Thermomechanical Fatigue Cycles Research Articles

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.

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

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

Predicting creep‐fatigue and thermomechanical fatigue life of Ni‐base superalloys using a probabilistic physics‐guided neural network

AbstractPredicting the life of thermomechanical fatigue (TMF) is challenging because there are several parameters describing the mechanical and thermal cycles including dwell periods within the cycle that can impact life. The relationships between these TMF history parameters and fatigue life are not always clear. This paper explores the use of a neural network (NN) with a probabilistic physics‐guided architecture to learn these relationships and predict the cycles to failure for a wide range of possible creep‐fatigue and thermomechanical fatigue histories. Using inputs of strain range, maximum and minimum temperature, the phasing between the thermal and mechanical cycles, cycling frequency, and dwell time in either tension or compression, the model predicts both the mean fatigue life and its confidence intervals (CI). The model is demonstrated from a comprehensive set of data on single‐crystal and directionally solidified Ni‐base superalloys extracted from the literature. The model is evaluated in several ways to determine the success of learning the relationship between the applied TMF histories and cycles to failure.

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.

Thermomechanical fatigue damage behavior and deformation mechanism of coke drum with Cr-Mo steel

ABSTRACT Damage behaviour and deformation mechanism of SA387Gr11CL2 (1.25Cr-0.5Mo) low alloy steel for coke drum in the thermomechanical fatigue regime are investigated. The experimental results indicate that a main macroscopic crack is detected in the gauge section of each testing specimen. Multiple crack initiation sources are observed in the fracture surface. There are a great deal of secondary cracks in the crack propagation region of the fracture surface. Longitudinal-section microcrack initiates on the specimen surface and its propagation path is predominantly transgranular. When the crack propagation arrives at the threshold with the cyclic number increase, crack bifurcation is obviously observed. Fatigue is the dominant damage of SA387Gr11CL2 low alloy steel under the thermomechanical fatigue loading. The dominant deformation mechanism of SA387Gr11CL2 low alloy steel during thermomechanical fatigue cycle is wavy slip of dislocations. The types and morphologies of carbides in SA387Gr11CL2 low alloy steel are not changed after plastic deformation.

Deformation and damage assessment in type 316 LN stainless steel weld joint under isothermal and thermomechanical cyclic loading

Present investigation is aimed at understanding the deformation behavior and the development of damage in a type 316 LN austenitic stainless steel (SS) weld joint (WJ) under isothermal low cycle fatigue (IF) and thermomechanical fatigue (TMF). In-phase (IP) and out-of-phase (OP) TMF tests were carried out by maintaining a phasing relation of 0° and 180° respectively, between the mechanical strain and temperature cycles. Dynamic strain ageing (DSA) was found to exert a strong influence on the cyclic stress response (CSR) of the joint under both IF and TMF cycling. The CSR was observed to be higher under TMF compared to IF cycling due to the activation of additional hardening mechanisms in the former tests. The difference in fatigue life under TMF and IF is rationalized based on the development of deformation and damage through optical and scanning electron microscopy (SEM) coupled with electron backscatter diffraction (EBSD) studies. A clear demarcation of the strain distribution in the base metal (BM) and weld metal (WM) region of the joint is achieved through detailed EBSD analysis of the tested specimens. Cyclic plastic deformation and the associated development of damage take place independently in the BM and the WM parts of the joint, competing to cause the failure. The build-up of damage under IF and TMF cycling is dictated by a combination of DSA and δ-ferrite transformation, depending on the applied strain amplitude and the strain-temperature phasing employed. The observed life variations have been rationalized in terms of the substructural evolution and fracture behavior under different testing conditions.

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.

