Thermomechanical Fatigue Life Research Articles

Strain-Controlled Thermal–Mechanical Fatigue Behavior and Microstructural Evolution Mechanism of the Novel Cr-Mo-V Hot-Work Die Steel

In response to the intensifying competition in the mold market and the increasingly stringent specifications of die forgings, the existing 55NiCrMoV7 (MES 1 steel) material can no longer meet the elevated demands of customers. Consequently, this study systematically optimizes the alloy composition of MES 1 steel by precisely adjusting the molybdenum (Mo) and vanadium (V) contents. The primary objective is to significantly enhance the microstructure and thermal–mechanical fatigue performance of the steel, thereby developing a high-performance, long-life hot working die steel designated as MES 2 steel. The thermal–mechanical fatigue (TMF) tests of two test steels were conducted in reverse mechanical strain control at 0.6% and 1.0% strain levels by a TMF servo-hydraulic testing system (MTS). The microstructures of the two steels were characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The results indicate that throughout the entire thermomechanical fatigue cycle, both steels exhibit initial hardening during the low-temperature half-cycle (tension half-cycle) and subsequent continuous softening during the high-temperature half-cycle (compression half-cycle). Furthermore, under the same strain condition, the cumulative cyclic softening damage of MES 1 steel is more pronounced than that of the newly developed MES 2 steel. The number, width, and length of cracks in MES 2 steel are smaller than those in MES 1 steel, and the thermomechanical fatigue life of MES 2 steel is significantly longer than that of MES 1 steel. The microstructures show that the main precipitate phase in MES 1 steel is Cr-dominated rod-shaped carbide. It presents obvious coarsening and is prone to inducing stress concentration, thus facilitating crack initiation and propagation. The precipitate phase in MES 2 steel is mainly MC carbide containing Mo and V. It has a high thermal activation energy and is dispersed in the matrix in the form of particles, pinning dislocations and grain boundaries. This effectively delays the reduction in dislocation density and grain growth, thus contributing positively to the improvement in thermomechanical fatigue performance.

Heat build-up and thermo-mechanical fatigue life optimization of aircraft tire using deformation index concept

Heat build-up and thermo-mechanical fatigue life optimization of aircraft tire using deformation index concept

A Modification of the Ostergren Model for Thermomechanical Fatigue Life Prediction of Die-Casting Die Steel.

The Ostergren model is simple in form and widely used in engineering practice, also serving as the modeling basis of both the damage differentiation and crack propagation models. However, the shortcomings of the Ostergren model are that the modeling process is affected by thermomechanical fatigue (TMF) test parameters. To establish a TMF life normalized model, a modified Ostergren model based on hysteresis energy damage and TMF data for H13 steel was proposed. The model was successfully applied to TMF life prediction for 4Cr5Mo2V steel. The band of predicted life and test life is basically within the factor of 1.5. In summary, the modified Ostergren model is suitable for the TMF life prediction of Cr-Mo-V-type die-casting die steel.

Thermo-mechanical fatigue behavior and life prediction of selective laser melted inconel 718

The thermo-mechanical fatigue (TMF) property and corresponding damage mechanisms of selective laser melted (SLM) Inconel 718 superalloy were investigated systemically. The results show that the TMF life under in-phase (IP) loading is lower than that under out-of-phase (OP) loading, and the life difference gradually decreases with decreasing the strain amplitude. The fatigue cracks mainly exhibit inter-granular cracking characteristics under IP loading, and the δ phase embedded to the grain boundary promotes the creep cavity formation and then causes the fatigue crack propagation. While under OP loading, the fatigue crack is mainly characterized by trans-granular cracking, the oxidation induced crack extends from the specimen surface to the interior. A parameter, known as the shape factor k, has been discovered to exhibit high stability in temperature variations. Given the high stability of the k value and the quantitative relationship between stress and plastic strain range across different loading modes, a rapid prediction method for TMF hysteretic energy based on low cycle fatigue (LCF) has been proposed. Finally, combined with the energy cumulative damage model, the TMF life is successfully predicted. This approach significantly reduces the experimental quantity and complexity required for the TMF life prediction process, demonstrating substantial industrial application value.

