Thermal Gradient Mechanical Fatigue Research Articles

Analysis of temperature gradients in thin-walled structures under thermomechanical fatigue loading conditions

In recent thermal gradient mechanical fatigue (TGMF) investigations, it is verified that the material fatigue lives are significantly affected by the temperature gradients. Determination of temperature gradients of thin-walled structures is always a difficult task. In the present work, tubular specimens were experimentally and computationally investigated under different conditions. The inverse heat conduction (IHCP) method was improved to predict the inner boundary from external measurements. Two models were finally developed, based on direct IHCP or modified Nusselt number estimation. The models are qualified to acquire temperature gradients in TGMF experiments with reasonable accuracy and versatility.

Mechanical Fatigue Testing Under Thermal Gradient and Manufacturing Variabilities in Nickel-Based Superalloy Parts with Air-Cooling Holes

BackgroundAs aerospace cast parts move to more complex geometries, the impact of process variabilities must be evaluated more precisely to provide new, less empirical acceptation criteria. However, experimental lifetime assessment with Thermal Gradient Mechanical Fatigue (TGMF) are still complicated to reproduce in a laboratory environment.ObjectiveThis paper investigates the impact of geometrical variability on thermo-mechanical fatigue life of Nickel-based superalloy single crystal parts.MethodsTo this end, hollow specimens with cooling holes drilled in their centre have been produced with parametrized variations of the wall thickness and have been characterized by X-ray tomography. An experimental fatigue setup operating at 1100\(^\circ\)C, with a thermal gradient of 50\(^\circ\)C across the wall, has been developed to assess the life of each specimen.ResultsExperimental lifetime assessment were conducted on a sample of specimens containing geometrical defects. Optical observations during the tests have permitted to study crack initiation and propagation in the vicinity of the cooling hole, an area of high thermal and stress gradients where damage localize. Axial strains (in the loading direction) were also gathered and compared to prove the impact of process variabilities on mechanical behaviour.ConclusionsSignificant mechanical differences are observed during the tests which can related to the geometrical variations of the specimens and their interaction with the shaped hole. Tests results will be further used to calibrate a creep-fatigue model for lifetime predictions.

A method for predicting the delamination life of thermal barrier coatings under thermal gradient mechanical fatigue condition considering degradation characteristics

To establish a method for predicting the delamination life under thermal gradient mechanical fatigue (TGMF) conditions considering thermally grown oxides and sintering behavior, degradation tests were conducted on a thermal barrier coating (TBC) under various degradation conditions. Accordingly, damage factors were defined, and a life prediction method was developed by applying them to the constant term of the strain–life diagram. For validation, a TGMF test was performed on a specimen in which the same TBC was applied to a different type of substrate (CM247LC). The actual and predicted lives were similar, thus confirming the validity of the prediction method.

Assessment of thermo-mechanical fatigue in a nickel-based single-crystal superalloy CMSX-4 accounting for temperature gradient effects

Turbine blades are working under complex thermal gradient mechanical loading conditions. In the present study, a nickel-based single-crystal superalloy CMSX-4 is experimentally and computationally investigated under the thermo-mechanical fatigue (TMF) and thermal gradient mechanical fatigue (TGMF) loading conditions of the in-phase and out-of-phase thermal-mechanical angel. The fatigue life of TGMF with and without the thermal barrier coating was studied. It was confirmed that the lifetime of TGMF is affected by the temperature gradient significantly and substantially differs from conventional isothermal high temperature fatigue and thermo-mechanical fatigue. Since conventional fatigue models fail to characterize the TGMF, a modified model considers temperature gradient effects and thermal-mechanical phase angle and can describe the TGMF lifetime of the single-crystal superalloy reasonably.

