Thermomechanical Fatigue Behaviour of a Nickel Base Superalloy

The isothermal low cycle fatigue (LCF)and thermomechanical fatigue (TMF) behaviourof a Ni-base superalloy was investigated. Theresults show that temperature plays an importantrole in both LCF and TMF. The alloy shows thelowest LCF fatigue resistance in the intermediatetemperature range (～760℃). For strain-controlledTMF, in-phase (IP) cycling is more damagingthan out-phase (OP) cycling. The high tempera-ture exposure in the TMF cycling influencesthe deformation behaviour at the low temperature.LCF lives at different temperatures, and IPand OP TMF lives are successfully correlatedby using the hysteresis parameter Δσ·Δε_p.     

Cyclic stress-strain response during isothermal and thermomechanical fatigue

Isothermal high-temperature low-cycle fatigue and in-phase and out-of-phase thermomechanical fatigue tests were carried out on 316L austenitic stainless steel specimens controlled by computer. A non-linear kinematic hardening model with internal variables was used to simulate the cyclic stress-strain behaviour of isothermal fatigue. This model was modified by considering thermal cyclic effects in order to describe the cyclic stress-strain behaviour of thermomechanical fatigue (TMF) using only isothermal fatigue data and the material performance data. A very good approximation of the hysteresis loops was obtained by comparing with experiments of both in-phase and out-of-phase cases. The thermomechanical fatigue behaviour described by isothermal fatigue data gives the possibility of developing the TMF lifetime prediction technique.     

Fatigue design of structures under thermomechanical loadings

This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast‐iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context.     

Isothermal and thermomechanical fatigue behaviour of a high temperature titanium alloy

Abstract

Isothermal and thermomechanical fatigue (TMF) behaviour (including cyclic stress response and number of cycles to failure) of a Ti – 5.6Al – 4.8Sn – 2.0Zr – 1.0Mo – 0.32Si – 0.8Nd (wt-%) hightemperature titanium alloy was examined. The purpose of the present investigation was to understand the effect of temperature fluctuation on the cyclic behaviour and fatigue life of this alloy and to test the suitability of lifetime prediction based on isothermal laboratory data. The results indicated that both the level of peak stress and fatigue life were decreasing with increasing test temperature from 400°C to 650°C in isothermal fatigue (IF) tests. In TMF tests run between 400°C and 600°C, the peak stresses corresponding to 600°C coincide well with that found in IF tests run at 600°C, while a slight increase in cyclic hardening was found for peak stress corresponding to 400°C compared to that found in a 400°C/IF test. This increase in cyclic hardening became more pronounced when the maximum temperature increased to 650°C. Fatigue life in 'out of phase' (OP) condition was found to be shorter than under an equivalent 'in phase' (IP) condition, and this gap increased with decreasing mechanical strain amplitude. The results indicate that lifetime prediction based on isothermal laboratory data may lead to non-conservative results if thermal fluctuations are present in components made of the present alloy.     

Modeling of cyclic stress–strain behavior and damage mechanisms under thermomechanical fatigue conditions

A multi-component model was applied to predict the cyclic stress–strain response of different alloys under thermomechanical fatigue conditions based upon isothermal hysteresis loops. A ductile AISI 304 L-type stainless steel and two high strength alloys, the near-α titanium alloy IMI 834 and the nickel-base superalloy IN 100, were chosen as test materials. These represent alloys with rather different dislocation slip modes, stress–strain characteristics and damage mechanisms. Model predictions are compared with experiments and the differences in cyclic stress–strain response and damage mechanisms under isothermal and thermomechanical fatigue conditions, respectively, are discussed based upon microstructural observations.     

Small crack growth in combined bending–torsion fatigue of A533B steel

Crack growth rate data from bending, torsional and in-plane and 90° out-of-phase combined bending–torsional fatigue tests of A533B steel are presented. Crack growth was monitored from initial sizes generally in the range of 50–300  μm to final sizes of several millimetres. Crack growth rate was found to vary linearly with crack size. Two approaches for correlating the A533B crack growth rate were evaluated, an effective strain-based intensity factor range and a method based on total cyclic strain energy density. The approaches were also evaluated using small crack growth data from the literature for SAE 1045 steel and Inconel 718 specimens tested under axial–torsional loadings. Predicted crack growth lives using both approaches were found to agree within a factor of two of observed lives for nearly all of the data examined.     