Fatigue damage and life prediction for AISI H13 steel under cyclic thermomechanical loading

Based on strain-controlled thermomechanical fatigue (TMF) experiments, this study conducts a comprehensive analysis of the TMF behavior of AISI H13 hot work die steel. Moreover, a life prediction model for the TMF behavior of AISI H13 steel has been developed and validated. The experimental results reveal that, under the in-phase (IP) and out-of-phase (OP) TMF conditions, the stress–strain response curves of AISI H13 steel under different mechanical strain amplitudes exhibits the similar evolution tendency. However, it is worth noting that in the stable TMF cycle, the hysteresis loop area is enlarged with the increase of the number of cycles, which can be attributed to the cyclic softening characteristics of the AISI H13 steel under cyclic thermomechanical loading. When examining different TMF conditions, it is found that at higher strain amplitudes and under OP TMF conditions, the hysteresis loop area significantly expands, leading to a substantial reduction in the TMF life of AISI H13 steel. From a microstructural perspective, the thermal–mechanical coupling effect makes the recovery of martensitic matrix and the coarsening of carbide precipitation, which substantiates the deterioration of mechanical properties of AISI H13 steel. Finally, a modified Ostergren model by integrating the hysteresis loop area has been developed to assess the TMF life of AISI H13 steel under complex thermomechanical loading conditions, and this refined model exhibits strong agreement with experimental data. An evaluation using scatter band shows that the predicted TMF life of AISI H13 steel are within 1.2 times the experimental values, which illustrates a high reliability and validity.

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.

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.

Deformation behavior and crack mechanism of a first generation single-crystal Ni-based superalloys under thermomechanical fatigue loading

A research investigation was conducted to analyze thermomechanical fatigue (TMF) in a first-generation nickel-based single-crystal superalloy. The analysis was conducted within a temperature range of 450–950 °C while employing a strain ratio of −1 during two phases, in-phase (IP) and out-of-phase (OP). TMF life data were collected at varying mechanical strain amplitudes. The IP and OP TMF lifetimes were compared under identical strain conditions. A detailed analysis of the fractures and microstructures of the tested specimens was performed to investigate the crack mechanism. In the IP specimens, the formation of cracks closely followed the direction perpendicular to the principal stress within the material, which was primarily associated with the presence of micropores. Internal cracks tended to develop at locations with strain concentrations and subsequently propagate outward from the surface. The main mechanism behind these cracks was the interaction between creep and fatigue damage. Conversely, in the OP TMF specimens subjected to an oxide scale formed on the fracture surface, leading to fatigue cracking at multiple locations. The cracks gradually extended within the material before transitioning into the (111) crystallographic planes, where localized deformation bands occurred, affecting an area depleted of the γ' phase. This crack mechanism predominantly involved interactions between oxidation and fatigue. Among the various fatigue life prediction models, the Ostergren model exhibited strong agreement with the predicted data points for the fatigue life.

Prediction of thermomechanical fatigue life in RuT450 compacted graphite cast iron cylinder heads using the Neu/Sehitoglu model

The increase in engine power has resulted in a significant rise in both thermal and mechanical loads on cylinder heads, leading to the emergence of thermomechanical fatigue (TMF) as one of the significant issues. This paper proposed a parameter identification method for the Neu/Sehitoglu model, utilizing multi-objective optimization (MOO), and applied it to predict the TMF life of compacted graphite cast iron (CGI) material used in cylinder heads. The study assessed the TMF behavior of RuT450 material through various experiments, including tensile, low cycle fatigue (LCF), creep, TMF, and oxidation experiments. Employing the MOO, the unknown model parameters are estimated and optimized to predict the TMF life of RuT450 materials. Results demonstrated that over 95% of the predicted TMF life falls within twice the error band of experimental values. Furthermore, plastic strain amplitude, temperature, and strain rate significantly influenced the damage mechanism and life prediction. The TMF life under out-of-phase (OP) loading conditions was found to be smaller than that of in-phase (IP) loading. Notably, under out-of-phase thermomechanical fatigue (OP TMF), oxidation damage and fatigue damage together contributed to over 95% of the total damage in RuT450 materials. Moreover, variations in oxidation damage parameters had a considerable impact on the predicted life. This study might offer an effective method for determining Neu/Sehitoglu model parameters and establish material boundaries for TMF life assessment of cylinder heads.

