Out-of-phase Thermomechanical Fatigue Research Articles

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.

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

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

Thermo-mechanical fatigue deformation and fracture mechanisms of nickel-based powder metallurgy superalloy

Thermo-mechanical fatigue (TMF) failure is one of the major concerns for aero-engine turbine discs under complex service loading conditions. By conducting both in-phase (IP) and out-of-phase (OP) TMF tests, the stress-strain hysteresis loops and the cyclic stress responses were analyzed for the third-generation nickel-based powder metallurgy (P/M) superalloy FGH4098. To reveal the failure mechanism, advanced microscopy analysis was carried out to characterize the fracture surface and microstructure of the alloy after failure. The phase angle between alternating mechanical and thermal loads plays an important role for the magnitude and evolution of mean stress. According to SEM fractography, the crack growth exhibits intergranular fracture under IP TMF and transgranular fracture under OP TMF. From the electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) characterization results, the geometrically necessary dislocation (GND) density is higher for IP TMF, indicating more severe plastic deformation. The material shows cyclic softening in the high-temperature half-cycle, but cyclic hardening in the low-temperature half-cycle. The cyclic deformation is mainly related to dislocation activities under IP loading conditions, for which fatigue damage dominates the failure process. Under OP loading conditions, the deformation behavior is mainly associated with the generation of stacking faults (SF), and its failure mechanism includes both fatigue and oxidation damage.

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.

Effect of Mg on elevated-temperature low cycle fatigue and thermo-mechanical fatigue behaviors of Al-Cu cast alloys

The effects of Mg addition on the cyclic deformation and fatigue life behaviors of Al-Cu 224 cast alloys were investigated under isothermal low cycle fatigue (LCF) at 300 °C and out-of-phase thermo-mechanical fatigue (OP-TMF) in the temperature range 60–300 °C. The results show that all tested alloys experienced cyclic softening during both LCF and OP-TMF, whereas the softening ratio of Mg-containing alloys was lower than that of the base alloy free of Mg because of the lower coarsening rate of θ'-Al2Cu precipitates. Under both LCF and TMF, near-surface porosity and brittle intermetallic particles were considered sources of crack initiation. TMF lives were inferior to LCF lives under the same strain amplitude because of the combined damage effect of the cyclic mechanical and thermal loadings. Under LCF, the addition of Mg enhanced the fatigue performance because of the co-existing θʹ and θʺ precipitates and the higher thermal resistance of θʹ; under TMF, it led to a slight reduction in fatigue performance. The hysteresis energy model was successfully applied to predict the LCF and TMF lifetimes. The predicted results agree well with the experimentally measured LCF and TMF lifetimes.

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.

Experimental characterization of the short crack growth behavior of a ductile cast iron (DCI GJS-500) affected by intergranular embrittlement at temperatures nearby 400 [formula omitted

In this investigation, the fatigue and crack growth behavior of the ductile cast iron material (DCI) GJS-500 was experimentally characterized by performing isothermal low cycle fatigue (LCF) tests, out-of-phase thermomechanical fatigue (OPTMF) tests as well as low cycle fatigue tests combined with the replica testing method (LCF-R) within the temperature range RT–500 °C. The studied material exhibits an embrittlement at temperatures nearby 400 °C, leading to a significant higher fatigue crack growth rate and a reduced lifetime, when compared with RT and 500 °C. A possible explanation for the observed higher fatigue crack growth rate coupled with a significant lifetime reduction, is intergranular embrittlement.

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.

A comparative study between in- and out-of-phase thermomechanical fatigue behaviour of a single-crystal superalloy

In this study, the difference between in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) cycling at 100–850 °C of a single-crystal superalloy is investigated both from a mechanical response and resulting microstructure perspective. Results indicate that there is no significant difference in fatigue lives between IP and OP TMF when similar strain ranges and crystal orientations are considered. The deformation mechanisms occurring during IP and OP TMF are similar where the main deformation mechanism for this alloy is localized deformation bands and crack initiation is preferred to these bands. Other TMF mechanisms, such as recrystallization and oxidation, are also discussed.

Effect of constraint factor on the thermo-mechanical fatigue behavior of an Al-Si eutectic alloy

The thermo-mechanical fatigue (TMF) behaviors and corresponding damage mechanisms of Al-Si piston alloy were investigated in the temperature range of 120~425 °C under different constraint factors (η). The TMF hysteresis loops exhibit asymmetrical, because the yield strength and elastic modulus decrease significantly with the increasing temperature. Taking the phase angles into consideration, the mean tensile stress for out of phase TMF (OP-TMF, η<0) and mean compressive stress for in-phase TMF (IP-TMF, η>0) can be found. The fatigue life of IP-TMF is longer than that of OP-TMF except for higher constraint factor, and the life of TMF decreases with the increasing absolute value of η. There are two different crack initiation mechanisms under the constrain TMF: primary phases broken mainly for OP-TMF and matrix deboned mainly for IP-TMF. The oxidation may have only little influence on the crack behavior and TMF life, considering the obvious damage by deboned or broken primary phases. The tensile stress is key factor for fatigue crack initiation and growth in primary phases (Si and intermetallic) under cyclic deformation. The TMF life under different constrain factors can be predicted by the strain energy model with good precision.

