Thermomechanical Fatigue Loading Research Articles

Creep-Fatigue Interaction on Small Crack Propagation in a Single Crystal Superalloy under Thermomechanical Fatigue Condition

Recently, the high-efficiency gas turbine combined cycle power generation has become essential in operation as a backup power supply of renewable energy sources. In turbine blades and vane of the gas turbine operated as a backup power supply, there is growing concern about the damage due to thermo-mechanical fatigue (TMF) loading because of expecting increased starting/stopping cycle and frequent load fluctuation. This study investigated the small crack propagation behavior in single crystal Ni-based superalloy under the in-phase TMF loading cycle. The creep-fatigue interaction on the small crack propagation behavior was discussed based on the relationship between the crack growth rate and the creep J-integral, considering the small-scale creep at the crack tip. The experimental results indicated that the crack growth rates under the in-phase TMF loading cycle were significantly higher than those under the out-of-phase TMF condition at a given fatigue J-integral range. In addition, introducing the tension holding at the maximum temperature in the in-phase TMF loading cycle accelerated the crack growth rates. The small-scale creep stress field at the crack tip might dominate the small crack propagation behavior under the in-phase TMF loading with the high-temperature holding as the creep-fatigue interaction.

A Temperature-Dependent Viscoplasticity Model for the Hot Work Steel X38CrMoV5-3, Including Thermal and Cyclic Softening under Thermomechanical Fatigue Loading.

In this paper, a temperature-dependent viscoplasticity model is presented that describes thermal and cyclic softening of the hot work steel X38CrMoV5-3 under thermomechanical fatigue loading. The model describes the softening state of the material by evolution equations, the material properties of which can be determined on the basis of a defined experimental program. A kinetic model is employed to capture the effect of coarsening carbides and a new isotropic cyclic softening model is developed that takes history effects during thermomechanical loadings into account. The temperature-dependent material properties of the viscoplasticity model are determined on the basis of experimental data measured in isothermal and thermomechanical fatigue tests for the material X38CrMoV5-3 in the temperature range between 20 and 650 ∘C. The comparison of the model and an existing model for isotropic softening shows an improved description of the softening behavior under thermomechanical fatigue loading. A good overall description of the experimental data is possible with the presented viscoplasticity model, so that it is suited for the assessment of operating loads of hot forging tools.

Viscoplastic constitutive model considering strain rate sensitivity under multiaxial thermomechanical fatigue loading

AbstractStrain rate sensitivity will change the cyclic mechanical response of materials under multiaxial thermomechanical fatigue loading; thus, it will lead to errors in fatigue life prediction if strain rate sensitivity is not considered in constitutive simulation. In order to more accurately express the time/rate dependence of viscoplastic behavior, a strain rate sensitivity factor is proposed to modify the viscoplastic function of Chaboche model first. On this basis, a viscoplastic constitutive model considering strain rate sensitivity under multiaxial thermomechanical fatigue loading is proposed, which can describe the varying influence of strain rate sensitivity on cyclic mechanical behavior. Meanwhile, the exponential isotropic hardening rule with the linear term is adopted to characterize the initial rapid softening and subsequent stable softening of the material under fatigue loading at elevated temperature. Moreover, the material‐dependent nonproportional hardening coefficient and the loading path‐dependent rotation factor are used in the kinematic hardening rule to consider the effect of nonproportional additional hardening on the cyclic mechanical behavior of materials. Finally, the proposed method is verified by the stress–strain data of titanium alloy TC4 and Ni‐based superalloy GH4169 under uniaxial and multiaxial thermomechanical fatigue loadings, and a good agreement is obtained.

Influence of T6 heat-treating and over-ageing on out-of-phase thermo-mechanical fatigue behaviors of Al-Si-Cu alloy

