Thermomechanical Fatigue Conditions Research Articles

Experimental investigation of thermomechanical fatigue behavior in directionally solidified Ni-based superalloy under in-phase and out-of-phase conditions

The fatigue behavior of a directionally solidified (DS) Ni-based superalloy was investigated under in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) conditions. Specimens were tested under strain control with a temperature range of 450–850 °C. In some tests, an optional 5 min out-of-phase dwell (OPD) time was introduced. In the case of IP TMF, the primary damage manifestation was predominantly intergranular cracks, whereas in the OP tests, the damage was mainly transgranular cracks. The fatigue life of IP TMF was found to be significantly shorter than that of OP TMF. This indicated that the intergranular cracks propagated faster than the transgranular cracks. The dwell time did not influence the dominant damage form; however, it led to an increase in the density of the microcracks. This increase resulted in a reduced lifetime, leading to lower stress amplitudes and higher plastic strain amplitudes compared with the continuous cycle tests. In addition, cycle hardening became apparent during the initial stages of cyclic loading. The microstructures and damage evolution were examined to provide insights into the changes in fatigue life. A model for predicting the fatigue life of DS Ni-based superalloy under given test conditions has been assessed. The proposed fracture mechanics model, which outlines the failure mechanism of directionally solidified Ni-based superalloy under TMF conditions, has proven to be effective in predicting fatigue life.

Constitutive model of an additively manufactured combustor material at high-temperature load conditions

ABSTRACT In this paper, the high-temperature constitutive behaviour of an additively manufactured ductile nickel-based superalloy is investigated and modelled, with application to thermomechanical fatigue, low-cycle fatigue and creep conditions at temperatures up to 800 ∘ C. Thermomechanical fatigue tests have been performed on smooth specimens in both in-phase and out-of-phase conditions at a temperature range of 100 − 800 ∘ C, and creep tests at 625 ∘ C, 700 ∘ C, 750 ∘ C and 800 ∘ C. Additionally, low-cycle fatigue tests at different strain ranges and load ratios have been performed at 700 ∘ C, and tensile tests have been performed at 600 ∘ C, 700 ∘ C and 800 ∘ C. A clear anisotropic mechanical response is obtained in the experiments, where the anisotropic effects are larger at high stress levels in creep loadings. To capture this behaviour, a rate-dependent strain based on a double-Norton model has been adopted in the model, by which the creep and mid-life response of the thermomechanical fatigue tests can be simulated with good accuracy.

Generalization of crack growth mechanisms under isothermal and thermomechanical fatigue by COD and ERR parameters

An experimental-computational study of the crack opening displacement (COD) was performed to evaluate the ability of this parameter through the hysteresis loop area and energy release rate (ERR) to characterise fatigue crack growth under isothermal and thermomechanical conditions. To this end, a range of experimental crack growth tests were carried out including isothermal fatigue (IF), creep-fatigue interaction (CFI), in-phase (IP), and out-of-phase (OOP) thermomechanical fatigue (TMF). The crack growth experimental results in terms of COD and ERR were interpreted based on multi-physics finite element analyses of the stress–strain and displacement fields performed by incorporating Maxwell 3D, Fluent, and the transient structural modules of ANSYS. As a result of measurements and computations, the COD on the outer surface of the SENT specimen at the location of the original notch was found to be directly related to the crack tip opening displacement (CTOD). To gain insight into the fatigue crack growth mechanism operating under isothermal and TMF conditions, the fracture surfaces of all the tested specimens were examined using a scanning electron microscope to compare the morphology and changes in the dominant failure mechanisms. It was found that propagating cracks under isothermal and thermomechanical conditions interact with the microstructure of the material, and supporting fractography evidenced subtle differences in fracture mechanisms in terms of COD and ERR parameters. The relationship between the experimentally determined energy release rate and degradation function in the fatigue phase-field fracture methodology is discussed.

Numerical and Experimental Analysis of Strength Loss of 1.2709 Maraging Steel Produced by Selective Laser Melting (SLM) under Thermo-Mechanical Fatigue Conditions.

