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The effect of phase angle on crack growth mechanisms under thermo-mechanical fatigue loading

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Abstract

The current paper describes TMF crack growth behaviour in an advanced nickel-based superalloy. Changes in behaviour are examined which occur as a function of the phase angle between applied stress and temperature. The fractography of the failed specimens reveals changes from transgranular to intergranular growth between high and low phase angle tests as a result of the onset of high temperature damage mechanisms. More targeted testing has also been undertaken to isolate the contributions of these mechanisms, with specific transitions in behaviour becoming clear in 90° diamond cycles, where dynamic crack growth and oxidation strongly interact.

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... During pre-cracking and TMF testing, crack growth was measured using direct current potential drop (DCPD) signals. Further information about the experimental setup can be found in [25]. ...
... The current state of the model is just able to capture the lower boundary region of the conducted IP tests. Microstructural evaluation conducted by John et al [25] showed a relationship between decreasing secondary γ' size distribution and increasing crack growth rate under IP-TMF loading for RR1000. In addition, Li et al [27] showed the influence of the microstructure, especially tertiary γ' for isothermal crack growth tests with different hold times. ...
... In addition, the transition between weakly and strongly coupled dislocation are given at 50 μm. Test TC026 shows the highest, TC010 the lowest crack growth rates under the same IP TMF test conditions [25] (see Fig. 13). ...
Article
The polycrystalline nickel-base superalloy RR1000 is used as turbine rotor material in Rolls-Royce aero engines and has to withstand a wide variety of load and temperature changes during operation. In order to maximize the potential of the material and to improve component design, it is of great interest to understand, and subsequently be able to accurately model the crack propagation caused by thermo-mechanical fatigue conditions. In this work, experimental data is analysed and used to inform unified modelling approaches in order to predict the crack propagation behaviour of RR1000 under a variety of stress-controlled thermo-mechanical fatigue conditions.
... From the above experimental investigation of different types of materials, it is found that the thermo-mechanical fatigue damage mechanism is material dependent. For creep damage dominated materials, such as nickel-based superalloy [22,23], cobalt-based superalloy [24], austenitic stainless steel [25] and martensitic steel [1], the activated damage mechanisms under IP-TMF loading are fatigue, creep and oxidation, and it is fatigue and oxidation for OP-TMF loading. Under IP-TMF loading, the creep damage can cause intergranular fracture, resulting in a drastic decrease in material failure life. ...
... Moreover, it can be seen from Fig. 12 (d) that the fatigue crack propagation area is oxidized, and the fatigue striation can only be seen vaguely, which means that the fatigue crack propagation is assisted by oxidation damage. Under fatigue loading at high temperature, when the crack opens, oxygen can penetrate into the crack tip, causing the material at the crack tip to become brittle, which is beneficial to crack propagation [22]. ...
Article
In this paper, the experimental investigation was carried out on the TC4 titanium alloy to understand the cyclic deformation behavior and fatigue damage mechanism of materials under multiaxial thermo-mechanical loading. The results showed that the fast cracking mode of the material may be activated by the combined action of high temperature, tensile stress and shear stress, inducing a fast degradation in the axial mechanical properties of the material, which results in a drastic decrease in the failure life of the material. Moreover, it is also found that the non-proportional additional hardening and the tensile mean stress can increase the fatigue and oxidation damages of the material, and the material damage behavior is temperature dependent and time dependent. It should be pointed out that the damage mechanisms identified in this paper can reasonably explain the life law under uniaxial isothermal fatigue loading with and without dwell time, uniaxial and multiaxial thermo-mechanical fatigue loadings.
... Ni-based superalloys [119][120][121], which is supported by the higher FCG rates associated with intergranular fractography under the in-phase (IP) loading conditions compared with out-of-phase (OP) testing associated with transgranular fractography, due to the increased temperature at peak stress and therefore increased time-dependent FCG. A few reviews of the interaction between environmental damage and fatigue in Ni-based superalloys can be found in [4,21,109]. ...