高温合金材料时间相关热机械疲劳寿命预测技术

本文在变温非线性运动强化规律所描述的高温合金材料热机械疲劳应力-应变循环特性的基础上,重点讨论了应变控制的时间相关热机械疲劳寿命预测技术。对于温度循环的影响,采用由应变能密度表示的损伤参数,并且引入了温度损伤系数。对于循环时间的影响,引入了蠕变─疲劳相互作用的损伤机制,采用韧性耗散损伤模型。在确定模型的一些参数时,采用等温力学试验和疲劳试验的数据,把等温疲劳研究成果推广到变温疲劳分析领域。     

Thermomechanical fatigue—damage mechanisms and mechanism-based life prediction methods

An existing extensive database on the isothermal and thermomechanical fatigue behaviour of high-temperature titanium alloy EVII 834 and dispersoid-strengthened aluminum alloy X8019 in SiC particle-reinforced as well as unreinv conditions was used to evaluate both the adaptability of fracture mechanics approaches to TMF and the resulting predictive capabilities of determining material life by crack propagation consideration. Selection of the correct microstructural concepts was emphasised and these concepts were, then adjusted by using data from independent experiments in order to avoid any sort of fitting. It is shown that the cyclic /-integral (δJ_eff concept) is suitable to predict the cyclic lifetime for conditions where the total crack propagation rate is approximately identical to pure fatigue crack growth velocity. In the case that crack propagation is strongly affected by creep, the creep-fatigue damage parameter δ_CF introduced by Riedel can be successfully applied. If environmental effects are very pronounced, the accelerating influence of corrosion on fatigue crack propagation can no longer implicitly be taken into account in the fatigue crack growth law. Instead, a linear combination of the crack growth rate contributions from plain fatigue (determined in vacuum) and from environmental attack is assumed and found to yield a satisfactory prediction, if the relevant corrosion process is taken into account.     

Thermomechanical fatigue behavior and lifetime prediction of niobium-bearing ferritic stainless steels

In this study, the thermomechanical fatigue (TMF) and isothermal low-cycle-fatigue (LCF) behaviors of niobium-containing ferritic stainless steels are presented for the temperature range from 100 °C to the maximum temperatures between 500 and 800 °C; furthermore, we propose a new fatigue failure criterion to predict the fatigue lives of the components for different thermal cycle ranges using the TMF condition. Higher maximum temperatures during TMF cycle resulted in shorter TMF life. By modifying the Coffin–Manson equation using the temperature factor, we obtained a new parameter that was successfully correlated with the life under different maximum temperatures. The deformation responses during fatigue cycling and the fatigue microstructure were compared to elucidate the different fatigue behaviors under the TMF and LCF conditions.     

Simulated flight fatigue of an alpha/beta titanium alloy

Simulated flight (FALSTAFF) fatigue tests have been carried out on precracked single edgenotch test-pieces of (Ti4Al4Mo2Sn0.5Si) IMI 550 titanium alloy. Predictions of simulated flight fatigue behaviour have been made from constant amplitude fatigue data, using a damage accumulation approach, with no allowance for load history. The predicted lives were conservative compared with the measured lives, and accurate within a factor of approximately two. Retardation of fatigue crack growth increased with increasing load amplitude. The microstructure produced by β-solution heat treatment at 1010°C, followed by ageing, was found to improve simulated flight fatigue lives by up to approximately 100% compared with standard solution treatment at 900°C, followed by ageing.     

Effect of Sc addition on low-cycle fatigue properties of extruded Al-Zn-Mg-Cu-Zr alloy

ABSTRACT

The influence of minor Sc addition on the low-cycle fatigue (LCF) properties of hot-extruded Al-Zn-Mg-Cu-Zr alloy with T6 state was investigated through performing the LCF tests at room temperature and air environment. The results indicate that two alloys show cyclic stabilisation, cyclic hardening and cyclic softening during fatigue deformation. The addition of Sc can significantly enhance the cyclic stress amplitude of the alloy. Al-Zn-Mg-Cu-Zr-Sc alloy shows higher fatigue lives at lower strain amplitudes, while has lower fatigue lives at higher strain amplitudes. For the two alloys, the density and movability of dislocations are related to the change of cyclic stress amplitudes. The existence of Al₃(Sc,Zr) phase can inhibit the appearance of cyclic softening phenomenon in the Al-Zn-Mg-Cu-Zr-Sc alloy.     

