Thermo-mechanical Low Cycle Fatigue Research Articles

Thermomechanical and isothermal fatigue properties of MAR-M247 superalloy

Thermomechanical low-cycle fatigue (TMF) tests were performed on polycrystalline cast nickel-based superalloy MAR-M247 under in-phase (IP) and out-of phase (OP) loading in the temperature range of 500–900 °C. A constant heating and cooling rate of 5 °C.s−1 was selected. Furthermore, isothermal low-cycle fatigue (ILCF) tests with the same cycle period as TMF cycle were carried out at the maximal temperature of TMF cycle (900 °C). All tests ran under strain control and fully reversed cycle on solid cylindrical specimens in laboratory air. High resolution scanning electron microscopy and transmission electron microscopy were utilized to characterize the damage mechanism occurring during TMF and ILCF loading. The cyclic deformation and lifetime behaviour as well as damage mechanisms dependent on loading regime were examined. Results show that the TMF loading has a detrimental effect on the lifetime of MAR-M247. Lifetime reduction is most significant under IP loading. The fracture surfaces are characterized by typical striation fields, where the largest spacing between striations was observed for IP loading with prevailing intercrystalline cracking followed by OP and isothermal ILCF loading with transcrystalline crack propagation. To improve discussion of obtained results, experimental data were compared with damage and life prediction models.

An Overview of the mechanism-based thermomechanical fatigue method (DTMF) in the design of automotive components

The DTMF damage Parameter described in the paper is the generalization of the creep fatigue parameter (DCF) proposed by Riedel, which quantifies the amount of damage produced by a non-isothermal loading cycle. The entire design methodology is outlined in the text, starting with the calibration of Chaboche's viscoplastic model and then the determination of the thermomechanical fatigue (TMF) parameters. Chaboche is generally a good choice for its ability to describe well both kinematic and isotropic hardening behaviors, also enabling stress relaxation and strain rate dependency to be considered. The DTMF method is based on the fracture mechanics concept of cyclic crack tip opening displacement (ΔCTOD) where the effective stress range takes the crack closure effect into account. This method is typically used to describe the thermomechanical low cycle fatigue behavior of cast irons, cast steels, stainless steels, Inconel, HiSiMo and nickel-based alloys. These materials are used in components that need to withstand high temperatures (higher than half of the melting point) in service as exhaust manifolds, cylinder heads, valves, turbochargers and disk brakes. Three failure mechanisms are detailed here: oxidation, creep and fatigue. Oxidation is a complex phenomenon that may occur when the material is hot under tensile in-phase loading or hot under compressive out-of-phase loading. Creep is a time and temperature dependent diffusion process which is incorporated into the DTMF parameter as a multiplicative factor. In this context fatigue life is defined as the number of cycles to propagate an existent microcrack up to an arbitrary size that is defined on a case-by-case basis.

Deformation and failure behaviour of 316 LN austenitic stainless steel weld joint under thermomechanical low cycle fatigue in as-welded and thermally aged conditions

Beneficial influence of prior thermal ageing on the cyclic life was demonstrated on a type 316 LN austenitic stainless steel weld joint under thermomechanical fatigue. Tests performed using different temperature intervals showed that the crack initiation occurs mostly in the heat affected zone in the as-welded joint. However, the failure location gets shifted to the weld region following thermal ageing. Mechanisms governing life variations between the as-welded and aged joints were identified. The localization of plastic deformation in different regions of the joints and its dependence on the microstructure was evaluated and explained on the basis of detailed EBSD investigations.

