Thermomechanical Fatigue Tests Research Articles

Reversible formation of stacking faults in a nickel-based single crystal TMS-82 superalloy

Abstract

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

Anisotropic TGO morphology and stress distribution in EB-PVD Y2O3-ZrO2 thermal barrier coating after in-phase thermo-mechanical test

An in-phase stress controlled thermo-mechanical fatigue test (IP-TMF) on an electron beam physical-vapor-deposition (EB-PVD) 7-wt.% Y2O3-ZrO2-coated CoNiCrAlY bond coat, called an IN-738LC thermal barrier coating (TBC) system, was carried out. The temperature gradient in the through-thickness direction was achieved. Examination of the system's deformation behavior showed clear creep and superimposed ratcheting behaviors similar to those of the substrate superalloy tested by the same IP-TMF test. The EB-PVD TBC also follows the creep deformation behavior of the substrate. Anisotropic undulation of the thermally grown oxide (TGO) layer was observed in polished section observations. The undulation behavior appeared only perpendicular to the loading direction and was not observed parallel to the loading direction. The average compressive stress distribution of the TGO layer depends on the undulation behavior. The compressive stress parallel to the loading direction is nearly constant and the value agrees with the estimated thermal stress, while the stress in the undulating section is distributed unevenly such that the compressive stress is enhanced at the bottom of concave region.

Large stress concentrations around micropore near a crack-tip induced deformation twinning in Ni-based single crystal superalloy

Thermo-mechanical fatigue (TMF) test was performed in a [0 0 1] oriented single crystal superalloy TMS-75. After TMF tests, detailed microstructural evolution were observed from the interior and outer surfaces of the specimens. The path of the crack initiation and propagation on the failure surface of the ruptured specimen has a concrete representation. It was found that the distribution of deformation twins occurs around the micropores near the crack-tip. Considering the twinning dislocation slip mechanism, the critical tensile stress value 1.34 GPa for twinning in single crystal superalloy was obtained by means of theoretical calculations. Associated with crack propagation in varying degrees, finite element method was performed in an effort to clarify the stress fields around the micropore near the crack-tip region. The finite element method analysis results reveal that the stress in the region between the micropore and crack-tip is larger than the critical tensile stress value to promote the nucleation and propagation of deformation twins. Both theoretical calculations and finite element method analysis results are well consistent with the experimental results. The stress concentration around the micropore near the crack-tip region results in a high density of deformation twins.

Thermo-Mechanical and Isothermal Low-Cycle Fatigue Behavior of Type 316L Stainless Steel in High-Temperature Water and Air

Two unique facilities for isothermal low-cycle fatigue (LCF) and in-phase (IP) or out-of-phase (OP) thermo-mechanical fatigue (TMF) testing of tubular specimens under boiling water reactor (BWR) and pressurized water reactor (PWR) coolant conditions were set up and successfully tested. The systems allow both strain- and stress-controlled fatigue experiments with very small strain amplitudes or complex stress/strain profiles with superimposed rather rapid temperature changes (100°C to 340°C) under flowing conditions. The present article introduces the new test facility and briefly discusses the first results of these experiments. In contrast to air, a strong effect of temperature on LCF and TMF lives was observed in deoxygenated and hydrogenated high-temperature water environment, where both lives decrease with increasing temperature above 100°C. The TMF life is between that of the isothermal LCF tests at minimum and maximum temperature. The IP TMF life is shorter than that of OP TMF and close to the LCF l...

Improvement of high temperature fatigue lifetime in AZ91 magnesium alloy by heat treatment

In the present paper, an improvement in high temperature fatigue properties of the AZ91 magnesium alloy with rare earth elements has been obtained by a typical heat treatment, denoted by T6. For this objective, out-of-phase thermo-mechanical fatigue, room temperature and high temperature low cycle fatigue tests are performed to compare lifetimes. Several rare earth elements are initially added to the AZ91 alloy during a gravity casting process in permanent molds. Also, the type of the heat treatment is examined. Results of specimens with only the solution (the T4 heat treatment) and the solution with the ageing process (the T6 heat treatment) are compared under isothermal fatigue loadings. Microstructural investigations are carried out, before and after fatigue experiments to demonstrate the heat treatment effect. Results showed that both low cycle fatigue and thermo-mechanical fatigue of the alloy at high temperatures increases tremendously after the T6 heat treatment. This behavior attributes to the variation of the ductility, which was a result of microstructural changes during the heat treatment and the varying temperature in fatigue tests.

