Thermomechanical Fatigue Tests Research Articles

Impact of Combined Zr, Ti, and V Additions on the Microstructure, Mechanical Properties, and Thermomechanical Fatigue Behavior of Al-Cu Cast Alloys

The effects of minor additions of the transition elements Zr, Ti, and V on the microstructure, mechanical properties, and out-of-phase thermomechanical fatigue behavior of 224 Al-Cu alloys were investigated. The results revealed that the introduction of the transition elements led to a refined grain size and a finer and much denser distribution of θ″/θ′ precipitates compared to that of the base alloy, which enhanced the tensile strength but reduced the elongation at both room temperature and 300 °C. Constitutive analyses based on theoretical strength calculations indicated that precipitation strengthening was the primary mechanism contributing to the strength of both tested alloys at room temperature and 300 °C. The out-of-phase thermomechanical fatigue test results showed that the addition of transition elements caused a slight decrease in the fatigue lifetime, which was mainly attributed to the reduced ductility and higher peak tensile stress at low temperatures. During the fatigue process, the transition element-added alloy exhibited a lower coarsening ratio, indicating higher thermal stability, which mitigated the negative impact of the reduced ductility on the fatigue performance to some extent. Considering their various properties, the addition of Zr, Ti, and V is recommended to improve the overall performance of Al-Cu 224 cast alloys.

Deformation and microregion fracture mechanisms in type 316L welded joint under isothermal and thermo-mechanical fatigue loadings

Isothermal (IF) and thermo-mechanical fatigue (TMF) tests were conducted on 316L welded joints to investigate the effects of phase angle (in-phase (IP), clockwise-diamond (CD), and out-of-phase (OP)) and mechanical strain amplitude (±0.4 %, ±0.5 %, ±0.6 %) on deformation and microregion fracture mechanisms. Results reveal that increasing strain amplitude induces a higher softening rate, reduced fatigue life, and more pronounced dynamic strain aging (DSA). Nevertheless, there is a complicated discrepancy in the cyclic deformation between various phase angles. At high strain amplitudes, local embrittlement along the austenite/δ-ferrite interface induces cracking in the weld metal (WM) under both IF and TMF loadings, with TMF life increasing with phase angle. At a lower strain amplitude of 0.4 %, however, fracture location migration and different TMF life were observed. Microstructural analysis reveals that during TMF a lower strain amplitude of 0.4 %, the microstructure in both base metal (BM) and WM evolves from simple planar structures to low-energy substructures, characterized by an increase in kernel average misorientation, geometrically necessary dislocations, and low-angle grain boundaries, together with a reduction in high-angle grain boundaries and twin boundaries. In the WM, increasing phase angle tends to reduce dislocation density and reproduces planar dislocation structures due to enhanced DSA, thereby lowering cellular structure maturity. Conversely, in the BM, dislocation proliferation and interaction intensify, enhancing cellular structure maturity and de-twinning effect, which reduces BM fracture resistance and causes fracture migration from the WM to the BM. Moreover, active DSA under CD loading improves deformation resistance in both microregions, prolonging fatigue life and contributing to the observed different TMF life.

Thermo-mechanical fatigue analysis of ferritic stainless steel STS444LM for exhaust manifold application

This work presents thermo-mechanical fatigue (TMF) life analysis of ferritic stainless steel STS444LM recently developed to enhance the durability of the automotive exhaust manifolds. To determine cyclic material properties, isothermal tensile and low cycle fatigue (LCF) tests at temperature ranging from 200 °C to 900 °C under two different strain rates of 0.1 %/s and 0.01 %/s are performed. Based on these tests, the unified Chaboche visco-plasticity model is determined. Then, the TMF test using the V-shaped specimen subjected to the temperature cycle from 250 to 850 – 950 °C with three different dwell times at the maximum temperature is performed. An energy-based model based on the Morrow model is determined using TMF test data by incorporating effects of the dwelling time and maximum temperature.

