Thermomechanical Fatigue Tests Research Articles

Delamination Behavior of Thermal Barrier Coatings under Thermo-Mechanical Fatigue

It has become common to apply thermal barrier coatings (TBC) to hot gas path parts in gas turbines to increase thermal efficiency. However, some uncertainty about the delamination life prediction method for TBC still remains. The influences of mechanical parameters on delamination behavior were investigated through thermo-mechanical fatigue (TMF) tests to improve TBC delamination life prediction. The parameters investigated were loading pattern, strain range, ceramic coating thickness, and high temperature exposure processing prior to TMF tests.Delamination behavior with a simulated loading pattern seemed to better correspond to behavior under out-of-phase (OP) loading than that with in-phase (IP) loading. The OP loading pattern was better for evaluating delamination lives because of its shorter test period. The delamination lives became shorter under larger strain ranges. Of the specimens tested, the specimen with a 0.9-mm-thick ceramic coating had the shortest delamination life. The specimen with a 0.6-mm-thick coating had almost the same or longer life than the specimen with a 0.3-mm-thick coating. These results strongly suggest that an optimum ceramic coating thickness keeps the cooling performance consistent with delamination strength. Thermally grown oxides (TGO) are generally thought to be the main cause of TBC degradation in long-term operated gas turbine blades. The growth behavior of TGO was observed by exposing specimens to a high-temperature environment and it was suggested that the Larson-Miller parameter (LMP) could predict TGO thickness well. In addition, some specimens were exposed to high-temperature environments prior to TMF tests to evaluate the effect of TGO thickness on delamination lives. This high temperature pre-exposure seemed to accelerate the delamination growth rate due to sintering and TGO.

Novel induction heating system for service cycle thermo-mechanical fatigue testing

Novel induction heating system for service cycle thermo-mechanical fatigue testing

熱機械疲労試験によるEB-PVD Y2O3-ZrO2熱遮蔽コーティングのき裂発生の解析

熱機械疲労試験によるEB-PVD Y2O3-ZrO2熱遮蔽コーティングのき裂発生の解析

Reduced softening of EUROFER 97 under thermomechanical and multiaxial fatigue loading and its impact on the design rules

Within the development of the test blanket module (TBM) for use in the ITER and DEMO fusion power plants, interest increasingly focuses on design rules for components built from EUROFER 97. One of the specific characteristics of EUROFER 97 as a ferritic–martensitic steel is its cyclic softening yielding to lower stresses under strain-controlled fatigue loading and, hence, longer lifetimes. Our thermomechanical and multiaxial fatigue tests, however, revealed lifetimes that were remarkably lower than those expected on the basis of isothermal uniaxial fatigue tests. The reduced cyclic softening observed in these experiments is believed to be one of the reasons of the shorter fatigue lifetimes. When applying the design rules derived for EUROFER 97 from isothermal uniaxial data considering the recommendations in the ASME and RCC-MR codes to our thermomechanical and multiaxial fatigue tests for verification purposes, strong loss of their conservatism was found. The lifetimes observed in some of the multiaxial experiments were even lower than the design lifetimes that had been supposed to be sufficiently conservative. To overcome this problem, new design rules will be proposed, which are based among others on the damage and lifetime prediction model developed lately for EUROFER 97.

Challenges in the constitutive modeling of the thermo-mechanical deformation and damage behavior of EUROFER 97

EUROFER 97, reduced activation ferritic martensitic (RAFM) steel, is a candidate as structural material for plasma facing components of future fusion reactors where it will be subjected to complex thermo-mechanical loading and high irradiation doses. Correct modeling of its deterioration under these loading conditions is a precondition of a sufficiently reliable lifetime prediction procedure. Therefore a coupled deformation damage model taking into account its complex non-saturating cyclic softening has been developed and successfully applied to describe its creep-fatigue behavior under isothermal cyclic loading. The model has been implemented in the finite element code ABAQUS allowing best estimation of the mechanical behavior of real structures built from EUROFER 97. For verification the implementation has been applied to thermo-mechanical fatigue tests performed on tube specimens. Previous evaluation of these tests showed that thermo-mechanical fatigue loading leads to remarkably reduced lifetime in comparison to isothermal fatigue loading with the same mechanical strain range. In this evaluation changes in the stress range as well as actual test conditions, e.g. temperature gradients, were not considered. However, when applying the coupled deformation damage model changes in stresses between isothermal and thermo-mechanical loadings will be a priori taken into account. Even though the simulations have provided a better description and thus a modified evaluation for the thermo-mechanical tests considered they showed at the same time the lack in describing partial phenomena among others the reduced softening observed in this type of tests. Consequently, further modifications of the deformation constitutive equations are required in order to predict the lifetime correctly. These results underline once more the complexity in modeling the thermo-mechanical material behavior and the need of comprehensive research in this area.

