Thermomechanical Fatigue Behavior Research Articles

A comparison of conventional and additively manufactured 316L under thermomechanical fatigue

The present study compares the thermomechanical fatigue (TMF) behaviour of 316L stainless steel manufactured conventionally by hot rolling and additively by laser powder bed fusion (L-PBF). Machined cylindrical specimens were tested under strain-controlled in-phase and out-of-phase TMF loading at a temperature range of 550–750 °C and total mechanical strain amplitudes of εamech = 0.2–0.6 %. While the conventional 316L significantly outperforms the L-PBF 316L under in-phase TMF, their lifetimes are comparable under out-of-phase TMF. Under in-phase TMF, creep damage in the form of intergranular crack networks occurs which is significantly more pronounced for the L-PBF 316L due to the higher amount of interfaces. Under out-of-phase TMF, the damage is mostly due to stress-assisted oxide cracking and the crack propagation is fatigue-dominated. TEM inspection revealed that L-PBF 316L exhibits a cellular dislocation structure in the initial state, which rearranges only slightly during cycling. For conventional 316L similar dislocation cells form during TMF cycling indicating that they represent a stable dislocation arrangement in 316L under TMF loading. This is evidenced by the rather stable cyclic stress response of the L-PBF material when compared to the conventional material.

Out-of-phase thermomechanical fatigue of a single crystal Ni-base superalloy

The present work studies the out-of-phase thermomechanical fatigue (OP-TMF) behavior of the precisely oriented [001] single crystal Ni-base superalloy ERBO/1 (CMSX-4 type). The OP-TMF tests are performed between minimum and maximum temperatures of 1023 and 1223 K and a cyclic mechanical strain amplitude of ±0.5 %. In accordance with previous findings, the present study confirmed that isothermal low cycle fatigue (LCF) tests at 1023 and 1223 K show significantly longer fatigue lives than the OP-TMF tests, specimens with rougher surfaces fail earlier than polished specimens and deformation bands and cracks form in the specimen surface. Two new results were obtained: First, when comparing OP-TMF cycles with fast cooling (tensile part of cycle) and fast vs. slow heating (compressive part of cycle) one finds that the slower heating/compressive cycle is more damaging. Second, analytical scanning electron microscopy of the flanks of an OP-TMF crack allows to study the diffusion-controlled growth kinetics of the oxide and the underlying zone, which is depleted by the oxide forming elements. These results are discussed in the light of the microstructural, mechanical and chemical results of the present study considering previous work on OP-TMF of superalloy single crystals (SXs).

Thermomechanical fatigue behavior of CF/PEKK composite under low and ultrasonic frequencies

This study aimed to extract the thermomechanical fatigue behavior of a CF/PEKK composite under low and ultrasonic frequency fatigue tests (LFFTs and UFFTs) in the presence of the self-heating effect. Preliminary increasing amplitude tests (IATs) were performed to obtain the minimum load level at which the self-heating is observable. Extracting the fatigue strength from T-σ curves for three mean stress levels resulted in constructing S-N curves for the LFFT regime at different load levels using a shift procedure. However, combining LFFT and UFFT results via S-N curve was impractical, while joining such results through the heat dissipation rate (q̇) was feasible for various constant amplitude tests and IATs. The derived σ-q̇ curves from combining LFFT and UFFT results and comparing the fracture mechanisms of CF/PEKK composite using fractography would make a step for bridging the transition zone between LFFTs and UFFTs and making the results transferable. The microscopy images obtained from fractography also confirmed the similarities of fatigue fracture mechanisms between LFFTs and UFFTs.

Thermomechanical fatigue behavior and its life prediction of AlSi9Cu3.5 cast alloy

Thermomechanical fatigue behavior and its life prediction of AlSi9Cu3.5 cast alloy

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.

A comprehensive study of a long-term creep thermo-mechanical fatigue behavior monitoring of BFRP composite pipeline using electrical capacitance sensors and deep learning algorithm