Investigation of Thermal Gradient Mechanical Fatigue Test Methods for Nickel-based Superalloys

Temperature gradients significantly affect the material fatigue process. A reliable and robust test procedure is needed for quantifying the effects of temperature gradients on the evolution of fatigue damage in nickel-based superalloys. The present study aims to develop a radiation heating system for universal material testing machine for simulating thermal gradient mechanical fatigue in turbines. The developed heating system mainly consists of halogen lamps, reflectors, cooling subsystems, and control units. Based on extensive experimental and computational investigations, a thermal model is developed for accurate thermo-mechanical fatigue testing under given temperature gradients in the heating system, and it is applied to variable temperature fatigue tests. The detail procedure for determining heat transfer coefficients is presented based on experimental and computational results. TGMF tests are successfully performed with the developed radiation heating system. The temperature gradients are found to reduce the TGMF life significantly in comparison to that of the TMF life. It is confirmed that the developed thermal gradient mechanical fatigue methodology can be applied to different thermo-mechanical fatigue tests with various temperature gradients.

Thermal gradient mechanical fatigue assessment of a nickel-based superalloy

In the present work, a radiation heating system was developed to simulate thermal gradient mechanical fatigue (TGMF) loads in turbines. The specimen is externally heated by radiation and internally cooled by compressed air. Experiments showed that the TGMF life of the nickel-based superalloy is significantly shorter than that of the thermo-mechanical and the isothermal fatigue, although the thermal stress is small. It was confirmed that the conventional fatigue models generated seriously deviations and could not catch effects of the thermal gradients. The modified TGMF model introduced a correction term of the temperature gradient effects and can describe the TGMF lifetime of Inconel 718 reasonably. The new model provides a uniform description of isothermal and complex thermo-mechanical fatigue.

Thermal Gradient Mechanical Fatigue Assessment of a Nickel-Based Superalloy

Turbine components generally work under thermomechanical loading conditions with varying temperature and significant temperature gradients. In the present work, a radiation heating system was developed to simulate thermal gradient mechanical fatigue (TGMF) loads in turbines. The specimen is externally heated by radiation and internally cooled by compressed air. Experiments showed that the TGMF life of the nickel-based superalloy is significantly shorter than that of the thermomechanical and the isothermal fatigue, although the thermal stress amplitude related to the temperature gradient is small. It was confirmed that the conventional fatigue models generated seriously deviations and could not catch effects of thermal gradients. The modified TGMF model introduced a correction term of the temperature gradient effects and can describe the TGMF lifetime of Inconel 718 reasonably. The new model provides a uniform description of isothermal and complex thermomechanical fatigue.

Mechanical Properties of Layered La2Zr2O7 Thermal Barrier Coatings

Lanthanum zirconate (La2Zr2O7) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La2Zr2O7 and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La2Zr2O7 + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La2Zr2O7 TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La2Zr2O7. The erosion rate of La2Zr2O7 TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La2Zr2O7. DCL porous 8YSZ + La2Zr2O7 had a lower erosion rate than other SCL and DCL La2Zr2O7 coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.

Thermomechanical and Thermal Gradient Mechanical Fatigue Lifetime of Thermal Barrier Coating Systems

Detailed damage analyses of an Y2O3 stabilized ZrO2 top coat (TC)–MCrAlY bond coat (BC)–superalloy thermal barrier coating (TBC) system during thermomechanical fatigue (TMF) and thermal gradient mechanical fatigue (TGMF) tests had been performed in present work. During tests, the lifetime of TBCs was strongly dependent on the strain ranges, pre-oxidation time and the thermal gradient in TBCs. Cracks were initiated in the TGO layer, propagated along the TC/TGO or TGO/BC interface, forming the delamination cracks. When the delamination cracks connected with the segmentation cracks which were initiated and propagated in TC, the TBCs spalled. The failure mechanism and stress were analyzed, which were significantly helpful to establish the TMF/TGMF lifetime prediction model for the TBCs.

The Thermal Gradient Mechanical Fatigue Behavior of Nickel-based Superalloy with Thermal Barrier Coatings

The Thermal Gradient Mechanical Fatigue Behavior of Nickel-based Superalloy with Thermal Barrier Coatings

Damage mechanisms in an EB-PVD thermal barrier coating system during TMF and TGMF testing conditions under combustion environment