Creep–fatigue interaction of the ODS superalloy PM 1000

This study reports on the creep–fatigue interaction of the oxide dispersion strengthened Ni-base superalloy PM 1000. Fully reversed symmetrical push–pull isothermal fatigue, thermomechanical fatigue (TMF), slow-fast and tensile hold time tests were conducted in the temperature range from 450 to 1050 °C. TMF tests resulted in unexpectedly low fatigue lives. Grain boundary cavitation observed in the in-phase TMF tests indicated that creep damage plays an important role under TMF loading conditions. Similarly, fatigue life dependence on wave-shape demonstrated that creep–fatigue tests exhibit the shortest cyclic life due to the additional creep damage in the tensile-going part of the cycle. Damage mechanisms were observed to vary substantially depending on the test mode. In the creep–fatigue tests, the grain boundaries of fine equiaxed grains triggered multiple internal crack initiation, thereby pointing to the adverse effects of these microstructural features. Cavitation occurs by a local stress directed diffusion and growth of voids, as evidenced by dispersoid free zones in areas near the transverse grain boundary cavities. Time-dependent damage occurring at and above 850 °C in the creep–fatigue tests is mostly due to creep whereas environmental attack plays only a minor role.

Thermomechanical fatigue behaviour of a third generation γ-TiAl intermetallic alloy

Thermomechanical fatigue (TMF) tests with loading conditions involving a combined thermal and mechanical strain cycle were carried out on a third generation γ-TiAl alloy with fine duplex microstructure. The experiments were conducted at a total strain amplitude of 0.6% in out-of-phase (OP) and in-phase (IP) modes. The temperature range was kept constant at 250 °C and the maximum temperatures applied were 700, 750 and 800 °C. The TMF results revealed that thermomechanical loading conditions must be considered as very crucial for the design of components made of γ-TiAl alloys. In particular, out-of-phase loading conditions reduce fatigue life tremendously as compared to the corresponding isothermal conditions. The very short out-of-phase loading life can be attributed to an accelerated oxidation-induced crack initiation during tension in the low-temperature part of the TMF cycle and is a consequence of the formation of an embrittled zone at the surface during the high-temperature part of the loading cycle.

Multi-Axial Thermo-Mechanical Fatigue of a Near-Gamma TiAl-Alloy

A material family to replace the current superalloys in aeronautical gas turbine engines is considered to be that of gamma Titanium Aluminide (-TiAl) alloys. Structural components in aeronautical gas turbine engines typically experience large variations in temperatures and multiaxial states of stress under non-isothermal conditions. The uniaxial, torsional and bi-axial thermo-mechanical fatigue (TMF) behaviour of this -TiAl alloy have been examined at 400 – 800oC with strain amplitudes from 0.15% to 0.7%. The tests were conducted at both in-phase (IP) and out-of-phase (OP). The effects of TMF on the microstructure were also investigated. For the same equivalent mechanical strain amplitude uniaxial IP tests showed significantly longer lifetimes than pure torsional TMF tests. The non-proportional multiaxial OP test showed the lowest lifetimes at the same equivalent mechanical strain amplitude compared to the other types of tests.

Crack initiation under thermal fatigue: An overview of CEA experience: Part II (of II): Application of various criteria to biaxial thermal fatigue tests and a first proposal to improve the estimation of the thermal fatigue damage

A Part I accompanying paper clearly showed that for a given level of strain range (Δ ɛ), the number of cycles required to achieve crack initiation is significantly lower in biaxial thermal fatigue than in uniaxial isothermal fatigue. Such discrepancy does not come from a thermal effect, as it is emphasized by a comparison between the uniaxial data, coming either from thermomechanical fatigue tests, or from isothermal low cycle fatigue tests. A detrimental effect of multiaxial loading on fatigue life is noticed in many cases as reported in the literature. In this frame, the goal of this paper is to propose criteria in order to take into account the reduction of fatigue life resulting from the multiaxiality. Among all the examined multiaxial criteria, a criterion based on strain proposed by Zamrik, and a new criterion based on energy proposed by “Ecole Polytechnique” give the best estimations. Furthermore, application of the Zamrik’s criterion using the RCC–MR method is very promising for the design engineer. Indeed, these two criteria exhibit a notable potential, since they are well adapted to all the available thermal fatigue experimental data, despite significant differences between approaches and processes developed by both accessible facilities. However, such investigation must be continued with achievement of additional thermal fatigue tests (for higher number of cycles…), and isothermal biaxial fatigue tests.