The effect of dwell on thermomechanical fatigue in superaustenitic steel Sanicro 25

AbstractSuperaustenitic steel Sanicro 25 has been subjected to out‐of‐phase thermomechanical fatigue cycles with 10‐min dwell at peak temperature in the temperature range from 250°C to 700°C. The effect of the dwells on the cyclic response, internal structure and damage mechanism was studied. Cyclic hardening/softening curves, cyclic stress–strain curves and fatigue life curves were evaluated. The internal structure and nanoparticles representing the obstacles for dislocation motion were analysed and identified by energy dispersive X‐ray spectroscopy in scanning transmission electron microscope. Scanning electron microscopy combined with focused ion beam and electron backscatter diffraction was adopted to reveal the respective mechanisms responsible for fatigue crack nucleation and propagation. Effect of dwells in in‐phase and in out‐of‐phase cycling on the fatigue behaviour, on the modification of internal structure and damage mechanisms, were analysed and discussed.

Semi in-situ observation of crack initiation in compacted graphite iron during thermo mechanical fatigue

Compacted Graphite Iron is a suitable material for the engine cylinder heads of heavy duty trucks. In the base of the cylinder head, the Valve Bridge is subjected to Thermo-Mechanical Fatigue as a result of daily start-up and shut-down operational cycles. With the aim to investigate crack initiation, a semi in-situ experiment was conducted by employing a disassemblable dog-bone sample. The procedure consisted of iteratively interrupting a thermo-mechanical fatigue test to observe the evolution of the microstructure by Scanning Electron Microscopy, and Electron Backscatter Diffraction. Finishing the first thermo-mechanical fatigue cycle, edge delaminated graphite particles were observed to be connected by cracks. Cyclic plastic strain computation showed that edge graphite particles can interact by overlapping plastic zones that arise at the tip of the delamination cracks and thus provide a crack propagation path. Finally, the electron backscattered diffraction technique allowed to observe the competition between recovery and plastic strain during the first few thermo-mechanical fatigue cycles.

The effect of temperature on TMF(HCF) crack initiation endurance

Thermo-mechanical fatigue (TMF) testing is increasingly used as a means of bench-marking the response of critical locations in high temperature components during operation by subjecting laboratory specimens to service and cyclic loading conditions involving the simultaneous action of thermal and mechanical strains. In this paper, evidence is presented to show the effect of maximum temperature in TMF cycles without and with superimposed high frequency (HCF) loading on crack initiation endurances for GJV-450 vermicular cast irons. For TMF cycles with maximum temperatures in the range 400–480 °C, there is little influence of superimposed higher frequency HCF strain amplitudes of up to ±0.03% on crack initiation endurance, whereas the effects of higher superimposed HCF loading are more significant.

Isothermal and thermomechanical fatigue behaviour of type 316LN austenitic stainless steel base metal and weld joint

Thermomechanical fatigue (TMF) studies were carried out on type 316 LN austenitic stainless steel (SS) base metal (BM) and its weld joint (WJ) with a temperature interval of 623–873 K under in-phase (IP) and out-of-phase (OP) combinations of mechanical strain and temperature. Tests were performed under mechanical strain control mode using the strain amplitudes in the range, ±0.25% to ±0.6%. Besides, isothermal low cycle fatigue (IF) tests were also conducted on the weld joint at 873 K. TMF cycling resulted in a slightly higher cyclic stress response (CSR) compared to IF loading for the weld joint. The IP TMF cycling yielded a higher plastic strain amplitude in comparison to OP TMF for both the base metal and the weld joint. Cyclic life of the weld joint is found to be inferior to that of the base metal. The fatigue lives exhibited by the base metal and the weld joint varied in the following sequence: IP TMF < IF < OP TMF. Preferential crack initiation in the weld region owing to embrittlement of transformed δ-ferrite was noticed with an increase in the mechanical strain amplitude under both TMF and IF cycling conditions. Crack initiation and propagation generally occurred in transgranular mode under IF and OP TMF and mixed (trans-plus intergranular) mode under IP TMF cycling. Electron back scattered diffraction (EBSD) measurements were employed to investigate the substructural development in terms of local misorientation distribution, inverse pole figure (IPF) maps and boundary dislocation density (ρ) in the weld metal region, under IF and IP TMF cycling. The dislocation density was found to decrease for both IF and IP TMF cycling compared to the as-welded condition, with the decrease being higher for IF cycling. The TMF data generated for the type 316LN SS base metal and weld joint at different strain ranges in the present study clearly demonstrated a lack of conservatism associated with the current design practice which is based on the IF data at the maximum temperature (Tmax) of the service cycle.

