Papers by Keyword: Creep Fatigue

Abstract: In this paper, a thermodynamically consistent formulation for the analysis of creep fracture and fatigue is presented. The model is described via two potential functions, the specific Helmholtz free energy and the complementary dissipation potential. Isotropic damage variable is used to describe the degradation of the material. In addition, the use of creep parameters, like Monkman-Grant and Larson-Miller parameters are discussed and their relation to the proposed model are derived. Developed model is implemented as a user subroutine to the commercial finite element code ANSYS and an example case of practical interest is shown.

Abstract: In some coal-fired power plants, pipeline elements have worked for over 200 000 hours and increased number of failures is observed. The paper discuses thermal wear processes that take place in those elements and lead to rupture. Mathematical model based on creep test data, and describing creep processes for analyzed material, has been developed. Model has been verified for pipeline operating temperature, lower than tests temperature, basing on Larson-Miller relation. Prepared model has been used for thermal-strength calculations based on a finite element method. Processes taking place inside of element and leading to its failure has been described. Than, basing on prepared mathematical creep model and FE model introduced to Ansys program further researches are made. Analysis of dimensions and shape of pipe junction and its influence on operational element lifetime is presented. In the end multi variable dependence of temperature, steam pressure and element geometry is shown, allowing optimization of process parameters in function of required operational time or maximization of steam parameters. The article presents wide range of methods. The creep test data were recalculated for operational temperature using Larson-Miller parameter. The creep strain were modelled, used equations and their parameters are presented. Analysis of errors were conducted. Geometry of failing pipe junction was introduced to the Ansys program and the finite element analysis of creep process were conducted.

Abstract: For the components working in high temperature and enduring fatigue loading, the fatigue fracture properties will be reduced remarkably when the working temperature is higher than the critical temperature of creep-fatigue interaction T_c of the material. In consequence, the damage mechanism from creep-fatigue interaction becomes more complex. A method is presented in this paper to determine the T_c of a nickel-based powder metallurgy superalloy. Pure fatigue crack growth and creep-fatigue crack growth tests were conducted in several different elevated temperatures. The fracture mechanism was investigated via observing the fractographic characteristics using scanning electron microscope (SEM). The test results show that the T_c of this superalloy is a little bit lower than a half of the melting temperature T_m.

Abstract: Due to the initiate failure from the interface of solder and singularity of stress and strain field in the area, new fatigue life laws based on equivalent intensity ranges are proposed. The thermal fatigue tests are carried out. With observation by scanning electron microscope (SEM), the cracks begin from the interface of the chip, and propagate along the interface and/or grow with zigzag shape in the solder. Relative tube voltage drop (TVD) as a parameter is conducted to determine the fatigue damage accumulation and the number of thermal cycles for initiate crack growth. The singular field from the interface edge of the chip is obtained from numerical analyses by sub-modeling technique. Two types of solder materials of Sn-3Ag-0.5Cu and Pb-5Sn with new viscous creep constitutive relationship are used. The constitutive model compose of linear curve for small stress and hyperbolic sine form for high stress, respectively. Two shocks are found in one cycle from numerical simulation. Compared the fatigue life from the experiment observation with the numerical prediction, it is noticed that the new fatigue laws do not depend on an artificial point near the interface edge in a chip, and give reasonable and reliable results.

Abstract: Creep-fatigue tests were carried out to study creep-fatigue property of P91 steel in many references. There are two kinds of loading control methods. One is controlled by stress, and the other is controlled by strain. In this study the creep-fatigue test data loading controlled by stress are summarized. Based on these data Supported Vector Machine(SVM) method is used to build up the models to predict the creep-fatigue lifetime of P91 steel. The results show SVM method can be used to predict the creep-fatigue life of P91 steel, and the value of insensitive factor is very important. When the value of insensitive factor is small and the training error of the model is small too, but the prediction ability decreases. On the contrary when insensitive factor is increased, the training error will become large, and the prediction ability will increase. So during the training, mean square error should not be regarded as the unique goal function.

Abstract: Modern 9-12%Cr steels are widely used for steam turbine components. For the design optimization and lifetime estimation of steam turbines, it is very important to investigate and describe the deformation as well as crack initiation behavior at critical location of steam turbine components under multiaxial service-type loading conditions. In this paper a phenomenological lifetime estimation method was validated by multiaxial loading. The applicability and reliability of this lifetime estimation model was confirmed by a recalculation of biaxial creep fatigue tests performed on cruciform specimens of rotor steel X12CrMoWVNbN10-1-1 successfully.

Abstract: A creep-fatigue life prediction model for P91 steel using Improved Elman Neural Network is founded based on the creep-fatigue experiment datas in this paper. The load keeping time is selected as the model input and the creep life or fatigue life as the output in the model. The prediction ability is validated from training the sample data. The results show the model has a very high prediction accuracy for training samples. But the model generalization ability is relatively weak, so the forecasting accuracy for test sample is lower than the training samples. Through increasing test sample number and decreasing the uncertain factors during the experiment in the follow-on working, the prediction accuracy and the model generalization ability will be increased.

Abstract: Lifetime prediction of steam turbine components under biaxial thermo-mechanical fatigue (TMF) loading of modern high chromium steel is prerequisite for design optimization. In this paper a phenomenological method which envelopes the synthesis of stress strain hysteresis loops and damage assessment under considering creep fatigue interaction is extended to multiaxial loadings. It is proposed as a post processing step depending on the results of a preceding finite element analysis based on a constitutive material model. Recalculation of biaxial service-type experiments on cruciform specimen of modern high chromium rotor steel 10CrMoWVNbN shows satisfactory results for lifetime estimation.

Abstract: Description of crack initiation behavior under thermo-mechanical fatigue (TMF) of modern high chromium steel is prerequisite for design optimization of steam turbine components. In this paper a phenomenological method which envelopes the synthesis of stress-strain hysteresis loops according to cycle counting methods and the individual assessment of creep fatigue damage is extended to TMF with superimposed creep. Recalculation of such service-type experiments on specimen of rotor steel 10CrMoWVNbN shows acceptable results for deformation description and lifetime estimation.

Abstract: Tools are exposed to severe working conditions during the hot extrusion process. In particular, dies and mandrels can be subjected to an excessive amount of deformation as a result of the developed high cyclic loads and temperatures. In this scenario, a physical experiment reproducing the thermo-mechanical conditions of a mandrel in a porthole die was performed with the Gleeble machine on the AISI H11 tool steel with the aim to investigate the mechanisms that influence the die deformation. The design of experiment consisted of 4 levels of temperature, 3 levels of stress and 3 types of load, i.e. pure creep, pure fatigue and creep-fatigue. In all the testing conditions, a comparable pattern of the mandrel displacement-time curve was found reproducing the 3 stages of softening typical of the strain evolution in a standard creep test but with a marked primary phase. Thus, with the aim to identify an easy-applicable equation to estimate the die deformation, the time hardening creep law was chosen. Coefficients of the time-hardening law were optimized, for each testing condition, on the basis of experimental data starting from values for similar alloys taken from the literature. Results in terms of mandrel displacement were then compared to experimental data for the creep-fatigue condition at different stress and temperature levels. The values found were validated against additional experimental data performed with different specimen geometries. A good average agreement was found between experimental and numerical results. The developed procedure was then applied to an industrial die.