Deformation and damage mechanisms during clockwise diamond and counter clockwise diamond thermomechanical fatigue in Timetal 834 alloy

In the present investigation, the thermomechanical fatigue (TMF) behavior of Timetal 834 alloy was studied under clockwise diamond (CWD) and counter clockwise diamond (CCD) loading conditions. These tests were conducted within 723 K ↔ 873 K temperature range at the strain amplitudes of Δεm/2 = ±0.6% and ±1.0%. The cyclic softening behavior observed under CWD-TMF condition at both strain amplitudes, is attributed to shearing of coherent Ti3Al precipitates. Contrarily, the alloy exhibits both cyclic hardening and softening responses under the CCD-TMF conditions. This additional hardening response is attributed to the occurrence of dislocation–dislocation interactions coupled with dynamic strain aging. The present study reveals that both phase angle and strain amplitude affect the TMF life. The alloy exhibited relatively lower fatigue life under CCD-TMF condition than under CWD-TMF condition, at both strain amplitudes. Transmission electron microscopy analyses of the post-deformed specimens reveal that the dislocation arrangements are similar in both loading conditions at Δεm/2 = ±0.6%, while being distinctively different at Δεm/2 = ±1.0%. At higher strain amplitude, the fatigue life is significantly reduced under CCD-TMF condition. Fractographic analyses indicate the involvement of oxidation damage during both the TMF loading conditions. However, its effect is more pronounced under CCD-TMF than under CWD-TMF condition.

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.

Effects of creep and oxidation to thermomechanical fatigue life assessment for nickel-based superalloy

The present work focuses on the quantification of creep and oxidation effects in the prediction of both isothermal and thermomechanical fatigue life for a nickel-based superalloy. The creep damage was evaluated using the time-fraction method and the ductility-exhaustion method. A diffusion-dominated environmental damage parameter was proposed based on the mechanism of oxygen-assisted intergranular cracking and applied to thermomechanical fatigue life assessment. The results revealed that creep damage appears to be negligible and environmental damage seems more dominant in the current study. The proposed model provides a unified description for both isothermal and thermomechanical fatigue.

Thermomechanical fatigue of Nickel-Based Single-Crystal superalloys

Nickel-based single-crystal superalloys are extensively used in aeroengine hot-end components owing to their unique crystal structure and outstanding high-temperature mechanical properties. In this study, a thermomechanical fatigue (TMF) test platform capable of realizing precise temperature control and coordinated control of mechanical and temperature loads was constructed. The TMF of a second-generation nickel-based single-crystal superalloy (DD6) was analyzed in a temperature range of 500–950 °C at a stress ratio of 0.1 in two phases (in-phase and out-of-phase); moreover, the TMF tests were conducted at various stress levels, and the TMF life of the single-crystal specimens was determined. The OP TMF lifetime was considerably longer than the IP TMF lifetime at identical stress levels. The failure mechanism of the material in different phases was elucidated. The fracture and microstructures of the specimens were analyzed by SEM and TEM. For the IP specimens, the fracture mode was determined to be a quasi-cleavage fracture, and the γ' phase exhibited N-type rafting prior to fracture. For the OP specimens, the fracture was relatively smooth and flat. Numerous fine secondary γ' phases are precipitated in the channels of the matrix phase.

