Thermomechanical Fatigue Loading Research Articles

Stress analysis of thermal barrier coating system subjected to out-of-phase thermo-mechanical loadings considering roughness and porosity effect

This paper presents the out-of-phase thermo-mechanical stress analysis of thermal barrier coating (TBC) system in real working conditions used as thermal barrier in diesel engine cylinder heads. The coating system in this research comprises 350μm zirconium oxide top coat (TC) and 150μm metallic bond coat (BC). These layers were deposited on the substrate, aluminum A356 alloy, by the aid of air plasma spray (APS) method. Afterwards, the specimen was subjected to thermo-mechanical fatigue (TMF) loadings. Based on the experimental conditions, FE simulations were performed by both time-independent and time-dependent substrate material properties in ABAQUS software. Simulation results related to heat transfer analysis demonstrate only about 10.5% comparative error compared to experimental results. Moreover, defining time-dependent properties, which were obtained from two-layer visco-plastic model, yields results with 15% less comparative error in comparison to the results based on time-independent material properties. In addition, the effects of roughness and porosity in coating layers and substrate were studied on three different models by the aid of a scanning electron microscopy image. Obtained results based on real geometry illustrate that consideration of porosity in TC layer has an effective role in the stress distribution of this layer. However, BC layer stress distribution is much more dependent on interface morphology.

Non-invasive temperature measurement and control techniques under thermomechanical fatigue loading

A non-invasive temperature measurement, control and profiling technique has been investigated for use with thermomechanical fatigue loading. The technique utilises an infrared thermography camera and Rolls–Royce developed thermal paint to control and monitor cyclic temperature. Thermal paint is used to maintain a stable surface emissivity upon the test piece. The accuracy of the technique is compared against type N thermocouples and a pyrometer for both temperature control and monitoring purposes. Diverse test specimen geometries and alloy compositions are used over a 100–700°C temperature range. Effects on temperature measurement accuracy such as thermocouple shadowing are highlighted and quantified. The non-invasive technique has proved accurate to within ±2°C of the reference thermocouples when in combination with the thermal paint coating.

Influence of Control Mode on Thermomechanical Fatigue Testing of Circumferentially-Notched Specimens

Abstract The aim of this study is to understand the influence of experimentally applied boundary conditions on circumferentially-notched specimens under thermomechanical fatigue loadings (TMF) in two Ni-base superalloys, one equiaxed polycrystalline, and one directionally-solidified. TMF experiments were performed under both force-controlled and displacement-controlled conditions. Notch severities ranged from kt = 1.3 to kt = 2. The notch response was evaluated by finite element analysis using temperature-dependent viscoplasticity. The experiments and analyses are used to assess the impact of the applied boundary conditions on calculated and observed fatigue crack initiation lives.

Electron back scattered diffraction characterization of thermomechanical fatigue crack propagation of a near α titanium alloy Timetal 834

Fatigue crack growth (FCG) mechanisms have been studied in light of the interaction of a propagating crack with local crystallographic orientations of primary alpha (αP) and secondary alpha (αS) colonies in a near α Timetal 834 Ti-alloy under thermomechanical fatigue (TMF) loading using electron backscattered diffraction (EBSD). FCG testing at in-phase (IP) and out-of-phase (OP) TMF loading have been carried out at two temperature intervals 300°C↔450°C and 450°C↔600°C. EBSD analysis and microhardness measurements have confirmed that larger cyclic plastic zone size at the crack tip leads to lower crack propagation under IP-TMFCG loading as compared to OP-TMFCG loading. EBSD analysis has also confirmed that crack dissipates more energy to propagate when it passes from a soft grain oriented with its c-axis normal to the loading direction and encounters a hard grain with its c-axis parallel with the loading direction.

Lifetime Prediction of EN-GJV 450 Cast Iron Cylinder Heads under Combined Thermo-Mechanical and High Cycle Fatigue Loading

Lifetime Prediction of EN-GJV 450 Cast Iron Cylinder Heads under Combined Thermo-Mechanical and High Cycle Fatigue Loading

Stress–strain time-dependent behavior of A356.0 aluminum alloy subjected to cyclic thermal and mechanical loadings

This article presents the cyclic behavior of the A356.0 aluminum alloy under low-cycle fatigue (or isothermal) and thermo-mechanical fatigue loadings. Since the thermo-mechanical fatigue (TMF) test is time consuming and has high costs in comparison to low-cycle fatigue (LCF) tests, the purpose of this research is to use LCF test results to predict the TMF behavior of the material. A time-independent model, considering the combined nonlinear isotropic/kinematic hardening law, was used to predict the TMF behavior of the material. Material constants of this model were calibrated based on room-temperature and high-temperature low-cycle fatigue tests. The nonlinear isotropic/kinematic hardening law could accurately estimate the stress–strain hysteresis loop for the LCF condition; however, for the out-of-phase TMF, the condition could not predict properly the stress value due to the strain rate effect. Therefore, a two-layer visco-plastic model and also the Johnson–Cook law were applied to improve the estimation of the stress–strain hysteresis loop. Related finite element results based on the two-layer visco-plastic model demonstrated a good agreement with experimental TMF data of the A356.0 alloy.

