Thermomechanical Fatigue Tests Research Articles

액체로켓엔진 연소기 열차폐코팅 내구성 시험 기술동향

액체로켓 연소기에 사용되는 열차폐코팅(TBC)의 내구성 시험 기술동향을 조사하였다. 표면 접합력 측정을 위한 기계적 시험, 레이저나 가열로, 버너나 플라즈마 등을 이용한 열피로 시험, 분사기를 이용한 소형 연소시험, 열적 기계적 피로시험 등의 많은 내구성 시험들이 있었다. 연소기에 사용하기 위해 이러한 시편 수준의 시험을 통해 내구성이 확보된 TBC를 결정할 수 있으며, 실제 연소시험을 통해 내구성을 검증할 수 있다. Durability testing method trends of the thermal barrier coating(TBC) for the combustion chamber of the liquid-propellant rocket engine have been investigated. Many types of the durability testing method such as the mechanical tests to measure surface cohesion force, the thermal fatigue tests with laser, furnace, burner or plasma, the small scale combustion tests using injectors, and the thermo-mechanical fatigue tests were observed. The TBC with sufficient durability can be selected for the use of combustion chamber through such specimen-level tests and the durability can be verified by the tests using the real scale combustion chambers.

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.

A Study on Fatigue Life Prediction of Ni-base Superalloy

A gas turbine blade rotates at high speed in a high-temperature flame, degradation environment under mechanical loading. It is also exposed to a thermo-mechanical fatigue environment caused by frequent starts and shutdowns. Therefore, the life of the blade material should be evaluated considering such various factors.In this study, the low cycle fatigue(LCF) and thermo-mechanical fatigue(TMF) tests were carried out by using a furnace, which directly transfer heat flux, for the life prediction of a Ni-base superalloy. LCF tests were conducted over a wide range of strain conditions considering the gas turbine operating condition. Also, IP (in-phase) and OP (out of-phase) TMF tests were conducted over a wide range of strain conditions. Finally, a fatigue life prediction model of the Ni-base superalloy was obtained from the LCF and TMF test results. And also, a comparison between the LCF and TMF test results was evaluated with respect to fatigue life.

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

AbstractThermo-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 mech...

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.

Microstructural Analysis of TMF Failure Mechanism of GTD-111 Applied to Gas Turbine Blades

Abstract Ni-based superalloys display excellent mechanical properties under high temperature conditions. Thermal barrier coating(TBC) technique protects the substrate from high temperatures and is, therefore, widely applied to industrial gas turbines and aircraft engines to enable them to endure severe enviornments. Thermo-mechanical fatigue(TMF) testing is a widely used method to evaluate the material properties and behaviors under the start-up and shut-down conditions of a gas turbine. In this study, to evaluate the microstructural failure mechanisms of Ni-based superalloys such as corrosion and metal diffusion at the surface and between coating layers, microstructural analysis was performed after TMF tests on specimens of GTD-111 applied to the 1st blades of industrial gas turbines. Also, the effect of TBC on the failure mechanism of GTD-111 was evaluated based on microstructural analysis of TMF tested specimens.

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.

Thermomechanical Fatigue Behaviour of a Modified 9Cr-1Mo Ferritic-Martensitic Steel

Thermomechanical fatigue (TMF) tests were carried out on a modified 9Cr-1Mo ferritic steel (P91) under a mechanical strain control mode using a strain amplitude of ±0.4% and a strain rate of 1.2×10−4 s−1. In-Phase (IP) and Out-of-Phase (OP) strain-time waveforms were employed for the tests which were performed under different temperature ranges in the interval, 573-923 K. For the sake of comparison, isothermal LCF (designated as IF) tests were also carried out at the maximum temperatures (Tmax) of TMF cycles on similar specimens and using the same strain amplitude and strain rate. Isothermal cycling was observed to be the most detrimental while IP TMF yielded the highest lives. However, with an increase in the Tmax of TMF cycling, the difference in lives was seen to narrow down. Also, lives under IP TMF and IF cycling were seen to reduce more drastically compared to OP cycling on account of a greater creep damage accumulation. A continuous cyclic softening characterized the stress response of the alloy under all testing conditions. The lower lives observed under OP cycling were rationalised in terms of oxidation damage and mean stress development. The observed behaviour was explained on the basis of detailed TEM investigations.

