Thermomechanical Fatigue Tests Research Articles

Phase-angle effects on damage mechanisms of thermal barrier coatings under thermomechanical fatigue

The damage mechanisms of thermal barrier coatings are investigated using in-phase (IP) and out-of-phase (OOP) thermomechanical fatigue (TMF) tests. During IP testing, TMF damage is governed by the susceptibility of the coating to buckling. In OOP tests, cracking of the substrate controls life. Cracks initiate either at the unprotected surface of the substrate or within the bond coat, nucleated at defects formed during thermal exposure.

Thermomechanical fatigue behaviour of nickel base superalloy IN738LC Part 1 – Comparison of two unified constitutive models for description of mechanical behaviour

The capabilities of two unified constitutive models to predict the mechanical behaviour of nickel base superalloy IN738LC under uniaxial loading conditions have been investigated over the temperature range 450–850°C. The material parameters of each model have been identified from an experimental investigation and complemented by available data from the literature. Mechanical responses from isothermal tests performed at 450 and 850°C (such as creep, monotonic, and fully reversed isothermal cyclic tests) were used for the identification of model parameters, and model capabilities were examined by comparison with in phase and out of phase thermomechanical fatigue and stress relaxation tests. Both models were found to capture all features of the material responses under uniaxial loading, although with varying degrees of accuracy.

Thermomechanical fatigue behavior of Cr–Ni–Mo cast hot work die steel

Under stress-controlled thermomechanical fatigue (TMF) and isothermal fatigue (IF) conditions, it is found that in-phase TMF lives are lower than those of IF. Tensile cyclic creep and compressive cyclic creep were found in TMF and IF tests, respectively, which is not usually observed in the more common strain-controlled TMF tests.

An overview of the damage approach of durability modelling at elevated temperature

Lifetime prediction techniques for components working at elevated temperature are revisited. Two damage approaches in which time effects at high temperature are introduced in different ways are discussed in greater detail. First, a creep–fatigue damage model considers the interaction of the two processes during the whole life before macrocrack initiation; and second, a creep–fatigue–oxidation model separates the fatigue life into two periods: during initiation the environment‐assisted processes interact with fatigue, although bulk creep damage only interacts during the micropropagation period. The second model is illustrated by its application to a coated single‐crystal superalloy used in aerojet turbine blades. Its capabilities are illustrated in a number of isothermal and thermomechanical fatigue tests. Anisotropy effects are also briefly discussed and a special test, introducing cyclic thermal gradients through the wall thickness of a tubular component, demonstrates the predictive capabilities for actual engine conditions.

Failure of thermal barrier coating systems under cyclic thermomechanical loading

The failure mechanisms of thermal barrier coating (TBC) systems applied on gas turbine blades and vanes are investigated using thermomechanical fatigue (TMF) tests and finite element (FE) modeling. TMF tests were performed at two levels of applied mechanical strain, namely five times and three times the critical in-service mechanical strain of an industrial gas turbine. TMF testing under the higher mechanical strain of air plasma-sprayed (APS) and electron beam–physical vapor deposition (EB-PVD) coated samples showed that both systems failed after a similar number of cycles by cracks that initiated at the bond coat/thermally grown oxide (TGO) interface and propagated through the bond coat to the substrate. When the applied mechanical strain was decreased, cracking of the bond coat in EB-PVD coated systems was suppressed, the life of the coated system increased significantly and delamination of the top-coat was observed. A subsequent FE analysis showed that, by subjecting the system to the higher mechanical strain, significant tensile stresses develop in the TGO and the bond coat that are thought to be responsible for the observed crack initiation and propagation. The FE model also predicts that cracking initiates at specific geometric features of the rough interface of a PS coated system, which was confirmed by metallographic examination of failed samples. The decrease of the applied mechanical strain and hence of the developed stresses led to the suppression of failure by bond coat cracking and activate delamination. These results outline the importance of designing TMF tests and selecting the appropriate mechanical loading in order to accelerate testing and still trigger the same failure mechanisms as observed in-service.