Crack growth characterisation of A356-T7 aluminum alloy under thermo-mechanical fatigue loading

This paper presents results and interpretations of several long crack thermo-mechanical fatigue tests on A356-T7 cast aluminum alloy used in automotive cylinder heads. These tests were conducted on CT16 and SEN specimens for different positive and negative load ratios, temperatures, frequencies and stress intensity factor values in order to establish the contribution of each parameter governing the fatigue crack propagation. It was shown that the decrease of the frequency causes significant increase of the crack growth rate especially at high temperatures and load ratios. The performing of negative load ratio tests had shown a primordial importance of the compressive load part of fatigue on the crack advance. In addition, SEM, optical microscopy and other metallographic techniques were used to examine fracture surfaces and to analyze particles cracking. For positive load ratios tests, it was shown that in stage II (Paris regime) and stage III (high crack growth rates) the number of cracked particles increases with Kmax and gradual changes in fracture surface appearance occur. These changes were associated with different crack growth mechanisms at the microstructural scale. The amplitude of the inelastic deformations and the extent of the plastic zone ahead of the crack tip were used to explain the observed changes in crack growth mechanisms. The crack growth under negative load ratio loads is especially analyzed, and a way of deriving this rate is given and shown to be relevant even for nonisothermal loading.

Fatigue lifetime of AZ91 magnesium alloy subjected to cyclic thermal and mechanical loadings

In the present paper, thermo-mechanical fatigue (TMF) and low cycle fatigue (LCF) or isothermal fatigue (IF) lifetimes of a cast magnesium alloy (the AZ91 alloy) were studied. In addition to a heat treatment process (T6), several rare elements were added to the alloy to improve the material strength in the first step. Then, the cyclic behavior of the AZ91 was investigated. For this objective, strain-controlled tension–compression fatigue tests were carried out. The temperature varied between 50 and 200°C in the out-of-phase (OP) TMF tests. The constraint factor which was defined as the ratio of the mechanical strain to the thermal strain, was set to 75%, 100% and 125%. For LCF tests, mechanical strain amplitudes of 0.20%, 0.25% and 0.30% were considered at constant temperatures of 25 and 200°C. Experimental fatigue results showed that the cyclic hardening behavior occurred at the room temperature in the AZ91 alloy. At higher temperatures, this alloy had a brittle fracture. But also, it was not significantly clear that the cyclic hardening or the cyclic softening behavior would be occurred in the material. Then, the high temperature LCF lifetime was more than that at the room temperature. The OP-TMF lifetime was the least value in comparison to that of LCF tests. At the end of this article, two energy-based models were applied to predict the fatigue lifetime of this magnesium alloy.

Failure behavior of thermal barrier coatings on cylindrical superalloy tube under thermomechanical fatigue

Failure behavior of thermal barrier coatings on cylindrical superalloy tube was investigated under thermomechanical fatigue (TMF). Two types of TMF tests, i.e. in phase (IP) and out of phase (OP), were performed in the temperature range of 450–850 °C. All tests were carried out under mechanical strain control at a given period of 300 s. The bond coat NiCrA1Y was produced by high velocity oxygen fuel (HVOF), and the top coat 7%Y2O3-ZrO2 was deposited by air plasma spraying (APS). The testing results showed that the OP TMF life was longer than the IP TMF one under the same mechanical strain amplitude. Observations of the fractured specimens revealed that the interface damage and cracking behavior in the two phasing conditions were different. In OP loading, the top coat was cracked and detached from the bond coat while no spallation was found in the IP loading.

Temperature gradients in flat thermomechanical fatigue specimens

This short report presents the investigation on measurement of dynamic thermal gradients of thin flat specimens for thermomechanical fatigue testing. The longitudinal as well as transverse thermal gradients of the gauge section of the flat specimen has been measured using two types of coil configuration in the temperature interval of 500 °C ↔ 850 °C at a heating and cooling rate of 5 °C/s. It eventually transpires from the measured gradients that the transversal electromagnetic field type coil configuration is suitable for carrying out thermomechanical fatigue testing with acceptable thermal gradients.