... TMF crack growth in PM Ni-Based superalloys is an emerging field. The effects of phase angle on TMF crack growth behaviours in two grain size variants of RR1000 were initially investigated[121,154,155] and a transition from intergranular to transgranular growth between low and high phase angle tests as a result of the onset of high temperature damage mechanisms (time-dependent FCG behaviours) is identified based on the fractography analysis. Similar to isothermal experiments, the finer grain size is related to higher TMF crack growth rate over all phase angles, especially under the IP loading, as there is less microstructural resistance to FCG. ...
Article
Full-text available
Powder metallurgy (PM) Ni-based superalloys are widely used for aeroengine turbine disc applications due to their excellent mechanical properties and good corrosion resistance at elevated temperatures. Understanding the fatigue crack growth (FCG) mechanisms of PM Ni-based superalloys is important for both disc alloy development and life prediction of disc components in these advanced aeroengines where damage tolerance design prevails. FCG in PM Ni-based superalloys is a complicated function of microstructure, temperature, loading conditions and environment and is usually a consequence of the synergistic effects of fatigue, creep and environmental damage. In this review, the mechanisms controlled by microstructural features including grain size, grain misorientation, γ′ size and distribution on short and long FCG behaviour in PM Ni-based superalloys are discussed. The contribution of creep and environmental damage to FCG has been critically assessed. The competing effects of mechanical damage (i.e. fatigue and creep) and environmental damage at the crack tip are microstructure-sensitive, and usually results in transition between transgranular, mixed-trans-intergranular and intergranular FCG depending on the contribution of environmental damage to FCG processes.
... [1][2][3][4][5][6][7][8][9] Moreover, the amount of these damages will change with the change of material type or loading path, which seriously affects the failure life of hightemperature equipment. [10][11][12] At present, modern aeroengines continue to pursue high thrust weight ratio, and their service reliability and safety need to be guaranteed; therefore, damage calculation and life assessment for MTMF have become a new hot issue. ...
Article
Strain rate sensitivity will change the cyclic mechanical response of materials under multiaxial thermomechanical fatigue loading; thus, it will lead to errors in fatigue life prediction if strain rate sensitivity is not considered in constitutive simulation. In order to more accurately express the time/rate dependence of viscoplastic behavior, a strain rate sensitivity factor is proposed to modify the viscoplastic function of Chaboche model first. On this basis, a viscoplastic constitutive model considering strain rate sensitivity under multiaxial thermomechanical fatigue loading is proposed, which can describe the varying influence of strain rate sensitivity on cyclic mechanical behavior. Meanwhile, the exponential isotropic hardening rule with the linear term is adopted to characterize the initial rapid softening and subsequent stable softening of the material under fatigue loading at elevated temperature. Moreover, the material‐dependent nonproportional hardening coefficient and the loading path‐dependent rotation factor are used in the kinematic hardening rule to consider the effect of nonproportional additional hardening on the cyclic mechanical behavior of materials. Finally, the proposed method is verified by the stress–strain data of titanium alloy TC4 and Ni‐based superalloy GH4169 under uniaxial and multiaxial thermomechanical fatigue loadings, and a good agreement is obtained. Highlights The time/rate dependence of viscoplastic behavior is expressed more accurately. Initial rapid cyclic softening and subsequent stable cyclic softening are described. The effect of nonproportional hardening on cyclic mechanical behavior is considered. The proposed model is verified by two materials with different damage types. The time/rate dependence of viscoplastic behavior is expressed more accurately. Initial rapid cyclic softening and subsequent stable cyclic softening are described. The effect of nonproportional hardening on cyclic mechanical behavior is considered. The proposed model is verified by two materials with different damage types.
... 曹 [82] 为了计算弹塑性体缺口根部非比例加载下的应力-应变响应,将多轴 [111][112][113] [114][115][116][117] [123][124][125] 和循环裂纹张开位移 [126,127] (1-86) ...