High temperature low cycle fatigue and thermo-mechanical fatigue of a 6061Al reinforced with SiC W

The high temperature isothermal low cycle fatigue (IF) and thermo-mechanical fatigue (TMF) of a 6061Al alloy reinforced with 28 vol.% silicon carbide whiskers were studied. IF experiments were conducted at a temperature of 300°C, and two types of TMF tests, i.e. in-phase (IP) and out-of-phase (OP), were performed with temperature cycling between 150 and 300°C. All experiments were carried out under mechanical strain range control. Experimental results of mechanical behavior and mean stresses for TMF and IF loadings are provided. It can be seen that both IP and OP-TMF show continuous cyclic softening until failure, while IF loading initially shows several cycles of cyclic hardening followed by cyclic softening until final fracture. A comparison of TMF and IF lives has been made and various damage mechanisms operative under different strain ranges and loading conditions have been explained. SEM micrographs show that damage processes under both IF and TMF conditions consist of initiation, growth and coalescence of voids in the matrix around whiskers. Due to the complexity of the relationship between IF and TMF damage, IF data cannot be used to evaluate TMF life.

Thermomechanical and isothermal fatigue behaviour of type 316 stainless steel base metal, weld metal, and joint

In phase thermomechanical fatigue (TMF) in the temperature range 573–973 K (300–700°C) and isothermal fatigue behaviour at 973 K in air were studied for type 316 stainless steel using smooth cylindrical specimens machined from base metal, weld metal, and the weld joint (cross-weld). In all joint specimens, fatigue failure occurred in the weld metal region. The lifetimes of weld metal and joint specimens were almost equal and were always inferior to those of base metal specimens. In the base metal, the effect of strain rate on the isothermal fatigue life was not very significant. Although TMF lifetimes were always a little shorter than the isothermal fatigue lifetimes in base metal, the difference was small for the same mechanical strain range and similar strain rate. This may be because the fracture mode for both types of loading was of a similar mixed type. Conversely, a dramatic reduction in lifetime was observed in weld metal and joint specimens under TMF in comparison with isothermal fatigue. This was attributed to the additional damage caused by many independent subsurface cracks at σ phase boundaries and linkage of these cracks with the surface crack, leading to rapid crack propagation. The δ ferrite in the weld metal completely transformed to σ phase in both the isothermal fatigue and TMF tests.

A Life Prediction Model for Metal Matrix Composites

Global deformation response of metal matrix composites (MMCs) to applied mechanical and thermally induced stresses depends upon (1) the mechanical and thermal properties of the constituent materials, (2) residual stresses in the constituents induced by the composite consolidation process, and (3) the nature and extent of micromechanical damage which may occur and accumulate over time. In the present work, a mechanistic model for predicting time dependent global deformation response of unidirectional metal matrix composite subjected to fiber direction stresses is used in conjunction with several viscoplastic models for the matrix, viz. power law, Bodner-Partom and GVIPS, and compared with laboratory test data. The model incorporates the effect of the micromechanical residual stresses in the fibers and the matrix induced by the composite fabrication process. The model also incorporates the effect of discrete existing and/or progressively occurring fiber damage on global MMC deformation response. The criterion for fiber fractures is based on statistical information on fiber strength. In this work strain predictions obtained using this model are compared with thermomechanical fatigue test data.

High temperature fatigue behaviour of TZM molybdenum alloy under mechanical and thermomechanical cyclic loads

High temperature isothermal mechanical fatigue and in-phase thermomechanical fatigue (TMF) tests in load control were carried out on a molybdenum-based alloy, one of the best known of the refractory alloys, TZM. The stress–strain response and the cyclic life of the material were measured during the tests. The fatigue lives obtained in the in-phase TMF tests are lower than those obtained in the isothermal mechanical tests at the same load amplitude. It appears that an additional damage is produced by the reaction of mechanical stress cycles and temperature cycles in TMF situation. Ratcheting phenomenon occurred during the tests with an increasing creep rate and it was dependent on temperature and load amplitude. A model of lifetime prediction, based on the Woehler–Miner law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature are introduced in the model so as to evaluate TMF lives in load control. With this method the lifetime prediction gives results corresponding well to experimental data.

