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Diguang Gong - One of the best experts on this subject based on the ideXlab platform.

  • A True-Stress Creep Model Based on Deformation Mechanisms for Polycrystalline Materials
    Journal of Materials Engineering and Performance, 2012
    Co-Authors: Xijia Wu, Steve Williams, Diguang Gong
    Abstract:

    A true-stress Creep Model has been developed based on well-recognized deformation mechanisms, i.e., dislocation glide, dislocation climb, and grain boundary sliding. The Model provides a physics-based description of the entire Creep deformation process with regards to the strain-time history (primary, secondary, and tertiary Creep), rupture strain and lifetime, which finds good agreement with experimental observations for Waspaloy. A deformation-mechanism map is constructed for Waspaloy, and a Creep failure criterion is defined by the dominant deformation mechanisms leading to intergranular/transgranular fracture. Thus, the Model is a self-consistent tool for Creep life prediction.

Mengxin Liu - One of the best experts on this subject based on the ideXlab platform.

  • a frozen soil Creep Model with strength attenuation
    Acta Geotechnica, 2017
    Co-Authors: Xiaoliang Yao, Mengxin Liu
    Abstract:

    A frozen soil Creep Model with shear strength attenuation was proposed based on a soft soil Creep Model. K 0 compression and triaxial shear tests were conducted to obtain the Model parameters. With triaxial Creep tests, the performance of soft soil Creep Model was verified at first. It was shown that different Creep stages can only be described separately, which was due to a constant failure line used in the Model. After incorporating strength attenuation into the original Model, relative position between stress point and failure line is time dependent, and the progressive development of Creep strain from primary to tertiary stage can be captured reasonably.

S T Tu - One of the best experts on this subject based on the ideXlab platform.

  • the influence of stress regime dependent Creep Model and ductility in the prediction of Creep crack growth rate in cr mo v steel
    Materials & Design, 2015
    Co-Authors: J W Zhang, G Z Wang, F Z Xuan, S T Tu
    Abstract:

    Abstract In this paper, the stress-regime dependent Creep Model and ductility have been implemented in a ductility exhaustion based damage Model, and their influence on Creep crack growth (CCG) behavior of materials have been analyzed. By using the stress-regime dependent Creep Model and ductility, the CCG rate in a Cr–Mo–V steel over a wide range of C * has been predicted by finite element analyses. The predicted CCG rates agree with the available experimental data in the literature. The analysis results show that with increasing C * , the Creep Model and ductility for determining crack-tip Creep damage accumulation change from the low-stress regime Model and ductility through a combination of low- and high-stress regime Model and ductility to high-stress regime Model and ductility. These changes lead to the line segments of the da / dt  −  C * curves. In CCG life analyses and designs of high temperature components, the stress-regime dependent Creep Model and ductility need to be used.

Ali P. Gordon - One of the best experts on this subject based on the ideXlab platform.

  • life fraction hardening applied to a modified theta projection Creep Model for a nickel based super alloy
    ASME Turbo Expo 2014: Turbine Technical Conference and Exposition, 2014
    Co-Authors: David W Day, Ali P. Gordon
    Abstract:

    This paper presents the application of a life fraction hardening rule to the analytical calculation of Creep in hot section components. Accurate prediction of Creep is critical to assuring the mechanical integrity of heavy-duty, industrial gas turbine (IGT) hardware. The accuracy of such predictions depend upon both the Creep Models assumed and how those Models are implemented in a finite element solution. A modified theta projection Creep Model for a nickel-based super alloy was presented in a previous paper as an accurate simulation of Creep behavior [1]. Application of such a user defined Creep law depends upon definition of a hardening rule in the form of either an explicit or an implicit integration scheme in order to calculate incremental strains during any time increment. Time hardening is the simplest and least computationally intensive of the two most common hardening rules, but does not correctly show the effect of changing stresses or temperatures. Strain hardening may provide the most accurate solution, but the Creep Models are too complex to invert, which results in highly iterative and computationally intensive solutions. A life fraction hardening rule has been presented in other works [2] as a compromise between time hardening and strain hardening. Life fraction hardening is presented here as a highly efficient and accurate means of calculating incremental Creep strain when applied to a modified theta projection Creep Model. A user Creep subroutine was defined using a state variable to represent the strain life fraction at any time. By using the time to tertiary Creep as the denominator for the life fraction, no new material constants are needed to relate to Creep failure. The start of tertiary Creep is effectively considered to be a failure. Additional design insight can be provided through the inclusion of other state variables to calculate temperature margins at current conditions. Material testing with changing stress levels will be used to help validate the technique. A simplified example of the technique is presented in the paper. More accurate Creep predictions allow our company to improve the structural integrity of its turbine blades and vanes.Copyright © 2014 by Alstom Technologie AG

  • A Modified Theta Projection Creep Model for a Nickel-Based Super-Alloy
    Volume 7A: Structures and Dynamics, 2013
    Co-Authors: Ali P. Gordon
    Abstract:

    Accurate prediction of Creep deformation is critical to assuring the mechanical integrity of heavy-duty, industrial gas turbine (IGT) hardware. The classical description of the Creep deformation curve consists of a brief primary, followed by a longer secondary, and then a brief tertiary Creep phase. An examination of Creep tests at four temperatures for a proprietary, nickel-based, equiaxed, super-alloy revealed many occasions where there is no clear transition from secondary to tertiary Creep. This paper presents a new Creep Model for a Nickel-based super-alloy, with some similarities to the Theta Projection (TP) Creep Model by Evans and all [1].The alternative Creep equation presented here was developed using meaningful parameters, or θ’s, such as: the primary Creep strain, time at primary Creep strain, minimum (or secondary) Creep rate, and time that tertiary Creep begins. By plotting the first and second derivative of Creep, the authors were able to develop a Creep equation that accurately matches tests. This Creep equation is identical to the primary Creep portion of the theta projection Model, but has a modified second term. An additional term is included to simulate tertiary Creep. An overall scaling factor is used to satisfy physical constraints and ensure solution stability. The new Model allows a constant Creep rate phase to be maintained, captures tertiary Creep, and satisfies physical constraints.The coefficients of the Creep equations were developed using results from 27 Creep tests performed at 4 temperatures. An automated routine was developed to directly fit the θ coefficients for each phase, resulting in a close overall fit for the material. The resultant constitutive Creep Model can be applied to components which are subjected to a wide range of temperatures and stresses. Useful information is provided to designers in the form of time to secondary and tertiary Creep for a given stress and temperature. More accurate Creep predictions allow PSM to improve the structural integrity of its turbine blades and vanes.© 2013 ASME

Srinivasan Swaminathan - One of the best experts on this subject based on the ideXlab platform.

  • damage mechanics based Creep Model for 9 10 cr ferritic steels
    Acta Materialia, 2011
    Co-Authors: Ramkumar Oruganti, M Karadge, Srinivasan Swaminathan
    Abstract:

    The key microstructural features that control Creep in advanced 9–10%Cr steels are elucidated. Conclusive evidence for the presence and coarsening of nano-scale MX carbonitrides which contribute to the long-term stability of these materials is presented. It is also shown that subgrain structures resulting from the martensitic transformation contribute to the basic strength by generating internal stresses through an anelastic bowing mechanism. Kinetic equations that describe the evolution of these microstructural features with time and strain, derived from exhaustive experimental measurements for two steels, are incorporated into a Creep equation set based on the damage mechanics philosophy. It is shown that the anomalously high activation energy reported in the literature for Creep of ferritic steels can be traced to the fact that temperature dependence of subgrain growth had been ignored hitherto. This Model is shown to be able to predict Creep data for two steels accurately up to tens of thousands of hours over which data are available.