The composite pipeline is a relatively new and viable alternative pipeline to the more commonly used traditional one due to its good mechanical and fatigue properties and lower production cost. For this purpose, it is critical to assess the mechanical and fatigue performance of composite pipeline material under various working conditions, particularly for monitoring long-term creep thermo-mechanical fatigue behavior. In this paper, a long-term creep thermo-mechanical fatigue behavior in a basalt fiber reinforced polymer laminated composite pipeline is detected through an integrated expert system consisting of the electrical capacitance sensors and a deep learning algorithm. First, a multi-physics finite element model is established for the simulation of a long-term creep thermo-mechanical fatigue behavior in basalt fiber reinforced polymer composite pipelines subjected to long-term fatigue loading of internal pressure and thermal effect. Second, theoretical model results of long-term creep thermo-mechanical fatigue compliance (Sf(t)) over the time of creep are analyzed in pipeline material using the modulus degradation approach. Finally, an electrical potential change between electrical capacitance sensors electrodes corresponding to Sf(t) over the time of creep for some levels of long-term creep thermo-mechanical fatigue (Rf%) is recorded and then used in these datasets for training of the novel deep neural network based on one of the most widely used of the deep neural network families is the convolutional neural network, to predict Sf(t) in pipeline for various Rf% not included in the previous finite element model evaluation (i.e. electrical capacitance sensors technique). In this paper first is detected the long-term creep thermo-mechanical fatigue behavior for Rf%=25%,50%, and 75% from a finite element model and modulus degradation approach, and then is predicted the long-term creep thermo-mechanical fatigue behavior for Rf%=15%,35%,60%, and 85% respectively via deep neural network. The proposed method results are in good agreement with the experimental results available in the literature, thus verifying the accuracy and reliability of the proposed technique and its applicability to other different composite structures.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

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.

An Overview of the mechanism-based thermomechanical fatigue method (DTMF) in the design of automotive components

The DTMF damage Parameter described in the paper is the generalization of the creep fatigue parameter (DCF) proposed by Riedel, which quantifies the amount of damage produced by a non-isothermal loading cycle. The entire design methodology is outlined in the text, starting with the calibration of Chaboche's viscoplastic model and then the determination of the thermomechanical fatigue (TMF) parameters. Chaboche is generally a good choice for its ability to describe well both kinematic and isotropic hardening behaviors, also enabling stress relaxation and strain rate dependency to be considered. The DTMF method is based on the fracture mechanics concept of cyclic crack tip opening displacement (ΔCTOD) where the effective stress range takes the crack closure effect into account. This method is typically used to describe the thermomechanical low cycle fatigue behavior of cast irons, cast steels, stainless steels, Inconel, HiSiMo and nickel-based alloys. These materials are used in components that need to withstand high temperatures (higher than half of the melting point) in service as exhaust manifolds, cylinder heads, valves, turbochargers and disk brakes. Three failure mechanisms are detailed here: oxidation, creep and fatigue. Oxidation is a complex phenomenon that may occur when the material is hot under tensile in-phase loading or hot under compressive out-of-phase loading. Creep is a time and temperature dependent diffusion process which is incorporated into the DTMF parameter as a multiplicative factor. In this context fatigue life is defined as the number of cycles to propagate an existent microcrack up to an arbitrary size that is defined on a case-by-case basis.

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.

3D Investigation of Damage During Strain‐Controlled Thermomechanical Fatigue of Cast Al–Si Alloys

The strain‐controlled thermomechanical fatigue behavior is investigated for three cast near‐eutectic Al–Si alloys with different Ni, Cu, and Mg contents. Synchrotron tomography and neutron diffraction experiments are used to correlate 3D microstructural features with damage initiation and evolution. The results show that the alloy with lower Cu, Ni, and Mg concentrations has up to 45% higher thermomechanical fatigue resistance for cooling/heating rates of 5 and 15 K s−1. In addition, this alloy also exhibits damage formation at later stages during thermomechanical fatigue and slower damage accumulation compared to other alloys. This difference in behavior is a consequence of its higher ductility, which is a result of the lower volume fraction and global interconnectivity of the 3D hybrid networks formed by Si and intermetallics and the absence of large primary Si clusters which act as preferred crack initiation sites during the early stages of thermomechanical fatigue.

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.

Improved Thermo-Mechanical Fatigue Resistance of Al-Si-Cu 319 Alloys by Microalloying with Mo.

Thermo-mechanical fatigue (TMF) is one of the most detrimental failures of critical engine components and greatly limits their service life. In this study, the out-of-phase TMF (OP-TMF) behavior in Al-Si-Cu 319 cast alloys microalloyed with Mo was systematically investigated under various strain amplitudes ranging from 0.1-0.6% and temperature cycling at 60-300 °C and compared with the base 319 alloy free of Mo. Cyclic stress softening occurred in both experimental alloys when applying the TMF loading, resulting from the coarsening of θ'-Al2Cu precipitates. However, the softening rate of the Mo-containing alloy was lower than that of the base 319 alloy because of its lower θ'-Al2Cu precipitate coarsening rate per cycle. The Mo-containing alloy exhibited a longer TMF lifetime than the base alloy at the same strain amplitude. Microalloying 319 alloy with Mo enhanced the TMF resistance mainly by slowing the coarsening of θ'-Al2Cu precipitates and providing supplementary strengthening from thermally stable Mo-containing α-dispersoids distributed in the Al matrix. The energy-based model was successfully applied for predicting the TMF lifetime with a low life predictor factor, which agreed well with the experimentally measured fatigue cycles.