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.

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.

Towards a temperature dependent and probabilistic lifetime concept for nodular ductile cast iron materials undergoing isothermal and thermo-mechanical fatigue

In this investigation, the fatigue behaviour of a ductile cast iron with high content of silicon and molybdenum, was experimentally characterized by performing isothermal low cycle fatigue (LCF) tests as well as out-of-phase thermomechanical fatigue (OPTMF) tests within the temperature range RT – 500 °C. The studied material shows an embrittlement at temperatures nearby 400 °C. A possible explanation for the observed lifetime reduction is intergranular embrittlement (IE). A mechanism based lifetime model is proposed for assessing the lifetime. The model is based on the assumption that the crack advance per cycle is correlated with the cyclic crack tip opening displacement (ΔCTOD) attributed to the crack tip blunting caused by accumulation of plastic and creep deformations ahead of the crack tip. Intergranular embrittlement is accounted for by introducing a temperature and strain rate dependent prefactor in the crack growth law, which only acts in a certain temperature range. The model is calibrated for a GJS material and successfully applied to predict the lifetime of this material when undergoing isothermal and non-isothermal mechanical loadings. A probabilistic interpretation of the scatter of the investigated material is presented in conjunction with the random nature of the initial defect size distribution.

Thermomechanical fatigue behavior of annealed Cu-Cr-Zr-Ti alloy in argon atmosphere

Strain controlled thermomechanical fatigue (TMF) behavior of Cu-Cr-Zr-Ti alloy was studied in the temperature range of 375–525°C in high purity argon atmosphere under both in-phase (IP) and out-of-phase (OP) loading. In the case of OP TMF tests, the material exhibited continuous hardening without any saturation of stress. On the other hand, it showed cyclic hardening for the first few cycles followed by saturation of stress in IP TMF tests. The material exhibited superior lives under OP TMF loading compared to that under IP TMF loading especially at lower strain amplitudes. This may be attributed to the fact that the material experienced peak tensile stress at lower temperatures in OP TMF tests, where the resistance to crack initiation is expected to be higher. Microstructural observations and fractographic studies indicated mixed mode of fracture involving Intergranular facets and transgranular cracking in both IP and OP TMF tests.

In‐phase and out‐of‐phase thermomechanical fatigue behavior of 4Cr5MoSiV1 hot work die steel cycling from 400 °C to 700 °C

AbstractThe hysteresis loops, stress and strain behavior, lifetime behavior and fracture characteristic of 4Cr5MoSiV1 hot work die steel at a wide range of mechanical strain amplitudes (from 0.5% to 1.3%) during the in‐phase (IP) and out‐of‐phase (OP) thermomechanical fatigue (TMF) tests cycling from 400 °C to 700 °C under full reverse strain‐controlled condition were investigated. Stress‐mechanical strain hysteresis loops of 4Cr5MoSiV1 steel are asymmetric, and stress reduction appears at high‐temperature half cycles owing to a decrease in strength with increasing temperature. 4Cr5MoSiV1 steel always exhibits continuous cyclic softening for both types of TMF tests, and the cyclic softening rate is larger in OP loading condition. OP TMF life of 4Cr5MoSiV1 steel is approximately 60% of IP TMF life at the same mechanical strain amplitude and maximum temperature. Lifetime determined and predicted in both types of TMF tests is adequately described by the Ostergren model. Fracture surfaces under IP TMF loading display the striation and tear ridge, showing quasi‐cleavage characteristics, and the cracks are less but longer. However, fracture surfaces under OP TMF loading mainly display the striation and dimple characteristics, and the cracks are more and shorter.

Characterization of fatigue crack growth, damage mechanisms and damage evolution of the nickel-based superalloys MAR-M247 CC (HIP) and CM-247 LC under thermomechanical fatigue loading using in situ optical microscopy

In this paper, the influence of different phase angles, load ratios and dwell times on the thermomechanical fatigue (TMF) crack growth of the nickel-based superalloys MAR-M247 CC (HIP) and CM-247 LC is studied. The thermomechanical fatigue crack growth tests are performed under in-phase (IP) and out-of-phase (OP) TMF loading between 300°C and 950°C. It is shown, that the applied load ratio plays a crucial role for the resulting fatigue crack growth rates and whether IP or OP TMF loading is more detrimental. A digital optical microscope is installed, which allows in situ observation of the fatigue crack growth, the active damage mechanisms and the damage evolution even at high temperatures. The in situ observations are accompanied by fractographic investigations, which confirm, that IP TMF loading preferentially leads to interdentritic fracture. Finally, first steps towards a quantification of the active damage mechanisms are taken using digital image processing.

