Thermomechanical Fatigue Properties Research Articles

Insight into the role of thermal on the thermomechanical fatigue properties and microstructural damage mechanism of 316L stainless steel

The role of thermal cycling on the deformation behaviour and microscopic damage mechanisms of 316L stainless steel are clarified through isothermal fatigue (IF) and thermomechanical fatigue (TMF) tests under various temperature ranges. Corresponding microscopic damage mechanism is revealed based on the analysis of dislocation structure, plastic deformation, and crack initiation mode. Results show that the introduction of thermal cycling, as well as an increase in average temperature and temperature difference, result in the saturation of cyclic peak stresses, a decrease in fatigue life, and an intensification of dynamic strain ageing (DSA). Notably, under TMF loading at the same average temperature, there is a significant recovery process compared to IF. High-density dislocations transform into low-energy dislocation structures through cross-slip, indicating a pronounced recovery phenomenon. At large temperature differences and high average temperatures, the accumulation of dislocation tangles at grain boundaries and the formation of dislocation cell structures lead to severe incoherence in plastic deformation and the accumulation of damage. Furthermore, the introduction of thermal cycling and an increase in temperature differences also induce severe creep damage. It is observed that the dominant crack initiation mode changes from fatigue-dominated transgranular cracking to oxidation-induced intergranular cracking as the average temperature increases. Finally, a modified life prediction model is developed by incorporating thermal plastic strain energy to reflect the effect of thermal cycling on TMF behaviour.

Molecular dynamics simulation of thermomechanical fatigue properties of Ni-based single crystal superalloys

In this paper, the thermomechanical fatigue (TMF) properties of Ni-based single crystal superalloys are studied by molecular dynamics simulations. Two different cyclic deformation mechanisms of superalloys are found under TMF loadings. The sample has a higher cyclic stress range, plastic strain energy density and a shorter fatigue life under Out-of-phase TMF loading than those under In-phase TMF loading. Moreover, the low-temperature tension half-cycle is more favorable for dislocations and stacking faults to cut into the γʹ precipitate phase, causing an earlier failure of superalloys under Out-of-phase TMF loading, which attracts more attention in actual operation.

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.

Architected Tunable Structure for Improved Capability in Extreme Environments

Abstract A novel patterned-void structure is developed to improve the fatigue life compared with conventional circular cooling holes typically used in gas turbine components exposed to high temperatures. The distinctive S-shape of the voids and their specific arrangement enable manipulation of the structure's macroscopic stiffness and Poisson's ratio. An investigation of the isothermal and thermomechanical fatigue (TMF) properties of the proposed structure is carried out in strain-controlled conditions. The testing is performed on tubular specimens machined from a Nickel-based superalloy commonly used in gas turbine combustion systems (HAYNES 230®). The isothermal fatigue tests, performed at 300 °C, 600 °C, and 800 °C, demonstrated an increase in crack-initiation life of the proposed structure by a factor of up to 28 compared with the standard circular holes. The thermomechanical fatigue tests, performed across temperature ranges 300 °C–750 °C and 300 °C–850 °C, and using in-phase (IP) and out-of-phase (OP) strain ratios, demonstrated an increase in crack-initiation life by a factor of up to 16. The life after crack initiation (crack-propagation mode) was also shown to be longer for the proposed structure, which is attributed to a crack-arresting behavior inherent to the structure.

Influence of electric current on the thermo-mechanical static and fatigue properties of shape memory NiTi wires

NiTi shape memory wires are fostered to be utilized as actuators due to their strain recovery capability involved with phase transformation and the high mechanical properties. Since large strains are involved with the phase transformation, fatigue is a crucial issue that needs to be carefully investigated. Fatigue of superelastic alloys is usually analyzed through S-N curves obtained by imposing alternated stresses. Differently, actuators based on shape memory alloys are usually subjected to a bias load and thermal cycles drive the actuation. This work presents an experimental characterization of the thermo-mechanical static and fatigue properties of shape memory NiTi wires intended for actuation purposes. A series of original results are presented. The wires’ stroke capability is found to depend on the heat rate and temperature associated with a varying magnitude of the current utilized to perform the thermal cycle exploiting the Joule’s effect. Alongside a reduction of the actuation time associated with increasing current, results show that the stroke increases with the current, and with it the alternated strain associated with a single thermal cycle. The ultimate stress under quasi-static loads is also found to be influenced by the current, as it linearly diminishes with it, hence with the operating temperature. Above all, the most interesting results relate to fatigue, as it is found that the influence of the applied current on thermo-mechanical fatigue is even larger than the effect of the applied bias load. Further, functional fatigue in terms of reduction of the recoverable strain upon thermal actuation has not been found for any loading condition.