Modeling thermomechanical fatigue life of high-temperature titanium alloy IMI 834

A microcrack propagation model was developed to predict thermomechanical fatigue (TMF) life of high-temperature titanium alloy IMI 834 from isothermal data. Pure fatigue damage, which is assumed to evolve independent of time, is correlated using the cyclic J integral. For test temperatures exceeding about 600 °C, oxygen-induced embrittlement of the material ahead of the advancing crack tip is the dominating environmental effect. To model the contribution of this damage mechanism to fatigue crack growth, extensive use of metallographic measurements was made. Comparisons between stress-free annealed samples and fatigued specimens revealed that oxygen uptake is strongly enhanced by cyclic plastic straining. In fatigue tests with a temperature below about 500 °C, the contribution of oxidation was found to be negligible, and the detrimental environmental effect was attributed to the reaction of water vapor with freshly exposed material at the crack tip. Both environmental degradation mechanisms contributed to damage evolution only in out-of-phase TMF tests, and thus, this loading mode is most detrimental. Electron microscopy revealed that cyclic stress-strain response and crack initiation mechanisms are affected by the change from planar dislocation slip to a more wavy type as test temperature is increased. The predictive capabilities of the model are shown to result from the close correlation with the microstructural observations.

Thermo-Mechanical Fatigue of a SiC<SUB>W</SUB>/6061Al Composite

In-phase (IP) and out-of-phase (OP) thermo-mechanical fatigue (TMF) experiments were undertaken on 6061 aluminum alloy reinforced with 15% volume fraction whiskers. Both TMF tests were conducted under mechanical strain range control with temperature cycling between 150 ∼ 300°C. The mechanical behavior and fatigue resistance of both TMF fatigue was investigated and discussed. The result shows that the two kinds of TMF cycling exhibited continuous cyclic softening, out-of-phase TMF loading gave rise to tensile mean stress, whereas in-phase TMF cycling led to compressive mean stress. It is also illustrated that at small strain range, the in-phase TMF life was longer than that of the out-of-phase TMF; on the contrary, out-of-phase TMF showed longer life at large strain range. This is tentatively explained by the various damage mechanisms with the help of SEM, which were predominantly operative under different strain ranges and different phasing conditions.

Fatigue Life Prediction of Single Crystals for Turbine Blade Applications

This paper describes a fatigue life prediction model accounting for high temperature applications of metallic materials. The formulation of the model is for general anisotropy and multiaxiality of loading. This phenomenological model distinguishes between an initiation phase and a propagation phase, and takes into account oxidation and creep effects on fatigue life. An application of the model is given for a coated single crystal superalloy for turbine blades. Model predictions are in good agreement with a large set of experimental data for mechanical loading including thermomechanical fatigue tests. A new experimental device, designed to produce a thermal gradient in the thickness of thin walled specimens, is also presented. This device has been used with polycrystalline and monocrystalline superalloys. Corresponding life predictions, performed by using recent anisotropic models, are presented for the single crystal superalloy.

Thermomechanical Fatigue Testing and Analysis of Solder Alloys

Thermomechanical fatigue (TMF) is one of the most common causes of failure in solder joints. TMF occurs due to the introduction of stresses arising from thermal expansion mismatch during thermal cycling caused by either internal heating from power dissipation, the external environment, or both. Due to its complicated nature including partial compliance of joints, evolution of microstructure, and multiple deformation mechanisms, testing a material’s resistance to TMF is not straightforward. A method developed at Rensselaer, based on an apparatus designed at North Carolina State University, allows the direct measurement of each stress-strain cycle during thermal cycling. Through further analysis, using spreadsheet macros, it is possible to consider the energy absorbed by the joint on a per cycle basis. The interpretation of such data as well as the cumulative energy absorption can provide insight into the failure process. The behavior of three alloys, eutectic Sn–Bi, Sn–Pb, and Sn–Ag, will be presented. These data will be compared with that generated by other methods. A comparison of the behavior of these alloys, as well as the apparatus design, test method, interpretation, and possible enhancements is discussed. [S1043-7398(00)01201-9]