Influence of molybdenum alloying on thermomechanical fatigue life of compacted graphite irons

In this study four compacted graphite irons (CGIs) and one grey cast iron (FGI) were produced and tested in the laboratory. The molybdenum content of the four CGI grades was varied between 0 and 1··01 wt-%. The purpose of the investigations was to examine the effect of the different molybdenum contents of the CGI on the thermomechanical fatigue (TMF) behaviour. The TMF tests were performed by cycling a constrained specimen between 110 and 600°C. For every material three tests were performed on specimens machined from a ∅20 mm cylinder. Other tests were performed on specimens machined from ∅55 mm and ∅85 mm cylinders respectively. The tests showed that additions of molybdenum improved the fatigue resistance of CGI. It was observed that additions of molybdenum refined the pearlite and that the specimens with a finer metallic matrix had a higher TMF resistance.

Thermo-mechanical fatigue properties of a ferritic stainless steel for solid oxide fuel cell interconnect

Thermo-mechanical fatigue (TMF) behavior of a newly developed ferritic stainless steel (Crofer 22 H) for planar solid oxide fuel cell (pSOFC) interconnect is investigated. TMF tests under various combinations of cyclic mechanical and thermal loadings are conducted in air at a temperature range of 25oC–800 °C. Experimental results show the number of cycles to failure for non-hold-time TMF loading is decreased with an increase in the minimum stress applied at 800 °C. There is very little effect of maximum stress applied at 25 °C on the number of cycles to failure. The non-hold-time TMF life is dominated by a fatigue mechanism involving cyclic high-temperature softening plastic deformation. A hold-time of 100 h for the minimum stress applied at 800 °C causes a significant drop of number of cycles to failure due to a synergistic action of fatigue and creep. Creep and creep–fatigue interaction mechanisms are the two primary contributors to the hold-time TMF damage. The creep damage ratio in the hold-time TMF damage is increased with a decrease in applied stress at 800 °C and an increase in number of cycles to failure.

Role of grain orientation in the failure of Sn-based solder joints under thermomechanical fatigue

A small Pb-free solder joint exhibits an extremely strong anisotropy due to the bodycentered tetragonal (BCT) lattice structure of fl-Sn. Grain orientations can significantly in∞uence the failure mode of Pb-free solder joints under thermomechanical fatigue (TMF) due to the coe‐cient of thermal expansion (CTE) mismatch of fl-Sn grains. The research work in this paper focused on the microstructure and damage evolution of Sn3.0Ag0.5Cu BGA packages as well as individual Sn3.5Ag solder joints without constraints introduced by the package structure under TMF tests. The microstructure and damage evolution in cross-sections of solder joints under thermomechanical shock tests were characterized using optical microscopy with cross-polarized light and scanning electron microscopy (SEM), and orientations of Sn grains were determined by orientation imaging microscopy (OIM). During TMF, obvious recrystallization regions were observed with difierent thermomechanical responses depending on Sn grain orientations. It indicates that substantial stresses can build up at grain boundaries, leading to signiflcant grain boundary sliding. The results show that recrystallized grains prefer to nucleate along pre-existing high-angle grain boundaries and fatigue cracks tend to propagate intergranularly in recrystallized regions, leading

Isothermal and thermomechanical fatigue studies on a modified 9Cr–1Mo ferritic martensitic steel

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) tests were carried out on a modified 9Cr–1Mo ferritic martensitic steel under a mechanical strain control mode employing a strain amplitude of ±0.4%. Different temperature ranges in the interval, 573–923K were employed for the tests which were carried out using a constant strain rate of 1.2×10−4s−1. Isothermal low cycle fatigue tests were also performed concurrently at the maximum temperatures (Tmax) of TMF tests on similar specimens and employing the same strain amplitude and strain rate. The life variation was seen to follow the sequence: IF<OP TMF<IP TMF. However, the difference in lives narrowed down with an increase in the Tmax of TMF/test temperature of IF cycling. The cyclic stress response behaviour of the alloy was characterized by a continuous softening under all testing conditions. The lower lives observed under OP TMF in comparison with IP cycling was attributed to a combined influence of oxide cracking and a higher tensile hysteresis loop energy in the former. Cyclic life under IP TMF was observed to reduce more drastically compared to OP cycling with increase in Tmax, owing to the increasing prominence of dynamic recovery effects coupled with creep deformation. Transmission electron microscopy (TEM) investigations revealed that IF cycling at the Tmax resulted in a significantly greater substructural recovery compared to that produced by TMF cycling as a consequence of which, the former led to lower lives compared to both the IP and OP TMF. Dynamic strain ageing (DSA) associated with serrated flow was observed as the deformation temperature under TMF cycling traversed the regime of 300–400°C.