In this study, the Influence of the T6 heat-treating and over-ageing process on the material behavior of cylinder head Al-Si-Cu alloy under out-of-phase thermo-mechanical fatigue at maximum temperatures of 250 °C to 350 °C was investigated. The specimen is subjected to a conventional T6 process that includes a 5 hours solution at 510 °C, an air quench, and 3 hours of ageing at 200 °C, some specimens (with and without the T6 process) were over-aged at 250 °C for 100 hours. The fracture surfaces of specimens under thermo-mechanical fatigue (TMF) conditions are examined using scanning electron microscopy (SEM). The Al-Si-Cu alloy exhibited a ductile characteristic, as shown by the SEM images. The quick cyclic softening happened early in the TMF cyclic stress response behavior, and afterward, the cyclic stress remained steady at decreasing strain amplitudes. Cracks in TMF damage behavior were mostly initiated by broken and debonded Si and intermetallic phases, and they commonly nucleated near the primary Si-matrix interface. The T6 treatment improved the fatigue lifetime considerably under TMF loadings for non-over-aged alloys. However, the T6 treatment had no significant effect on the over-aged alloy. • The material hardened and softened at <250°C, but softened at >250°C. • The A380-T6 alloy has more cyclic softening than the A380-T0 alloy (without heat-treating). • The reduction in crack initiations and the absence of fatigue striations indicated a slower propagation rate. • Temperature affected the softening rate due to dislocation annihilation and rearrangement. • T6 enhanced fatigue lifetime in non-over-aged alloys, but not in over-aged alloys.

Optimization of Thermo-Mechanical Fatigue Life for Eutectic Al–Si Alloy by the Ultrasonic Melt Treatment

The eutectic cast Al–Si alloys with excellent high-temperature and casting performance are widely used in engine pistons. During frequent starts and stops, the thermo-mechanical fatigue (TMF) is the most important failure cause. Ultrasonic melt treatment (UT) was chosen to compare and investigate the influence of micro-structures on fatigue life and damage mechanisms of as-cast (AC) eutectic Al–Si alloys under TMF loading. After UT, the grain size, primary Si, and intermetallic particles are reduced significantly in the alloy; fatigue life increases obviously. As a result of pilling-up of dislocations, the competitive effects of the critical strain/stress for fatigue crack nucleation can be found. There are two different crack initiation mechanisms under TMF: one is primary Si fracture for AC alloys with limited critical strain/stress for fatigue crack nucleation at fractured Si particles, and the other is primary Si debonding for UT alloys with increasing critical fracture strain/stress. After the crack initiation, the fractured or debonded primary phases provide the advantages for the further development of main cracks for both alloys. The UT alloy (805 ± 253 cycles) has about twice the TMF life of the AC alloy (403 ± 98 cycles). The refinement of micro-structures is instrumental in improving the fatigue resistance and life of TMF for the UT alloy.

Thermomechanical fatigue damage behavior and deformation mechanism of coke drum with Cr-Mo steel

ABSTRACT Damage behaviour and deformation mechanism of SA387Gr11CL2 (1.25Cr-0.5Mo) low alloy steel for coke drum in the thermomechanical fatigue regime are investigated. The experimental results indicate that a main macroscopic crack is detected in the gauge section of each testing specimen. Multiple crack initiation sources are observed in the fracture surface. There are a great deal of secondary cracks in the crack propagation region of the fracture surface. Longitudinal-section microcrack initiates on the specimen surface and its propagation path is predominantly transgranular. When the crack propagation arrives at the threshold with the cyclic number increase, crack bifurcation is obviously observed. Fatigue is the dominant damage of SA387Gr11CL2 low alloy steel under the thermomechanical fatigue loading. The dominant deformation mechanism of SA387Gr11CL2 low alloy steel during thermomechanical fatigue cycle is wavy slip of dislocations. The types and morphologies of carbides in SA387Gr11CL2 low alloy steel are not changed after plastic deformation.

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

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

Active Crack Obstruction Mechanisms in Crofer® 22H at 650 °C.

Increased cyclic loading of components and materials in future thermal energy conversion systems necessitates novel materials of increased fatigue resistance. The widely used 9–12% Cr steels were developed for high creep strength and thus base load application at temperatures below 620 °C. At higher temperature, these materials present unstable grain structure, prone to polygonization under thermomechanical fatigue loading and limited resistance to steam oxidation. This seminal study compares thermomechanical fatigue resistance and long crack propagation of the advanced ferritic-martensitic steel grade 92 and Crofer® 22H, a fully ferritic, high chromium (22 wt. %) stainless steel, strengthened by Laves phase precipitation. Crofer® 22H features increased resistance to fatigue and steam oxidation resistance up to 650 °C. Both thermomechanical fatigue (crack initiation) and residual (crack propagation) lifetime of Crofer® 22H exceeded that of grade 92. The main mechanisms for improved performance of Crofer® 22H were increased stability of grain structure and “dynamic precipitation strengthening” (DPS). DPS, i.e., thermomechanically triggered precipitation of Laves phase particles and crack deflection at Laves phase-covered sub-grain boundaries, formed in front of crack tips, actively obstructed crack propagation in Crofer® 22H. In addition, it is hypothesized that local strengthening may occur near the crack tip because of grain refinement, which in turn may be impacted by testing frequency.

