Papers by Keyword: Low Cycle Fatigue

Abstract: By the test of rotational bending for bar of 45 steel with V notch in low cycle fatigue, the bar’s fatigue life is studied under strain-controlled condition. The characteristics of several kinds of specimens' crack propagations are analyzed. The accumulative effect of crack propagation is discussed to find the appropriate load for quick fracture. Based on the theory of continuum medium damage mechanics, the damage evolution model in low cycle fatigue is obtained. The predicted curves and strain-life curves agree with experimental data for medium carbon steel well in this work. These results are very important for the life estimation of medium carbon steel.

Abstract: In this study, failure behavior of hot gas casing for gas turbine was investigated. The microstructure and damage mechanism of serviced hot gas casing were examined. Also low cycle fatigue tests of the Inconel 617 super alloy is used for structural material of hot gas casing were performed. To predict the low cycle fatigue life, Coffin-Manson and strain energy density methods were used.

Abstract: Behavior of thermo-mechanical fatigue (TMF) failure of a single crystal Ni-base superalloy, CMSX-4, was studied and, compared with isothermal low-cycle fatigue (ILCF) of it. Strain-controlled TMF and ILCF tests of CMSX-4 were carried out under various test conditions, where the experimental variables were strain rates, strain ratio, test temperature and the range, and strain/temperature phase angle. At first it was shown from the experiments that the TMF and LCF failures associated with some noteworthy characteristics were rarely seen in the traditional pollycrystalline heat-resistant alloys. These phenomena could be explained inadequately, on the basis of the macroscopic parameters and the historical failure criteria; e.g., Manson-Coffin law and Ostergren approach. A new micromechanics model is proposed to predict the TMF and LCF lives, based on the Eshelby's theory.

Abstract: The isothermal low cycle fatigue (LCF) behaviors of a directionally solidified (DS) nickel base superalloy, coated with a NiCrAlYSi coating were studied. The study concerned NiCrAlYSi coating formed by an arc-discharged physical vapor deposition (PVD) process for protection against high-temperature corrosion and oxidation of gas turbine blades. The effect of protective coating on LCF life of coating/substrate system was investigated at high temperatures and compared with uncoated alloy. The test results show that coating has no or less effect on LCF life under high strain range and the LCF life is governed by the fatigue behavior of substrate at different temperatures. However, when strain range is smaller, crack initiation and propagation are observably affected by temperature, which leads to a shorter LCF life of coating/substrate system at 500°C and a longer LCF life at 760°C or 980°C.

Abstract: The objective of this study is to examine the feasibility of the X-ray diffraction method for the fatigue life assessment of high-temperature steel pipes used for main steam pipelines, re-heater pipelines and headers etc. in power plants. In this study, X-ray diffraction tests were performed on the specimens simulated for low cycle fatigue damage, in order to estimate fatigue properties at the various stages of fatigue life. As a result of X-ray diffraction tests, it was confirmed that the full width at the half maximum (FWHM) decreased with an increase in the fatigue life ratio, and that the FWHM and the residual stress due to fatigue damage were algebraically linearly related to the fatigue life ratio. From this relationship, a direct assessment of the remaining fatigue life was feasible.

Abstract: Cyclic strain control tests have been performed on cylindrical specimens of cast polycrystalline Inconel 792-5A superalloy at 23, 500, 700 and 800 °C in laboratory atmosphere to study the effect of temperature on the fatigue behavior. Cyclic hardening-softening curves and fatigue life curves were measured. Scanning electron microscopy was used to investigate the surface relief. Low amplitude straining was characterized by saturation of the stress amplitude. In room temperature high amplitude straining cyclic hardening was followed by marked saturation. Pronounced continuous hardening until failure was observed at 500 °C. Initial cyclic hardening was followed by softening at 800 °C. A systematic shift of the Manson-Coffin and of the Basquin curves to lower fatigue lives was found when temperature was increased. Fatigue cracks were observed to initiate from surface slip markings at all temperatures.

Abstract: Gray cast iron shows large asymmetrical features by the graphite flake when tensile and compressive stresses are applied. The plastic strain rage which is used in low-cycle fatigue life prediction by many researchers is hardly defined and gives very different values by the Standards in this case. From the results of this study, it is not reliable to use the plastic strain range as a low-cycle fatigue damage parameter. Therefore, the plastic strain energy density which is uniquely defined was suggested as a damage parameter and it showed good correlation in low-cycle fatigue in gray cast iron.

Abstract: In this work, the formation of fatigue cracks is considered as a nucleation process due to the development of a characteristic microstructure formed just beneath the specimen surface. Strain controlled cyclic tests were carried out at room temperature at total strain ranges εt = 0.8 and 1.2% in flat specimens of SAF 2507 Duplex Stainless Steel (DSS). The results show that for this DSS, at εt = 0.8%, the correlation between phases (Kurdjumov-Sacks crystallographic relation) plays an important role in the formation of microcracks. On the other hand, at εt = 1.2%, microcracks initiate in the ferritic phase and the K-S relation does not seem to affect the formation of the cracks.

Abstract: Low cycle fatigue behavior of Sn-3.8Ag-0.7Cu solder was investigated under fully reversed cyclic loading, with particular emphasis on microstructural effects. The LCF behavior of the solder with equiaxed microstructure was found to differ greatly from that of the solder with a dendrite microstructure. At a given total strain amplitude, the dendrite microstructure exhibited a much longer fatigue life than the equiaxed microstructure. Such a strong microstructural effect on fatigue life arose from the difference in cyclic deformation and fracture mechanisms between the two microstructures. A large number of microcracks along grain boundaries of the equiaxed structure solder developed with increasing cycling, while for the dendrite structure solder, cyclic deformation took place along the direction of the maximal shear stress during fatigue tests and microcracks initiated and propagated along shear deformation bands. Besides, the fatigue behavior of the dendritic microstructure was very sensitive to cyclic frequency whereas the fatigue behavior of the equiaxed microstructure showed less sensitivity to cyclic frequency.