Modern high pressure turbine blades and vanes of aero-engines are cooled and thermally protected using a thermal barrier coating (TBC) system. TBC systems are submitted to very complex service loadings, with a combination of mechanical and thermal fatigue under complex thermal gradients, in addition to the contribution of oxidation to the damage processes.The high temperature durability under combustion environment of the AM1/NiAlPt/EB-PVD TBC system used for high temperature blades and vanes has been investigated performing out of phase load-controlled thermo-mechanical fatigue (TMF) and thermal-gradient-mechanical fatigue (TGMF) tests in the 500–1100°C temperature range. This characterization has been performed using the MAATRE burner where massive or hollow samples can be mechanically loaded under hot gas flow conditions. This unique test rig allows for gas testing temperatures up to 1600°C and internal cooling of hollow samples.The damage mechanisms under different TMF conditions are presented and the impact of a through thickness thermal gradient (induced by means of internal cooling) is analyzed. Moreover, a prior thermal over-aging of the whole system is investigated to mimic service degradation. It is shown in this work that depending on the tensile stress applied at low temperature, a change in the main degradation mechanism of the TBC system is observed. Moreover, the thermal over-aging before TMF tests favors cycling ratcheting in compression during thermo-mechanical cycling due to the degradation of the substrate microstructure.

Evaluation of thermal gradient mechanical fatigue characteristics of thermal barrier coating, considering the effects of thermally grown oxide

Gas turbines of aircraft and power generation plants operate at a high turbine inlet temperature (TIT) or combustion temperature higher than 1,000°C to obtain high thermal efficiency. The parts directly in contact with the high temperature flame are made of a nickel-base superalloy, which has strong heat resistance. In addition, the thermal barrier coating (TBC) technique is applied to increase the heat resistance. The TBC prevents direct heat transfer from the high temperature flame to the metallic substrate. Thus, the TBC technique reduces the substrate surface temperature by approximately 100~170°C. The delamination caused by the growth of thermally grown oxide (TGO) decreases the life of the TBC system. In addition, gas turbine blades experience centrifugal forces owing to high speeds of approximately 3600 rpm and they experience low cycle fatigue because of frequent startup and shutdown. Therefore, the integrity of the TBC system should be evaluated under the thermal gradient mechanical fatigue condition. In this study, finite element analysis (FEA) was performed for the TBC model considering TGO, and the effect of TGO on TBC systems was evaluated under the thermal gradient mechanical fatigue condition.

Synchrotron X-Ray Diffraction Measurements Mapping Internal Strains of Thermal Barrier Coatings During Thermal Gradient Mechanical Fatigue Loading

An understanding of the high temperature mechanics experienced in thermal barrier coatings (TBC) during cycling conditions would be highly beneficial to extending the lifespan of the coatings. This study will present results obtained using synchrotron X-rays to measure depth resolved strains in the various layers of TBCs under thermal mechanical loading and a superposed thermal gradient. Tubular specimens, coated with yttria stabilized zirconia (YSZ) and an aluminum containing nickel alloy as a bond coat both through electron beam-physical vapor deposition (EB-PVD), were subjected to external heating and controlled internal cooling generating a thermal gradient across the specimen's wall. Temperatures at the external surface were in excess of 1000 °C. Throughout high temperature testing, 2D high-resolution XRD strain measurements are taken at various locations through the entire depth of the coating layers. Across the YSZ, a strain gradient was observed showing higher compressive strain at the interface to the bond coat than toward the surface. This behavior can be attributed to the specific microstructure of the EB-PVD-coating, which reveals higher porosity at the outer surface than at the interface to the bond coat, resulting in a lower in plane modulus near the surface. This location at the interface displays the most significant variation due to applied load at room temperature with this effect diminishing at elevated uniform temperatures. During thermal cycling with a thermal gradient and mechanical loading, the bond coat strain moves from a highly tensile state at room temperature to an initially compressive state at high temperature before relaxing to zero during the high temperature hold. The results of these experiments give insight into previously unseen material behavior at high temperature, which can be used to develop an increased understanding of various failure modes and their causes.