Comparison of fatigue characteristics and failure analysis of Ni-based superalloy subjected to thermomechanical fatigue under different phase conditions

In this research, the thermomechanical fatigue (TMF) characteristics of a second-generation Ni-based single-crystal superalloy were investigated. The combined effects of temperature cycling and mechanical stress were examined using a strain-controlled approach under different TMF testing conditions. The results indicated that cyclic softening occurred during the early stages during in-phase (IP) loading. In contrast, out-of-phase (OP) tests exhibited cyclic hardening, as evidenced by an increase in the stress range, which intensified at higher mechanical strain amplitudes. This resulted in a shorter fatigue life compared to the IP TMF case. The crack distribution patterns and damage mechanisms on the cross-sectional and fracture surfaces of the failed specimens were analyzed. In the OP TMF tests, oxidation-assisted cracking was identified as the primary damage mechanism. The dominance of oxide layers was a key distinguishing feature. Additionally, twinning-related deformation and topologically close-packed (TCP) phase particles were observed in specimens subjected to a higher mechanical strain of 0.6%, where they ran parallel to the main crack, contributing to a reduction in lifetime. Conversely, the IP TMF cases were mainly dominated by creep damage mechanisms, with some signs of oxidation penetration observed during testing. The application of the Ostergren model to the fatigue life prediction of a Ni-based single-crystal superalloy under both conditions was evaluated. As this model was insufficient, a modified Ostergren model was proposed by incorporating the maximum and amplitude stresses in the power law forms.

Isothermal and thermomechanical fatigue crack growth behavior and modelling of 316LN stainless steel with the superposition of HCF loading

This work comparatively investigates the effect of superposition of high-cycle fatigue (HCF) loading on the crack growth behavior of 316LN stainless steel in isothermal fatigue (IF) and thermomechanical fatigue (TMF) tests. Results show that a deflection of the crack growth rate is observed in the tests without HCF loading. The superposition of HCF loading leads to a marked increase of crack growth rate in IF and out-of-phase TMF tests while a minimal variation in in-phase TMF test. Moreover, the crack growth rate in in-phase TMF test is smaller than the IF and out-of-phase TMF tests due to the different shape and evolution behavior of plastic zone at crack tip. Finally, the model based on the cumulative damage rule of time-dependent and cycle-dependent components is proposed, which considers the characteristics of plastic zone size and grain size and leads to a satisfactory prediction result.

Fatigue life prediction considering conversion of mean stress for titanium alloy under multiaxial thermo-mechanical random loading

This paper presents a thermo-mechanical fatigue (TMF) life prediction method for titanium alloy TA15 under multiaxial random amplitude loading. Through strain-controlled TMF tests, it was found that under full-reversed cyclic loading, there was a significant presence of mean stress in the stress response. The induced mean stress led to the anomalous phenomenon where the fatigue life of thermal phase angle out-of-phase (TOP) was shorter than that of thermal phase angle in-phase (TIP). Therefore, a new damage calculation model was proposed, which can take into account the pure fatigue damage considering mean stress conversion and the cumulative environmental damage. Uniaxial TMF tests, constant amplitude multiaxial TMF tests, and random amplitude multiaxial TMF tests are conducted to verify the proposed method. The predicted results for all types of tests are in a good agreement with the experimental data.

The High Temperature Strength of Single Crystal Ni‐base Superalloys – Re‐visiting Constant Strain Rate, Creep, and Thermomechanical Fatigue Testing