Material and fatigue life models for thermo‐mechanical loaded components

AbstractToday, specific material data, mainly from static material tests, are not sufficient to design parts of a product to assess lifetime behaviour, especially when local complex loading occurs like low cycle fatigue (LCF) and thermo mechanical fatigue (TMF). In this work, specimen tests are carried out under LCF and TMF loading with different materials, where mechanical and thermal properties are considered. The investigated materials are an aluminium alloy, cast iron, copper alloy and nickel. The TMF tests are performed by using a TMF test rig, designed by the Chair of Mechanical Engineering (AMB). The results of these tests allow a description of the material behaviour depending on the mechanical and thermal boundary conditions. Commercial Software for Finite Element Analysis (FEA) enables a fit with the test results to calculate stress and strain values for different loading conditions. These results are the input data for the final lifetime calculation. A number of approaches are given to determine the lifetime for the investigated materials. Finally, a special modified energy method – “unified energy approach” – is presented and compared with conventional approaches.

Comparison of the Temperature and Pre-Aging Influences on the Low Cycle Fatigue and Thermo-Mechanical Fatigue Behavior of Copper Alloys (CuCoBe/CuCo2Be)

Abstract Typically thermo-mechanical fatigue (TMF) loaded components in power plants and steel works are made of copper alloys and are used in different applications (e.g. cooling systems in continuous casting, transformers). So a careful analysis and comparison of the experimental results and microstructure of thermo-mechanical loaded copper components, which are based on a systematic variation of the relevant influence factors, have been conducted to develop empirical models for computing the fatigue life. At specific low cycle fatigue (LCF) and thermo-mechanical fatigue test series on the copper alloys CuCoBe and CuCo2Be, the influence of pre-aging and temperature were investigated by means of the cyclic deformation and lifetime behavior. Based on stress-strain loops from LCF tests at different temperatures and aging conditions a non-linear combined material model was adopted to describe the cyclic deformation behavior. The simulated loading parameters of stress and strain were the basis for the subsequent lifetime simulation. A new energy based damage parameter for copper alloys has been developed to fulfill the requirements of lifetime simulation.

Biaxial Thermomechanical-Fatigue Life Property of a Directionally Solidified Ni-Base Superalloy

High-temperature components in thermal power plants are subjected to creep-fatigue loading where creep cavities initiate and grow on grain boundaries. Development of life assessment methods of high-temperature components in gas turbine for maintenance and operating cost reduction is strongly demanded by Japanese utilities. Especially, first row blades are subjected to complicated thermomechanical-fatigue (TMF) loading during start, steady state, stop cycles. Therefore it is important to clarify the TMF life property of blade materials to develop a life assessment procedure. In this study, tension-torsion biaxial TMF tests have been performed between 450°C and 870°C on a Ni-base directional solidified superalloy. Strain ratio ϕ was defined as shear strain range, Δγ, to normal strain range, Δε, and ϕ varied from 0 to infinity. The “Blade wave form,” which simulated temperature and strain condition of the blade surface, was employed. The biaxial TMF tests were also carried out on coated specimens with CoCrAlY. Fatigue life under the biaxial TMF loading showed strain ratio dependency giving shorter life with increasing ϕ. Considering biaxial stress effect on the failure life, an equivalent shear strain range was derived based on the Γ-plane theory, and the biaxial TMF life was well correlated with the equivalent shear strain range. The biaxial TMF life was reduced by introducing strain hold duration at the maximum temperature. The maximum stress increased by introducing the hold time due to increasing mean stress level in the Blade wave form. It was concluded that creep damage gradually accumulated during cycles resulting in reduction in the TMF life. The nonlinear creep-fatigue damage accumulation model was applied to predict failure life of the hold time tests. As a result, the failure lives were predicted within a factor of 1.5 on the observed life. It was found that the fatigue life of CoCrAlY coated material reduced 1∕2 to 1∕3 from that of the substrate. From observation of the longitudinal section of the coated specimens, many cracks started from the coating surface and penetrated into the substrate. It was concluded that the CoCrAlY coating reduced the biaxial TMF life due to acceleration of crack initiation period in the substrate.