The role of HCl-impurities in the environment and Y for the high temperature corrosion of CoNiCrAl(Y)-coatings on a Ni-base-superalloy under complex loading conditions

The influence of small concentrations of HCl in synthetic air on the high-temperature corrosion of an Y-free and an Y-modified CoNiCrAl-coating on the directly solidified Ni-base superalloy CM247LC DS was investigated at 1,000°C under isothermal and thermocyclic conditions, accompanied by acoustic emission measurements. Furthermore, thermomechanical fatigue tests were carried out. Both coatings formed an Al2O3-layer. Y had a positive influence on the scale adherence and changed the growth direction of the Al2O3-scale. Y-modified coatings showed substantial internal oxidation. Pore formation at the oxide/metal interface took place in the case of the Y-free coating during oxidation in synthetic air as well during oxidation in HCl-containing atmospheres leading to a decrease of scale adherance. No crack closure by oxidation was observed under thermomechanical fatigue conditions in the case of Y-modified coatings on the alloy CM247LC DS. For the Y-free coating in HCl-containing atmospheres scale damage could be detected by accoustic emission measurements even under isothermal oxidation.

Delamination Crack Growth of Unidirectional CFRP Laminates under Variable Temperature (2nd Report, 180.DEG.C. Cure Type Epoxy Resin Matrix).

The delamination crack growth behavior under iso-thermal and thermo-mechanical fatigue were investigated with unidirectional CF/epoxy laminates. Tests were conducted either in air or in water with double cantilever beam specimens. The crack growth tests were conducted under constant ΔK conditions. In air, the delamination crack growth rates for the iso-thermal fatigue tests were higher for higher temperature, and the crack growth rates for the thermo-mechanical fatigue tests were lower than those for the iso-thermal fatigue tests. The crack growth rates for the in-phase thermo-mechanical tests were lower than those for the out-of-phase thermo-mechanical tests. In water, the crack growth rates for the iso-thermal fatigue tests were higher for higher temperature under high stress ratio. The rates, however, were not monotonically changed with test temperature under low stress ratio. The rates for the themo-mechanical fatigue test were almost the same value as those for the iso-thermal fatigue test whose test temperature was equal to that at the maximum load for the themo-mechanical fatigue test. The most remarkable difference in the growth behavior between present material and the material employed for the 1st report appeared in the thermo mechanical fatigue crack growth tests in air

Thermomechanical fatigue behavior of the high-temperature titanium alloy IMI 834

The isothermal and thermomechanical fatigue (TMF) behavior of the titanium alloy IMI 834 was studied between 350 °C and 650 °C in air and vacuum, respectively. Transmission electron microscopy (TEM) observations revealed that the microstructure established in the TMF tests was governed by the maximum temperature within the cycle. However, if the maximum temperature does not exceed 600 °C, planar dislocation slip prevails and similar microstructures are formed regardless of the test temperature and the testing mode (TMF and isothermal, respectively). As a result, the stress-strain response in TMF tests can be assessed from the corresponding isothermal data. Wavy dislocation slip was found to determine the stress-strain behavior if the maximum test temperature exceeded 600 °C. Moreover, in TMF tests with a maximum test temperature of 650 °C, the dislocation arrangement formed in the high-temperature part of the hysteresis loop was found to be stable throughout the cycle and to affect significantly the stress-strain response at the low temperatures. Although in-phase (IP) and out-of-phase (OP) TMF tests led to an almost identical microstructure, OP loading was always found to be most detrimental. The interaction between the embrittled subsurface layer, caused by oxygen uptake, and the high tensile stresses developing in the low-temperature part of the hysteresis loop in OP tests eases crack initiation and initial crack propagation and results in reduced fatigue life.