The present work takes a new look at the high temperature strength of single crystal (SX) Ni‐base superalloys. It compares high temperature constant strain rate (CSR) testing, creep testing, and out‐of‐phase thermomechanical fatigue (OP TMF) testing, which represent key characterization methods supporting alloy development and component design in SX material science and technology. The three types of tests are compared using the same SX alloy, working with precisely oriented <001>‐specimens and considering the same temperature range between 1023 and 1223 K, where climb controlled micro‐creep processes need to be considered. Nevertheless, the three types of tests provide different types of information. CSR testing at imposed strain rates of 3.3 × 10−4 s−1 shows a yield stress anomaly (YSA) with a YSA stress peak at a temperature of 1073 K. This increase of strength with increasing temperature is not observed during constant load creep testing at much lower deformation rates around 10−7 s−1. Creep rates show a usual behavior and increase with increasing temperatures. During OP‐TMF loading, the temperature continuously increases/decreases in the compression/tension part of the mechanical strain‐controlled cycle (±0.5%). At the temperature, where the YSA peak stress temperature is observed, no peculiarities are observed. It is shown that OP‐TMF life is sensitive to surface quality, which is not the case in creep. A smaller number of cycles to failure is observed when reducing the heating rate in the compression/heating part of the mechanical strain‐controlled OP‐TMF cycle. The results are discussed on a microstructural basis, using results from scanning and transmission electron microscopy, and in light of previous work published in the literature.

Thermomechanical and isothermal fatigue properties of MAR-M247 superalloy

Thermomechanical low-cycle fatigue (TMF) tests were performed on polycrystalline cast nickel-based superalloy MAR-M247 under in-phase (IP) and out-of phase (OP) loading in the temperature range of 500–900 °C. A constant heating and cooling rate of 5 °C.s−1 was selected. Furthermore, isothermal low-cycle fatigue (ILCF) tests with the same cycle period as TMF cycle were carried out at the maximal temperature of TMF cycle (900 °C). All tests ran under strain control and fully reversed cycle on solid cylindrical specimens in laboratory air. High resolution scanning electron microscopy and transmission electron microscopy were utilized to characterize the damage mechanism occurring during TMF and ILCF loading. The cyclic deformation and lifetime behaviour as well as damage mechanisms dependent on loading regime were examined. Results show that the TMF loading has a detrimental effect on the lifetime of MAR-M247. Lifetime reduction is most significant under IP loading. The fracture surfaces are characterized by typical striation fields, where the largest spacing between striations was observed for IP loading with prevailing intercrystalline cracking followed by OP and isothermal ILCF loading with transcrystalline crack propagation. To improve discussion of obtained results, experimental data were compared with damage and life prediction models.

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.

Thermo-mechanical fatigue behavior and microstructure evolution of 4Cr5Mo3V hot work die steel

Thermo-mechanical fatigue (TMF) testing was used to study the fatigue behavior of 4Cr5Mo3V hot work die steel to reflect the in-service conditions. The synergistic effect of cyclic temperature and mechanical load was investigated using a strain-controlled method. Our results confirmed that the cyclic softening degree of 4Cr5Mo3V steel increased as the mechanical strain amplitude get bigger, leading to a reduction of the fatigue life. The microstructure evolution was studied before and after the TMF testing, and it was found the maximum texture strength in as-received steel decreased from 7.22 to 2.34 when the strain amplitudes was set as ± 1.1 %. Experimental evidence showed that martensite decomposition, oxidation, dislocation density reduction and carbide segregation all contributed to the final TMF failure. Severe deformation existed in the vicinity of the newly precipitated M2C carbide during the TMF process, acting as fatigue crack initiation sources due to stress concentration. Four different models were interrogated including Basquin-Coffin-Manson model, Smith-Watson-Topper (SWT) model, Ostergren model and 3SE energy model; and it was found that SWT model yielded the best fatigue life description. Nevertheless, all the four models showed reasonable fitting capability within the 2x scatter band, which proved the reliability of the experiments established in this study.