Coupled Thermomechanical Fatigue Tests for Simulating Load Conditions in Cooled Turbine Parts

Modern heavy-duty gas turbines operate under hot gas temperatures that are much higher than the temperature capability of nickel superalloys. For that reason, advanced cooling technology is applied for reducing the metal temperature to an acceptable level. Highly cooled components, however, are characterized by large thermal gradients resulting in inhomogeneous temperature fields and complex thermomechanical load conditions. In particular, the different rates of stress relaxation due to the different metal temperatures on hot gas and cooling air exposed surfaces lead to load redistributions in cooled structures, which have to be considered in the lifetime prediction methodology. In this context, the paper describes coupled thermomechanical fatigue (CTMF) tests for simultaneously simulating load conditions on hot and cold surfaces of cooled turbine parts (Beck et al., 2001, “Experimental Analysis of the Interaction of Hot and Cold Volume Elements During Thermal Fatigue of Cooled Components Made From AISI 316 L Steel,” Z. Metallkunde, 92, pp. 875–881 and Rau et al., 2003, “Isothermal Thermo-mechanical and Complex Thermo-mechanical Fatigue Tests on AISI 316 L Steel—A Critical Evaluation,” Mater. Sci. Eng., A345, pp. 309–318). In contrary to standard thermomechanical fatigue (TMF) testing methods, CTMF tests involve the interaction between hot and cold regions of the parts and thus more closely simulates the material behavior in cooled gas turbine structures. The paper describes the methodology of CTMF tests and their application to typical load conditions in cooled gas turbine parts. Experimental results are compared with numerical predictions showing the advantages of the proposed testing method.

Thermo-mechanical stress analysis of thermal barrier coating system considering thickness and roughness effects

Cast aluminium–silicon alloy, A356.0, is widely used in automotive and aerospace industries because of its outstanding mechanical, physical, and casting properties. Thermal barrier coatings can be applied to combustion chamber to reduce fuel consumption and pollutions and also improve fatigue life of components. The purpose of the present work is to simulate stress distribution of A356.0 under thermo-mechanical cyclic loadings, using a two-layer elastic-visco-plastic model of ABAQUS. The results of stress–strain hysteresis loop are validated by an out of phase thermo-mechanical fatigue test. Different thicknesses from 300 to 800μm of top coat and also roughness of the interfaces are simulated to get best stress gradient. Results show that increasing top coat thickness causes stress increase. The realistic interface model is useful for identifying critical areas in stress development. Two important factors having considerable effect on development of high stress in TBC, are the severity of undulations relating to amplitude and wavelength of interface waves; and the thickness of BC layer relating to mutual positioning of either interfaces. However the realistic model has some limitations including long calculation time and difficulties of generating a suitable mesh. To diminish these limitations, after recognizing critical area, in second stage of the study, a periodic unit cell is used instead . Eight models considering different mutual positioning of interfacial asperities along with different penetration in adjoining layers are simulated and compared. Results show that detachment of the thermal barrier coating system from substrate is more probable Results show that IP positioning of mutual waves produce more severe stress but contour pattern is less likely to promote crack propagation.

Thermo-mechanical fatigue testing and simulation using a viscoplasticity model for a P91 steel

An experimental programme of cyclic thermo-mechanical testing for a P91 power plant steel, under isothermal, and in-phase and out-of-phase thermo-mechanical, temperature-strain cycle conditions, has been implemented. Using the experimental data, an optimisation procedure has been developed for the accurate determination of the material constants under isothermal conditions, in which the Chaboche model is employed to describe material responses. The material was found to exhibit cyclic softening throughout the full life cycles, which is believed to be related to the evolution of microstructure and the propagation of micro-cracks. The model developed shows good predictive capability of cyclic stress–strain behaviour and cyclic softening.