Thesis
High temperature components, such as aero engine turbine disks, are usually subjected to the random multiaxial thermo-mechanical fatigue loading during service process. Therefore, it is of great theoretical and practical significance to investigate the damage mechanism and fatigue life prediction method for high temperature structure durability design. Based on the investigations of damage mechanism and deformation behavior of superalloys under the multiaxial thermo-mechanical fatigue loading, the variable amplitude multiaxial thermo-mechanical fatigue life prediction method of the material and structural level was proposed in this paper. Firstly, through the constant/variable amplitude uniaxial/multiaxial thermo-mechanical fatigue tests of Ni-based superalloy GH4169, the damage mechanism of the material was revealed under multiaxial thermo-mechanical fatigue loading. It is found that the tensile stress can cause creep voids between grains at high temperature, and shear stress can increase creep damage by tearing the voids. The continuous evolution process of creep voids can induce intergranular fracture, which can lead to the sharp decrease of failure life. It is also found that non-proportional additional hardening behavior can increase the stress response, which can increase the fatigue, creep and oxidation damages, and result in the failure life to be dramatically decreased. Secondly, based on the microstructure observation, the deformation behavior of the material is further investigated under multiaxial thermo-mechanical cyclic loading. Due to the temperature dependence of mechanical properties, the stress response of the material at low temperature is larger than that at high temperature, resulting in a mean stress biased towards low temperature under symmetrical strain loading. It is also found that the strengthening phases were elongated under dynamic strain aging, which can increase the pinning of dislocation motion and cause cyclic hardening. Thirdly, the path dependent ration factor and the non-proportional hardening coefficient were introduced in the kinematic hardening rule to consider the effect of non-proportional additional hardening on cyclic mechanical behavior, in which the rotation factor was used to characterize the path dependence of hardening degree. At the same time, the dynamic strain aging influence factor was introduced to consider the caused cyclic hardening behavior. Based on the above modification, a viscoplastic constitutive model considering non-proportional additional hardening and dynamic strain aging was proposed. The stress-strain data of multiaxial thermo-mechanical fatigue tests were used to verify the proposed model, and the prediction error is between -1.51% and 7.54%. Forthly, the relationship between pseudo stress increment and actual stress increment was proposed by analyzing stress-strain curves of the material and structure. Then, combined with the proposed viscoplastic constitutive model, the notch stress-strain evaluation method was proposed, which can consider the influence of temperature change on notch correction. The results of thermal-structural non-linear finite element analysis for fir tree structural specimen were used to verify the proposed method, and the prediction error ranges from -4.98% to 6.44%. Fifthly, a multiaxial thermo-mechanical cycle counting method considering temperature history was proposed to process loading history in real time. At the same time, based on the mechanism study, the multiaxial fatigue-oxidation-creep damage was characterized, especially considered the effect of non-proportional additional hardening on damage. Then, combined with the notch stress-strain evaluation method, the variable amplitude multiaxial thermo-mechanical fatigue life prediction method was proposed, which is suitable for the actual structure, and a multiaxial thermo-mechanical fatigue life prediction system was developed. The life results of constant/variable amplitude multiaxial thermo-mechanical fatigue tests were used to verify the proposed method, and the prediction errors were within a factor of 2. Finally, the turbine disk structure of an aero engine was analyzed by the proposed method. The turbine disk structure was simulated by thermal-structural non-linear finite element analysis based on secondary development, and the location of the dangerous point was determined. Then, the stress-strain history and temperature history of the dangerous point were extracted to evaluate the multiaxial fatigue-oxidation-creep damage at this point. It was found that the reason for the low life of the point may be that the larger tensile stress to cause more creep damage.
... The oxide layer can be formed on the surface of the structure exposed to the high temperature, and it can be further broken and peeled off during the deformation of the component [16][17][18][19]. It has been experimentally reported in [20,21] that the creep and oxidation mainly depend on high temperature exposure, applied stress and holding time, which means that the relative contribution of each fatigue, oxidation and creep can be changed with different service conditions [22][23][24]. In terms of the deformation behavior, the rate dependent viscous response can be activated by the creep in addition to the plastic deformation, resulting in stress relaxation during the stable operation phase at high temperature [25,26]. ...