The result of the development of additive manufacturing (AM) methods is the increasing use of the selective laser melting (SLM) method as a technique for producing tooling for injection moulds and die casting pressure moulds from maraging steel powders. The mould components are subjected to varying thermo-mechanical loads during these operations. This paper presents a numerical model that is used to predict the fatigue life of a material that is loaded with a time-varying temperature field according to the classic and modified Coffin test. Using a computational model, the temperature changes in the resistance-heated specimen and the stress and strain fields that are caused by this phenomenon were determined. Using three different multiaxial fatigue criteria, the fatigue life of SLM steel was determined. Numerical calculations were verified using experimental thermal fatigue tests on 1.2709 SLM steel that was aged at 490 °C as well as via metallographic tests. The numerical model was used to predict the durability of the same steel aged at 540 °C. The effect of specimen clamping conditions on the fatigue life of SLM steel was determined numerically. The value of the decrease in strength of SLM steel as a result of the increasing number of cycles of temperature changes was determined experimentally; a great influence of ageing temperature on fatigue life was found. Changes in the structure of steel occurring during cyclic changes in temperature are presented.

Isothermal and thermo-mechanical fatigue-crack-growth analysis of XH73M nickel alloy

In service-power engineering and aviation, gas-turbine engine structures are operated at elevated temperatures and under extensive variable mechanical loading, which is classified as thermomechanical fatigue (TMF). This coupling effect leads to flaw initiation and growth in components fabricated from thermally resistant nickel-based alloys. This paper presents experimental crack-growth data for isothermal pure fatigue, in-phase and out-of-phase TMF conditions. A crack-growth testing method is developed using inductive heating/forced convective cooling and direct crack-tip opening displacement techniques. The crack-growth experimental results are interpreted based on finite element analyses of the stress–strain rate fields at the crack tip under TMF conditions. Therefore, multi-physics numerical calculations are employed for the interaction of the heat loss from magnetic-field eddy currents with forced convective air cooling through a computer fluid dynamics model, which presents a nonuniform mechanical elastic–plastic stress distribution. Consequently, the dependence of the new nonlinear stress intensity factors on the crack length are obtained for each of the isothermal and thermomechanical cyclic deformation conditions. The polycrystalline XH73M nickel-based alloy tests performed show that, from the crack-growth acceleration viewpoint, the following order of arrangement of fatigue fracture diagrams is formed: isothermal pure fatigue, in-phase TMF, out-of-phase TMF, and isothermal pure fatigue at T = 400 °C and T = 23 °C. The differences and corresponding conclusions based on the interpretation of the experimental results of the crack growth rate characteristics at elevated temperatures in terms of elastic and nonlinear fracture resistance parameters are discussed.

Thermomechanical fatigue of round tube specimens manufactured by precision directional solidification casting method

AbstractIn the present study, a superalloy round tube specimen prepared by directional precision casting without mechanical turning is experimentally investigated under the thermomechanical fatigue conditions of the in‐phase and out‐of‐phase thermomechanical angels from 500°C to 1000°C. The deformation behavior and damage mechanism of the round tube specimens under TMF load are discussed. It was confirmed that the tension–compression asymmetry under TMF cycling is affected by temperature–load phasing and mechanical strain range. Moreover, the damage to round tube specimens includes creep, fatigue and oxidation. Due to the concentrated stress, the rough surface of the specimen provides suitable locations for crack initiation. A novel life prediction approach that combines the Neu–Sehitoglu model and modified Manson–Coffin model was proposed in response to the traditional life model's insufficient prediction performance. This method may explain the TMF life of precision cast specimens.

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.

Accounting for anisotropic, anisothermal, and inelastic effects in crack initiation lifing of additively manufactured components

AbstractThe crack initiation life of a ductile additively manufactured nickel‐based superalloy is studied and modeled for low‐cycle fatigue and thermomechanical fatigue conditions up to 600°C. Isothermal experiments were performed on smooth specimens at temperatures up to 600°C with different applied strain ranges. Additionally, thermomechanical fatigue experiments at 100–450°C and 100–600°C were performed on smooth specimens under in‐phase and out‐of‐phase conditions. A life prediction model accounting for the anisotropy was developed, where the temperature cycle is accounted with a ‐functionality, generating good agreements with the experiments. The model was also validated on notched specimens undergoing thermomechanical fatigue conditions at 100–500°C using simplified notch correction methods.