The effect of dwell on thermomechanical fatigue behaviour of Ni-base superalloy Inconel 713LC

Ni-base superalloy Inconel 713LC has been subjected to in-phase and out-of-phase thermomechanical fatigue loading in the temperature range from 500 °C to 900 °C. Both constant strain rate cycling and cycling with 10 min dwell at peak temperature were applied. The effect of phase shift and dwells on the cyclic stress response and damage mechanism was studied. The cyclic stress-strain curves and fatigue life curves were evaluated. The microstructure was investigated in SEM and TEM and the effect of dwells on thermomechanical fatigue characteristics is documented. The acquired results further discussion on the damage mechanism operating under specific loading conditions.

Modelling of Strain-Controlled Thermomechanical Fatigue Testing of Cast AlSi7Cu3.5Mg0.15 (Mn, Zr, V) Alloy for Different Aging Conditions

Thermomechanical fatigue loadings (TMF) applied on components in a certain temperature range with a variable state of stress (tensile and/or compression) produce a localized concentration of plastic strains that results in crack initiation and propagation. The time evolution of plastic strains must be known a priori to predict the lifetime of a part submitted to TMF loadings, which requires an extensive campaign of mechanical characterization conducted at different temperatures and aging conditions. Such a campaign was proposed for the aluminum alloy AlSi7Cu3.5Mg0.15 (Mn, Zr, V), which is recognized as being creep resistant. Combined isothermal low-cycle fatigue and isothermal creep tests were performed on this alloy to determine the constitutive parameters based on the Lemaître and Chaboche (LM&amp;C) viscoplastic model. These laws were implemented within the finite element simulation software (Z-set) to model the response of the alloy to a thermomechanical fatigue test. The results of TMF Z-Set simulations, using the LM&amp;C model adapted for two aging conditions, were then compared with results obtained from “Out of Phase” thermomechanical fatigue testings (OP-TMF) performed on a Gleeble 3800 machine. The modelling of the OP-TMF test revealed the complexity of the mechanical behavior of the material induced by the temperature gradient prevailing along with the cylindrical specimen. It was found that a better prediction of the evolution of plastic strains requires taking into account a larger range of plastic strain rates conditions for the determination of the constitutive law and eventually includes the role of the microstructure in the evolution of the material behavior, starting first with the yield stress.

Zirconia Cantilever Fixed Dental Prostheses Supported by One or Two Implants: An In Vitro Study on Mechanical Stability and Technical Outcomes

To evaluate the mechanical stability of highly translucent zirconia (Zr) cantilevered fixed dental prostheses (cFDPs) and to investigate the influence of the number of implants (one versus two) supporting cFDPs with different restorative materials on their mechanical stability and load-bearing capacity. Thirty-two specimens consisting of implant-supported prostheses embedded in resin blocks were fabricated. Sixteen specimens received one implant (bone-level implant, 4.1-mm diameter, 13-mm length; Straumann) to support two-unit cement-retained cFDPs with one extension unit and the other 16 received two implants (bone-level implant, 4.1-mm diameter, 13-mm length; Straumann) positioned corresponding to the missing maxillary central incisors to support three-unit cement-retained cFDPs with one extension unit. Two different prosthetic materials, chromium-cobalt (Cr-Co; Wirobond C+, Bego) and highly translucent Zr (Lava Plus, 3M ESPE) were selected to fabricate the two- and three-unit cFDPs. Standardized twoand three-unit Cr-Co frameworks (CC-I, n = 8; CC-II, n = 8) and highly translucent Zr frameworks (Zr-I, n = 8; Zr-II, n = 8) with a 6-mm cantilever extension were fabricated using CAD/CAM (EOS M 290). Following thermomechanical fatigue loading, the specimens were tested for fracture resistance under static loading. The influence of restoration material and number of supporting implants on fracture resistance were tested using two-way analysis of variance (ANOVA). The level of statistical significance was set below 5% (α < .05). All specimens survived aging. The mean (± standard deviation) fracture resistance values were 416.25 (± 42.71) N for Zr-I, 548.75 (± 75.41) N for Zr-II, 601.0 (± 41.51) N for CC-I, and 664.5 (± 37.59) N for CC-II. CC and Zr group specimens showed significantly different fracture resistance results (P < .001). The number of implants significantly influenced the fracture resistance of Zr groups (P = .001), whereas the influence was not significant for CC groups (P = .089). Within the limitations of this in vitro study, highly translucent zirconia cFDP frameworks demonstrated the potential to withstand reported physiologic occlusal forces applied in the anterior region. The increase in the number of implants supporting zirconia cFDPs significantly contributed to achieving higher fracture resistance values.