Experimental and numerical modeling approach for thermomechanical low cycle fatigue analysis of cyclically non-stabilized steels

The widely used fatigue life prediction models, such as the Coffin–Manson model or S–N curve related models are based on the assumption that the response of a material experiencing low cycle fatigue loading is stabilized during some period. However, for many materials such a stabilized state is hardly observed, and the activated mechanisms for cyclic hardening or softening depend on test conditions. In general, the selected test conditions (stress or strain control) should depend on the intended use of the obtained material data. If testing conditions do not correspond to the operation mode of the considered mechanical facilities, the above mentioned life prediction models will produce inaccurate results. Hence, selecting and identifying proper fatigue parameters, which would represent the state of a material during the whole fatigue life, is extremely important in reliability evaluation of structures.In the case of non-stabilizing steels, the common challenges are:•Selecting and performing a suitable set of experimental tests to recognize various aspects of the material behavior under low-cycle thermomechanical fatigue;•Adjusting a proper constitutive modelling, reflecting the real physical phenomena taking place in the material microstructure;•Effective numerical implementation and optimal parameter identification.

Modeling of cyclic thermo-elastic-plastic behaviour of P91 steel

Thermomechanical low cycle fatigue behaviour of P91 steel used in power industry applications has been extensively investigated. The constitutive model of Armstrong-Frederick, extended with temperature rate effects, has been applied to describe the behaviour of the thermo-elastic-plastic material. The proposed model has been successfully implemented in simulation of low cycle fatigue of the examined steel in two different temperatures.

New energy-based low cycle fatigue model for reactor steels

Abstract A new low cycle fatigue criterion is presented based on the stored energy, which accumulates in the material’s microstructure during fatigue loading. The new damage parameters are based on the assumption that only the stored part of the introduced energy causes the changes in the microstructure, while the other part does not contribute to the fatigue process. Moreover the dissipated part may depends on the fatigue conditions, especially on the strain rate, which could affect the lifetime prediction accuracy of the applied models. To demonstrate the prediction capability of the proposed model a comprehensive experimental work were conducted on two types of reactor pressure vessel structural material. Investigation on the fraction of plastic work dissipated to heat was carried out to provide information on thermo-mechanical behavior on the tested materials, which can be used as input parameters of the new model. Furthermore isothermal and thermo-mechanical low cycle fatigue tests were performed with in-service loading conditions. The result is higher prediction accuracy than by the classical strain amplitude and strain energy based approaches. The developed model may provide a useful analytical tool for the low cycle fatigue evaluation of reactor components.

An internal variable dependent constitutive cyclic plastic material description including ratcheting calibrated for AISI 316L

The purpose of this work is to provide a reduced complexity constitutive model capable of accurately reproducing the material response of stainless steels under conditions of thermal stress-induced deformation occurring in the primary cooling circuit of a light water nuclear reactor. The proposed approach consists of a Chaboche-type material description, modified to allow the kinematic hardening parameters to vary as a function of a set of internal variables. The methodology implemented to calibrate the model using low cycle fatigue and strain-controlled ratcheting experiments performed on a stainless steel of grade AISI 316L is presented in detail. The improvement provided by the internal variable constitutive model is evaluated by a comparison with simulations carried out by means of an unmodified Chaboche description and with experimental data for strain-controlled, stress-controlled and thermo-mechanical low cycle fatigue tests.

F303 THERMAL STRESS CONCENTRATION FACTOR (TSCF) IN TURBINE CASINGS AND VALVES(Components-1)

An important issue for a fossil power plant is maintaining its reliability and safety against frequent start-ups and load changes. Unstable states arising during start-ups, shut-downs, and load changes give rise to unsteady temperature distribution with respect to time in steam turbine components. Thermal stresses caused by the rapid increase in temperature make the components susceptible to failure and reduce their remaining life. In particular, HP and IP casings undergo frequent cracking due to thermo-mechanical low-cycle fatigue at the nozzle fit corner radius or other stress concentration shapes. Thus, for effective maintenance, it is necessary to have accurate knowledge of transient thermal stresses at the critical position that is susceptible to failure. In this paper, thermal stress concentration factors for the inner surface of the casing and valve are defined to account for variations in the geometry using three-dimensional finite element analysis. In addition, the total strain-range is obtained to assess the fatigue life according to the life assessment procedure in Korea. Using this study, the life consumption of steam turbine inner-casing was obtained, and a guideline for effective maintenance is proposed.