Isothermal Low Cycle Fatigue Research Articles

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

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

Thermomechanical and isothermal fatigue properties of MAR-M247 superalloy

Thermomechanical low-cycle fatigue (TMF) tests were performed on polycrystalline cast nickel-based superalloy MAR-M247 under in-phase (IP) and out-of phase (OP) loading in the temperature range of 500–900 °C. A constant heating and cooling rate of 5 °C.s−1 was selected. Furthermore, isothermal low-cycle fatigue (ILCF) tests with the same cycle period as TMF cycle were carried out at the maximal temperature of TMF cycle (900 °C). All tests ran under strain control and fully reversed cycle on solid cylindrical specimens in laboratory air. High resolution scanning electron microscopy and transmission electron microscopy were utilized to characterize the damage mechanism occurring during TMF and ILCF loading. The cyclic deformation and lifetime behaviour as well as damage mechanisms dependent on loading regime were examined. Results show that the TMF loading has a detrimental effect on the lifetime of MAR-M247. Lifetime reduction is most significant under IP loading. The fracture surfaces are characterized by typical striation fields, where the largest spacing between striations was observed for IP loading with prevailing intercrystalline cracking followed by OP and isothermal ILCF loading with transcrystalline crack propagation. To improve discussion of obtained results, experimental data were compared with damage and life prediction models.

Failure-mode dependence on the formation of deformation twinning in the fourth-generation single crystal Ni-based superalloy at high temperatures

In this work, the twinning-related deformation mechanisms under different failure modes of a fourth-generation single crystal (SX) superalloy at high temperatures were investigated. Notably, the twinnability of alloy increased in the sequence as follows: pure tension, in-phase (IP) thermal mechanical fatigue (TMF), isothermal low cycle fatigue (LCF), hot compression, out-of-phase (OP) TMF. High-temperature compressive loading which facilitated the successive operation of {111}〈1 1 2〉 viscous slipping, was indispensable for the nucleation of micro-twins. However, this stimulative effect could be markedly offset by the co-existence of high-temperature tensile loading. Moreover, the activation of different {111}〈1 1 2〉 slipping systems as well as potential interactions between stacking faults played a significant role in the formation and thickening processes of deformation twins. In the experimental alloy system, the presence of deformation twins was attested to impair the fatigue resistance of the specimen. These findings provided new insights for ensuring the safe service of the fourth-generation SX superalloys.

A continuum damage coupled unified viscoplastic model for simulating the mechanical behaviour of a ductile cast iron under isothermal low-cycle fatigue, fatigue-creep and creep loading

A novel continuum damage coupled unified viscoplastic model is used for simulating the stress–strain responses of SiMo 4.06 cast iron under Low-Cycle Fatigue (LCF), fatigue-creep and creep loading for temperatures up to 650 °C. The advanced constitutive model that is developed within the framework of the Chaboche model allows the simulation of various effects including strain rate sensitivity, cyclic hardening and softening, static recovery and strain range dependency. The hyperbolic sine flow rule and the static recovery of kinematic hardening rule are proposed in order to effectively simulate both stress relaxation and creep strains. The isotropic damage variable is introduced into the constitutive equations to represent the effects of material degradation. Three main damage mechanisms are considered: fatigue, creep and ductile damage. In addition, the model is further modified to take into account the progressive microdefects closure effect. Finally, the prediction capability of the proposed model is illustrated for the experimental data obtained from the various uniaxial material tests. A good correlation was achieved between the simulated and the experimental results.

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.

Investigating the impact of weld dilution and local mismatch on the low-cycle fatigue failure of Alloy 182 dissimilar weld transition under intermediate isothermal condition

Dissimilar welded joints are the critical region that determines the operation lifetime of pressure vessels under severe cyclic deformation. The failure susceptibility of dissimilar welded joints is attributed to local strength mismatch and heterogeneous microstructures. This study evaluated the isothermal low-cycle fatigue performance of a dissimilar welded joint between nickel-based Alloy 182 and 1.25Cr-0.5Mo steel at 250 °C. During low-cycle fatigue tests, the Alloy 182 weld metal exhibited significant cyclic hardening over the 1.25Cr-0.5Mo steel base metal. The local strain variations in the weld transition were measured using the digital image correlation (DIC) technique to reveal the impact of material strength and hardening mismatch upon the cyclic behavior within the weld transition. Microhardness measurements of as-welded and cyclically deformed weld transition regions were performed to assess the hardening due to cyclic deformation. The WT failures occurred in the weld metal region adjacent to the fusion boundary. In addition, microstructural characterization shows that the crack initiation occurred within the weld dilution region, and the short crack growth was affected by microstructural heterogeneity in the weld metal.