THERMAL-MECHANICAL FATIGUE OF MAR-M 509 SUPERALLOY. COMPARISON WITH LOW-CYCLE FATIGUE BEHAVIOUR

Abstract— The thermal-mechanical fatigue behaviour of a cast cobalt based superalloy, MAR-M 509 was investigated. Hollow smooth specimens were submitted to temperature cycling between 600 and 1050°C. The mechanical strain vs temperature cycle shows a diamond shape with peak strains at intermediate temperatures. The stress-strain behaviour as well as fatigue life curves are reported. A significant part of total life is spent in crack initiation. Comparison with isothermal low cycle fatigue was made at the temperature where peak strains occurred. Isothermal low cycle fatigue life to crack initiation was shown to give a conservative estimate of thermal mechanical fatigue life provided extra strains due to differences in thermal expansion coefficients of the oxide scale and base alloy were taken into account.     

Thermomechanical design in the automotive industry

In this paper thermomechanical fatigue assessment in the automotive industry is discussed. The design strategy is based upon a consistent approach of the thermomechanical loading, the mechanical constitutive law of the material, the damage parameters and the fatigue strength criteria. The good understanding of these different steps allows one to perform predictive calculations of automotive parts subjected to thermomechanical loading. The main hypotheses and modelling choices are presented and results are illustrated by a series of computations on real 3D structures. Cracked area and lifetime prediction are described in the case of aluminium alloy cylinder heads subjected to transient thermal loadings.     

STRESS RESPONSE BEHAVIOUR OF A CAST NICKEL BASE SUPERALLOY SUBJECT TO COMBINED CREEP-FATIGUE

Abstract— An advanced nickel base alloy, MAR MOO2, was subject to creep-fatigue, strain controlled, cycling at temperatures of 750, 850 and 1000°C. Under continuous cycling the alloy exhibited cyclic stability at 750 and 850°C but not at 1000°C. The presence of a tensile or compressive dwell caused softening at 750 and 850°C compared with the pure fatigue case. At 1000°C a tensile dwell caused slight hardening and a compression dwell caused cyclic softening. The stress response for a balanced, tensile plus compressive, dwell was similar to that for continuous cycling. Unbalanced tensile dwells produced significant compressive mean stresses and compressive dwells resulted in large tensile mean stresses at 750 and 850°C. However, at 1000°C no significant mean stresses were produced whatever cycle was used. Stress relaxation behaviour was similar for both tensile-only and compressive-only dwells. In the balanced case the amount of stress relaxation was double that in each dwell of an unbalanced cycle. Reliable predictions of stress relaxation were possible using stress exponents from available creep data and the Gittus equation. Severe ageing (10⁴ h at 1000°C) does not destroy the cyclic stability of the alloy although some softening occurred in the compression-only dwell at 850°C. Tentative explanations for the observed stress response in terms of dislocation-precipitate interactions have been suggested.     

Low and high‐cycle fatigue properties of an ultrahigh‐strength TRIP bainitic steel

The scope of this study is to characterize the mechanical properties of a novel Transformation‐Induced Plasticity bainitic steel grade TBC700Y980T. For this purpose, tensile tests are carried out with loading direction 0, 45 and 90° with respect to the L rolling direction. Yield stress is found to be higher than 700 MPa, ultimate tensile strength larger than 1050 MPa and total elongation higher than 15%. Low‐cycle fatigue (LCF) tests are carried out under fully reverse axial strain exploring fatigue lives comprised between 10² and 10⁵ fatigue cycles. The data are used to determine the parameters of the Coffin–Manson as well as the cyclic stress–strain curve. No significant stress‐induced austenite transformation is detected. The high‐cycle fatigue (HCF) behaviour is investigated through load controlled axial tests exploring fatigue tests up to 5 × 10⁶ fatigue cycles at two loading ratios, namely R = ?1 and R = 0. At fatigue lives longer than 2 × 10⁵ cycles, the strain life curve determined from LCF tests tends to greatly underestimate the HCF resistance of the material. Apparently, the HCF behaviour of this material cannot be extrapolated from LCF tests, as different damage, cyclic hardening mechanisms and microstructural conditions are involved. In particular, in the HCF regime, the predominant damage mechanism is nucleation of fatigue cracks in the vicinity of oxide inclusions, whereby mean value and scatter in fatigue limit are directly correlated to the dimension of these inclusions.     

