Thermomechanical Fatigue Life Research Articles

Mechanical Fatigue Testing Under Thermal Gradient and Manufacturing Variabilities in Nickel-Based Superalloy Parts with Air-Cooling Holes

BackgroundAs aerospace cast parts move to more complex geometries, the impact of process variabilities must be evaluated more precisely to provide new, less empirical acceptation criteria. However, experimental lifetime assessment with Thermal Gradient Mechanical Fatigue (TGMF) are still complicated to reproduce in a laboratory environment.ObjectiveThis paper investigates the impact of geometrical variability on thermo-mechanical fatigue life of Nickel-based superalloy single crystal parts.MethodsTo this end, hollow specimens with cooling holes drilled in their centre have been produced with parametrized variations of the wall thickness and have been characterized by X-ray tomography. An experimental fatigue setup operating at 1100\(^\circ\)C, with a thermal gradient of 50\(^\circ\)C across the wall, has been developed to assess the life of each specimen.ResultsExperimental lifetime assessment were conducted on a sample of specimens containing geometrical defects. Optical observations during the tests have permitted to study crack initiation and propagation in the vicinity of the cooling hole, an area of high thermal and stress gradients where damage localize. Axial strains (in the loading direction) were also gathered and compared to prove the impact of process variabilities on mechanical behaviour.ConclusionsSignificant mechanical differences are observed during the tests which can related to the geometrical variations of the specimens and their interaction with the shaped hole. Tests results will be further used to calibrate a creep-fatigue model for lifetime predictions.

FEM simulation of thermo-mechanical stress and thermal fatigue life assessment of high-speed steel work rolls during hot strip rolling process

Work rolls used for hot strip rolling undergo the cyclic sequence of heating-cooling over the roll surface due to contact with the hot strip and the following cooling systems during every rolling revolution. Severe temperature gradient and thermal stress caused by cyclic heating-cooling loading are dominant factors for thermal fatigue failure of work rolls. Therefore, investigations on temperature and thermal stress-strain are essential for understanding the thermal fatigue of work roll. In this study, a thermo-mechanical analysis with elastic-plastic material parameters via a simplified finite element model is carried out to compute the stress-strain range experienced by the work roll during rolling and idling. Then, a model with the initial crack in the roll surface considering different depths and a model with the hot strip is conducted to better understand the thermal behaviors of the work roll during hot rolling respectively. Furthermore, the thermal fatigue life of work roll is assessed based on the modified Coffin-Manson relation, considering the effect of multi-axial stress state and temperature-dependent material properties caused by temperature fluctuations in work roll during hot rolling.

Multiaxial thermo-mechanical fatigue life prediction based on notch local stress-strain estimation considering temperature change

The modified incremental multiaxial Neuber rule by considering the temperature change was firstly proposed, which combined with the Chaboche viscoplastic constitutive model to approximately compute the notch local stress and strain under multiaxial thermo-mechanical loading. Based on the proposed method, a life prediction method that can take into account fatigue, oxidation, and creep damage was developed, which can quickly predict notch fatigue failure lifetime under variable amplitude multiaxial thermo-mechanical loading. The proposed method was verified by the coupled thermal-structural nonlinear FEA and the experimental life results for the tenon joint structure specimens under eight thermo-mechanical loading paths. The comparison of computed and analyzed stress-strain history showed that a good agreement can be obtained. Comparing with experimental life data, all predicted lives fall in a 2-factor scatter band.

In-phase thermomechanical fatigue studies on P92 steel with different hold time

Abstract The effect of hold time with 0, 20, and 40 s on in-phase thermomechanical fatigue (TMF) behavior and life of P92 steel is investigated in this study. TMF tests are carried out under mechanical strain control with strain amplitudes of 0.4 0.4 , 0.6 0.6 , and 0.8 % 0.8\text{\%} , and temperature range of 550–650°C which is closely relevant to the operating condition in power plant. TMF tests are performed in a mechanical strain ratio of R = − 1 R=-1 and cycle time of 120 s. The fatigue life variation follows the sequence of N f 0 s < N f 20 s < N f 40 s {N}_{\text{f}}^{0\hspace{.1em}\text{s}}\lt {N}_{\text{f}}^{20\hspace{.1em}\text{s}}\lt {N}_{\text{f}}^{40\hspace{.1em}\text{s}} for the same mechanical strain amplitude. In addition, the influence of hold time on fatigue life decreases with the increasing strain amplitude. A continuous softening can be observed from the cyclic stress response under all test conditions. Fractographic and microstructural tests indicate that the fracture surfaces are characterized by a multi-source cracking initiation and an oxidation phenomenon. Furthermore, a modified Ostergren model is used to predict the fatigue life and achieves a good predicted result.