Thermal cycling effects on the creep-fatigue interaction in type 316LN austenitic stainless steel weld joint

Thermomechanical fatigue (TMF) tests under in-phase temperature-mechanical strain combination were performed on a type 316 LN austenitic stainless steel weld joint using temperature ranges of 573–823 K, 623–873 K and 673–923 K employing a mechanical strain amplitude (Δεmech/2) of ±0.4%. Isothermal low cycle fatigue (designated as IF) tests were also carried out at the maximum temperatures (Tmax) of TMF cycling. The interaction of creep deformation with IF and TMF cycling was assessed by incorporating hold periods of 1 min and 5 min at the maximum tensile strain. TMF cycling resulted in considerably lower lives compared to IF, with the life reduction being more significant under hold conditions. Further, a clear difference in the cyclic stress response (CSR) was observed between creep-TMF and creep-IF cycling, depending on the hold duration and temperature range. The contribution of creep deformation to the overall damage was found to be higher under TMF compared to IF cycling at the Tmax. The observed variations in the cyclic lives under IF and TMF cycling with and without hold times were explained on the basis of the dominant fracture mechanisms identified through detailed microstructural characterization of the crack initiation and propagation modes. The extent of intergranular damage was seen to increase with an increase in the temperature range and the hold period. The role of oxidation in the development of damage was observed to be different under creep-TMF and creep-IF cycling. Numerous grain boundary microvoids and their inter-linkages caused enhanced crack propagation under creep-TMF, compared to creep-IF cycling. A comparison of the deformation behaviour, cyclic life and damage developed in the material under IF and TMF suggested that the performance of the weld joint under TMF cycling cannot be satisfactorily represented through IF tests at the Tmax.

Thermo-Mechanical Fatigue Lifetime Assessment of Spheroidal Cast Iron at Different Thermal Constraint Levels

In previous work on the thermo-mechanical fatigue (TMF) of compacted graphite iron (CGI), lifetimes measured under total constraint were confirmed analytically by numerical integration of Paris’ crack-growth law. In current work, the results for CGI are further validated for spheroidal cast iron (SGI), while TMF tests at different constraint levels were additionally performed. The Paris crack-growth law is found to require a different CParis parameter value per distinct constraint level, indicating that Paris’ law does not capture all physical backgrounds of TMF crack growth, such as the effect of constraint level. An adapted version of Paris’ law is developed, designated as the local strain model. The new model considers cyclic plastic strains at the crack tip to control crack growth and is found to predict TMF lifetimes of SGI very well for all constraint levels with a single set of parameters. This includes not only full constraint but also over and partial constraint conditions, as encountered in diesel engine service conditions. The local strain model considers the crack tip to experience a distinct sharpening and blunting stage during each TMF cycle, with separate contributions to crack-tip plasticity, originating from cyclic bulk stresses in the sharpening stage and cyclic plastic bulk strains in the blunting stage.

Thermo-mechanical fatigue evaluation of a thermal barrier coating bond-coatless system