Insights into thermomechanical fatigue behavior of nitrogen alloyed 316LN stainless steel under various temperature cycling ranges

Thermomechanical fatigue (TMF) failure has become one of the most important life-limiting factors of components exposure to the high temperature in the fourth-generation sodium-cooled fast reactor (SFR) nuclear power plant. Therefore, TMF tests at different temperature cycling ranges are performed to study the cyclic deformation behavior and damage mechanism of a nitrogen alloyed 316LN stainless steel. Moreover, the isothermal fatigue (IF) tests at the maximum temperature of TMF cycling are conducted for comparison. The cyclic hardening and softening response are analyzed, taking into account the phenomenon of dynamic strain aging which occurs at the temperature range of 350 °C–650 °C. Furthermore, the critical strain corresponding to the occurrence of serrated stress flow in engineering stress-strain curves keeps decreasing from 28.94% at 350 °C to 0.09% at 650 °C. The distribution of plastic deformation and the degree of strain localization are evaluated by the characterization results of kernel average misorientation map, high/low angle grain boundary map and dislocation configuration. The dominant damage mechanism and its corresponding cracking behavior in both IF and TMF tests are analyzed based on the observation of fracture surface morphology. In addition, the sequence order of fatigue life is determined as IF < IP-TMF < OP-TMF. Moreover, the maximum difference of fatigue life between IF test and TMF test occurs at the temperature range of 400 °C–600 °C, in which the IP and OP life is about 2.91 and 3.65 times larger than the IF life. The results in this work provide reliable data support and theoretical guidance for the fatigue design of high-temperature components in SFR.

Thermo-mechanical fatigue deformation and fracture mechanisms of nickel-based powder metallurgy superalloy

Thermo-mechanical fatigue (TMF) failure is one of the major concerns for aero-engine turbine discs under complex service loading conditions. By conducting both in-phase (IP) and out-of-phase (OP) TMF tests, the stress-strain hysteresis loops and the cyclic stress responses were analyzed for the third-generation nickel-based powder metallurgy (P/M) superalloy FGH4098. To reveal the failure mechanism, advanced microscopy analysis was carried out to characterize the fracture surface and microstructure of the alloy after failure. The phase angle between alternating mechanical and thermal loads plays an important role for the magnitude and evolution of mean stress. According to SEM fractography, the crack growth exhibits intergranular fracture under IP TMF and transgranular fracture under OP TMF. From the electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) characterization results, the geometrically necessary dislocation (GND) density is higher for IP TMF, indicating more severe plastic deformation. The material shows cyclic softening in the high-temperature half-cycle, but cyclic hardening in the low-temperature half-cycle. The cyclic deformation is mainly related to dislocation activities under IP loading conditions, for which fatigue damage dominates the failure process. Under OP loading conditions, the deformation behavior is mainly associated with the generation of stacking faults (SF), and its failure mechanism includes both fatigue and oxidation damage.

Thermomechanical fatigue behavior and failure mechanism of a nickel-based directional solidification column crystal superalloy

Thermomechanical fatigue (TMF) is a primary cause of turbine blade failure. TMF tests of a nickel-based directional solidification superalloy, which is a typically used material for turbine blades, are performed out in the surface temperature range of 400 °C–700 °C, two phases (in-phase and out-of-phase, i.e. IP and OP, respectively), three stresses, and two stress ratios. The test results idicate that the TMF life of the OP exceeds that of the IP. An increase in the stress ratio prolongs the TMF life. Scanning electron microscopy and transmission electron microscopy are performed to investigate the failure mechanism. The TMF failure behavior is complex and mainly includes fatigue, creep, and oxidation damage, with a quasi-cleavage fracture mode. Based on the theory of crystal plasticity as well as comprehensively considering the failure mechanism and focusing on the statistical characteristics of the thermal–stress interaction, a TMF constitutive model is established, which explains the differences in TMF behavior between the IP and OP conditions from a theoretical perspective.