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 fatigue properties and microstructural damage of nitrogen alloyed 316LN stainless steel

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) tests are carried out to investigate the cyclic deformation behavior, microstructural damage and cracking behavior of 316LN stainless steel. The result shows that dynamic strain aging (DSA) and oxidation are the two principal deformation and damage mechanisms. DSA occurs within the temperature range of 250 °C-450 °C, and the action intensity of which increases from 250 °C to 450 °C. Oxide layers preferentially form along surface persistent slip markings, which accelerates the microcrack nucleation and crack propagation, leading to an oxidation-assisted transgranular failure. The fatigue life under IP-TMF and OP-TMF loadings are approximately equal and longer than that under IF-450 °C loadings.

Influence of microstructure on the thermo-mechanical fatigue behavior and life of vermicular graphite cast irons

Cylinder head is subjected to the combination changes of temperature and mechanical loads in service. In such harsh condition, thermo-mechanical fatigue (TMF) failure is a main problem. In this study, two kinds of vermicular graphite cast irons were selected to reveal the TMF properties and life prediction. Vermicular graphite casts with different microstructures have different ferrite cluster morphologies, which result in different fracture mechanisms. The hysteresis loops of both materials display obvious asymmetry. Hysteresis energy and mechanical strain amplitude were selected as damage parameters to determine the fatigue life. It is found that the TMF performance of vermicular graphite cast iron with higher pearlite content is better; meanwhile, the life of TMF at different constraint ratios was also studied. Hysteretic energy related to constrain ratio is in a parabolic form, and the life of TMF decreases with the absolute value of constraint ratio increasing. Finally, the quantitative relationship between constraint ratio and TMF life was also discussed.

Thermomechanical fatigue life prediction method for nickel-based superalloy in aeroengine turbine discs under multiaxial loading

The multiaxial thermomechanical fatigue properties for nickel-based superalloy GH4169 in aeroengine turbine discs are investigated in this paper. Four types of axial–torsional thermomechanical fatigue experiments were performed to identify the cyclic deformation behavior and the damage mechanism. The experimental results showed that the creep damage can be generated under thermally in-phase loading while it can be ignored under thermally out-of-phase loading, and the responded stress increasing phenomenon, that is, non-proportional hardening, can be shown under the mechanically out-of-phase strain loading. Based on the analysis of cyclic deformation behavior and damage mechanism, a life prediction method was proposed for multiaxial thermomechanical fatigue, in which the pure fatigue damage, the creep damage, and the interaction between them were simultaneously considered. The pure fatigue damage can be calculated by the isothermal fatigue parameters corresponding to the temperature without creep; the creep damage can be calculated by the principle of subdivision, and the creep–fatigue interaction can be determined by creep damage, fatigue damage, and an interaction coefficient which is used to reflect the creep–fatigue interaction strength. The predicted results showed that the proposed method is reasonable.

Thermo-mechanical fatigue behavior of nickel-based 23Cr-45Ni-7W alloy for boiler pipes and tubes

This study focuses on thermo-mechanical fatigue (TMF) behaviors of nickel-based 23Cr-45Ni-7W alloy for pipes and tubes in high-efficiency power boilers. TMF tests (maximum temperature = 700 °C; minimum temperature = 100 °C) were conducted under in-phase (IP) and out-of-phase (OP) conditions. The lives under the IP condition is 0.30–0.44 times of that under the OP condition when same total strain range, Δεt, is applied. The TMF properties obtained via the tests are compared with the results of isothermal fatigue (ITF) tests conducted at 700 °C and bithermal fatigue (BTF) tests with maximum and minimum temperatures of 700 °C and 100 °C, respectively. The fracture morphology for each type of TMF, ITF, and BTF is similar under the test conditions of this study. Intergranular cracking is predominant under application of cyclic tensile creep strain during the fatigue tests, and transgranular cracking is predominant under application of cyclic compressive creep strain during the fatigue tests. The inelastic strain range–fatigue life (Δεin–Nf) relation in the TMF test and the inelastic strain range components–life (Δεij–Nij) relation determined using the strain range partitioning method show a good agreement within a factor of 1.5 scatter band. These results indicate the possibility of predicting the TMF life of the 23Cr-45Ni-7W alloy using the results of the ITF test by considering the amount and direction (tension or compression) of creep strain applied to the alloy.