Multiaxial thermo-mechanical fatigue damage mechanism of TC4 titanium alloy

In this paper, the experimental investigation was carried out on the TC4 titanium alloy to understand the cyclic deformation behavior and fatigue damage mechanism of materials under multiaxial thermo-mechanical loading. The results showed that the fast cracking mode of the material may be activated by the combined action of high temperature, tensile stress and shear stress, inducing a fast degradation in the axial mechanical properties of the material, which results in a drastic decrease in the failure life of the material. Moreover, it is also found that the non-proportional additional hardening and the tensile mean stress can increase the fatigue and oxidation damages of the material, and the material damage behavior is temperature dependent and time dependent. It should be pointed out that the damage mechanisms identified in this paper can reasonably explain the life law under uniaxial isothermal fatigue loading with and without dwell time, uniaxial and multiaxial thermo-mechanical fatigue loadings.

Using machine learning to predict lifetime under isothermal low-cycle fatigue and thermo-mechanical fatigue loading

In this article, machine learning is used to predict lifetime under isothermal low-cycle fatigue and thermo-mechanical fatigue loading, both of which represent the most complex loadings that couple creep, fatigue and oxidation damage. A uniaxial fatigue and fatigue–creep dataset, which was obtained for temperatures of between 300°C and 600°C for a low-alloy martensitic steel, is utilized in this study. Two different machine learning based approaches to lifetime prediction are demonstrated. The first approach is based only on a shallow neural network, whereas the second approach is proposed as a combination of a sequence learning based model – either long short-term memory network or gated recurrent unit – with the shallow neural network. A good correlation between the experiment and the prediction suggests that lifetime under complex thermo-mechanical loading can be reasonably predicted via the proposed machine learning based damage models.

Investigation of damage mechanisms and short fatigue crack growth during thermomechanical fatigue loading of the nickel‐based superalloy Inconel 100

AbstractGrowing demands on gas turbines, meaning lower consumption and higher efficiency, is accompanied by higher temperatures inside the turbine. Better lifetime prediction concepts for thermomechanical fatigue (TMF) loading of gas turbine components are needed to ensure safe operation. The influence of the TMF loading history—either in‐phase (IP) or out‐of‐phase (OP)—on crack initiation and short crack growth is studied for the cast nickel‐based super alloy IN100, which is commonly used as blade material. Therefore, smooth specimens are loaded at two different mechanical strain levels with, respectively, two phase relationships, using a temperature cycle T = 300–950°C and T = 300–850°C. The replica technique is used to detect cracks in the order of tens of micrometers. The differences in crack initiation and short crack growth are studied for the different loading conditions and the development of the crack length evolving from multiple short cracks is investigated.

Constitutive model for thermomechanical fatigue conditions of an additively manufactured combustor alloy

In this study, the mechanical response of an additively manufactured nickel-based combustor alloy, subjected to thermomechanical fatigue (TMF) loadings has been investigated and modelled. TMF tests were performed in both in-phase and out-of-phase conditions with different strain ranges and temperature ranges of 100 °C–450 °C and 100 °C–600 °C, respectively. The smooth specimens were manufactured in two different orientations to study the influence of anisotropy, and the specimens were machined to final dimensions with conventional techniques. A constitutive model with focus on describing the mid-life behaviour was developed where the total inelastic strain was divided into one plastic (rate-independent) and one creep (rate-dependent) part, to be able to describe both the rate-dependent effects from TMF conditions as well as rate-independent responses. A cycle jumping procedure was used, which enables to simulate the mid-life response of the material for TMF as well as low-cycle fatigue conditions within three simulated loading cycles.