Thermo-mechanical fatigue life prediction based on the simulated component of cylinder head

As the vulnerable component of diesel engine, cylinder head is subjected to the thermo-mechanical coupling loads more and more heavily. For establishing a simple and effective technology to evaluate the damage degree, a prediction method based on simulated component is developed in the present work. By the comparison of temperature and stress at measured points, it shows that the simulation and testing results have a good agreement, verifying the effectiveness of simulated component. Applying Sehitoglu theory and loading spectrums, the failure locations are predicted successfully. Then damage characteristics of thermo-mechanical fatigue are analyzed at failure locations. It indicates that mechanical damage takes a leading role compared with oxidative damage and creep damage, and thermal load is more sensitive to mechanical damage in three damage types. This study may provide an effective way to estimate the service life of cylinder head.

Thermomechanical fatigue life prediction of metallic materials by a gradient‐enhanced viscoplastic damage approach

AbstractFatigue failure plays an important role in engineering applications, especially when structural components experience significant cyclic thermal loading and complex force loading simultaneously. During the last decades, several post‐processing techniques have been developed based on empirical investigations of experimental evidence to predict the fatigue life of materials. The work at hand postulates a conventional continuum damage theory for thermomechanical fatigue failure modeling. In particular, an implicit gradient‐enhanced approach is employed to address the ill‐posedness of the partial differential equation system when the damage onsets. An internal fatigue variable is phenomenologically defined based on the accumulation of viscoplasticity. In the sequel, a regularized fatigue variable is obtained to further yield the damage softening function, which straightforwardly applies to the stress, material tangent, and viscoplastic dissipation. A multi‐field problem, consisting of the strain field, the temperature, and the non‐local variable, is taken into consideration, leading to a fully coupled system. This numerical methodology is consistently derived and implemented into the context of the finite element method. Several representative and demonstrative examples are performed, which yield good numerical stability and agreement with experimental data. Conclusive findings and further perspectives close this article.

Thermomechanical Fatigue Life Prediction Method of the Trailing Edge Holes in the Turbine Blade for Turboshaft Engine

In this paper, a thermomechanical fatigue (TMF) life analysis method considering stress relaxation was established for turbine blade in a turboshaft engine. The nonlinear creep deformation of a superalloy is predicted by coupling creep damage in the framework of viscoplastic theory. The results revealed that the calculation error of the model for the elastic-plastic stress-strain curve was less than 5%, while the simulation accuracy for the creep curve is within the dispersion range of the inherent properties of material creep deformation. Based on the elastic-plastic creep analysis of a turbine blade, the creep damage and its evolution law of the leading edge and trailing edge of the blade are clarified, and a new method is provided for determining the local dangerous points on the blade. With the help of the linear damage cumulative life theory, the fatigue creep life of the trailing edge hole of a turbine blade considering the stress relaxation is obtained, which provides a more reasonable and better engineering application method for the blade life evaluation.

Thermomechanical fatigue behavior of CGI 450 and CGI 500 cast irons

The vermicular cast iron grade 450 is widely used in engineering applications, primarily by the automotive industry in the fabrication of internal combustion engines. However, in recent years, the vermicular cast iron grade 500 has been developed, and new studies searching for the replacement of the grade 450, with the aim of increasing the high-performance internal combustion engines life, as well as reducing their weight, are in development. Due to their operational characteristics, these engines are subjected simultaneously to thermal and mechanical cycles, which influence their useful life. Therefore, the characterization of the resistance to these types of loads for materials selection for these engines, is very important. Thus, the objective of this work was to study the thermomechanical fatigue life of two vermicular cast irons in conditions close to those to which internal combustion engines are subjected. The materials were characterized by chemical and metallographic analyses and tensile tests. The thermomechanical fatigue tests were carried out at temperatures ranging from 50 ºC to 420 ºC, and a dwell time of 180 seconds. The vermicular cast iron grade 500 exhibited higher tensile strength, with higher ductility when compared to the grade 450, as well as higher fatigue strength. These properties were related to the size and distribution of eutectic cells (graphite) in the matrix.