A phenomenological model to predict the crack growth in single crystal superalloys at high temperature

This paper presents the formulation of a phenomenological model to predict the crack growth in single crystal superalloy at high temperature. The proposed model relies on an extensive experimental study performed on AM1 single crystal superalloy at temperatures ranging from 650 °C to 950 °C. Tests carried out in fatigue and creep–fatigue regimes investigate the effects of time on crack growth rates. The crack growth model follows the framework of classical linear elastic fracture mechanics. Time effects at high temperature are captured by creep–fatigue and oxidation–fatigue interactions. The specific model formulation for nonisothermal conditions is attractive for identifying parameters on a large temperature domain and for predicting complex Thermo-Mechanical Fatigue (TMF) tests. Model predictions are then compared with a large set of experimental results including TMF tests. The application of this model, which accounts for a better understanding and modeling of physical phenomena such as the environmental or creep effects on crack growth rate, should improve the prediction of crack growth regime in single crystal superalloys that are used to design critical components such as turbine blades.

Cyclic behaviour of structures under thermomechanical loadings: Application to exhaust manifolds

A structure subjected to thermomechanical cyclic loadings may exhibit various cyclic behaviours: perfect elasticity, elastic shakedown, plastic shakedown and ratchetting. The quantitative characterization of these different asymptotic states is addressed in this paper, in the context of the Generalized Standard Materials (GSM). Criteria are established to identify them and capture the trend line of their evolution, in the context of numerical simulation. The definitions are extended to the case of temperature dependent mechanical properties. Simple strain paths are first considered to illustrate the relevancy of the proposition. Finally, as an application, the example of an exhaust manifold subjected to a classical thermomechanical fatigue test is analyzed.

Influence of Protective Coatings on Damage and Lifetime of Alloy 247 DS in Thermomechanical Fatigue and Bending Tests

Abstract The present work was focused on the damage mechanisms and lifetime of a directionally solidified, oxidation protection coated Ni-base superalloy under out-of-phase thermomechanical fatigue (OP TMF) loading. The main scope of the work was the analysis of crack formation and propagation in the coating depending on its fracture strain. Two coating systems applied by low pressure plasma spraying were investigated: (i) MCrAlY-coating and (ii) duplex-coating consisting of the same MCrAlY coating and a NiAl top coat. The fracture strains of the coatings were determined at T = 20°C up to 600°C by four-point bending with acoustic emission analysis. The MCrAlY-coating did not influence significantly the TMF lifetime of the superalloy, whereas duplex-coatings reduced it drastically. This correlates with the fracture strains of the coatings that are for the MCrAlY-coating distinctly higher and for the duplex coating lower than the mechanical strain range of the TMF tests. Thus, the damage process was controlled by propagation of cracks formed after few TMF cycles in the duplex coated material and by slower initiation and propagation of fatigue cracks in the MCrAlY coated alloy. Moreover, crack branching at the interface between coating and substrate alloy affects crack penetration into the substrate. In addition to measurement of TMF life, the early stages of crack initiation and propagation in the duplex coating were studied in more detail after TMF tests that were interrupted after defined cycle numbers as well as during in situ bending tests in the scanning electron microscope. Furthermore, the influence of pre-oxidation on crack propagation is discussed.

Thermo-mechanical fatigue behaviour of a duplex stainless steel

In this paper the thermo-mechanical fatigue (TMF) behaviour of the duplex stainless steel X2CrNiMoN22-5-3 is presented. TMF tests were carried out in the temperature range 100–350 °C on the duplex steel and on single-phase ferritic and austenitic steels comparable to the two phases of the duplex steel. The cyclic stress response curves of the duplex steel are showing pronounced primary and secondary hardening, where the former is in agreement with the cyclic hardening regime of the single-phase austenitic steel and the latter with that of the ferritic steel. For reference, additional isothermal low cycle fatigue (LCF) tests were performed at two strain rates at minimum and maximum temperature of the TMF temperature range, proving dynamic strain ageing effects occurring at 350 °C.