Creep-Fatigue Lifetime Assessment with Phenomenological and Constitutive Material Laws (original) (raw)
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On Creep Fatigue interaction of components at elevated temperature
Journal of Pressure Vessel Technology, 2015
The accurate assessment of creep–fatigue interaction is an important issue for industrial components operating with large cyclic thermal and mechanical loads. An extensive review of different aspects of creep fatigue interaction is proposed in this paper. The introduction of a high temperature creep dwell within the loading cycle has relevant impact on the structural behavior. Different mechanisms can occur, including the cyclically enhanced creep, the creep enhanced plasticity and creep ratchetting due to the creep fatigue interaction. A series of crucial parameters for crack initiation assessment can be identified, such as the start of dwell stress, the creep strain, and the total strain range. A comparison between the ASME NH and R5 is proposed, and the principal differences in calculating the aforementioned parameters are outlined. The linear matching method (LMM) framework is also presented and reviewed, as a direct method capable of calculating these parameters and assessing a...
2013
This paper presents a novel direct method, within the Linear Matching Method (LMM) framework, for the direct evaluation of steady state cyclic behaviour of structures subjected to high temperature – creep fatigue conditions. The LMM was originally developed for the evaluation of shakedown and ratchet limits. The latest extension of the LMM makes it capable of predicting the steady state stress strain solutions of component subjected to cyclic thermal and mechanical loads with creep effects. The proposed iterative method directly calculates the creep stress and cyclically enhanced creep strain during the dwell period for the assessment of the creep damage, and also creep enhanced total strain range for the assessment of fatigue damage of each load cycle. To demonstrate the efficiency and applicability of the method to assess the creep fatigue damage, two types of weldments subjected to reverse bending moment at elevated temperature of 550C are simulated by the proposed method conside...
2018
This paper presents a novel direct method, within the Linear Matching Method (LMM) framework, for the direct evaluation of steady state cyclic behaviour of structures subjected to high temperature – creep fatigue conditions. The LMM was originally developed for the evaluation of shakedown and ratchet limits. The latest extension of the LMM makes it capable of predicting the steady state stress strain solutions of component subjected to cyclic thermal and mechanical loads with creep effects. The proposed iterative method directly calculates the creep stress and cyclically enhanced creep strain during the dwell period for the assessment of the creep damage, and also creep enhanced total strain range for the assessment of fatigue damage of each load cycle. To demonstrate the efficiency and applicability of the method to assess the creep fatigue damage, two types of weldments subjected to reverse bending moment at elevated temperature of 550C are simulated by the proposed method conside...
The prediction of the creep behaviour and life of components of aeronautic engines like high pressure turbine blades is still a challenging issue due to non-isothermal loadings. Indeed, certification procedures of turboshaft engines for helicopters consist of complex thermomechanical histories, sometimes including short and very high temperature excursions close to the γ'-solvus (T~1200°C) of the blade alloy. A better design of those components could be gained using a model that takes into account non-isothermal loadings inducing microstructural changes. Most of the commonly used models consider only a nearly constant (or slowly evolving) microstructure, i.e. far from the rapid microstructure evolutions encountered during close γ'-solvus overheatings where a rapid dissolution/precipitation of the γ'-phase and fast recovery mechanisms were observed by . A new constitutive modelling approach was hence recently proposed in a crystal viscoplasticity framework to capture the transient effects of such rapid microstructure evolutions on the creep behaviour and life ). In this article, an updated version of this model is detailed. Special attention will be paid to the effect of the accumulated plastic strain on the microstructure evolution, (ii) the introduction of an additional damage formulation, and (iii) the creep strain at failure. The performances of the model are illustrated on the basis of isothermal or complex non-isothermal creep experiments performed on nearly [001] oriented samples.
Journal of Engineering Materials and Technology, 2016
Hastelloy X (HX) and 304 stainless steel (304SS) are widely used in the pressure vessel and piping industries, specifically in nuclear and chemical reactors, pipe, and valve applications. Both alloys are favored for their resistance to extreme environments, although the materials exhibit a rate-dependent mechanical behavior. Numerous unified viscoplastic models proposed in literature claim to have the ability to describe the inelastic behavior of these alloys subjected to a variety of boundary conditions; however, typically limited experimental data are used to validate these claims. In this paper, two unified viscoplastic models (Miller and Walker) are experimentally validated for HX subjected to creep and 304SS subjected to strain-controlled low cycle fatigue (LCF). Both constitutive models are coded into ansys Mechanical as user-programmable features. Creep and fatigue behavior are simulated at a broad range of stress levels. The results are compared to an exhaustive database of ...
Modeling Creep Damage Based on Real Microstructure
Practical Failure Analysis, 2002
This paper outlines a method to model creep failure of polycrystalline materials based on a real microstructure taken from an optical microscope. The creep failure is simulated in 304 stainless steel and the simulation is based on Norton's creep law. By treating the grain boundaries and the grains differently and adopting the void nucleation process proposed by Shewmon, the creep strain energy density can be used as a failure criterion. The result of the simulation confirmed the results of conventional methods used in a high-temperature remnant life assessment. The intermediate results of the simulation process allow calculation/monitoring of stiffnesses degradation as the material undergoes creep failure.
Creep deformation and failure under cyclic thermal loading
Nuclear Engineering and Design, 1989
The purpose of the present paper is to demonstrate how the results of limit load and shakedown analyses can be utilised in the prediction of the deformation characteristics and life of structures subjected to cyclic thermal loading. The structural response is described in terms of a material test conducted at a constant reference stress and a cyclic reference temperature history. Simple expressions for these quantities are given for deformation which require a knowledge of the limit load and shakedown boundary and the resulting mechanisms of collapse and incremental deformation beyond shakedown. A procedure is described for evaluating the time-to-failure, where the results depend on the shape of the isochronous surface in stress space. For an effective stress material the resulting reference quantities are the same as those for deformation.
Materials Science and Engineering: A, 2010
The accumulation of creep-fatigue damage over time is the principal damage mechanism which will eventually lead to crack initiation in critical high temperature equipment. A model has been developed that assumes on a macroscopic level that the energy dissipated in the material may be taken as a measure of the creep damage induced in the material and hence the creep damage is directly proportional to absorbed internal energy density. The model developed is derived from considerations of mechanistic cavity growth and is based on rupture elongation to failure data using true strain. The predictions of the energy density exhaustion approach are compared with the results of creep-fatigue tests on low alloy ferritic steels. The predicted results of the energy density model are found to have good correlation with the measured creep-fatigue lives.