Planar-Biaxial Thermo-mechanical Fatigue Behavior of Nickel-Base Superalloy Inconel 718 Under Proportional Loading

The planar-biaxial thermo-mechanical fatigue behavior of nickel-base superalloy Inconel 718 was studied for selected proportional loading conditions, in particular biaxial strain ratios of 1.0, 0.6, and − 1.0. The cyclic temperature loading with minimum and maximum temperatures of 400 °C and 630 °C and a duration of 250 seconds was either In-Phase or Out-of-Phase to the mechanical axes. Besides the multiaxial tests, uniaxial thermo-mechanical fatigue tests were conducted In-Phase and Out-of-Phase with the same temperature cycle and cycle duration. The performed thermo-mechanical fatigue tests were analyzed regarding the deformation and lifetime behavior and compared with high-temperature isothermal low-cycle fatigue tests from a previous work of the authors. On the side of the lifetime description, a strain- and a stress-based approach were presented. For the planar-biaxial tests, the crack initiation mechanism and crack paths were shown.

Unified viscoplasticity model for type 316 stainless steel subjected to isothermal and anisothermal-mechanical loading

316FR-type low carbon stainless steel is usually used for many advanced gas cooled reactor (AGR) power plants which are subjected to operating temperatures in the range of 500–650 °C under thermo-mechanical loading, which may lead to isothermal low cycle fatigue (LCF) and thermal mechanical fatigue (TMF) damage. The strain controlled thermo-mechanical fatigue test with and without holding time was carried out for 316FR-type stainless steel under the temperature cycling between 500 °C and 650 °C, and the strain ranges are Δε = ± 0.4%, ±0.8%, ±1.0% and ±1.2%, respectively. Within the framework of Abdel Karim and Ohno model, an improved unified viscoplastic constitutive model (UVM) is proposed to simulate the cyclic plastic behavior of 316FR stainless steel. The improved UVM includes strain rate sensitivity, temperature rate dependence, static recovery, average stress evolution and strain range dependence of cyclic hardening. And then based on the radial return mapping and backward Euler integration methods, the implicit stress integration algorithm and the new expressions of the consistent tangent modulus are derived respectively, the improved UVM is implemented by a User Material Subroutine into ABAQUS software again. The good correlation between simulation results and experimental data is obtained.

Modelling of Strain-Controlled Thermomechanical Fatigue Testing of Cast AlSi7Cu3.5Mg0.15 (Mn, Zr, V) Alloy for Different Aging Conditions

Thermomechanical fatigue loadings (TMF) applied on components in a certain temperature range with a variable state of stress (tensile and/or compression) produce a localized concentration of plastic strains that results in crack initiation and propagation. The time evolution of plastic strains must be known a priori to predict the lifetime of a part submitted to TMF loadings, which requires an extensive campaign of mechanical characterization conducted at different temperatures and aging conditions. Such a campaign was proposed for the aluminum alloy AlSi7Cu3.5Mg0.15 (Mn, Zr, V), which is recognized as being creep resistant. Combined isothermal low-cycle fatigue and isothermal creep tests were performed on this alloy to determine the constitutive parameters based on the Lemaître and Chaboche (LM&C) viscoplastic model. These laws were implemented within the finite element simulation software (Z-set) to model the response of the alloy to a thermomechanical fatigue test. The results of TMF Z-Set simulations, using the LM&C model adapted for two aging conditions, were then compared with results obtained from “Out of Phase” thermomechanical fatigue testings (OP-TMF) performed on a Gleeble 3800 machine. The modelling of the OP-TMF test revealed the complexity of the mechanical behavior of the material induced by the temperature gradient prevailing along with the cylindrical specimen. It was found that a better prediction of the evolution of plastic strains requires taking into account a larger range of plastic strain rates conditions for the determination of the constitutive law and eventually includes the role of the microstructure in the evolution of the material behavior, starting first with the yield stress.

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.

Using machine learning to predict lifetime under isothermal low-cycle fatigue and thermo-mechanical fatigue loading

In this article, machine learning is used to predict lifetime under isothermal low-cycle fatigue and thermo-mechanical fatigue loading, both of which represent the most complex loadings that couple creep, fatigue and oxidation damage. A uniaxial fatigue and fatigue–creep dataset, which was obtained for temperatures of between 300°C and 600°C for a low-alloy martensitic steel, is utilized in this study. Two different machine learning based approaches to lifetime prediction are demonstrated. The first approach is based only on a shallow neural network, whereas the second approach is proposed as a combination of a sequence learning based model – either long short-term memory network or gated recurrent unit – with the shallow neural network. A good correlation between the experiment and the prediction suggests that lifetime under complex thermo-mechanical loading can be reasonably predicted via the proposed machine learning based damage models.