Article
In this paper, a new life assessment framework is proposed based on the elastic-viscoplastic modeling and the damage behavior for the structural component under multiaxial non-proportional loading at high temperature. The viscoplastic constitutive model with the ability to capture the non-proportional hardening effect is used to obtain the stress-strain fields, in which a new method based on the rotation of strain axis is proposed to evaluate the notch rotation factor. The damage model, which can comprehensively capture the multiaxial fatigue-oxidation-creep behaviors, is used to assess the failure life. In addition, the proposed framework is evaluated by the life results under proportional and non-proportional fatigue loadings at 650°C, and the errors are found to be within a factor of 2.
... Loureiro et al. [12,13] conducted TMF tests on nickel-based high-temperature alloy IN792 and found that the crack closure phenomenon has a significant influence on crack propagation. Jones et al. [14] demonstrated that the crack propagation rate of TMF is associated with the grain size of material. Birol et al. [15] further pointed out that a solution heat treatment not only degrades the fatigue properties but also reduces the impact toughness. ...
Article
Thermomechanical fatigue (TMF) loading is inevitable during the operation of ultra-supercritical power plants. In the present work, TMF tests are carried out at strain rates of 5×10-5/s to 4×10-4/s and in-phase (IP) and out-of-phase (OP) to investigate the cyclic deformation and damage mechanisms of P92 steel. The results reveal that the increase in strain rate leads to a significant improvement in fatigue life under both IP-TMF and OP-TMF, in which OP-TMF induces more severe damage to fatigue life than IP-TMF. It was found that the fatigue life reaches a maximum after a strain rate of 1×10-4 s-1. It is important to note that the action of dynamic strain ageing (DSA) is strain rate-, fatigue cycle- and phase angle-dependent. It was also demonstrated that the effect of the strain rate on the fatigue life is a combined effect of DSA, fatigue cracks, creep voids and oxidation. Additionally, the predictive abilities of various traditional life prediction methods under various loading conditions are comparatively assessed.
... It has been shown in the literature that the stress amplitude ratio and phase angle have a significant impact on the fatigue behavior of low-carbon and medium-carbon steels (Ref [11][12][13] and some other materials as well. Although theoretical studies on the effects of the loading condition on the fracture mode and short fatigue crack behavior have been comprehensively reported (Ref [14][15][16][17][18], articles which can provide a suitable analysis of the short fatigue crack behavior and fracture mode of railway axles are limited. ...
Article
The loads on railway axles in actual service conditions are complex and variable, which may lead to local changes in the fatigue performance, which then affect the fatigue reliability and safety evaluation results of the components and structures. At present, studies on the short fatigue crack behavior and fracture failure mode of EA4T steel, a common material for high-speed trains, are limited. Therefore, fatigue tests on smooth funnel solid round bar specimens under different stress amplitude ratios and different phase angles were performed. Replica technology was used to record the initiation and propagation information of short fatigue cracks. Due to the homogenous and fine microstructure of all the specimens, the same short fatigue crack behavior was observed regardless of the loading condition. The fracture morphology observed by a scanning electron microscope shows that the cracks initiated on the surface of the specimens, and the specific fracture morphology was exceedingly distinct in each region. In addition, the short fatigue crack growth rate and fatigue life were compared and analyzed. The results show that the stress amplitude ratio and phase angle have a considerable influence on the fatigue life and short crack growth rate of the specimens. The results of this research can provide a reference for practical engineering applications.
Article
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a b s t r a c t This paper presents the formulation of a phenomenological model to predict the crack growth in single crystal superalloy at high temperature. The proposed model relies on an extensive experimental study performed on AM1 single crystal superalloy at temperatures ranging from 650 °C to 950 °C. Tests carried out in fatigue and creep–fatigue regimes investigate the effects of time on crack growth rates. The crack growth model follows the framework of classical linear elastic fracture mechanics. Time effects at high temperature are captured by creep–fatigue and oxidation–fatigue interactions. The specific model formu-lation for nonisothermal conditions is attractive for identifying parameters on a large temperature domain and for predicting complex Thermo-Mechanical Fatigue (TMF) tests. Model predictions are then compared with a large set of experimental results including TMF tests. The application of this model, which accounts for a better understanding and modeling of physical phenomena such as the environ-mental or creep effects on crack growth rate, should improve the prediction of crack growth regime in single crystal superalloys that are used to design critical components such as turbine blades.