Multiaxial isothermal and thermomechanical fatigue behavior of 316LN stainless steel

Effects of multiaxial loading on the isothermal fatigue (IF) and thermomechanical fatigue (TMF) behavior of 316LN stainless steel were comparatively studied. Results showed that the shape of axial stress-strain hysteresis loops was asymmetric in the tensile and compressive half-cycle while the shear hysteresis loops were symmetric. The kernel average misorientation value and low-angle grain boundaries fraction significantly increased, however, the fraction of Σ3 twin boundaries drastically decreased under multiaxial loading, where the coincidence site lattice value (Σ) represented the reciprocal density of coincident lattice sites between the two adjoining grains. The multiaxial non-proportional loading produced much more damage in comparison to uniaxial loading, consequently a significant reduction in fatigue life, regardless of the IF and TMF loadings. Furthermore, the IF life was the shortest, and the longest life occurred in out-of-phase TMF tests.

Near-component testing of materials for cylinder heads to determine thermomechanical fatigue under superimposed high-frequency mechanical loads

Abstract Due to higher combustion chamber temperatures and pressures in efficient combustion engines, both the high-cycle and thermomechanical fatigue loads on service life-critical components, such as the cylinder head, are increasing. Material comparisons and analysis of damage behavior are very expensive and time-consuming using component tests. This study therefore develops a test method for cylinder head materials that takes into account the combined loading conditions from the above-mentioned loads and allows realistic temperature transients and gradients on near-component samples. The near-component cylinder head sample represents the failure-critical exhaust valve crosspiece and is tested in a test rig specially designed with the aid of conjugate heat transfer simulations. In the test rig, the sample is subjected to thermal stress by a hot gas burner and to mechanical stress by a high-frequency pulsator. Optical crack detection allows permanent observation of fatigue crack growth and crack closure during the test. Fractographic and metallo-graphic examinations of the fracture areas as well as analyses of the damage patterns show that loads close to engine operation can be set in this way and their influences on the damage can be monitored.

Thermomechanical fatigue life due to scatter in constitutive parameters

For critical component application, such as aerospace turbine rotors, it is imperative to be able to make accurate in-service material behaviour and component life predictions for both design and monitoring of component life. The development of such predictive capability is dependent on the quality of the experimental data from which the material parameters are derived. This paper shows the effect that scatter which may be present within experimental data, manifesting itself within the constitutive parameters derived from this data, has on the resulting fatigue crack initiation life of the nickel-based superalloy RR1000. Industrial relevance was added to this investigation by the use of flight representative thermomechanical fatigue loading cycles and state of the art material behaviour and fatigue crack initiation models used within the finite element simulations conducted. The effect of the ‘scatter in’ to the modelling approach on the outcoming predictions is made via a Monte-Carlo analysis. This analysis consisted of running the same simulation several times, but with the experimentally determined and validated ‘baseline’ constitutive parameters varied via correction factors built into the model, for each run via a singular value decomposition procedure. It was found that small ‘scatter in’ has only a very localised ‘scatter out’ effect on the crack initiation predictions under the flight representative loading.

Thermomechanical fatigue behaviour and damage mechanisms in a 9% Cr steel: Effect of strain rate

Thermomechanical fatigue (TMF) loading is inevitable during the operation of ultra-supercritical power plants. In the present work, TMF tests are carried out at strain rates of 5 × 10−5/s to 4 × 10−4/s and in-phase (IP) and out-of-phase (OP) to investigate the cyclic deformation and damage mechanisms of P92 steel. The results reveal that the increase in strain rate leads to a significant improvement in fatigue life under both IP-TMF and OP-TMF, in which OP-TMF induces more severe damage to fatigue life than IP-TMF. It was found that the fatigue life reaches a maximum after a strain rate of 1 × 10−4 s−1. It is important to note that the action of dynamic strain ageing (DSA) is strain rate-, fatigue cycle- and phase angle-dependent. It was also demonstrated that the effect of the strain rate on the fatigue life is a combined effect of DSA, fatigue cracks, creep voids and oxidation. Additionally, the predictive abilities of various traditional life prediction methods under various loading conditions are comparatively assessed.

Influence of Load Interaction between Creep and TMF on the Life of Single Crystal Nickel-based Superalloy

Experimental investigation on the influence of load interaction on the life of Single Crystal Nickel-based Superalloy is conducted. Three kinds of load spectrums considering single and coupled failure mode are designed. Life tests are carried out under creep, thermo-mechanical fatigue (TMF), and creep-TMF interaction loading. The test results show that test lives, under the creep-TMF interaction loading, are lower than the life predictions given by the linear damage accumulation (LDA) rule, indicating that the load coupling can accelerate the damage evolution process. The microstructure of fractured specimens shows that under the creep-TMF interaction loading, rafting cause more dislocations to accumulate in the at γ/γ′ phase boundary, which could be the evidence of life decrease.