Influence of notch severity on thermomechanical fatigue life of a directionally solidified Ni‐base superalloy

ABSTRACTThe aim of this work is to understand the influence of notches under thermomechanical fatigue (TMF) in a directionally solidified Ni‐base superalloy. Experiments were performed utilizing linear out‐of‐phase and in‐phase TMF loadings on longitudinally oriented smooth and cylindrically notched specimens. Several notch severities were considered with elastic stress concentrations ranging from 1.3 to 3.0. The local response of the notched specimens was determined using the finite element method with a transversely isotropic viscoplastic constitutive model. Comparing the analysis to experiments, the locations observed for crack nucleation in the notch, which are offset from the notch root in directionally solidified alloys, are consistent with the maximum von Mises stress. Various local and nonlocal methods are evaluated to understand the life trends under out‐of‐phase TMF. The results show that a nonlocal invariant area‐averaging method is the best approach for collapsing the TMF lives of specimens with different notch severities.

A constitutive model for inelastic behavior of casting materials under thermo-mechanical loading

High-temperature components, for example turbochargers, are often subject to complex thermal and mechanical loading paths. Non-uniform temperature distribution and constraints by neighboring components result in complex timely varying stress and strain states during operation. The aim of this paper is to analyze inelastic behavior of a casting material Ni-resist D-5S in a wide stress, strain rate and temperature ranges. The material model including a constitutive equation for the inelastic strain rate tensor and a non-linear kinematic hardening rule is discussed. To calibrate the model, experimental databases from creep and low cycle fatigue tests are generated. They include creep curves for temperatures within the range 600–800 °C and stress levels from 10 to 150 MPa. The low cycle fatigue data collect a family of hysteresis loops for the strain rate of 10−3 1/s, the strain amplitude from 0.4% to 2% and temperature levels within the range 200–800 °C. For the verification of the model, simulations of the material behavior under uniaxial thermo-mechanical fatigue loading conditions are performed. The results for the stress response are compared with experimental data.

Dissipated energy-based fatigue lifetime calculation under multiaxial plastic thermo-mechanical loading

The paper concerns the metallic material lifetime calculation under arbitrary multiaxial thermo-mechanical fatigue loading. An energy-based damage operator approach ([Formula: see text]) is introduced. In such an approach, fatigue lifetime is continuously computed by the Prandtl type damage operator. The dissipated energy of plastic deformation is applied as a damage parameter that is obtained at any moment by the recently developed multiaxial energy operator approach. Strain-life curves are transformed into energy-cycle damage curves in order to associate the damage parameter with the fatigue lifetime. The present approach is validated on turbocharger housing. A satisfactory evaluation of the fatigue lifetime is obtained even though visco-plasticity and mean stress correction are not addressed yet.

Lifetime, Cyclic Deformation and Damage Behaviour of MAR-M247 LC under Thermo-Mechanical Fatigue Loading with 0°, 180°, -90° and +90° Phase Shift between Strain and Temperature

Thermo-mechanical fatigue tests under 0° (in-phase), +180° (out-of-phase), +90° (clockwise diamond) and -90° (counterclockwise diamond) phase shift between mechanical loading and temperature were conducted on nickel-based superalloy Mar-M247 LC. Tests were carried out under total strain control with a temperature range of 100 – 850°C and a heating and cooling rate of 5K/s. Mechanical strain amplitudes were 0.3 to 0.6% with a strain ratio of Rɛ = -1. Results show, that for the strain amplitudes tested, lifetime varies significantly with strain-temperature phase. In-phase loading gives the shortest lives followed by out-of-phase and the diamond-phase loadings which show comparable lifetimes. Metallographic examination indicates that the dominant damage mechanisms are creep damage at higher temperatures and early cracking of oxide layers at lower temperatures. Both mechanisms occur primarily under tensile stress. Hence, the portion of each mechanism varies with the phase angle.