Thermomechanical fatigue behaviors and failure mechanism of 2.5D shallow curve-link-shaped woven composites

Thermomechanical fatigue (TMF) performance is one of the most challenges for aero-engine materials. This work experimentally elaborated the in-of-phase (IP) TMF behaviors and failure mechanism of high-temperature resistant 2.5D woven composites (2.5DWCs) for the first time. In this study, both fatigue load and thermal load varying from 160 °C to 260 °C are synchronously imported in the form of trapezoidal wave. An experimental platform consisting of quasi-static mechanical tester, strain collection device, heating/cooling equipment and temperature control system was firstly established. The relationship between stress level and TMF life was obtained. The fracture morphologies were investigated using scanning electron microscope (SEM) and X-ray computed tomography (Micro-CT). Results show that the TMF behaviors of 2.5DWC exhibit a low cycle fatigue characterization with a fatigue limit of 0.75 stress level. The hysteresis loops gradually shift to the right, confirming an elongation of specimen during the TMF. It indicates a reduction of inclination angle of warp yarns. Particularly, the failure behavior is fiber-dominated brittle fracture mode, without clear necking phenomenon. The fiber pull-out length and the extensive debonding are more severe than those failed in the static loading. Finally, a probable failure mechanism for the TMF behaviors of 2.5DWC is proposed.

Influence of Ru on the thermomechanical fatigue deformation behavior of a single crystal superalloy

The deformation mechanisms of a single crystal nickel-base superalloy with and without Ru-doped have been investigated under out-of-phase thermomechanical fatigue. The Ru-doped alloy exhibits a thermomechanical fatigue life more than twice as high compared to the Ru-free alloy and a difference in thermomechanical fatigue behavior is also displayed. Microstructure studies by scanning electron microscopy and transmission electron microscopy revealed that the deformation mechanism of the Ru-free alloy in the initial stage is the movement of dislocations in the γ matrix. In the later stage of the thermomechanical fatigue test, large amounts of twins are formed in the material, and a large number of stacking faults and dislocations are sheared into the γ' precipitates. By comparing with the Ru-free alloy, the Ru-doped alloy has a higher matrix strength due to the solid solution strengthening effect of Ru, and is also prone to different deformation mechanisms. For example, the stacking faults are formed in the initial thermomechanical fatigue cycles and remain in the matrix throughout the entire thermomechanical fatigue process. The formation of twins, on the other hand, is suppressed by Ru addition. Such effects are believed to extend the thermomechanical fatigue life effectively.

Prediction of work-rolls failure in hot ring rolling process

The work-rolls of the hot ring rolling process are subjected to various damages due to the contact with hot metal. Thermo-mechanical fatigue is one of these damages. In order to predict failure due to the thermo-mechanical stress in the work-rolls, the developed code in ABAQUS has been used. The comparison between three- and two-dimensional models and, also, thermal and thermo-mechanical response of work-rolls with variable boundary conditions has been investigated. The results have shown that by applying mechanical and thermal loads separately or simultaneously, the response of work-rolls is completely different. In the mandrel, the location of the maximum equivalent stress is on the surface, while the location of equivalent maximum stress is in the subsurface of the main-roll. By making use of cumulative damage rules and the stress life method, the thermo-mechanical fatigue life was estimated. The cumulated damage in the mandrel’s surface was higher than subsurface regions. In contrast to the mandrel, the cumulated damage in the main-roll’s subsurface was higher than surface regions. In hot ring rolling machines, these locations are prone to crack initiation as a result of the thermo-mechanical fatigue in the work-rolls.

Modelling of elastocaloric regenerators with enhanced heat transfer structures

The elastocaloric refrigeration utilizing Shape Memory Alloys (SMAs) has been actively pursued as an alternative cooling technology to the traditional vapor compression. Many research efforts have been focusing on the thermomechanical cycles, material properties, and fatigue life of the SMAs. However, less effort has been made to achieve optimal heat transfer performance of the entire system. In this work, for the first time, we introduce a porous regenerative heat exchanger to a tube-based compression-loaded elastocaloric cooling device for enhanced cooling performance without compromising its fatigue life. A convective regenerator made of a porous structure is proposed for better thermal coupling between the nickel-titanium (Ni-Ti) tube and the heat transfer fluids. A quasi-1D numerical model has been developed to predict the elastocaloric cooling performance and optimize the heat transfer structures. We demonstrate, at specific operating conditions, the optimized elastocaloric regenerator design can improve the Specific Cooling Power (SCP) and Coefficient of Performance (COP) by 50% and 45%, respectively, as compared to a bare tube design. The numerical framework developed in this work offers a systematic investigation of complex regenerative heat exchanger systems and a detailed understanding of the design parameters that are vital for prototype development.