Cyclic deformation mechanism and fracture behaviour of 316L stainless steel under thermomechanical fatigue loading

Thermomechanical fatigue (TMF) behaviour of 316L is investigated based on the internal stress and microstructural characterization at various cyclic stages. Results reveal that the cyclic stress response shows initial hardening followed by cyclic softening at a high strain amplitude, while presenting continuous cyclic hardening as the strain amplitude decreases. Among these, Out-of-phase TMF always shows cyclic softening irrespective of strain amplitude. It is important to note that the hardening of back stress and friction stress contribute to the initial and subsequent cyclic hardening. In contrast, the subsequent cyclic softening is mainly dominated by the reduced friction stress. It is observed that the proliferation of dislocations at grain boundaries and the enhanced dislocation interactions within the grains are primarily responsible for the hardening of back stress and friction stress, respectively. Specifically, the cyclic softening of friction stress is attributed to the annihilated dislocations as a result of cross-slip. It is also found from the hysteresis curves that the intensity of dynamic strain ageing is related to strain amplitude, loading history and phase angle, which correlates to the occurrence of cross-slip, the diffusion rate of the solute atom, the density of point defect and the mobility of vacancies. Finally, the effect of loading conditions on fracture behaviour is demonstrated from the point view of crack sources, secondary cracks, creep cavities and oxidation damage.

Weft direction pseudo-thermomechanical fatigue behaviors of T300/ Polyimide-380 2.5D woven composites

This work elaborates the weft direction in-phase thermomechanical fatigue (IP TMF) behaviors of T300/PI-380 2.5D-WCs for the temperature variation between 100 and 300 °C for the first time. Results show that the IP TMF responses of 2.5D-WCs exhibit a low cycle fatigue characterization, which approximately linear decreases with the increase of cycle number. The hysteresis loops shifted to the right confirm an elongation of specimen during the IP TMF process. The residual stiffness and residual strength behaviors are also investigated in the case of TMF, and the corresponding numeric models are proposed based on the damage mechanics. Moreover, fracture morphologies of 2.5D-WCs under the IP TMF loading have been explored by means of SEM and X-rays observations. Finally, a probable failure mechanism for the weft direction IP TMF behaviors of 2.5D-WCs is proposed. This work provides a new benchmark in studying high-temperature fatigue properties of resin matrix 2.5D woven composites.

A comparison on isothermal and thermomechanical fatigue behavior of 316LN stainless steel with various tension dwell time

Isothermal and thermomechanical fatigue tests with various tension dwell time (designated as creep-IF and creep-TMF respectively) were conducted. The cyclic stress response and stress relaxation behavior were qualitatively and quantitatively discussed based on TEM and EBSD characterization. The fatigue dominated transgranular crack initiation and propagation led to specimen fracture in all creep-IF tests. The transition of dominant damage mechanism from fatigue to creep-fatigue interaction occurred when dwell time increased to 1800 s in creep-TMF test, which caused the intergranular fracture mode and a much shorter life than creep-IF. Moreover, the life prediction model based on tension strain energy was proposed.

Thermo-Mechanical Fatigue Behavior and Resultant Microstructure Evolution in Al-Si 319 and 356 Cast Alloys.

The out-of-phase thermo-mechanical fatigue (TMF) behavior of the two Al-Si cast alloys most widely used for engine applications (319 and 356) were investigated under temperature cycling (60-300 °C) and various strain amplitudes (0.1-0.6%). The relationship between the microstructural evolution and TMF behavior was closely studied. Both alloys exhibited asymmetric hysteresis loops with a higher portion in the tensile mode during TMF cycling. The two alloys showed cyclic softening behavior with regard to the maximum stress, but an earlier inflection of cyclic stress was found in the 356 alloy. The TMF lifetime of the 319 alloy was generally longer than that of the 356 alloy, especially at higher strain amplitudes. All the precipitates (β'-MgSi in 356 and θ'-Al2Cu in 319) coarsened during the TMF tests; however, the coarsening rate per cycle in the 356 alloy was significantly higher than that in the 319 alloy. An energy-based model was applied to predict the fatigue lifetime, which corresponded well with the experimental data. However, the parameters in the model varied with the alloys, and the 356 alloy exhibited a lower fatigue damage capacity and a higher fatigue damage exponent.

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.