Thermal barrier coating systems are often subjected to complex thermo-mechanical loading involving a combination of mechanical stresses under complex thermal gradients, in addition to the contribution of oxidation to the damage processes. Recently, a thermal-barrier coating bond-coatless system has been developed using prior-surface treatments. Atmospheric plasma spray (APS) Yttria-Stabilized-Zirconia (YSZ) is deposited on the AM1 nickel-based single crystalline superalloy substrate. The substrate has been either grit-blasted or laser textured before thermal spraying. The durability of these systems are investigated performing out-of-phase load controlled thermo-mechanical fatigue (TMF) tests in the 500–1100 °C and 120–700 MPa temperature and stress ranges. The contribution of a prior thermal over-aging of 200 h at 1100 °C has also been investigated. The damage mechanisms during out-of-phase TMF of the different TBC bond-coat-less systems have been studied and compared to the ones obtained during isothermal creep at 1100 °C/120 MPa. It is shown that a prior thermal over-aging favors cycling ratcheting during TMF cycling due to the degradation of the substrate microstructure. In addition, the coating damage mechanisms are observed to depend on the prior-surface treatment and thermal over-aging. Cracks perpendicular to the interface in the top coat have been observed whatever the specimens and microstructural state. Top coats are debonded for grit-blasted substrates while crack bifurcation above key holes is observed for laser textured substrates, leading to a better top coat TMF durability.

Lifing the Effects of Crystallographic Orientation on the Thermo-Mechanical Fatigue Behaviour of a Single-Crystal Superalloy.

Thermo-mechanical fatigue (TMF) is a complex damage mechanism that is considered to be one of the most dominant life limiting factors in hot-section components. Turbine blades and nozzle guide vanes are particularly susceptible to this form of material degradation, which result from the simultaneous cycling of mechanical and thermal loads. The realisation of TMF conditions in a laboratory environment is a significant challenge for design engineers and materials scientists. Effort has been made to replicate the in-service environments of single crystal (SX) materials where a lifing methodology that encompasses all of the arduous conditions and interactions present through a typical TMF cycle has been proposed. Traditional procedures for the estimation of TMF life typically adopt empirical correlative approaches with isothermal low cycle fatigue data. However, these methods are largely restricted to polycrystalline alloys, and a more innovative approach is now required for single-crystal superalloys, to accommodate the alternative crystallographic orientations in which these alloys can be solidified.

The Effect of Phase Angle on the Thermo-Mechanical Fatigue Life of a Titanium Metal Matrix Composite.

The thermo-mechanical fatigue (TMF) behaviour of a Ti-6Al-4V matrix composite reinforced with SCS-6 silicon carbide fibres (140 μm longitudinal fibres, laid up hexagonally) has been investigated. In-phase and out-of-phase TMF cycles were utilized, cycling between 80–300 °C, with varying maximum stress. The microstructure and fracture surfaces were studied using electron backscatter diffraction (EBSD), energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), profilometry, and optical microscopy. The results have shown the damaging effect of out-of-phase cycling with crack initiation occurring earlier than in in-phase tests and crack propagation rates being accelerated in out-of-phase cycles. Fatigue crack initiation has been shown to be sensitive to crystallographic texture in the cladding material and thermo-mechanical fatigue test results can be considered according to a previously proposed conceptual framework for the interpretation of metal matrix composite fatigue.

Thermomechanical fatigue crack growth in a single crystal nickel base superalloy

Thermomechanical fatigue crack growth in a single crystal nickel base superalloy was studied. Tests were performed on single edge notched specimens, using in phase and out of phase thermomechanical fatigue cycling with temperature ranges of 100–750 °C and 100–850 °C and hold times at maximum temperature ranging from 10 s to 6 h. Isothermal testing at 100 °C, 750 °C and 850 °C was also performed using the same test setup. A compliance-based method is proposed to experimentally evaluate the crack opening stress and thereby estimate the effective stress intensity factor range ΔKeff for both isothermal and nonisothermal conditions. For in phase thermomechanical fatigue, the crack growth rate is increased if a hold time is applied at the maximum temperature. By using the compliance-based crack opening evaluation, this increase in crack growth rate was explained by an increase in the effective stress intensity factor range which accelerated the cycle dependent crack growth. No significant difference in crack growth rate vs ΔKeff was observed between in phase thermomechanical fatigue tests and isothermal tests at the maximum temperature. For out of phase thermomechanical fatigue, the crack growth rate was insensitive to the maximum temperature and also to the length of hold time at maximum temperature. The crack growth rate vs ΔKeff during out of phase thermomechanical fatigue was significantly higher than during isothermal fatigue at the minimum temperature, even though the advancement of the crack presumably occurs at the same temperature. Dissolution of γ′ precipitates and recrystallization at the crack tip during out of phase thermomechanical fatigue is suggested as a likely explanation for this difference in crack growth rate.