The effect of dwell times and minimum temperature on out-of-phase thermomechanical fatigue crack propagation in a steam turbine steel—Crack closure prediction

Exploring crack growth behaviour is needed to establish accurate fatigue life predictions. Cracked specimens were tested under strain-controlled out-of-phase thermomechanical fatigue conditions. The tests included dwell times and three different minimum temperatures. Higher minimum temperature gave faster crack growth rates while the additions of dwell times showed no effects. Crack closure was observed in all the tests where the addition of dwell times and change in minimum temperature displayed little to no effect on crack closure stresses. Finite element models with a sharp stationary crack and material parameters switching provided acceptable predictions for the maximum, minimum, and crack closure stresses.

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.

Assessment of thermo-mechanical fatigue in a nickel-based single-crystal superalloy CMSX-4 accounting for temperature gradient effects

Turbine blades are working under complex thermal gradient mechanical loading conditions. In the present study, a nickel-based single-crystal superalloy CMSX-4 is experimentally and computationally investigated under the thermo-mechanical fatigue (TMF) and thermal gradient mechanical fatigue (TGMF) loading conditions of the in-phase and out-of-phase thermal-mechanical angel. The fatigue life of TGMF with and without the thermal barrier coating was studied. It was confirmed that the lifetime of TGMF is affected by the temperature gradient significantly and substantially differs from conventional isothermal high temperature fatigue and thermo-mechanical fatigue. Since conventional fatigue models fail to characterize the TGMF, a modified model considers temperature gradient effects and thermal-mechanical phase angle and can describe the TGMF lifetime of the single-crystal superalloy reasonably.

Effects of thermal cycling and microstructure on the fatigue crack propagation in forged titanium–aluminide alloys under thermomechanical fatigue conditions

The mechanical properties and fatigue strength of titanium–aluminide (TiAl) alloys are sensitive to the environmental conditions, such as temperature, and their microstructures can be controlled by thermomechanical processing. In this study, two samples of a forged TiAl alloy were manufactured through high-temperature forging followed by different heat treatments to obtain a near-lamellar microstructure and a triplex microstructure, which contains lamellar and equiaxed γ and β grains. The fatigue crack propagation tests were conducted under isothermal low-cycle fatigue (LCF) and the out-of-phase type thermomechanical fatigue (OP-TMF) conditions. The experimental results indicated that the microstructure strongly affects the crack propagation behavior because the near-lamellar microstructure had a higher resistance to fatigue crack propagation compared to the triplex microstructure. This also revealed that the fatigue crack was remarkably accelerated by the OP-TMF conditions compared to the LCF conditions. The oxygen diffusion into the β phase occurred at the crack tip and lead to the transformation of the β phase into the brittle α phase. The results of the scanning electron microscope (SEM), energy dispersive X-ray (EDX), and electron backscatter diffraction (EBSD) analyses indicated that this transformation induced the acceleration of crack propagation under the OP-TMF loading conditions.

A TMF crack propagation model considering time dependency and load sequence effects

Due to increasing temperatures and mechanical loads in aero engine turbine components, the prediction of thermomechanical life requires accurate crack propagation calculations under thermomechanical Fatigue (TMF) conditions. Based on a linear elastic crack propagation model which considers pure cyclic crack propagation behavior only, a crack propagation model was developed to accurately predict TMF crack propagation life. It is considering short crack behavior by an El Haddad approach, time dependent effects by a combined creep and oxidation model, as well as cyclic and static time dependent load sequence effects described by a modified Willenborg model. Furthermore, an approach was developed to calculate the time dependent threshold behavior. The model can be applied to arbitrary mission profiles.A total of 29 parameters are required to describe all considered effects. As an example, these were calibrated by isothermal cyclic crack propagation, threshold and creep crack propagation tests on Inconel 718 at three temperatures. TMF crack propagation tests were carried out to validate the capability of the model. It is shown that the calculated crack propagation rates and thresholds, and consequently the predicted crack propagation life, match the measured data very well for isothermal as well as for TMF conditions.