The fracture behavior of premolar teeth with class II cavities restored by both direct composite restorations and endodontic post systems

Purpose The aim of the study was to evaluate the influence of different posts on the fracture load and fracture mode of endodontically treated premolars with class II cavities and direct composite restorations in an ex vivo setting. Methods Forty-eight single-rooted human premolars were endodontically treated and prepared with standardized MO (mesio-occlusal) cavities. Eight teeth each received either no posts or were restored with titanium screws (BKS), glass fiber posts (DentinPost), or quartz fiber posts (DT Light SL). Sixteen teeth were restored with zirconium dioxide posts (CeraPost). BKS-screws and eight zirconium dioxide posts were cemented conventionally with glass ionomer cement; Panavia F resin cement was used for all others. The specimens were restored with direct composite restorations. Eight sound premolars served as the controls. After thermomechanical fatigue testing, the samples were loaded until fracture occurred at an angle of 45°. All specimens were evaluated for fracture lines. Results The sound teeth showed the significant highest fracture load (792.50 ± 210.01 N). The group restored with quartz fiber posts differed not significantly from the control. In the groups with fiber posts and titanium screws significant higher fracture load values occurred as in the group with direct composite restorations without posts. The groups with fiber posts did not show a more favorable fracture mode than the other groups. Clinical significance The use of an intraradicular post in premolars with class II cavities can significantly increase the resistance towards extra-axial forces.

Thermomechanical fatigue mechanism in a modern single crystal nickel base superalloy TMS-82

Thermomechanical fatigue (TMF) in a 〈0 0 1〉 oriented nickel base single crystal TMS-82 superalloy was studied in an effort to clarify the mechanisms of stress relaxation and failure. Detailed observations of the microstructural evolution from the interior and outer surfaces of the specimens after TMF tests were made using transmission electron microscopy, scanning electron microscopy and optical microscopy. The stress relaxation took place during a hold time in compression at 900 °C, and the associated mechanisms varied with the following cycles. During TMF cycling, three stages of stress relaxations were identified: (1) primary stress relaxation; (2) steady stress relaxation; and (3) tertiary stress relaxation; each stage exhibits a distinct microstructural evolution. The first stage is related to the filling of dislocations in the γ channels; the second stage involves dislocation annihilation; and the final stage is associated with the de-twinning of deformation twins. The main crack was found to originate from the intersection of deformation twin plates with the specimen surface, and oxidation then assists the growth of the crack. The stress concentration at the crack tip results in a high density of deformation twins, and the propagation of the crack along the twin boundaries can lead to TMF failure of the specimen.

Thermomechanical fatigue evaluation and life prediction of 316L(N) stainless steel

An attempt has been made to understand the thermomechanical fatigue (TMF) behaviour of a nitrogen-alloyed type 316L austenitic stainless steel under different temperature domains. Smooth, hollow specimens were subjected to in-phase (IP) and out-of-phase (OP) thermal–mechanical cycling in air under a mechanical strain control mode, at a strain rate of 6.4 × 10 −5 s −1 and a strain amplitude of ±0.4%. For the sake of comparison, total strain controlled low cycle fatigue (LCF) tests were also performed at the peak temperatures of TMF cycling on similar specimens employing the same strain rate and strain amplitude. Life was found to depend on the thermal/mechanical phasing and temperature. Creep was found to contribute to life reduction in IP tests when the peak temperature of cycling was above 600 °C. A few TMF tests were performed in vacuum in order to assess environmental influence on life. Thermomechanical fatigue cycling led to the development of significant amounts of mean stresses and the stress response was generally higher compared to that of LCF tests at the peak cyclic temperatures. Also, the isothermal tests at the peak temperature of TMF cycling resulted in lower lives compared to those obtained under TMF. An attempt was made to predict the TMF life using the isothermal database and satisfactory predictions were achieved using the Ostergren’s frequency modified damage function (FMDF) approach.

Effect of HVOF Sprayed MCrAlY Coating on Thermomechanical and Isothermal Fatigue Life of Superalloy M963

A cast nickel based superalloy M963 was coated by high-velocity oxy-fuel (HVOF) spraying process. The effect of HVOF MCrAlY coating on thermo-mechanical fatigue (TMF) and isothermal fatigue (IF) in M963 was studied to understand fatigue life and failure mechanisms in coated and uncoated M963 alloy. Two types of TMF tests, i.e. in-phase (IP) and out-of-phase (OP), were performed in temperature range of 450~900°C, and IF tests were conducted at 900°C. It was found that the coating had a detrimental effect on fatigue life under OP TMF, while a beneficial effect of the coating existed under IP TMF and IF. Crack initiation time in the coated specimen was shorter than that in the uncoated specimen and the former’s crack density was higher than the latter’s one under OP TMF. The relationship of deformation and fracture response with fatigue life was discussed based on microscopic analysis.