Effect of surface integrity generated by machining on isothermal low cycle fatigue performance of Inconel 718

Critical aero-engine components such as turbine discs must withstand severe cyclic stresses, which can eventually lead to low cycle fatigue (LCF) failures. These components are made of difficult-to-cut alloys and machining conditions that are employed in the last stage of the manufacturing chain must be correctly defined to avoid the generation of an adverse surface condition (tensile residual stresses, excessive surface roughness or microstructural defects) that will accelerate fatigue failure. The analysis presented in this paper is aimed at understanding the isothermal LCF behaviour of turned Inconel 718 workpieces and finding quantitative correlations with the surface integrity. For this purpose, two Inconel 718 forged discs were face turned at cutting conditions employed in the aero-engine manufacturing industry using two different tools (1.2 mm and 4 mm nose radius respectively). The surface integrity produced by the face turning process was characterised in both discs: surface topography, residual stresses and surface layer anomalies. Specimens extracted from both discs were tested in load control at 450 °C to obtain LCF fatigue behaviour. Importantly, interrupted isothermal LCF fatigue tests were conducted and residual stresses were measured by the X-ray diffraction technique at the surface of the tested specimens to study the role of residual stresses in the LCF fatigue behaviour. For the tested conditions, the specimens machined with 4 mm nose radius showed 1.3–1.4 times longer fatigue lives than the specimens turned with 1.2 mm nose radius. These results are in agreement with the better surface integrity generated with the 4 mm nose radius, mainly because it induced lower tensile residual stresses. A novel local approach was implemented to understand the influence of surface integrity (surface roughness, surface residual stresses and altered stress-strain properties of the surface layer) on the isothermal LCF fatigue behaviour of both machined discs. Interestingly, a satisfactory correlation was found between the maximum applied stress in the surface layer and the isothermal LCF fatigue life employing the local approach.

Use of Prandtl operators in simulating the cyclic softening of Inconel 718 under isothermal low-cycle fatigue loading

In this article, a new approach is proposed for modelling the stress–strain response of the Inconel 718 super-alloy under isothermal Low-Cycle Fatigue (LCF) loading. The proposed constitutive model is based on the Prandtl operator approach, in which a set of modifications is introduced in order to simulate strain range dependent cyclic softening. A new simulation capability is introduced by evolving the yield strains of the individual hysteresis operators with an accumulated plastic strain. In addition, the effect of the strain range dependency of cyclic softening is introduced into the proposed constitutive model by coupling its parameters with the concept of the plastic strain memory surface. These introduced modifications preserve the main advantages of the Prandtl operators, such as a small number of model parameters, their fast determination from the cyclic stress–strain curve, and a high computational speed, when used to simulate complex non-linear mechanical behaviour. Finally, the prediction capability of the proposed model is illustrated by various strain controlled tests performed at 500 ∘C, including block spectrum loading and variable strain amplitude loading.

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.

Constitutive modelling for isothermal low-cycle fatigue and fatigue-creep of a martensitic steel

An improved unified viscoplastic model is developed within the framework of the Chaboche model in order to simulate the mechanical behaviour of a martensitic steel under Low-Cycle Fatigue (LCF) and Low-Cycle Fatigue-Creep (LCFC) loadings at 550 °C. The effects of strain rate sensitivity, static recovery, strain range-dependent cyclic softening, along with the effect of additional hardening under non-proportional loading are incorporated into the constitutive equations. The non-linear kinematic hardening rule is modified in such a way that it enables the simulation of the evolutionary behaviour of mean stress and the effect of decelerating stress relaxation. Finally, the prediction capability of the proposed model is illustrated for the experimental data obtained from the various strain-controlled axial-torsional tests.