Determining a critical strain for APS thermal barrier coatings under service relevant loading conditions

Despite the huge progress made in recent years in analysing the degradation behavior and the reliability of thermal barrier coating systems, there is still some deficit in the capability to predict damage evolution in terms of crack initiation and crack growth, which ultimately leads to macroscopic delamination and spallation of the coating system. In order to obtain this prediction capability, a fundamental understanding of the damage evolution processes under isothermal, thermo-cyclic and under thermo-mechanical loading conditions has to be developed.The aim of the presented work is to determine the critical strain, i.e. the strain at which cracking initiates, and to analyse the evolution of a network of cracks for widely used atmospheric plasma sprayed (APS) thermal barrier coating (TBC) systems. The TBC system has been exposed in our study to service relevant loading conditions, namely to thermal gradient mechanical fatigue (TGMF). TGMF tests for in-phase as well as out-of-phase loading cycles were performed on hollow cylindrical specimens made of the single crystal super alloy CMSX-4, loaded mechanically in 〈001〉 orientation, and being coated with a duplex system comprised of a CoNiCrAlY bond coat and a 8wt.% Yttria partially stabilized Zirconia (YSZ) TBC. The CoNiCrAlY bond coat was deposited by Low Pressure Plasma Spraying (LPPS), while the ceramic top coat was deposited using the APS process. The loading cycles were chosen to represent an industrial gas turbine engine. Critical strains measured for delamination (within the ceramic coating or at the CoNiCrAlY – TBC interface) and through cracking, i.e. segmentation of the ceramic top coat was determined using a special compression test equipped with in situ acoustic emission technique. The mechanical testing was performed at room temperature after TGMF exposure. In order to study the impact of thermally grown oxide (TGO), specimens have been TGMF tested in the “as received” conditions as well as after isothermal aging (up to 3000h at 1000°C). To correlate the signal obtained by acoustic emission (AE) with the evolution of (micro-) cracks, the specimens have been carefully sectioned and investigated by standard metallographic means.The measured critical strains are used as a data basis for a strain-based lifetime model developed for isothermal and cyclic oxidation as well as thermo-mechanical loading. The lifetime model considers two failure modes, namely delamination and (vertical) through cracking.Metallographically obtained crack patterns within the TBC system have been incorporated into finite element models to quantify stress–relaxation as a consequence of damage evolution in the TBC system.The observations show that thermal gradient fatigue loading under in-phase loading leads to a shorter lifetime compared to out-of-phase loading.For the delamination mode, the critical strain values of the model are in good agreement with the experimental data of the TGMF experiments. The modeled critical strain for through cracking, on the other hand, is consistently lower than the experimentally determined failure strains, implying that the model describes the failure situation in a conservative manner.

On TGO creep and the initiation of a class of fatigue cracks in thermal barrier coatings

The initiation of a class of fatigue cracks observed in thermal barrier coatings (TBCs) subjected to thermal gradient mechanical fatigue testing is investigated. The coating system is based on a NiCoCrAlY bond coat and a partially yttria stabilized zirconia top coat. To explain the development of the cracks of interest, the thermo-mechanical response of the bond coat and the thermally grown oxide (TGO) is examined and quantified through finite element analyses. The models include non-linear and time-dependent behavior such as creep, TGO growth stress, and thermo-mechanical cyclic loading. The simulations suggest that stress-redistribution due to creep can lead to tensile stresses in the TGO during TGMF testing that are large enough to initiate the cracks investigated.

Advanced thermal gradient mechanical fatigue testing of CMSX-4 with an oxidation protection coating

Frequently, turbine blades are cooled internally, which generates thermal gradients over the blade wall and, consequently, multi-axial stresses in addition to stresses due to centrifugal forces. In order to study these conditions, a new thermal gradient mechanical fatigue (TGMF) testing equipment with a lamp furnace has been developed. The advantages of this furnace are high power (16 lamps a 1000 W), controlled thermal gradients, high heating and cooling rates, and lamp lives exceeding 8000 thermal cycles. The studied material system was the single-crystalline superalloy CMSX-4 with a NiPtAl oxidation protection coating. Different TGMF tests with a maximal surface temperature of 1050 °C, mechanical loads up to 400 MPa, and cycle numbers up to 9000 resulted in microstructural changes and defects, reflecting in each case the particular temperature, thermal gradient, and local stresses. For example, phase evolution of the metal coatings and rafting of the γ/γ′ substrate morphology was investigated. Furthermore, cracks at the inner specimen surface and at substrate pores were detected. The observed morphology and defects were related to the applied thermomechanical loads using finite element calculations.