Influence of endodontic posts on the fracture behavior of crowned premolars with class II cavities

Aim The aim of the study was to evaluate the influence of different posts and types of cementation on the fracture load and fracture mode of crowned, endodontically treated premolars with class II cavities in an ex vivo setting. Methods Forty-eight single-rooted human premolars were endodontically treated and prepared with standardized MO (mesio-occlusal) cavities and a circular chamfer preparation. Eight teeth each received either no posts or were restored with screws (BKS), glass fiber posts (DentinPost), or quartz fiber posts (DT Light SL). Sixteen teeth were restored with zirconium dioxide posts (CeraPost). BKS-screws and eight zirconium dioxide posts were cemented conventionally with glass ionomer cement; Panavia F was used for all others. The specimens were restored with composite cores and crowns cast from a non-precious metal. Eight sound premolars served as the controls. After thermomechanical fatigue testing, the samples were loaded until fracture occurred at an angle of 45°. All specimens were evaluated for fracture lines. Results The sound teeth showed the significant highest fracture load (792.50 ± 210.01 N). Conventionally cemented zirconium dioxide posts showed the lowest fracture load (327.00 ± 45.84 N); the highest fracture load occurred with quartz fiber posts (421.75 ± 90.19 N). Only the difference between these two groups was statistically significant. With glass fiber posts and conventionally cemented zirconia posts, restored teeth failed mostly in an “unfavourable” mode. Conclusions With respect to the fracture load, there was no statistical difference between the restoration of non-vital premolars with class II cavities with crowns and posts or crowns alone.

Identification and validation of a continuum damage model applied for the lifetime prediction of H11 tool steel

This paper describes the work performed to identify and validate a non-isothermal continuum damage model for a widely used hot-work tool steel H11 at a hardness of 47 HRc. The investigation is based on an extensive high-temperature low-cycle fatigue database performed under strain rate controlled tests with and without dwell times in the range of 300?C?600?C. The approach gives a good description of the thermomechanical behaviour of H11 tool steel under various test conditions and constitutes a very good prediction tool for a very large amount of fatigue tests. This work is divided into three stages. First, isothermal fatigue tests are performed on flat specimens in order to identify lifetime model parameters. Then, thermo mechanical fatigue tests are considered in order to validate the model in a non isothermal case. Lastly, low-cycle fatigue tests are carried out on notched specimens in order to investigate the model capabilities in a multiaxial condition.

Estimating fatigue damage model constants with maximum likelihood method

Obtaining fatigue damage model constants for different damage models have long been of interest to many scientists and researchers interested in evaluating the reliability of materials. The conventional technique is to test the material for many numbers of cycles and fit the model to the stress-number of cycles to failure (S-N) curve. This manuscript proposes a new methodology to statistically obtain the fatigue model constants using Weibull based maximum likelihood (ML) function. Maximum likelihood theory for different stress levels is introduced. The ML function is then elaborated using the Weibull distribution as underlying distribution. Weibull based ML function is used to obtain the damage model constants. The proposed approach is then tested Coffin-Manson model using the data obtained from a thermo-mechanical fatigue test conducted for BGA solder joints. The model constants obtained using this approach is compared with available model constants and showed a good correlation.

Thermo-mechanical fatigue property of a surface-hardened AC4CH cast aluminum alloy

The thermo-mechanical fatigue life and microstructural change under a compression-tension thermo-mechanical cyclic loading of a surface-hardened Al-Si-Mg alloy casting are investigated. A surface cold working technique using steel balls is utilized for the surface hardening. Out-of-phase type thermo-mechanical fatigue tests have been performed with the temperature range of 323–523 K and the applied mechanical strain range of 0.75–2.0%. The surface-hardened material exhibits better thermo-mechanical fatigue property especially in a low cycle regime, together with higher generated stress, suggesting that it is even superior under stress-controlled loading. The differences in the stress–strain hysteresis loops and hardness variation near specimen surface between the materials with and without surface hardening have not vanished until final fracture. The observed difference in the thermo-mechanical property is attributed to some microstructural differences. The effects of damaging at the silicon particles to this difference are also investigated using the high-resolution microtomography technique. It has been clarified by microstructural observations that recovery and recrystallization are suppressed in the surface-hardened material. It can be inferred that such thermal stability is attributed to the multi-axial deformation by a combination of the surface hardening and the thermo-mechanical loading.