New insights into thermomechanical fatigue behavior of AISI Type 316 LN SS weld joint

AbstractCyclic deformation and fracture behavior of a type 316 LN austenitic stainless steel (SS) weld joint (WJ) were investigated under thermomechanical fatigue (TMF) and isothermal low cycle fatigue (IF) cycling at the maximum temperature (Tmax) of TMF. A higher cyclic stress response (CSR) and reduced cyclic softening were observed under TMF compared with IF tests. In‐phase (IP) TMF resulted in lower lives compared with IF cycling at the Tmax and out‐of‐phase (OP) TMF, which was attributed to the more pronounced creep‐induced intergranular damage. Characterization of microstructural features and microhardness variations revealed that the accumulation of damage and associated failure depends on the strength and microstructural gradient, together with the deformation incompatibility. The crack propagation is found to depend on the individual and synergistic interactions of the microstructural transformations, creep, and oxidation, depending on the type of fatigue cycle (IF and IP/OP TMF), strain amplitude, and thermal cycling effects.

Influence of Grain Orientation Distribution on the High Temperature Fatigue Behaviour of Notched Specimen Made of Polycrystalline Nickel-Base Superalloy

Two different material batches made of random and textured orientated polycrystalline nickel-base superalloy René80 were investigated under isothermal low cycle fatigue tests at 850 °C for a notched specimen geometry. In contrast to a smooth specimen geometry, no significant improvement in fatigue behaviour of the notched specimen could be observed for the textured material. Finite element simulations reveal an area along the notch where high stiffness evolves for the textured material, which lead to nearly similar shear stresses in the slip systems compared to a random orientation distribution and therefore to no distinct differences in the lifetime.

Unified constitutive modeling of Haynes 230 including cyclic hardening/softening and dynamic strain aging under isothermal low-cycle fatigue and fatigue-creep loads

A unified viscoplastic constitutive model with wide temperature adaptability is developed based on the basic framework of the Chaboche type model to capture the intricate cyclic viscoplastic behaviors of Haynes 230 subjected to isothermal low-cycle fatigue and fatigue-creep loads. By introducing effective aging time into drag stress in the hyperbolic sine viscosity function, the critical value of back stress and the additional hardening model, a rate-dependent model based on the dimensionless concentration of solute atoms or precipitates is further improved to realize simulation of dynamic strain aging effects. A multistage evolutionary approach to determining asymptotic value of back stress influenced by strain range and cumulative plastic strain is proposed to respond to cyclic hardening or softening evolved differently with the variation of temperature. For the fatigue-creep interaction, a rate or time dependent directional hardening cumulative model combined with the existing mean stress model is established. After the multi-step material parameter determination method is given, the capabilities of the proposed model are fully validated to accurately depict the mechanical behaviors of Haynes 230 for five representative temperatures.

Cyclic behavior and fatigue resistance of AISI H11 and AISI H13 tool steels

The present experimental work investigates the isothermal low cycle fatigue resistance of two well-known tool steels, AISI H11 and AISI H13, at four different temperatures: ambient, 200 °C, 350 °C and 550 °C. The characterization of the steels included tensile and creep tests too. The tensile, the creep and the cyclic properties are in fact often employed to assess the thermal fatigue of a material. The fatigue tests up to 350 °C resulted in similar strain-cycles to fracture curves, whereas the ones performed at 550 °C were significantly different. A deep analysis was hence carried out to distinguish the contribution of the fatigue, of the creep and of the oxidation. It was found that at 550 °C fatigue and oxidation are the main driving mechanisms. Finally, the fracture surfaces were observed both by stereo and scanning electron microscopes. Multiple crack nucleation was observed, especially for the tests at high temperature.

Effects of thermal cycling and microstructure on the fatigue crack propagation in forged titanium–aluminide alloys under thermomechanical fatigue conditions

The mechanical properties and fatigue strength of titanium–aluminide (TiAl) alloys are sensitive to the environmental conditions, such as temperature, and their microstructures can be controlled by thermomechanical processing. In this study, two samples of a forged TiAl alloy were manufactured through high-temperature forging followed by different heat treatments to obtain a near-lamellar microstructure and a triplex microstructure, which contains lamellar and equiaxed γ and β grains. The fatigue crack propagation tests were conducted under isothermal low-cycle fatigue (LCF) and the out-of-phase type thermomechanical fatigue (OP-TMF) conditions. The experimental results indicated that the microstructure strongly affects the crack propagation behavior because the near-lamellar microstructure had a higher resistance to fatigue crack propagation compared to the triplex microstructure. This also revealed that the fatigue crack was remarkably accelerated by the OP-TMF conditions compared to the LCF conditions. The oxygen diffusion into the β phase occurred at the crack tip and lead to the transformation of the β phase into the brittle α phase. The results of the scanning electron microscope (SEM), energy dispersive X-ray (EDX), and electron backscatter diffraction (EBSD) analyses indicated that this transformation induced the acceleration of crack propagation under the OP-TMF loading conditions.