Effects of post-processing on the thermomechanical fatigue properties of laser modified NiTi

The multifunctional capabilities needed for advanced shape memory alloys (SMA) actuators has been shown to be achievable by locally tuning the properties through laser processing. Before the wide-spread use of these SMAs is realized, a detailed understanding on the long-term stability and functional life span of these material must be achieved. The current study systematically investigates the effects of thermomechanical treatment on laser modified NiTi wires, while comparing them to the original base material. Surface analysis was done using a scanning electron microscope (SEM), while microstructure analysis was performed using transmission electron microscopy (TEM). Mechanical properties were assessed using standard tensile tests and a custom built thermomechanical fatigue. Results showed that the coarse-grains, large inclusions and surface defects associated with as laser modified NiTi resulted in reduced mechanical performance. However, subsequent thermomechanical treatment restored the refined microstructure and mechanical performance similar to the base material while providing the added functionality, thus allowing for laser processed NiTi to be used for manufacturing multiple memory NiTi actuators.

Effect of Strain Ranges and Phase Angles on the Thermomechanical Fatigue Properties of Thermal Barrier Coating System

Abstract Thermal barrier coatings (TBCs) are the key material of components used in elevated temperature of gas turbines, and their mechanism of delamination and failure under service conditions has been the hot spot of research for a long time. The influences of strain ranges and phase angles on the thermomechanical fatigue (TMF) properties of samples with TBCs were investigated. It is shown that under the same phase angles, the TMF lifetime decreases with the increase of strain ranges. Under the same strain range, the in-phase tests have longer TMF lifetime than out-of-phase tests. In both samples, cracks are initiated in thermally grown oxide (TGO) layer, and then propagate along the bond coat/ceramic top coat, forming the delamination cracks. When the delamination cracks connect with the segmentation cracks initiated in ceramic coat, the TBCs spall. A TMF lifetime model concerning strain ranges and phase angles is established, and an exponential law exists between TMF lifetime and the maximum stress.

Thermo-mechanical fatigue property and life prediction of vermicular graphite iron

Thermo-mechanical fatigue (TMF) failure is the major problem of the cylinder head subjected to combined variations in temperature and loading during operation. This study mainly focuses on the TMF property and the life prediction of vermicular graphite iron (VGI). The ferrite clusters can be easily found from the microstructure and fractography images. Compared with the TMF experimental data testing at 125–400℃ and 125–500℃, significant cyclic hardening occurs in the former and slight hardening does in the latter. Depending on the difference in the damage mechanism between TMF and iso-thermal low-cycle fatigue (LCF), based on the hysteresis energy, a life prediction method has been proposed at first. By the minimum amount of LCF and TMF tests, the present method can predict the TMF life rapidly, accurately and cheaply. And based on the difference in the fatigue crack propagation thresholds between pearlite and ferrite, the fracture mechanism of TMF was also discussed.

Lifetime Assessment of Cylinder Heads for Efficient Heavy Duty Engines Part I: A Discussion on Thermomechanical and High-Cycle Fatigue as Well as Thermophysical Properties of Lamellar Graphite Cast Iron GJL250 and Vermicular Graphite Cast Iron GJV450

Cast iron materials are used as materials for cylinder heads for heavy duty internal combustion engines. These components must withstand severe cyclic mechanical and thermal loads throughout their service life. While high-cycle fatigue (HCF) is dominant for the material in the water jacket region, the combination of thermal transients with mechanical load cycles results in thermomechanical fatigue (TMF) of the material in the fire deck region, even including superimposed TMF and HCF loads. Increasing the efficiency of the engines directly leads to increasing combustion pressure and temperature and, thus, lower safety margins for the currently used cast iron materials or alternatively the need for superior cast iron materials. In this paper (Part I), the TMF properties of the lamellar graphite cast iron GJL250 and the vermicular graphite cast iron GJV450 are characterized in uniaxial tests and a mechanism-based model for TMF life prediction is developed for both materials. The model can be used to estimate the fatigue life of components by means of finite-element calculations (Part II of the paper) and supports engineers in finding the appropriate material and design. Furthermore, the effect of the elastic, plastic and creep properties of the materials on the fatigue life can be evaluated with the model. However, for a material selection also the thermophysical properties, controlling to a high level the thermal stresses in the component, must be considered. Hence, the need for integral concepts for material characterization and selection from a multitude of existing and soon-to-be developed cast iron materials is discussed.

Thermomechanical fatigue of post-weld heat treated NiTi shape memory alloy wires

Integration of NiTi shape memory alloys (SMAs) into a growing range of applications will rely on the development of welding and joining techniques. Successful adoption of welded NiTi SMA actuators into these applications requires the characterization of the joint fatigue properties. Few investigations have been attempted to characterize the mechanical fatigue properties of NiTi joints, and to the authors’ knowledge there have been no previous investigations on the thermomechanical fatigue properties of these joints. The current work investigated the thermomechanical fatigue properties of Nd:YAG pulsed laser welded, and post-weld heat treated NiTi wires. The welded wires maintained over 86% of the base metal ultimate tensile strength; however, they had reduced actuation stability and stroke, and had significantly reduced cycle life. Use of a post-weld heat treatment successfully increased both the actuation stability and the cycle life by an order of magnitude compared to the welded wires.