Strengthening mechanism in thermomechanical fatigue properties of Ti6Al4V titanium alloy by laser shock peening

In this study, the role of laser shock peening (LSP) on the thermomechanical fatigue properties of Ti6Al4V titanium alloy was investigated. The effects of LSP with different coverage layers on the microstructural evolution, residual stresses distribution, and thermomechanical fatigue lives were systematically analyzed by scanning electron microscopy, transmission electron microscope, residual stresses tests, microhardness tests, and thermomechanical fatigue tests. Results showed that the thermomechanical fatigue lives of the LSP1 and LSP2 specimens at room temperature increased by 48.00 % and 114.95 %, respectively. While the fatigue life of the LSP2 specimen increased by 68.91 % even under high-temperature heat cycle. Meanwhile, the locations of crack initiations in the LSPed specimens were significantly shifted to the sub-surface with a smaller fatigue initiation region area compared to the unLSPed specimen. Furthermore, the fracture surface was essentially occupied by equiaxed dimples and small-spaced fatigue striations. LSP-induced considerable depth of severe plastic deformation in the near-surface of Ti6Al4V titanium alloy, generating gradient compressive residual stresses, high-density dislocation structures (dislocation lines, dislocation tangles, dislocation walls, and dislocation cells), and grain refinement (nanograins and nanotwins). These three main factors contributed to inhibiting the sprouting and propagation of cracks during the thermomechanical fatigue tests and played a primary role in improving the thermomechanical fatigue properties.

Thermomechanical Fatigue Mechanical Behavior and Life Prediction of Coke Drum with Cr-Mo Steel

Thermomechanical Fatigue Mechanical Behavior and Life Prediction of Coke Drum with Cr-Mo Steel

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.

Isothermal and thermomechanical fatigue behavior of 316LN stainless Steel: Effects of strain amplitude

Effects of strain amplitude on the isothermal fatigue (IF) and thermomechanical fatigue (TMF) behaviors of 316LN stainless steel were comparatively studied. The physical mechanisms for the evolution of cyclic stress response were revealed based on microstructure characterization. The manifestations of dynamic strain aging (DSA), dependence of DSA intensity on loading modes and influence of DSA on deformation properties were discussed. The dominated damage mechanism changed at the strain amplitude of about 0.27% at which the IF and TMF life curves intersected. The maximum stress was the appropriate damage parameter for life prediction with predicted life falling into the scatter band of 1.5.

Research on thermomechanical fatigue life prediction of regenerative cooling thrust chamber

The ratcheting effect of the liner wall structure in the thrust chamber of liquid rocket engine under cyclic load is studied, and its service life is predicted. Firstly, the 3-D heat transfer analysis of the wall of the thrust chamber is carried out while explaining the mechanism of its "dog room" failure. Based on the results of the heat transfer analysis, the stress-strain evolution of the structure under cyclic load is analyzed by nonlinear plane strain finite element method. According to the finite element analysis results, the ratcheting effect of the liner wall structure is described and the ratcheting damage is calculated. The low cycle fatigue damage is calculated by using modified Morrow model, and the creep damage is also calculated using Norton model. Finally, the total damage of thrust chamber liner wall is calculated by using Miner linear cumulative damage law and the thermomechanical fatigue life of the structure is estimated. The result shows that the middle point of the lower surface of the liner wall will fail first, and the calculated service life is 33 times, which is good agreement with its actual service life. In the total damage of the weak point, ratcheting damage, creep damage and low cycle fatigue damage account for 52%, 32% and 16% respectively, indicating that the ratcheting effect is the main reason for the failure of the thrust chamber structure. The research results of this paper provide important engineering reference for the structural optimization design and rapid life prediction of regenerative cooling thrust chamber of liquid rocket engine and the future reusable aerospace power.

Thermomechanical fatigue behavior and lifetime modeling of powder metallurgy superalloy considering phase angle effect

In the present work, the thermomechanical fatigue behavior and life prediction of PM superalloy under various mechanical strain amplitudes and phase angles are investigated. Results demonstrate that the thermomechanical fatigue life is phase angle dependent and the tensile mean temperature is the main factor affecting the TMF life of alternative phase angle. A modified energy-based life prediction model is proposed by considering the effects of phase angle and tensile mean temperature. Moreover, machine learning methods were used for life prediction, and the TMF lifetime at various phase angles was predicted by combining it with RF regression and modified energy model.