cycles. Then we draw the two fatigue strength plots on probability papers, and we calculate the two coefficients of correlation. The separation line is shifted and the previous procedure is repeated. The optimum separation is the one for which the sum of these coefficients reaches its maximum. Using this proposed method, a computational system to decide the S-N curve to be used in the design has been developed. To validate the proposed method, we applied the computational system to the fatigue test data of different materials. The agreement of this separation with that of obtained by SEM observation allow us to judge positively our method and to confirm the accuracy of the obtained S-N curves.
A new method is proposed to assess the fatigue reliability of gas equipment subjected to a load with a randomly fluctuating stress amplitude in a relatively small range. The Langevin equation for fatigue damage evolution was derived on the basis of the Palmgren-Miner's rule and the S-N diagram under constant amplitude loading. A stochastic partial differential equation, called the Fokker-Planck equation, for the evolution of the probability density function of cummulated fatigue damage was obtained from the Langevin equation. The probability density function of cummulated dàmage was expressed in a Gaussian distribution function. The mean of the remaining fatigue life can be evaluated on the basis of the concept of first passage time. The new method was applied to the fatigue data of an aluminum alloy subjected to a random loading. The scatter of the fatigue life can be obtained by introducing a magnification factor of the intensity of actual randomly fluctuating stress. The magnification factor corresponds to the degree of the contributions by factors other than the fluctuating load.
This paper is concerned with the temperature dependence of flexural fatigue strength of pitch/PAN hybrid unidirectional CFRP laminates. The hybrid laminates were constructed by replacing few compressive surface layers of the pitch-based CFRP composites by PAN-based layers. Three point bending fatigue tests for the hybrid laminates were carried out for a temperature range from -20°C to 120°C which is below the glass transition temperature, 143°C, of matrix resin. The flexural fatigue strength for the hybrid laminates is significantly higher than that for the pitch-based laminates at all the temperatures tested. These experimental results were compared with the predicted ones obtained from the mechanical properties of pitch and PAN based unidirectional CFRP laminates.
An investigation was performed on the interfacial contorol of aramid/epoxy composite system using the oxygen plasma treated aramid fiber. In particular, it was shown that the oxygen plasma treatment was effective for the improvement on the interfacial adhesion between the aramid fiber and the epoxy resin from the results of a single fiber pull-out test. It was clarified from a single fiber tensile test that the tensile strength of aramid fiber decreased after the oxygen plasma treatment. The formation of functional groups such as the carboxyl groups, carbonyl groups and hydroxyl groups was found by XPS analysis on the surface of aramid fiber treated by oxygen plasma. Then, the interfacial shear strength obtained from a single fiber pull-out test was increased by oxygen plasma treatment and that maximum value was found at the treatment time 10min. under the plasma treatment condition; the treatment pressure 100Pa and the treatment power 70W. It became clear from DCB test that the energy release rate increased at the treatment time 10min. as same as the interfacial shear strength and the fiber bridging was observed for this sample.
It is known that lightweight concretes made with fully-saturated expanded shale aggregates exhibited an extremely high chloride permeability, when they were subjected to freezing and thawing. In order to improve the chloride permeability of lightweight concretes under freezing and thawing, the effect of absorbed moisture of expanded shale aggregates and mix proportions on this property of the concretes was investigated by using the AASHTO T277 chloride permeability test method. The results showed that the chloride permeability of lightweight concretes increased conspicuously with the repeated cycles of freezing and thawing up to about 10bcycles at both curing periods of 28bdays and 3bdays, when the levels of absorbed moisture of an expanded shale sand and an expanded shale coarse aggregate were higher than 9% and 27%, respectively. It was found that at the moisture levels of at most 5% and 2% for the sand and 14% and 7% for the coarse aggregate at 28bdays and 3bdays, respectively, the lightweight concretes showed an almost constant chloride permeability up to 34cycles of freezing and thawing. The test results also indicated that air entrainment up to about 11% and decrease in water-cement ratio from 55% to 45% were little effective in suppressing an increase in the chloride permeability of lightweight concretes exposed to freezing and thawing at the age of 28bdays but yielded great benefit in terms of the chloride permeability of the concretes which had undergone freezing and thawing at the age of 3bdays.