ITER baffle module small-scale mock-ups: First Wall thermo-mechanical testing results

The EU-Home Team is in charge of the R&D related to the ITER Baffle First Wall. Five small-scale mock-ups, using Be, CFC and W tiles and different armour/heat-sink material joints under development, have been fabricated and thermomechanically tested in FE200 (Le Creusot) and JUDITH (Jülich) Electron Beam facilities. The small-scale mock-ups have been submitted to thermo-mechanical fatigue tests (up to failure using accelerating techniques). The objective was to determine the performances of the armour material joints under high heat flux cycles.

High temperature isothermal and thermomechanical fatigue on a molybdenum-based alloy

High temperature isothermal fatigue and in-phase thermomechanical fatigue (TMF) tests in stress control were performed on a molybdenum-based alloy, TZM. Two temperature levels of 350 and 500°C were chosen for isothermal fatigue tests and a temperature interval of 350–500°C was applied to thermomechanical fatigue tests. The stress–strain response and the life cycle of the material were measured during the tests. It appears that temperature variations influence the fatigue life of the alloy and the material is further damaged with combined stress and temperature cycles. The analysis of the hysteresis loops showed that a ratcheting phenomenon and a cyclic softening behavior of the material occurred during each kind of the fatigue tests. The observations of the microstructure of the damaged specimens demonstrated that the fracture of the alloy was, on the whole, a brittle process under higher cyclic stress conditions.

A Finite Element Procedure of a Cyclic Thermoviscoplasticity Model for Solder and Copper Interconnects

A finite element procedure using a semi-implicit time-integration scheme has been developed for a cyclic thermoviscoplastic constitutive model for Pb-Sn solder and OFHC copper, two common metallic constituents in electronic packaging applications. The scheme has been implemented in the commercial finite element (FE) code ABAQUS (1995) via the user-defined material subroutine, UMAT. Several single-element simulations are conducted to compare with previous test results, which include monotonic tensile tests, creep tests, and a two-step ratchetting test for 62Sn36Pb2Ag solder; a nonproportional axial-torsional test and a thermomechanical fatigue (TMF) test for OFHC copper. At the constitutive level, we also provide an adaptive time stepping algorithm, which can be used to improve the overall computation efficiency and accuracy especially in large-scale FE analyses. We also compare the computational efforts of fully backward Euler and the proposed methods. The implementation of the FE procedure provides a guideline to apply user-defined material constitutive relations in FE analyses and to perform more sophisticated thermomechanical simulations. Such work can facilitate enhanced understanding thermomechanical reliability issue of solder and copper interconnects in electronic packaging applications.

Delamination Crack Growth of Unidirectional CFRP Laminates under Variable Temperature. 1st Report. 130.DEG.C. Cure Type Epoxy Resin Matrix.

The delamination crack growth behavior under iso-thermal and thermo-mechanical fatigue were investigated with unidirectional CF/epoxy laminates. Tests were conducted either in air or in water with double cantilever beam specimens. The crack growth tests were conducted under constant ΔK conditions at the load ratio of 0.5 with a loading frequency of 1/180 Hz. The test temperature was between 20°C and 70°C. In air, the effect of test temperature on the crack growth behavior was not observed in the iso-thermal fatigue tests, and the crack growth rates for the thermomechanical fatigue tests were almost identical to those for the iso-thermal fatigue tests. In water, however, the crack growth rates for the iso-thermal fatigue tests were higher for lower temperature, and the rates for the thermomechanical fatigue test were almost the same value as those for the isothermal fatigue test whose test temperature was equal to that at the maximum load for the thermomechanical fatigue test. The effect of environment was not observed for the iso-thermal fatigue test at 20°C. For the iso-thermal fatigue test at 70°C and in-phase thermo-mechanical fatigue test, however, the crack growth rates in water were lower than those in air.