Verification and validation of multi-physics numerical analysis of thermomechanical fatigue test conditions under induction heating and forced convection

This paper presents the validation and verification of thermomechanical fatigue multi-physics numerical analysis. To determine the local thermomechanical stress–strain state and displacement fields, an algorithm for the multi-physics numerical calculations is developed and implemented incorporating the Maxwell 3D, Fluent, and Transient Structural modules of ANSYS 2021R1. Single-edge-notch tension specimen heating is modelled using a rectangular induction coil. The temperature distribution in the specimen under forced and natural convection is considered. The nonlinear solid mechanics are analysed considering the temperature nonuniformity under in-phase and out-of-phase loading cycles with a triangular waveform and temperature range of 400–650 °C. Sensitivity analysis is performed to explore the effects of the mesh density. To complement the multi-physics computations, crack-opening displacement and infrared thermography temperature distribution measurements are employed for the thermomechanical fatigue test control in a single-edge-notch tension specimen produced from a nickel-based alloy XH73M. Based on the interconnected verification and validation of coupled numerical analysis, recommended ranges of numerical model parameters have been found to provide a stable solution.

Isothermal and thermomechanical fatigue behavior of Ti-2Al-2.5Zr titanium alloy

In-phase and out-of-phase thermomechanical fatigue (TMF) tests in the temperature range of 200–400 °C and isothermal fatigue (IF) tests at 400 °C were conducted to investigate cyclic deformation behavior and fatigue life of Ti-2Al-2.5Zr alloy. The back stress and the friction stress were extracted, and the internal stress competition mechanism underlying the cyclic hardening/softening behavior was clarified. The fatigue life of TMF and IF exhibited slight differences at strain amplitudes greater than 0.6%, whereas it depended on loading modes at the strain amplitude of 0.4% owing to the increase in the relative deviation in the plastic strain energy density and in the peak stress.

Can Martensitic Phase Transformation Measured by Magnetic Methods be an Indicator of Fatigue Damage in Austenitic Steel at Elevated Temperature and Thermo-Mechanical Loading?

The phenomenon of phase transformation from paramagnetic γ austenite to ferromagnetic α' martensite in metastable austenitic stainless steels under fatigue loading is well investigated, especially at ambient temperature. Generally, it can be said that at ambient temperature, the amount of martensite is proportional to the accumulated plastic strain and, therefore, can be an early indicator of fatigue damage. The problem arises at elevated temperatures when the thermal energy can severely or even completely inhibit the phase transformation. In this case, the amount of α'-martensite can no longer be a good indicator of fatigue damage. The question is, where is the limit the deformation-induced martensite can still be used as a fatigue damage indicator? To answer this question, several fatigue tests have been performed on AISI 347 (X6CrNiNb18-10), a metastable austenitic stainless steel often used in the piping of European nuclear powerplants. Tests included isothermal fatigue tests as well as thermo-mechanical fatigue (TMF) loading in the range of 25 to 320 °C. Deformation-induced martensite was measured in situ by means of a uniaxial magnetic balance, from which the rate of phase transformation was obtained. Fatigue specimens were then cut to investigate the distribution of martensite in the gauge length by means of magnetic force imaging. The results showed that isothermal fatigue loading results in a steady decrease in phase transformation rate with temperature. Initial martensite nucleation was observed to be different at elevated temperatures and occurred only at the surface, where sufficient shear deformation was present. TMF tests showed higher than isothermal rates of phase transformation. Peak temperatures above 300 °C were nevertheless enough to severely reduce the formation of α' martensite, even when cooled back to ambient temperature in the other half of the TMF cycle.

Elastic modulus data for additively and conventionally manufactured variants of Ti-6Al-4V, IN718 and AISI 316 L

This article reports temperature-dependent elastic properties (Young’s modulus, shear modulus) of three alloys measured by the dynamic resonance method. The alloys Ti-6Al-4V, Inconel IN718, and AISI 316 L were each investigated in a variant produced by an additive manufacturing processing route and by a conventional manufacturing processing route. The datasets include information on processing routes and parameters, heat treatments, grain size, specimen dimensions, and weight, as well as Young’s and shear modulus along with their measurement uncertainty. The process routes and methods are described in detail. The datasets were generated in an accredited testing lab, audited as BAM reference data, and are hosted in the open data repository Zenodo. Possible data usages include the verification of the correctness of the test setup via Young’s modulus comparison in low-cycle fatigue (LCF) or thermo-mechanical fatigue (TMF) testing campaigns, the design auf VHCF specimens and the use as input data for simulation purposes.