Experimental fatigue lifetime of coated and uncoated aluminum alloy under isothermal and thermo-mechanical loadings

This paper presents the fatigue lifetime of an aluminum–silicon–magnesium alloy, widely used in diesel engine cylinder heads, both with and without a thermal barrier coating (TBC) system. The coating system in this study consists of two layers including a 150 μm thick metallic bond coat and a zirconium oxide top coat 350 μm thick. These coating layers were applied on the substrate of A356.0 alloy by air plasma thermal spraying. The isothermal fatigue tests were conducted in low cycle fatigue (LCF) regime at various temperatures. Out-of-phase thermo-mechanical fatigue (OP-TMF) tests were also performed at different maximum temperatures and constraint factors. Experimental results demonstrate that at high temperature the coating increases the LCF lifetime of A356.0 alloy at higher strain amplitude and decreases it at lower strain amplitude. At LCF situation, thicker coating has more detrimental effect. However, the coating increases significantly the OP-TMF lifetime. To analyze the failure mechanisms, the fracture surfaces of specimens were investigated by scanning electron microscopy. The fractography of surfaces showed that the failure mechanism of TBC system in coated A356.0 alloy was separations of coating layers in the interface of the substrate and the bond coat layer. This was observed on both high temperature LCF and OP-TMF conditions.

Coupled thermomechanical high cycle fatigue in a single crystal Ni-base superalloy

Thermomechanical fatigue (TMF) is the life-limiting damage phenomenon occurring in gas turbine components. Laboratory TMF tests are traditionally used to simulate the operating conditions inside of the turbine. However, typical LCF laboratory TMF test conditions do not account for the superposition of a HCF waveform which occurs during operation. This work studies the effects of the superposition of a HCF waveform during a high temperature hold occurring in both in-phase and out-of-phase bithermal fatigue TMF tests performed on a single crystal Ni-base superalloy. Results indicate that an atypical inelastic deformation–environmental degradation interaction is caused by the modification of the waveform due primarily to the change in the mean strain during HCF loading. The effect on life was found to be severe under certain conditions. Analysis of the deformation response and the extent of environmental degradation, in the form of γ′ depletion, are presented. An existing physics-based life model (Amaro, R.L., Antolovich, S.D., Neu, R.W. Mechanism-based life model for out-of-phase thermomechanical fatigue in single crystal Ni-base superalloys. Fatigue Fracture Eng Mater Struct; 2012) [1], which explicitly accounts for inelastic–environmental interactions, is used to provide insight to the likely dominant deformation modes.

Thermo-mechanical and low cycle fatigue behaviour of a nickel-base superalloy at elevated temperatures

Isothermal low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests were performed to investigate the deformation behaviour and fatigue life of the nickel-base Superalloy René 80 at elevated temperatures. The LCF test temperature was 950 °C. The TMF tests were performed in the temperature ranges 100–950 °C and 750–950 °C and both out-of-phase (OP) and in-phase (IP) shifts of mechanical and thermal loading. All LCF and TMF tests were conducted with a mechanical strain ratio Rε,me = −1, which allows using the mechanical strain range to compare the material behaviour under LCF and TMF stresses. With respect to the mechanical strain range, the material showed the shortest lifetime under TMF conditions for broad temperature ranges, whereas the TMF tests with narrow temperature ranges resulted in similar lifetimes to those in the LCF tests. Additionally, damage parameters were applied to assess the mean stress development during cycling and the different maximum stresses during IP and OP TMF testing. The Smith–Watson–Topper damage parameter is proposed for the assessment of fatigue life prediction because this parameter provides reasonable trends over the entire lifetime. Nevertheless, LCF and TMF cannot be combined in a single 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.

Thermo-mechanical fatigue tests on MarM-247 nickel-based superalloy using the direct resistance method

Thermo-mechanical fatigue tests were carried out in MarM-247 nickel-based superalloy with a system that uses the direct resistance method for heating, instead of the common induction system. Therefore, this paper has two aims: (1) to validate the test facility via an inter-comparison exercise (carried out at BAM) and (2) to give an overview of the material behaviour based on cyclic response, lifetime and some microstructural aspects. In-phase (IP), out-of-phase (OP) and in-phase test with 2 minutes of dwell period (IP dwell) were executed. Ostergren and Zamrik life prediction approaches were found to be suitable tools for life assessment regardless of the test-type. In fact, some modifications in the mentioned models allow predicting life within a factor of two of the unit correlation line. Apart from that, microstructural investigations reveal that IP tests are prone to intergranular fracture whereas in OP tests trangranular fracture prevails given a clear indication of the prevailing damage mechanisms.