Elastic and Mechanical Fatigue at High Temperatures of High-Alumina Castables with Addition of Partially Stabilized Zirconia

This work evaluates the impact of added zirconia on the thermomechanical and thermoelastic fatigue properties of high-alumina refractory castables. Three testing scenarios are presented in the study: 1) Resonant Frequency Damping Analysis (RFDA) after cyclic temperature changes with and without mechanical load, 2) Refractoriness under Load (RuL) tests, and 3) results of abrupt temperature shifts between two high temperatures in a special thermal shock furnace. The tested refractory formulations were a reference castable based on tabular alumina and two castables each containing 13.75 mass-% of zirconia. The formulations of the zirconia castables had an addition of either fully-stabilized zirconia doped with 8 mol-% yttria (Y-FSZ) or partially-stabilized zirconia doped with 3 mol-% calcia (Ca-PSZ). The Y-FSZ castable displayed elastic and mechanical behaviour by means of RFDA and RuL similar to the reference sample and had good recovery after thermal shock testing. The Ca-PSZ formulation showed some beneficial toughening through energy dissipation of progressing cracks, but after cooling exhibited the matrix fracturing influence of the martensitic zirconia phase transformation which would negatively impact service life of the refractory.

Damage evolution in compacted graphite iron during thermomechanical fatigue testing

Thermomechanical fatigue properties of a compacted graphite iron in an out of phase configuration are investigated for different maximum temperatures and mechanical strain ranges. Furthermore, the stress–strain hysteresis loops are analysed, and, in particular, the unloading modulus, i.e. the elastic modulus measured during specimen unloading, is obtained from each cycle. This material parameter has earlier been explicitly related to the amount of microcracking in cast irons. The results show that the unloading modulus linearly declines with the numbers of cycles in all tests performed. In addition, the rate of change of the unloading modulus is closely related to the number of cycles to failure. Accordingly, it is concluded that microcracks are independently propagated by fatigue until a point of rapid crack linking resulting in ultimate failure. This is supported by microstructural analyses consisting of optical microscope images taken at different stages throughout the life of a specimen.

Thermomechanical Fatigue Behavior of SS304

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) properties of SS304 stainless steel were evaluated in the temperature range of 300–600 °C. All tests were strain controlled and the strain rate was kept constant. Material exhibited cyclic hardening followed by saturation of cyclic stress. Fatigue life reduced in OP testing compared to IP testing as the strain amplitude increased. Specimens failed with multiple cracks on the surface in OP testing whereas a single crack had predominantly grown inward and weakened the section in IP testing. Nanoindentation studies showed higher hardness around the crack tip compared to the uncracked regions. TEM showed a well developed dislocation cell structure in OP TMF while IP specimens showed planar slip bands with pile up of dislocations.

Microstructure, Creep and Thermomechanical Fatigue of Novel Solid Solution and Laves Phase Strengthened Cr2O3 and Al2O3 Forming Ferrites for Car Engine Exhaust and Heat Exchanger Systems

Increasing efficiency of internal combustion engines by downsizing leads to a rise in exhaust gas temperature up to 1000 °C accompanied with higher mechanical loading of exhaust systems, approaching the limits of currently used Cr2O3 forming ferrites. Similarly, the conditions in solid oxide fuel cell (SOFC) heat exchanger systems exceed the limits of commercial Al2O3 forming heat resistant ferrites with respect to temperature and thermomechanical fatigue (TMF) loading. Aim of the presented work was the development of Cr2O3 and Al2O3 forming ferrites with improved mechanical strength and TMF properties up to 900–1000 °C. Baseline material was the Crofer® 22H alloy, which should be further hardened by solid solution and Laves phase precipitates. The paper gives an overview of the key results achieved at Research Center Julich on correlation between microstructure evolution, deformation and lifetime in hot tensile, creep and TMF tests. Increasing Laves phase dissolution temperature elevates short time stability of precipitates and long term stability of the grain structure up to 950 °C. Furthermore, a lower Laves phase content of the Al2O3 forming material could be compensated by dynamic strain ageing via Al diffusion resulting in superior high temperature strength, creep behavior and TMF life.

Thermo-mechanical fatigue test of a wrought Co-based alloy as potential tooling material for die casting

Thermo-mechanical fatigue (TMF) test of a wrought Co–Cr–Mo alloy was investigated and Co-based alloy showed superior TMF properties than the control sample of a hot work tool steel. During TMF test, high oxidation resistance and increased hardness were observed in the Co-based alloy in contrast to the low oxidation resistance and decreased hardness in the tool steel. The γ-phase-to-ε-phase transformation of the Co-based alloy was considered as the main hardening factor during the TMF test.