Impacts of nano-clay particles and heat-treating on out-of-phase thermo-mechanical fatigue characteristics in piston aluminum-silicon alloys

Abstract. In this article, the effect of nano-clay particles and heat-treating on thermo-mechanical fatigue (TMF) behaviors and failures of piston aluminum-silicon (AlSi) alloys was investigated. For this purpose, thermo-mechanical fatigue tests were conducted under out-of-phase (OP) loading conditions. Two loading conditions were checked based on different maximum temperatures (250, 300, and 350 °C) and various thermo-mechanical loading factors (100, 125, and 150%). The minimum temperature was constant in all tests at 50 °C under a heating/cooling rate of 10 °C/s and a dwell time of 5 s. Results showed that the nano-composites had a longer fatigue lifetime, at least 2 times higher, compared to the Al alloy, when the maximum temperature was 250 °C and the thermo-mechanical loading factor was 100%. However, no effective change was seen for the stress value and the plastic strain. At higher maximum temperatures, the change in the material behavior was lower. The fracture analysis by scanning electron microscopy (SEM) demonstrated that both materials had a brittle behavior due to cleavage and quasi-cleavage marks. The damage mechanism was also due to the Si-rich phase and intermetallics, respectively for the crack propagation and the micro-crack initiation.

Creep–Fatigue Interaction of Inconel 718 Manufactured by Electron Beam Melting

Electron beam melting of Ni‐base superalloy Inconel 718 allows producing a columnar‐grained microstructure with a pronounced texture, which offers exceptional resistance against high‐temperature loading with severe creep–fatigue interaction arising in components of aircraft jet engines. This study considers the deformation, damage, and lifetime behavior of electron‐beam‐melted Inconel 718 under in‐phase thermomechanical fatigue loading with varying amounts of creep–fatigue interaction. Strain‐controlled thermomechanical fatigue tests with equal‐ramp cycles, slow–fast cycles, and dwell time cycles are conducted in the temperature range from 300 to 650 °C. Results show that both dwell time and slow–fast cycles promote intergranular cracking, gradual tensile stress relaxation, as well as precipitate dissolution and coarsening giving rise to cyclic softening. The interplay of these mechanisms leads to increased lifetimes in both dwell time and slow–fast tests compared to equal ramp tests at higher strain amplitudes. Conversely, at lower mechanical strain amplitudes, the opposite is observed. A comparison with results of conventional Inconel 718 indicates that the electron‐beam‐melted material exhibits superior resistance against strain‐controlled loading at elevated temperatures such as thermomechanical fatigue.

Isothermal low-cycle fatigue, fatigue–creep and thermo-mechanical fatigue of SiMo 4.06 cast iron: Damage mechanisms and life prediction

A series of strain-controlled Low-Cycle Fatigue tests (LCF) and Thermo-Mechanical Fatigue tests (TMF) were performed on silicon-molybdenum ductile cast iron SiMo 4.06. LCF tests were performed for various mechanical strain amplitudes at a high strain rate of 3×10−3/s for various temperatures between 20 °C and 750 °C. In addition, several LCF tests were performed for a low strain rate of 1×10−4/s and as fatigue–creep tests with a tensile strain dwell. TMF tests were performed as out-of-phase and fully constrained between 100 °C and 650 °C. The investigation of the damage mechanisms revealed that the predominant failure mode is a combination of graphite-matrix debonding and transgranular crack propagation through ferritic matrix. Finally, a novel damage model based on the dissipated hysteresis energy is proposed in order to effectively predict the lifetime of SiMo 4.06 under LCF and TMF loading. Creep and fatigue damage are each calculated separately and the oxidation effect is taken into account indirectly.