Thermo-mechanical behaviours of light alloys in comparison to high temperature isothermal behaviours

In this article, out-of-phase thermo-mechanical fatigue (TMF) behaviours of light alloys were investigated in comparison to their high temperature low cycle fatigue (LCF) behaviours. For this objective, strain based fatigue tests were performed on the A356 aluminium alloy and on the AZ91 magnesium alloy. Besides, TMF tests were carried out, where both strain and temperature changed. The fatigue lifetime comparison demonstrated that the TMF lifetime was less than that one under LCF loadings at elevated temperatures for both light alloys. The reason was due to severe conditions in TMF tests in comparison to LCF tests. The temperature varied in TMF test but it was constant under LCF loadings. As the other reason, the tensile mean stress occurred under TMF loadings, in comparison to the compressive mean stress under LCF loadings. At high temperatures, the cyclic hardening behaviour occurred in the AZ91 alloy and the A356 alloy had the cyclic softening behaviour.

Lifing the thermo-mechanical fatigue (TMF) behaviour of the polycrystalline nickel-based superalloy RR1000

Microstructural damage and subsequent failures resulting from thermo-mechanical fatigue (TMF) loading within the temperature range 300–700 ∘ C are investigated for the polycrystalline nickel superalloy, RR1000. Strain controlled TMF experiments were conducted over various mechanical strain ranges, encompassing assorted phase angles, using hollow cylindrical test pieces. The paper explores two scenarios; the first where the mechanical strain range is held constant and comparisons of the fatigue life are made for different phase angle tests, and secondly, the difference between the behaviour of In-phase (IP) and − 180 ∘ Out-Of-Phase (OOP) tests over a variety of applied strain ranges. It is shown that different lifing approaches are currently required for the two scenarios, with a mean stress based approach being more applicable in the first case, whereas a Basquin-type model proves more appropriate in the second.

Multiaxial cyclic viscoplasticity model for high temperature fatigue of P91 steel

This paper presents a novel multiaxial, cyclic viscoplasticity material model for high temperature low cycle fatigue of P91 power plant steel. The model incorporates mechanisms-based variable strain-rate sensitivity and the key high temperature cyclic deformation phenomena of cyclic softening and non-linear kinematic hardening. The model has been calibrated to accurately represent the cyclic high temperature constitutive behaviour of ‘as received’ P91 steel. Details on the material Jacobian, with the consistent tangent stiffness for finite element implementation, are presented. The multiaxial implementation is applied to a notched specimen under strain-controlled loading at 600°C and a thin walled pipe under representative pressurised thermomechanical fatigue loading conditions. It is shown that the model for variable strain-rate sensitivity of the present paper predicts significantly different Coffin–Manson notch fatigue life compared to the Chaboche power law model. Ratchetting is shown to be a key candidate failure mechanism for next generation thermomechanical power plant loading conditions, for thin walled pressurised pipes.

Evaluation on Thermo-Mechanical Fatigue Life of IN738LC Using Finite Element Analysis

In this paper, thermo-mechanical fatigue tests were performed for the nickel-based super alloy IN738LC, after which the thermo-mechanical fatigue life was evaluated using finite element analysis. Nickel-based super alloy is used as the main material of turbine blades, which are important equipment in thermal power generation plants. In general, such materials receive three types of damage under thermo-mechanical fatigue loading. In the case of low-cycle fatigue behavior in which large plastic deformation mainly occurs, the lifetime can be decided by its relationship with the plastic strain amplitude. In order to obtain the plastic strain amplitude from the measured strain amplitude, a hysteresis loop should be derived. However, low-cycle fatigue tests are difficult. Moreover, precise experimental techniques are required to obtain the hysteresis loops. In this study, after thermo-mechanical fatigue tests were performed, thermal mechanical fatigue tests on IN738LC were simulated using finite element analysis. The results of analysis were verified by comparing with the hysteresis loops of an experiment

Numerical simulations of cyclic behaviors in light alloys under isothermal and thermo-mechanical fatigue loadings

In this article, numerical simulations of cyclic behaviors in light alloys are conducted under isothermal and thermo-mechanical fatigue loadings. For this purpose, an aluminum alloy (A356) which is widely used in cylinder heads and a magnesium alloy (AZ91) which can be applicable in cylinder heads are considered to study their stress–strain hysteresis loops. Two plasticity approaches including the Chaboche’s hardening model and the Nagode’s spring-slider model are applied to simulate cyclic behaviors. To validate obtained results, strain-controlled fatigue tests are performed under low cycle and thermo-mechanical fatigue loadings. Numerical results demonstrate a good agreement with experimental data at the mid-life cycle of fatigue tests in light alloys. Calibrated material constants based on low cycle fatigue tests at various temperatures are applied to models to estimate the thermo-mechanical behavior of light alloys. The reason is to reduce costs and the testing time by performing isothermal fatigue experiments at higher strain rates.

