Papers by Keyword: Creep Curve

Abstract: Creep characteristics of alloys and compounds have been evaluated mainly by the minimum creep rate or the steady-state creep rate, and by its stress and temperature dependences. In some cases, however, direct comparison of the minimum creep rate or the steady-state creep rate are not practically easy due to difficulties of experiment, i.e., a long duration of primary stage of creep deformation. The minimum creep rates are not always precise representative value, which is directly evaluated from experiments. It should be valuable, if one could estimate the minimum creep rate from creep curve in primary stage. I have proposed a method of quantitative evaluation of creep curve based on the evaluation of strain rate change and its strain dependence during creep [1-3]. The value that reflects a shape of creep curve is named “Strain Acceleration and Transition Objective-Index (SATO-Index)” [4]. SATO-Index and related differential equation show a strain dependence of strain rate and lead entre creep curve by numerical integration. This concept provides quantitative information of shape of each creep curve, and information of the entire creep curve. In this paper, examples of evaluation and extrapolation of creep rate from primary stage in compression are presented. It is concluded that the extrapolation with the concept of SATO-Index reasonably provides imaginal minimum creep rate. Usability of extrapolation of creep curve by the concept is presented.

Abstract: The estimation of the lifetime for equipments and installations working at high temperatures represent an actual problem. In the last years, there have been proposed a lot of methods in order to evaluate the lifetime for the equipments working under creep conditions. In the frameworks of the paper, some results regarding the behavior of pipes belonging to a methane gas cracking reactor are presented. Pipes worked on about 160.000 hours under a pressure of 14 at and a temperature of 800°C. The behavior of the pipes under above mentioned pressure has been calculated and plotted. Creep tests were performed at 650 and 800°C, and on these bases was evaluated the creep strength of the material. With Larson-Miller method the results were prolonged for spans shorter than 10.000 hours. The creep strength variation curves, drawn for 1000 and 100000 hours can be used for predictions about the lifetime at different.

Abstract: Resin concrete is a new material which can be made into machine bed instead of the traditional pieces of gray cast iron as the machine base, it can improve the dynamic stiffness of machine tools and the quality of machined parts, and extend the campaign life, reduce noise and improve efficiency. However, due to the long-term effect of load of the resin concrete, the elastic deformation occurs in its component, and the strain will increase over time. Thus it can affect the resin concretes service life, and the calculation of creep has become an urgent task in structure design and use, which should be taken seriously. In this paper, the bending creep properties of resin concrete beam were studied and analyzed by using four-point bending test method. The creep curve under different load levels were obtained, and the viscoelastic properties were analyzed.

Abstract: Considering the erosion effect of underground water deteriorate the micro structure of rock which results in the time effect of rock deform, the paper developed a uniaxial compression equipment for rock creep under chemical and stress coupling action. It adopts good corrosion prevention design, can conveniently test the rock uniaxial compression creep effect with erosion. Take limestone as study object, the paper carried out uniaxial compression creep tests with different PH environment. Compared the results of different tests and found that limestone has obvious creep effect in acid solution, the smaller the PH value is, the greater the creep deform is. For the same test specimen, the creep deform of primary stage is larger than that of later stage. In each level load, the creep velocity decreases along with the time increasing. Analyzed result shows that the equipment has provided basement for studying the rock creep mechanism with chemistry and stress interaction.

Abstract: An isochronous stress-strain curve (ISSC) needs to be generated for a creep design application for high-temperature materials. To generate the ISSC for type 316LN stainless steel (SS), a series of creep data, which was obtained from creep tests with different stress levels at 600oC, was used. Creep curves were modeled by means of a nonlinear least square fitting (NLSF) of the Garofalo model. In the fitting of the creep curve, a secondary creep region was separated into first and second phases, and its fitting range was suitable to use for the first phase. The Garofalo model revealed a good agreement with the experimental creep data, and its parameters, P1, P2 and P3 revealed a good linear relationship as a function of a stress. The ISSCs for type 316LN SS at 600oC were successfully generated up to 300,000 hours.

Abstract: This study aimed to model the long-term creep curves above 105 hours by implementing a nonlinear least square fitting (NLSF) of the Kachanov-Rabotnov (K-R) model. For this purpose, the short-term creep curves obtained from a series of creep tests at 950oC were used. In the NLSF of their full creep curves, the K-R model represented a poor match to the experimental curves, but the modified K-R one revealed a good agreement to them. The Monkman-Grant (M-G) strain represented the behavior of a stress dependency, but the 􀁏 parameter was constant with a stress independency. The 􀁏 value in the modified K-R model was 2.78. Long-term creep curves above 105 hours from short-term creep data were modeled by the modified K-R model.

Abstract: Creep tests and microstructural observations in several high Cr ferritic steels bearing V and Nb were carried out in order to investigate effects of addition of V and Nb on
, which is a coefficient of
method. Creep strength was improved with increasing the V content and was slightly lowered by increasing the Nb content.
in all the steels bearing V or Nb was lager than that in fundamental steel. The dependence of
on the amount of additions was different between V and Nb. In steels bearing V,
became larger with increasing V content. In steels bearing Nb,
became smaller. These results revealed that
corresponds to the creep strength, indicating that the value of
can be estimated using the strengthening effects of the additive element.

Abstract: To design HTGR components for up to 1000oC, their creep curves are necessary during a design process. In this study, the full creep curves were modeled by the nonlinear least square fitting method using the Kachanov-Rabotnov (K-R) creep model. A series of creep data was obtained experimentally under various stress levels for Hastelloy-X at 950oC, and the data was used to model the creep curves. The K-R model gave a poor description of modeling creep curves, but the modified K-R one, which has another variable, K in the K-R model, was in better agreement than the K-R one. It was found that the λ parameter in the K-R model was constant regardless of the stress variations. The λ value was about 3.9 for the K-R model and about 5.8 for the modified one.