Temperature-Dependent Fatigue Characteristics of P91 Steel

Abstract In this paper, the experimental investigations, constitutive description and numerical modelling of low-cycle fatigue behaviour of P91 steel in non-isothermal conditions are presented. First, experimental tests are performed to recognise different aspects of material behaviour. Then, an appropriate constitutive model is developed within the framework of thermodynamics of irreversible processes with internal state variables. The model describes two phases of cyclic softening, related to plastic mechanisms. An important goal of the presented research is to include thermomechanical coupling in the constitutive modelling. Next, the model parameters are identified based on the available experimental data. Some parametric studies are presented. Finally, numerical simulations are performed, which indicate the significant influence of thermomechanical coupling on the response of the constitutive model in thermomechanical fatigue conditions.

The prediction of crack propagation in coarse grain RR1000 using a unified modelling approach

The polycrystalline nickel-base superalloy RR1000 is used as turbine rotor material in Rolls-Royce aero engines and has to withstand a wide variety of load and temperature changes during operation. In order to maximize the potential of the material and to improve component design, it is of great interest to understand, and subsequently be able to accurately model the crack propagation caused by thermo-mechanical fatigue conditions. In this work, experimental data is analysed and used to inform unified modelling approaches in order to predict the crack propagation behaviour of RR1000 under a variety of stress-controlled thermo-mechanical fatigue conditions.

Effect of microstructure on fatigue crack propagation in a titanium-aluminide alloy under thermomechanical fatigue conditions - Current activities in the subcommittee of Titanium-Aluminide alloys in the Society of Materials Science, Japan (JSMS) -

Titanium aluminide (TiAl) alloys have attracted to considerable interest as a material of blade in the low-pressure turbine section of aero engines since their superior specific strength. The mechanical properties and strengths of TiAl alloys are strongly sensitive to their microstructure controlled with thermo-mechanical processing. The collaborative research has been started from 2017 by the subcommittee on Titanium-Aluminide alloys, JSMS Committee on High Temperature Strength of Materials, in order to get basic information about the influence of microstructure on the high-temperature strength. This paper is a part of the collaborative research. The crack propagation tests were carried out under the load controlled out-of-phase type TMF (OP-TMF) loading condition with temperature range 400 ℃ -760 ℃ . The effect of microstructure on fatigue crack propagation behavior in was discussed.

Toward a Better Understanding of Crack Growth in Nickel-Cast Alloys Under Creep-Fatigue and Thermo-Mechanical Fatigue Conditions

Abstract To ensure the feasibility of gas turbines, despite rising commodity prices and emission restrictions, an enhancement of both their efficiency and flexibility is necessary. The consequential higher loading of components at high temperature conditions calls for an increased use of damage tolerant design approaches. To still guarantee a safe operation, a sound understanding and reliable estimations for crack growth under service conditions are indispensable. In this paper, the results from several projects in this field conducted at the TU Darmstadt and involved partners are summarized to identify and describe the various influences on crack growth under creep-fatigue and thermo-mechanical fatigue (TMF) loading. The activation of damage mechanisms under TMF loading and interactions between them is dependent on the temperature cycle and the respective load phasing. Depending on the type of loading (force versus strain control), contrary influences of the phase shift on the TMF crack growth rates are found. This can partially be attributed to the differences in mean stress evolution. Crack initiation and propagation under creep-fatigue and TMF conditions are also often connected with significant scattering of initiation sites and crack growth rates. One reason for this nonuniform behavior is the interaction of geometric discontinuities with the microstructure. To investigate the role of the local grain structure for crack initiation and propagation, in situ observation techniques for crack tip movement and local strain fields were applied. Harsh gradients in the local deformation behavior were identified as origins of secondary crack initiation. To describe crack growth under creep-fatigue and TMF conditions, the linear accumulation model “O.C.F.” was developed. It is based on the contributions of fatigue, creep, and oxidation to crack growth per load cycle. This model is capable of reproducing the effects of time-dependent damage, different load ratios, TMF phase shifts, as well as component geometry. Substantial advantages of this method are its independence from empiric correction factors to assess changing load cycle forms and the possibility to give analytic estimations without the need of extensive data processing. The model is currently validated for three nickel cast alloys, also including single-crystalline and directionally solidified cast variants, different creep-fatigue and TMF loading scenarios, and crack geometries. The model's linear formulation allows assessing the dominant driver of crack growth at each stage of an experiment. These predictions are compared with fractographic investigations and in situ observations of crack paths to identify the mechanisms of crack growth under different TMF load cycle forms.