Thermomechanical fatigue damage mechanism and life assessment of a single crystal Ni-based superalloy

The thermomechanical fatigue (TMF) of single-crystal air-cooled turbine blades is critical for accurately evaluating the lifetimes of advanced aero-engines. The present work focuses on the mechanical behavior and damage mechanism of a single-crystal Ni-based superalloy (DD6) under stress-controlled TMF loading, and in-phase (IP) and out-of-phase (OP) mode of TMF were conducted and compared with low cycle fatigue (LCF) loading. A ratcheting effect is observed during the deformation of DD6 under TMF loading, and the direction and size of the ratcheting strain are considerably influenced by the phase angle and mechanical load. The ratcheting strain increases with mechanical load and dwell time at high temperature, consequently shortening the lifetime of the material. The key factors affecting the TMF damage of DD6 are identified through a SEM analysis, which shows that the damage under IP TMF loading mainly comes from creep and fatigue, whereas that under OP TMF loading is dominated by oxidation and fatigue. Based on the critical plane approach, a fatigue life prediction model is proposed considering the ratcheting effect to predict the fatigue life of DD6 under TMF loading. The good agreement between the proposed model and experimental data indicates that the model has the potential to predict the fatigue life of DD6 under TMF loading.

Cyclic deformation behavior and life prediction of P92 steel welded joints under thermomechanical fatigue loadings

In the present work, thermomechanical fatigue behavior and life prediction of P92 steel welded joints under various mechanical strain amplitudes and phase angles are investigated. Results demonstrate that the welded joints exhibit significant cyclic softening during fatigue process and the thermomechanical fatigue life is strain amplitude and phase angle dependent. The decrease in strain amplitude and the increase in phase angle induce the increase in fatigue life. Fracture analysis indicates that the fracture position changes significantly with the strain amplitude and phase angle. Finally, a modified energy-based life prediction model is proposed by considering the effects of phase angle and mean stress.

On the correlation between microstructural parameters and the thermo-mechanical fatigue performance of cast iron

Strain-controlled out-of-phase thermo-mechanical fatigue tests at 100–500 °C and various strain ranges were conducted on five cast iron grades, including one lamellar, three compacted and one spheroidal graphite iron. Investigations of graphite morphology and matrix characteristics were performed to associate parameters, such as geometrical features of graphite inclusions and the matrix microhardness, to the thermo-mechanical fatigue performance of each grade. From this, thermo-mechanical fatigue life as a function of maximum stress at half-life, is found to decrease consistently with increasing average graphite inclusion length irrespective of the graphite content. In contrast, no evident correlation between the fatigue life and the matrix microhardness is observed.

The effect of dwell times on the thermomechanical fatigue life performance of grade P92 steel at intermediate and low strain amplitudes

Results of an extended TMF test program on grade P92 steel in the temperature range of 620 °C–300 °C, comprising in-phase (IP) and out-of-phase (OP) tests, partly performed with symmetric dwells at Tmax/Tmin, are presented. In contrast to previous studies, the low-strain regime is also illuminated, which approaches flexible operation in a power plant with start/stop cycles. At all strain amplitudes, the material performance is characterized by continuous cyclic softening, which is retarded in tests at lower strains but reaches similar magnitudes in the course of testing. In the investigated temperature range, the phase angle does not affect fatigue life in continuous experiments, whereas the IP condition is more detrimental in tests with dwells. Fractographic analyses indicate creep-dominated and fatigue-dominated damage for IP and OP, respectively. Analyses of the (micro)hardness distribution in the tested specimens suggest an enhanced microstructural softening in tests with dwell times for the low- but not for the high-strain regime. To rationalize the obtained fatigue data, the fracture-mechanics-based DTMF concept, which was developed for TMF life assessment of ductile alloys, was applied. It is found that the DTMF parameter correlates well with the measured fatigue lives, suggesting that subcritical growth of cracks (with sizes from a few microns to a few millimeters) governs failure in the investigated range of strain amplitudes.