THERMAL FATIGUE OF MAR-M509 SUPERALLOY—I. THE INFLUENCE OF SPECIMEN GEOMETRY

Abstract— The thermal fatigue behaviour of a cast cobalt superalloy was investigated. Wedge type specimens were tested on a rig using flame heating. A standard cycle from 200 to 1100°C was used for most tests and the specimen geometry was changed to investigate a broad range of thermal fatigue lives. The maximum temperature was also varied for a single specimen geometry. The thermal fatigue life to create a 1 mm deep crack ranges from a few tens to one thousand cycles. Crack initiation occurs early in life at oxidised interdendritic carbides as in high-temperature, low-cycle fatigue. Precipitation of small chromium-rich carbides was found to occur in the dendrites during thermal fatigue cycling. An identical precipitate distribution could be induced by a two stage thermal treatment as shown by quantitative metallography. The isothermal stress-strain behaviour at high temperature was so shown to be almost insensitive to the microstructure changes induced by thermal fatigue.     

Microstructure degradation in high temperature fatigue of TiAl alloy

Low cycle fatigue properties of lamellar TiAl with 8 at.% Nb were studied at four temperatures: room temperature, 700, 750 and 800 °C. Up to 750 °C, stable cyclic behaviour is observed while cyclic softening is characteristic for 800 °C. The strength of the alloy is still high even at 800 °C. The TEM observation did not reveal any substantial changes in the microstructure due to the cycling at RT. At 750 °C, the lamellar structure was in some places destroyed by cyclic plastic straining and pure γ-phase islands with high density of dislocation debris were formed. At 800 °C, the domains without lamellar structure cover about 10% of volume and are almost dislocation free. The destruction of lamellar microstructure and possible annealing of dislocation debris is the reason for marked cyclic softening at 800 °C.     

Effect of heating temperature and magnesium content on the thermal cyclic failure behaviour of ductile irons

Abstract

This study elucidates the effect of residual magnesium content and heating temperature on the thermal cyclic failure behaviour of ductile irons by applying repeated heating and cooling cycles. Five irons with different residual magnesium contents ranging from 0.038 to 0.066 wt-% were obtained by controlling the amount of nodulariser additions. The thermal fatigue cracking behaviour was investigated during thermal cycling from 25°C to 650, 700, 750, and 800°C, respectively. Experimental results indicate that the thermal fatigue cracking resistance of ductile iron decreases with increasing residual magnesium content. The maximum heating temperatures of 700°C and 750°C led to the most severe thermal fatigue cracking in the specimens containing 0.054 wt-% and 0.060 wt-% residual magnesium content. Recrystallisation of ferrite grain occurred when the thermal cycles exceeded a certain number after testing at 800°C, which deferred the initiation of thermal fatigue cracking.     

Fatigue design of structures under thermomechanical loadings

This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast‐‐iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context.     

HIGH TEMPERATURE FATIGUE DAMAGE IN THREE AUSTENITIC ALLOYS

Abstract— A series of cyclic strain controlled tests have been carried out at 600°C on three high temperature austenitic iron-based alloys. These alloys were AISI type 316 stainless steel, Alloy 800 H and Sandvik 253 MA. The tests were carried out under constant total strain control using a constant strain rate of 0.005 s⁻'. Damage mechanics was applied to the results in order to follow the accumulation of damage. By consideiing the changes in modulus throughout the life of each specimen it was found that damage evolution could be successfully predicted as a function of plastic strain range despite the fact that each alloy had been chosen because of a different stress response at 600°C, namely cyclic saturation, hardening, and softening followed by hardening for the AISI 316, 253 MA and Alloy 800 H respectively. Although each alloy accumulated fatigue damage in a similar manner the longer lives of Sandvik 253 MA and Alloy 800 H at a given total strain range were due to a smaller plastic strain component and a reduced stage I crack propagation rate. In the 253 MA alloy. slip was predominantly planar with some cells occasionally forming at high strain ranges. Slip was localized in Alloy 800 H due to the shearing of small γ precipitates. In the AISI 316 stainless steel, dislocation cells formed at all strain ranges. It is concluded that all these alloys accumulate damage similarly, independent of their deformation behaviour.