Thermo-mechanical fatigue testing of a rhenium-free single-crystalline super alloy

Due to high temperatures and mechanical loads, cracks are initiated in aero engine turbine blades which limit the cyclic life of these components. The materials used for components which underlie high thermal and mechanical load are single crystalline (SX) nickel based super alloys that in most cases contain a certain amount of rhenium. Dramatically increasing Re prices lead to the development of Re-free alloys.In this work, low-cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) tests were carried out on the Re-free single crystal M-247LC SX. The test results are shown and a model based on crack propagation was used to predict LCF and TMF life. It was shown, that the modeling results fit properly for out-of-phase TMF and LCF life while for in-phase TMF differences between calculated life and experiments occur due to a different mechanism of fracture.

Development of fatigue crack growth testing under thermo-mechanical fatigue conditions

As the need for the prediction of component life and maintenance interval schedules becomes more demanding, there is an increasing requirement for thermo-mechanical fatigue (TMF) test data including fatigue crack growth rates under such conditions. The test equipment requirements to meet this challenge are discussed and finally a conventional servo-electric load frame is utilised in combination with a radiant lamp furnace to generate the desired thermal cycles. The radiant lamp furnace enables reasonably consistent temperature gradients to be achieved with notched test pieces.The temperature calibration method to achieve the desired thermal cycle will be briefly described, along with some considerations for ensuring reproducibility in the thermal cycles applied during tests. The measurement of crack growth under TMF conditions will also be considered. Such measurements are challenging because of the changing thermal conditions and the effect on conventional potential difference (PD) electrical methods. These effects make it difficult to continuously monitor crack size during the thermal/load cycles. Thus, convenient dwells within the TMF cycles, where both the load and temperature are held constant for a brief period, have been utilised to make time-averaged crack size measurements using PD methods. The performance of these experimental methods has been demonstrated with some trials on an advanced nickel base superalloy, RR1000.

Cyclic deformation and lifetime of Alloy 617B during thermo-mechanical fatigue

Different heats of the nickel-base Alloy 617B are tested under in-phase and out-of-phase thermo-mechanical fatigue (TMF) conditions at temperatures between 50 and 900 °C. During one of the TMF tests the growth of microcracks is observed using the replica technique. After the tests, some of the specimens are inspected by scanning electron microscopy in order to analyse the prevailing damage mechanisms compared with those observed in isothermal low-cycle fatigue (LCF) tests. In addition, a Chaboche-type model and a fracture-mechanics-based lifetime model are employed to describe the cyclic viscoplastic deformation and fatigue lifetime. The Chaboche model adjusted to isothermal data is found to reasonably predict the cyclic viscoplastic behaviour of thermo-mechanically loaded specimens. Lifetime data of TMF tests fall into a common scatter band with LCF tests at temperatures above 400 °C if the test results are analysed based on the introduced lifetime model.

Scanning and transmission electron microscopy investigations of defect arrangements in a two-phase γ-TiAl alloy

Different methods of scanning and transmission electron microscopy (SEM, TEM) were applied on a γ-TiAl alloy TNB-V5 after a thermo-mechanical fatigue test. Electron channelling contrast imaging (ECCI) and electron backscattered diffraction were carried out on bulk specimen. In addition, ECCI and scanning transmission electron microscopy in the SEM were carried out on a TEM foil in the electron opaque and the electron transparent region, respectively. The investigations were completed by transmission electron microscopy in the form of standard bright field imaging as well as by taking corresponding diffraction patterns. The results demonstrate in an impressive way that the ECCI technique applied in scanning electron microscopy can successfully supplement or in some cases replace imaging of dislocation arrangements in TEM.

Stress relaxation of compacted graphite iron alloyed with molybdenum

In a previous study, the thermomechanical fatigue resistance of four compacted graphite irons (CGIs) and one grey cast iron was investigated. The molybdenum content of the four CGIs varied between 0 and 1·01 wt-%. It was observed that during thermal cycling, the maximum value of the compressive stress continuously decreased while the value of the maximum tensile stress continuously increased. The continuous decrease in compressive stresses showed that stress relaxation occurs at elevated temperatures during thermal cycling. The goal of the present investigation was to investigate the phenomenon of stress relaxation at elevated temperatures. The tests were performed at 350 and 600°C respectively. The results of the stress relaxation tests performed at 600°C showed the same trend observed at thermomechanical fatigue testing. The tests showed that additions of molybdenum improved the fatigue resistance of CGI by lowering the stress relaxation rate.