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The surface and through crack propagation of three high temperature steels in low cycle thermal-mechanical and isothermal fatigue at elevated temperatures was investigated. The rate of crack propagation obtained was correlated with the range of cyclicJ-integral, ΔJf. It was found that there is a linear relationship on alog-log plot regardless of materials, test conditions, and crack configurations. Furthermore, fatigue life predicted by integrating the equation of crack propagation was compared with experimental results.
Article
Thermo-mechanical fatigue (TMF) testing plays an increasingly important role in the design, the reliability assessment and the lifecycle management of safety critical components used, for instance, for power generation, in the process industry and in aeronautical and automotive applications, with a view to increasing the fuel efficiency, safety and service intervals, while reducing production (and material) costs. In a European Commission funded research project (acronym: TMF-Standard) of the 5th Framework Programme, 20 European laboratories have undertaken a joint research effort to establish a validated code-of-practice (CoP) for strain-controlled TMF testing. Starting from a survey of the testing protocols and procedures previously used by the partners, a comprehensive pre-normative research activity into various issues has been completed, addressing the dynamic temperature control, the effects of deviations in nominal temperatures and phase angles, the influences of temperature gradients, as well as the practicalities of test interruption and restart procedures. Meaningful allowable tolerances for the various test parameters were identified and practical recommendations as to the test techniques were formulated. From this a preliminary CoP was compiled and used to guide an extensive round robin exercise among the project partners. From the statistical analysis of that exercise, a validated CoP was derived dealing with strain-controlled constant amplitude TMF of nominally homogeneous metallic materials subjected to spatially uniform temperature fields and uniaxial mechanical loading. It is intended to give advice and guidance on the appropriate test setup, testing procedures and the analysis of results, in particular for newcomers in the field of strain-controlled TMF. This paper highlights some of the results of the TMF-Standard project. Moreover, commonalities and differences of the present CoP with respect to the standard documents for strain-controlled TMF, which have been developed at ISO and ASTM levels, are presented in this paper.
Article
The application of several fracture mechanics data correlation parameters to predicting the crack propagation life of turbine engine hot section materials was evaluated. A survey was conducted to determine the conditions where conventional fracture mechanics approaches may not be adequate to characterize cracking behaviour. Both conventional linear and non-linear fracture mechanics analyses were considered. Isothermal and thermal-mechanical (variable temperature) crack propagation tests were performed in Hastelloy-X, and B-1900 + Hf materials. The crack growth data were reduced using the stress intensity factor, the strain intensity factor and the . None of these three parameters successfully correlated the crack growth data. All three parameters showed strain range effects and significant scatter between the various testing conditions (in-phase, out-of-phase, isothermal). The parameter which showed the most effectiveness in correlating high temperature and variable temperature crack growth was a modified stress intensity factor () computed using the measured load, the closing bending moment caused by the increase compliance with crack length and with the effective opening stress. The was shown to rationalize the effect of mode of testing (stress vs strain controlled) and the mode of fracture. Furthermore, the isothermal data at and where shown to provide an upper and lower bounds for the thermal-mechanical fatigue data if plotted in terms of .
Article
An improved theory is proposed for the crack-growth analysis of cyclic-loaded structures. The theory assumes that the crack tip stress-intensity-factor range, ΔK, is the controlling variable for analyzing crack-extension rates. The new theory, however, takes into account the load ratio, R, and the instability when the stress-intensity factor approaches the fracture toughness of the material, Kc . Excellent correlation is found between the theory and extensive experimental data. A computer program has been developed using the new theory to analyze the crack propagation and time to failure for cyclic-loaded structures.
Code of practice for force-controlled thermo-mechanical fatigue testing
  • Brookes