Propagation Behavior of Naturally Initiated Small Crack in Austenitic Heat Resistant Steel under Thermo-Mechanical Fatigue Loading

In this paper, the crack propagation behavior of naturally initiated small crack in in low-carbon/medium-nitrogen 316 stainless steel under thermo-mechanical fatigue loading was investigated. The experimental results indicated that the importance of the investigation of small crack propagation behavior because the information on the basis of physically long crack growth rate provides a dangerous evaluation on reliability to actual components. The small crack exhibits high growth rate under the In-phase TMF loading because of irreversible creep and plastic strains. However, the growth rates of small crack under the Out-of-phase TMF loading were lower because the effect of creep deformation became negligible in such condition.

Two lifetime estimation models for steam turbine components under thermomechanical creep–fatigue loading

The flexibility of steam turbine components is currently a key issue in terms of the fluctuations in the power supply due to regenerative energy. Conventional steam power plants must run at varying utilization levels. Life estimation methods according to standards, e.g. ASME Code N47 and TR, assess the influences of creep and fatigue separately under the assumption of isothermal conditions at the maximum operating temperature. The influence of thermomechanical fatigue (TMF) loading still requires a significant number of experimental studies. Further, the interaction of creep and fatigue is not adequately taken into account. Thus, new lifetime estimation methods are required for the monitoring, re-engineering and new design of power plant components. In this paper, both a phenomenological and a constitutive crack initiation lifetime estimation model for steam turbine components are introduced. The effectiveness of each method is shown by recalculation of uniaxial as well as multiaxial service-type creep–fatigue experiments on high-chromium 10%Cr stainless rotor steel. Finally, the two models are compared with respect to different aspects, such as the type and number of necessary experiments to determine model parameters, the prerequisite for the application and the limitations of each model.

Thermo-mechanical fatigue of a wrought nickel based alloy

The nickel-based alloy Haynes 230 was investigated under low-cycle fatigue (LCF) and thermo-mechanical fatigue loading at different temperatures and with different phase shift. Whereas the yield strength is low, causing high plastic strains in every cycle, appreciable cyclic hardening up to double the yield strength was observed. The crack propagation rates were evaluated at different temperatures, with different mean stress and a standard waveform. A dwell signal was also tested. The crack propagation was modelled assuming a plasticity controlled fatigue part, which was correlated to the crack tip opening displacement by modified cyclic J-integral. At higher temperatures, an additional thermally activated oxide embrittlement increased the crack growth rate. Both mechanisms were considered separately in the model as they work independently. This model enabled us to describe the experimental growth rate, but cannot account for the threshold effects. As the cyclic life was found to be controlled by crack propagation starting from carbides or oxide spikes, life was predicted based on crack propagation. The results compare well for LCF within a scatterband of factor 2 at the lower cycles to failure. For higher cycle numbers, the measured lives were higher, which was assumed to be caused by the lack of a crack growth threshold in the model.

Influence of protective coatings on damage mechanisms and lifetime of Alloy 247 DS in thermo-mechanical fatigue tests

The degradation process and lifetime of the directionally solidified Ni-base superalloy Alloy 247 DS was investigated under out-of-phase thermo-mechanical fatigue (OP-TMF) loading in relation to the properties and microstructure of oxidation protection coatings. The influence of two coating systems applied by low pressure plasma spraying (i) MCrAlY coating and (ii) duplex-coating consisting of the same MCrAlY coating and a NiAl-topcoat was studied.The MCrAlY coating did not cause significant changes in failure behaviour and TMF life of the superalloy, whereas duplex coating reduced TMF life significantly because of accelerated fatigue crack initiation and propagation into the substrate material. Due to much lower fracture strains of the duplex coating than of MCrAlY coating the crack formation in the duplex coatings was already observed at very early stages of lifetime. Furthermore, crack propagation was found to be significantly affected by crack branching at the interface between coating and substrate alloy.

Influence of protective coatings on damage mechanisms and lifetime of Alloy 247 DS in thermo-mechanical fatigue tests

The degradation process and lifetime of the directionally solidified Ni-base superalloy Alloy 247 DS was investigated under out-of-phase thermo-mechanical fatigue (OP-TMF) loading in relation to the properties and microstructure of oxidation protection coatings. The influence of two coating systems applied by low pressure plasma spraying (i) MCrAlY coating and (ii) duplex-coating consisting of the same MCrAlY coating and a NiAl-topcoat was studied.The MCrAlY coating did not cause significant changes in failure behaviour and TMF life of the superalloy, whereas duplex coating reduced TMF life significantly because of accelerated fatigue crack initiation and propagation into the substrate material. Due to much lower fracture strains of the duplex coating than of MCrAlY coating the crack formation in the duplex coatings was already observed at very early stages of lifetime. Furthermore, crack propagation was found to be significantly affected by crack branching at the interface between coating and substrate alloy.