Papers by Keyword: Creep Rupture

Abstract: In the material’s creep failure analysis, the difficulty of assessing the applied thermo-mechanical boundary conditions makes it critically important. Numerous creep laws have been established over the years to predict the creep deformation, damage evolution and rupture of the materials subjected to creep phenomena. The omega model developed by the American Petroleum Institute and Material Properties Council is one of the most commonly used creep material models for numerical analysis over the years. It is good in defining the fitness of mechanical equipment for service engineering evaluation to ensure the reliable service life of the equipment. The Omega model, however, is not readily accessible and specifically incorporated for creep evaluation in FEA software codes and creep data is always scarce for the complete analysis. Therefore, extrapolation of creep behavior was performed by fitting various types of creep models with a limited amount of creep data and then simulating them, beyond the available data points. In conjunction with the Norton Bailey model, based on API-579/ASME FFS-1 standards, a curve fitting technique was employed called regression analysis. From the MPC project omega model, different creep strain rates were obtained based on material, stress and temperature-dependent data. In addition, as the strain rates increased exponentially with the increase in stresses, regression analysis was used for predicting creep parameters, that can curve fit the data into the embedded Norton Bailey model. The uncertainties in extrapolations and material constants has highlighted to necessitate conservative safety factors for design requirement. In this case study, FEA creep assessment was performed on the material SS-304 dog bone specimen, considered as a material coupon to predict time-dependent plastic deformation along with creep behavior at elevated temperatures and under constant stresses. The results indicated that the specimen underwent secondary creep deformation for most of the period.

Abstract: P92 heat-resistant steel was used to demonstrate that creep rupture life evaluation period could be shorted by the assistant of the creep data from short-period stress relaxation test without reducing the prediction precision. Research showed that the minimum creep rate and the relaxation creep rate were exchangeable, and the stress exponent and the apparent activation energy analysis of the constant strain creep and the constant stress creep showed a similar deformation mechanism at the condition of T and . The creep rupture life predicted through the combination of these two kinds of creep data was closer to the real creep data than that evaluated by the traditional method based on the time to rupture only, and the precision of the evaluated creep strength increased at last 14.5 %.

Abstract: N5 single crystal was transient liquid phase (TLP) diffusion bonded under different conditions in vacuum environment using Ni-base insert metal. Scanning electron microscopy (SEM) was used to observe the microstructure of the joints. Results show that the typical TLP joint could be divided into rapidly solidified zone (RSZ), isothermally solidified zone (ISZ) and diffusion zone (DZ). The widths of RSZ decreases with the increase of holding time at any established bonding temperature, amount of bulk compounds decrease at the same time. Bonding temperature has a complex influence on TLP joint. High temperature creep rupture tests were performed, results reveal that the sample fractured in the joint interface and the fracture mechanism has been studied.

Abstract: We present a theoretical study of the creep rupture of heterogeneous materials based on a fiber bundle model which provides a direct connection between the microscopic fracture mechanisms and the macroscopic time evolution. In the model, material elements fail either due to immediate breaking or undergo a damage accumulating ageing process. We found that on the micro-level the competition of the two failure modes gives rise to bursts of breakings with power law distributed size and waiting time between events. We demonstrate that approaching macroscopic failure the system accelerates which can be fully described as a non-homogeneous Poissonian process for long range load sharing, however, when localization occurs breaking events get clustered. Bursts are composed of sub-avalanches which lead to a non-trivial temporal shape comparable to measurements. The pulse shape proved to be sensitive to the range of load sharing.

Abstract: Influences of aging on the creep rupture properties of super-clean 9%CrMoV steel and 1%CrMoV steel, the heat resistant steels for steam turbine rotors of thermal power plants, are investigated. Using the as-received and the aging-treated materials of the two steels, creep rupture tests are carried out at 566°C. Creep rupture lives, creep fracture modes as well as the microstructural changes of the specimens are examined. It is made clear that the creep strength and the microstructural stability of super-clean 9%CrMoV steel are superior to those of 1%CrMoV steel in long-term services.

Abstract: The present paper reports the results of microcreep tests and microstructure of alloy 617 foils of 100μm thick specimens at 800 and 900 oC in air. Before each test, the specimens were annealed in vacuum at 950 oC for 17 hrs. The dynamic recrystallizations were observed during creep test. The voids and cracks were observed along grain boundaries. The maximum elongation of grains was found at the lowest applied stress along the loading direction. The average grain size decreases from 20 μm to 2 μm as applied stress increases from 48 to 120 MPa at 800 oC and from 50 μm to 5 μm as the applied stress increases from 35 to 60MPa at 900 oC. The steady state creep rates were increased as applied stress increased in all the specimens.

Abstract: The eutectic Sn3.8Ag0.7Cu alloy is widely considered a leading Pb-free replacement for the eutectic Pb-Sn solder alloy in electronic packaging where creep deformation and rupture is a major concern. In this study, creep rupture behavior of Sn-Ag-Cu solder alloy was investigated under the isothermal condition. Creep tests were conducted under a range of stresses and temperatures. Creep lifetime data were analyzed by the combined time-temperature equations following the Sherby, Larsen-Miller, and Manson-Haferd approaches. From these analyses, a series of material parameters were obtained from the experimental data. The results showed that the Manson-Hanferd method provided a better correlation with the creep rupture data. The mechanisms of creep deformation and rupture at different time-temperature combinations are discussed.

Abstract: Recent researches have shown the premature breakdown of creep rupture strength in long term creep region of advanced high Cr ferritic steels. As safe operation of power plants becomes a serious problem we should be able to detect and predict the breakdown transition of creep rupture strength. Some methods for detecting the breakdown transition have been presented till now like the measurement of reduction of area after creep rupture and particle size of laves phase. However it will be more economic if we make use of non-destructive tests, for example, hardness testing. In this paper 3 types of ferritic steels with different Cr concentration have been studied. The results suggest that the hardness of aged structures is constant independently of exposure time in short term region, whereas the hardness breaks down in long term region. The boundary of breakdown in hardness coincides with that of breakdown in creep rupture strength.

Abstract: The creep properties of type 316LN SS welded by the SAW process have been evaluated. The creep tests for both the base and weld metals were conducted with different stress levels at 550oC and 600oC. The creep-rupture time of the weld metal did not show a large difference when compared to that of the base one, though it exhibited a little lower value at 600oC. The creep rate of the weld metal was lower than that of the base one at the same stress and rupture-time conditions. Especially, the creep-rupture ductility of the weld metal is found to be decreased by about 60%, compared to the base one. This is due to the decreasing of tensile elongation and the increasing of the yield stress in the weld metals.

Abstract: The initiation and growth of micro-defects such as micro cracks and voids usually causes the failure of long term operated structural components at high temperature. In this study, the creep characteristics and void nucleation and growth characteristics of P92 steel which is used as main steam pipe material in power plant were investigated at several temperatures and loading conditions. The area fraction of void increased with increase of test temperature, stress, and load holding time. In case of internal defect presence, micro-voids initiated in the early stage of loading period and resulted in the increased load line displacement and crack growth rate. The microvoids were found to form along the prior austenite grain boundaries and at the martensite packet boundaries.