Probabilistic Fatigue Software Platform (ProbFat) for Optimizing Life-Cycle Decisions

Fatigue is inherently uncertain. Thus, the use of technology for prediction of fatigue damage to ensure the long-term reliability of equipment must embrace such uncertainty. The proposed probabilistic fatigue software platform (ProbFat) accomplishes just that. The stochastic engine behind ProbFat is a collection of Bayesian Decision Networks (BDN) that assess risk to recommend the optimal life-cycle strategy for maximizing overall return (i.e. return = benefit – cost). Benefit is a measure of profitability (e.g. annualized revenue) and cost is a measure of expense (e.g. design, inspection, repair, replacement, failure, et cetera). To optimize the return, the BDN must incorporate all aspects of LCM including: (i) design and construction, (ii) monitoring, inspection, and maintenance scheduling, (iii) identification of damage, (iv) assessment of observed damage, and (v) decision strategies to either run, repair, rerate, replace, or retire the component(s) of interest at the best possible times. BDNs are structured as graphical cause-effect relationships, represented by probabilistic nodes with directional links. Populating conditional probabilities associated with nodes is accomplished through assessment of laboratory and site-specific data, contributions from subject matter experts and simulations from in-house deterministic and stochastic fatigue models. In this paper, two industry examples are used to demonstrate the effectiveness of ProbFat: (i) low cycle thermo-mechanical fatigue of a coke drum, and (ii) very-high cycle vibration fatigue of a piping system. For both applications the fatigue methods available in API 579-1/ASME FFS-1 are utilized for deterministic/probabilistic prediction of fatigue damage accumulation and remaining life. Methods available in API RBI are used for inspection planning and maintenance scheduling.

Proposal of Characterization Procedure of Metal⁻Graphite Interface Strength in Compacted Graphite Iron.

Compacted graphite iron is the material of choice for engine cylinder heads of heavy-duty trucks. Compacted graphite iron provides the best possible compromise between optimum mechanical properties, compared to flake graphite iron, and optimum thermal conductivity, compared to spheroidal graphite iron. The vermicular-shaped graphite particles, however, act as stress concentrators, and, as a result of delamination from the metal matrix, they are responsible for crack initiation during the thermomechanical fatigue cycles occurring through engine startup and shutdown cycles. Scratch tests driven over the matrix and into the graphite particles were performed in order to characterize the strength of the metal–graphite interface. Samples extracted from a cylinder head in as-cast condition were compared to samples subjected to a heat-treatment at 700 °C for 60 h. The former samples were composed of a primarily pearlitic matrix and graphite particles (~11.5 vol %), whereas, after annealing, a certain pearlite fraction decomposed into Fe and C, producing a microstructure with graphite–ferrite interfaces, exhibiting a partially spiky morphology. The scratch test revealed that the ferrite–graphite interfaces with spiky nature exhibited a stronger resistance to delamination compared to the ferrite–graphite interfaces with smooth morphology. One reason for the high interface strength is the mechanical interlocking between graphite spikes and ferrite, increasing the contact area between the two phases.

Service-like TMF tests for the life assessment of an SX GT blade

Thermo-mechanical fatigue (TMF) is the main source of failure of gas turbine (GT) components, owing to thermal gradients and complex geometry. A GT-component life assessment procedure has to consider TMF and the effect of metallic coatings. A single crystal (SX) blade is analysed by FE simulation to identify the critical locations and their thermo-mechanical conditions during a ‘cold’ startup/shutdown transient. The defined TMF cycle is applied to SX superalloy specimens with two different coatings. The results highlight a significant life reduction for specimens with the coating having the highest Re content due to its microstructure. The experimental results are confirmed by field feedback from operated blades.This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.