Life Fraction Hardening Applied to a Modified Theta Projection Creep Model for a Nickel-Based Super-Alloy
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
Elevated-temperature shear creep evolution and life prediction of sintered nano-silver lap-shear joint
Nano-silver paste, as a state-of-the-art die-attach material, has great application potential on elevated temperature, high power wide-band semiconductor packaging. During operation of semiconductor devices, sintered nano-silver bond-lines have to withstand creep deformation caused by elevated operating temperature and high switch frequency. In this study, the shear creep behavior of nano-silver sintered lap-shear joint was studied over the shear stress range from 3.5 to 8 MPa and the elevated temperature range from 498 to 598 K. Four kinds of constitute equations, namely three-parameter theta projection concept, Kachanov damage evolution, Larson–Miller parameter relationship and Monkman–Grant relationship, were employed to describe the shear creep behavior and predict the shear creep rupture life of the joint. For nano-silver sintered lap-shear joint, the three-parameter theta projection could quantitatively describe the shape of individual creep curves at any specified stress and temperature. The creep damage evolution of the joint, which was investigated by Kachanov damage evolution, was temperature dependent. The prediction accuracy of creep rupture life using these four kinds of constitutive equations was also evaluated.
Statistical scatter and variability of creep property estimates in θ projection method
Abstract The prediction of long term creep properties from short term tests is discussed in terms of the θ projection method. The method has been shown to yield reasonable extrapolation functions and the present paper investigates the characterisation of creep scatter and the procedures required to give estimates of variability of creep predictions. The nature of errors in individual creep curves and in multiple curves obtained from various specimens under differing test conditions are discussed and formulae are developed that allow an assessment of the reliability of derived parameters. These methods are then extended to estimated creep properties (both interpolated and extrapolated) so that the variances of predicted quantities can be calculated. The analysis is carried out for general creep curve functions, but is illustrated with reference to the double exponential description of creep strain ξ with time: ξ=θ1(1−e−θ2t) + θ3(eθ4t − 1). Numerical examples relating to the creep properties of IN 100 are g...