Isothermal and thermomechanical fatigue behaviour of type 316LN austenitic stainless steel base metal and weld joint

Thermomechanical fatigue (TMF) studies were carried out on type 316 LN austenitic stainless steel (SS) base metal (BM) and its weld joint (WJ) with a temperature interval of 623–873 K under in-phase (IP) and out-of-phase (OP) combinations of mechanical strain and temperature. Tests were performed under mechanical strain control mode using the strain amplitudes in the range, ±0.25% to ±0.6%. Besides, isothermal low cycle fatigue (IF) tests were also conducted on the weld joint at 873 K. TMF cycling resulted in a slightly higher cyclic stress response (CSR) compared to IF loading for the weld joint. The IP TMF cycling yielded a higher plastic strain amplitude in comparison to OP TMF for both the base metal and the weld joint. Cyclic life of the weld joint is found to be inferior to that of the base metal. The fatigue lives exhibited by the base metal and the weld joint varied in the following sequence: IP TMF < IF < OP TMF. Preferential crack initiation in the weld region owing to embrittlement of transformed δ-ferrite was noticed with an increase in the mechanical strain amplitude under both TMF and IF cycling conditions. Crack initiation and propagation generally occurred in transgranular mode under IF and OP TMF and mixed (trans-plus intergranular) mode under IP TMF cycling. Electron back scattered diffraction (EBSD) measurements were employed to investigate the substructural development in terms of local misorientation distribution, inverse pole figure (IPF) maps and boundary dislocation density (ρ) in the weld metal region, under IF and IP TMF cycling. The dislocation density was found to decrease for both IF and IP TMF cycling compared to the as-welded condition, with the decrease being higher for IF cycling. The TMF data generated for the type 316LN SS base metal and weld joint at different strain ranges in the present study clearly demonstrated a lack of conservatism associated with the current design practice which is based on the IF data at the maximum temperature (Tmax) of the service cycle.

Lifing the Effects of Crystallographic Orientation on the Thermo-Mechanical Fatigue Behaviour of a Single-Crystal Superalloy.

Thermo-mechanical fatigue (TMF) is a complex damage mechanism that is considered to be one of the most dominant life limiting factors in hot-section components. Turbine blades and nozzle guide vanes are particularly susceptible to this form of material degradation, which result from the simultaneous cycling of mechanical and thermal loads. The realisation of TMF conditions in a laboratory environment is a significant challenge for design engineers and materials scientists. Effort has been made to replicate the in-service environments of single crystal (SX) materials where a lifing methodology that encompasses all of the arduous conditions and interactions present through a typical TMF cycle has been proposed. Traditional procedures for the estimation of TMF life typically adopt empirical correlative approaches with isothermal low cycle fatigue data. However, these methods are largely restricted to polycrystalline alloys, and a more innovative approach is now required for single-crystal superalloys, to accommodate the alternative crystallographic orientations in which these alloys can be solidified.

Isothermal and thermomechanical fatigue behavior of Inconel 718 superalloy

The fatigue behavior and damage mechanisms of Inconel 718 are investigated and discussed under thermomechanical fatigue (TMF) and isothermal fatigue (IF) test conditions. The experimental results show that cyclic temperature accelerates fatigue degradation compared to isothermal equivalents, and the fatigue life under in-phase (IP) TMF mode is shorter than that under out-of-phase (OP) mode. The S-N data can be fitted by Coffin-Manson equation under LCF condition, and TMF lifetime can be predicted by the model based on hysteresis loop energy with a comprehensive consideration of mechanical and thermal strains.