Dynamic Strain Aging and Oxidation Effects on the Thermomechanical Fatigue Deformation of Reduced Activation Ferritic-Martensitic Steel

Thermomechanical fatigue (TMF) behavior of a reduced activation ferritic-martensitic steel was investigated under in-phase (IP) and out-of-phase (OP) conditions under different mechanical strain amplitudes and temperature regimes. OP TMF was generally observed to result in the lowest cyclic lives compared to both IP TMF and isothermal low cycle fatigue (IF) at the maximum temperature (T max). The stress–strain hysteresis loops under TMF were marked by extensive serrations associated with dynamic strain aging (DSA) at the strain amplitudes of ±0.4 and ±0.6 pct. The serrations were noticed during the downward ramp of temperature that resulted in IP and OP TMF exhibiting jerky flow in the compressive and tensile portions, respectively. However, no evidence of serrated flow was seen under IF cycling at any of the temperatures within the TMF cycle. The stress response during IP TMF was marked by a near-saturation regime over 65 to 70 pct of life in contrast to continuous cyclic softening in the case of OP TMF. The marked life reduction observed under OP cycling at the strain amplitudes of ±0.4 and ±0.6 pct was attributed to the deleterious influence associated with oxidation, DSA, and tensile mean stress. The findings assume importance in the context of elevated temperature fatigue design, considering the fact that the IF data at T max are deemed adequately conservative in traditional design approaches.

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.

Comparison Between Isothermal and Non-Isothermal Fatigue Behavior in a Cast Aluminum-Silicon-Magnesium Alloy

In the present study, the out-of-phase thermomechanical fatigue (OP-TMF) behavior of a cast aluminum-silicon-magnesium alloy, the A356.0 alloy which has been widely used in diesel engine cylinder heads, is compared to room-temperature and high-temperature low cycle fatigue (RT-, HT-LCF) behaviors. For this purpose, strain/temperature-controlled isothermal and non-isothermal fatigue tests were performed based on realistic loading conditions in cylinder heads. Fatigue tests results showed that the plastic strain increased during cycles under constant mechanical strain amplitude, while the specimen failed. Under LCF loadings, the cyclic hardening occurred at low temperatures for the A356.0 alloy and the cyclic softening occurred at high temperatures, as it was also observed in TMF tests. The radial and longitudinal temperature gradients during TMF tests were almost 2 and 3°C, respectively. Comparing stress-strain hysteresis loops, tensile stresses at minimum temperatures under TMF loadings were more than tensile stresses at LCF cases, due to the out-of-phase loading condition in TMF tests. In this state, maximum temperatures occurred within compressive regimes and minimum temperatures occurred at tensile loads. In general, TMF lifetimes were less than LCF ones due to severe conditions and the temperature deviation in TMF tests.

Parameters influencing thermomechanical fatigue of a directionally-solidified Ni-base superalloy

Thermomechanical fatigue (TMF) is a low cycle fatigue process in which material life is correlated to the mechanical strain amplitude. However, it is well known that several other factors influence this life. This paper examines several of these parameters and their influence on life using experiments conducted on a second generation directionally-solidified (DS) Ni-base superalloy. The parameters considered include the influence of the temperature extremums (Tmax of either 750 or 950°C and Tmin of either 100 or 500°C), strain ratio (R∊), the strain-temperature phasing (in-phase (IP) and out-of-phase (OP)), the influence of dwells at the high temperature end of the cycle resulting in a creep–fatigue (CF) interaction, and material anisotropy associated with the grain growth direction (longitudinal versus transverse). Results suggest that the phasing has a primary role in controlling the mechanism of degradation. IP TMF is dominated by crack formation in volumes surrounding debonded carbides for both continuously cycling (CC) and CF at 950 and 750°C, while OP TMF is dominated by surface oxidation and repetitive cracking of the oxide that reforms at the crack tip at 950°C. Decreasing the Tmax to 750°C the environmental and creep effects are reduced resulting in virtually pure fatigue exposure under OP conditions. With decreasing Tmin from 500°C to 100°C was observed an increase in inelastic strain amplitude and corresponding decrease in life. Variations in R∊ were found to have no significant influence on life or stabilized stress behavior. TMF loading in the transverse orientation resulted in life reductions over the longitudinal orientation due to cracks propagating in a transgranular manner. Lastly, only in material exposed to CF with a Tmax of 950°C rafting of the γ′ precipitates was observed.