Impact of inter-metallic compound thickness on thermo-mechanical reliability of solder joints in solar cell assembly

This study evaluates the impact of intermetallic compound (IMC) thickness on thermo-mechanical reliability of lead-free SnAgCu solder joints in crystalline silicon solar cell assembly with regard to fatigue life. Finite element modelling is used to simulate the non-linear thermo-mechanical deformation of the joints. Five geometric models of solar cell assemblies with different IMC thickness layers in the range of 1 to 4 μm are utilized. The models were subjected to accelerated thermal cycling from 40 °C to 85 °C employing IEC 61215 standard for photovoltaic panels. Creep response of each of the assembly's solder joints to the induced thermal load were simulated using Garofalo-Arrhenius creep model. Simulation results indicate that when IMC thickness grows incrementally to 1, 2, 2.5, 3 and 4 μm, thermo-mechanical fatigue life of solder joints diminishes to 13,800, 11,800, 10,600, 9400 and 7800 cycles to failure respectively. Thus, solder joint fatigue life decreases as the IMC thickness increases during service lifetime. Therefore, proper design of solder joint in crystalline silicon solar cell assembly must include consideration of IMC layer thickness to prevent premature failure and to ensure fulfilment of desired functional lifetime of 13,688 cycles to failure (25 years).

Experimental investigation of fatigue life of Inconel 617 at elevated temperature

The design of the turbine has many complex terms and its efficiency is always related to its material selection and performance. Gas turbine efficiency is affected by the crucial limiting factors like temperature, thermalstress, and fatigue. The most common problems of components will be at different magnitudes of temperature, which results in the selection of the material of different criteria of various gas turbines. Inconel617 has the properties of withstanding high-temperature conditions due to which it has possible applications in gas turbines. In this paper, the experimental investigation was conducted for thermo-mechanical fatigue life prediction ofInconel617 at elevated temperature. The low cycle fatigue (LCF) test andthermo-mechanicalfatigue (TMF) test were utilized to comprehend the possible application of this material in the gas turbine combustion chamber.TMFand LCF tests were conducted using different strain parameters, these strain parameters are tested by in-phaseTMFand out-phaseTMFtests. This study is expected to provide an important role in the development of gas turbine material at elevated temperatures.

Experimental and computational study on thermo‐mechanical fatigue life of aluminium alloy piston

AbstractTo assess the life of a new diesel aluminium alloy piston under thermal shock loads, thermo‐mechanical fatigue (TMF) testing was conducted to characterise the TMF properties of the piston alloy, and an empirical model based on the constraint ratio concept was proposed to predict the TMF life of the piston. Considering that the empirical model required expensive experimental support, a platform with high‐frequency induction heating was established to simulate the force on the piston under thermal shock loads to calculate the piston life using the thermal shock test. Additionally, a finite element method was developed to compute the distributions of temperature, strain, and stress during this process. The characteristics of crack initiation and propagation in TMF test rods and piston mock‐ups were also investigated. The results showed that the TMF test rod suffered brittle fracture with brittle quasi‐cleavage features. The microcracks mainly occurred in primary Si particles due to stress concentration around the primary Si particles induced by the difference between the thermal expansion coefficients of Si and Al. From a macro perspective, the piston initially cracked at the rim above the pinhole, where the stress is larger than that along other directions. From a micro perspective, the protrusions of various sizes on the piston rim were induced by the compression stresses at high temperature. The piston cracks usually initiate around primary Si particles, propagate along the edge of primary Si in a straight line, bifurcate and then stop at a certain depth. If the piston was only heated, cracks or plastic deformations were not produced. The piston life can be assessed using the proposed empirical model based on the constraint ratio concept or thermal shock testing based on the developed platform. The difference between the predicted and experimental life was not greater than 7%.

Thermomechanical properties and fatigue life evaluation of SnAgCu solder joints for microelectronic power module application

In this paper, a new constitutive model of SnAgCu lead-free solder, which covered the low-temperature and elevated-temperature creep properties, was constructed according to the thermomechanical property test under a wide range of temperatures (−40 to 120°C). This model was then implemented to the finite element model for simulation of the cyclic creep deformation of Sn3Ag0.5Cu solder in a chip inside the microelectronic power module (MPM). The accumulated creep strain of the solder layer was calculated and used to predict the thermo-mechanical fatigue life of the MPM. The applicability of the life prediction method was evaluated through a cross-check of the present results with that obtained by a double power model in the literature, as well as the thermo-mechanical fatigue test results. It was found that the thermo-mechanical fatigue lives, estimated by the accumulated creep strain and creep strain energy density from the established hyperbolic sine power law, are closer to the test results than that estimated by the double power law. The presented model can be seen as an improvement on the existing constitutive models of SnAgCu lead-free solder.