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The effect of phase angle on crack growth mechanisms under thermo-mechanical fatigue loading
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.
... 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). ...
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]. ...
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. ...
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. ...
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) ...
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]. ...
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. ...
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. ...
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.
The presented methodology in this study is addressed to in-phase (IP) and out-of-phase (OOP) loading cycles in stationary and transient thermo-mechanical fields. The subject of the numerical and experimental study is a single edge notch tension (SENT) specimen produced from a high-temperature nickel-based alloy ХН73М. In order to determination a local thermo-mechanical stress-strain rate and displacement fields a new algorithm for the multi-physics numerical calculations developed and implemented incorporates Maxwell 3D, Fluent and Transient Structural modules of ANSYS 2021R1. The employment of proposed algorithm to represent the cyclic history associated with the TMF conditions in the experiments, multi-physics finite element (FE) modelling of the stress, strain and displacement fields in the SENT specimen was performed. Additionally, time dependent non-uniform temperature fields were determined with the same cyclic variations and magnitudes as in the experimental OOP and IP cycling. As a complement to the FEM computations, the infra-red thermography temperature distribution measurements was implemented for the TMF state in the experiments in the SENT specimen. The comparison multi-physics FE-analysis and direct measurements shown in the present study is intended to contribute to a better understanding of the different mechanisms driving TMF crack growth and the address the outstanding questions associated with basic methodology.
Thermomechanical fatigue in distinct crystal orientations of nickel-base single-crystal superalloy was studied. Crack initiation modes and damage mechanisms were characterized on fracture surfaces and longitudinal sections by scanning electron microscope and optical microscope. Creep damage nucleated from casting pores in IP-TMF and propagated under mode I. The creep facets were controlled by the activated slip systems, whereas fatigue damage was orientation-dependent and developed by slipping along different crystallographic planes. Oxidation-assisted cracking in OP-TMF tests was non-crystallographic for all crystal orientations. Multi-layer structures near the crack tip revealed successive growth of oxide and the crack grew within the oxidized material.
Investigating fatigue crack growth behavior of materials is crucial to the life design and safety evaluation of engineering structures. During the practical service condition, the temperature keeps changing, which cannot be studied by the traditional fatigue crack growth test method at constant temperature. In this study, we focus on constructing a new thermomechanical fatigue (TMF) crack propagation testing method, developing a thermal deformation compensation method, and proposing an equivalent compliance method suitable for variable temperature conditions based on ASTM E647 standard to measure the crack length. Furthermore, the load‐ and strain‐controlled TMF tests under different phase angles are carried out to prove the validity of the new method. A temperature compensation method for the variable temperature test is proposed. A modified compliance method for crack length measurement is developed. The thermomechanical fatigue (TMF) crack propagation testing method was standardized. Only the stress intensity factor range ΔK cannot be used to characterize the fatigue crack growth rate.
Thermo-Mechanical Fatigue (TMF) is one of the most complex mechanical phenomena that couples creep, fatigue and oxidation. So far, developing simple empirical relationships like those that exist in isothermal equivalents have proven elusive. This work presents a study on the TMF behaviour of the aerospace nickel based superalloy RR1000 using TMF fatigue crack growth rates, strain controlled TMF results, studies of the fracture morphologies and empirical lifing models. This paper takes a holistic approach to TMF lifing, specifically focusing on the impact of phase angle. As a result, this work develops an elastic modulus normalisation technique that when a Coffin-Manson type relationship is applied, predicts life of material under interim TMF phase angles.
The article is an overview of the complex nature of thermo-mechanical fatigue (TMF) loading scenarios. The diverse types of TMF phasing that can exist and their prevalence within the hot sections of a gas turbine engine are discussed. The resultant damage mechanisms are investigated and attributed to the phase shift between thermal and mechanical strains within the employed cycle, unique to TMF loading.
Thermo-mechanical fatigue data is critical for the generation of appropriate lifing methodologies for a range of in-service applications where non-isothermal conditions are prevalent. Recently the development of more standardised testing approaches through appropriate code of practice documents and international standards has proved crucial. In the current paper, several methods of undertaking TMF testing are explored, with the benefits and pitfalls of each test type investigated. It is shown that bespoke test setups are often required, dependent on material, TMF cycle and specimen type. Further developments are suggested, along with a suggested methodology for TMF crack growth tests.
This paper describes the advantages and enhanced accuracy thermography provides to high temperature mechanical testing. This technique is not only used to monitor, but also to control test specimen temperatures where the infra-red technique enables accurate non-invasive control of rapid thermal cycling for non-metallic materials. Isothermal and dynamic waveforms are employed over a 200–800 °C temperature range to pre-oxidised and coated specimens to assess the capability of the technique. This application shows thermography to be accurate to within ±2 °C of thermocouples, a standardised measurement technique. This work demonstrates the superior visibility of test temperatures previously unobtainable by conventional thermocouples or even more modern pyrometers that thermography can deliver. As a result, the speed and accuracy of thermal profiling, thermal gradient measurements and cold/hot spot identification using the technique has increased significantly to the point where temperature can now be controlled by averaging over a specified area. The increased visibility of specimen temperatures has revealed additional unknown effects such as thermocouple shadowing, preferential crack tip heating within an induction coil, and, fundamental response time of individual measurement techniques which are investigated further.
Thermo-mechanical fatigue (TMF) tests including 0°, 90°, -90°, 45° -135° and -180°, phasing (φ) between mechanical loading and temperature were undertaken on a polycrystalline nickel-based superalloy, RR1000. Mechanical loading was employed through strain control whilst 300-700 °C and 300-750°C thermal cycles were achieved with induction heating and forced air cooling. Mechanical strain ranges from 0.7 to 1.4% were employed. Results show that, for the strain ranges tested, TMF life is significantly affected by the employed phase angle. Furthermore the strain range and peak cycle temperature used has a substantial influence on the significance of dominant damage mechanisms, and resultant life. Various metallographic examination techniques have outlined that the dominant damage mechanisms are creep deformation at higher temperatures and early cracking of oxide layers at lower temperatures.
Non-isothermal conditions during flight cycles have long led to the requirement for thermo-mechanical fatigue (TMF) evaluation of aerospace materials. However, the increased temperatures within the gas turbine engine have meant that the requirements for TMF testing now extend to disc alloys along with blade materials. As such, fatigue crack growth rates are required to be evaluated under non-isothermal conditions along with the development of a detailed understanding of related failure mechanisms. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180° OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles.
Thermomechanical fatigue (TMF) crack growth testing has been performed on the polycrystalline superalloy IN792. All tests were conducted in mechanical strain control in the temperature range between 100 and 750 °C. The influence of in-phase (IP) and out-of-phase (OP) TMF cycles was investigated as well as the influence of applying extended dwell times (up to 6 hours) at the maximum temperature. The crack growth rates were also evaluated based on linear elastic fracture mechanics and described as a function of the stress intensity factor KI. Without dwell time at the maximum temperature, the crack growth rates are generally higher for the OP-TMF cycle compared to the IP-TMF cycle, when equivalent nominal strain ranges are compared. However, due to the fact that the tests were conducted in mechanical strain control, the stress response is very different for the IP and OP cycles. Also the crack closure level differs significantly between the cycle types. By taking the stress response into account and comparing the crack growth rates for equivalent effective stress intensity factor rages ΔKeff defined as Kmax − Kclosure, very similar crack growth rates were actually noticed independent of whether an IP or OP cycle were used. While the introduction of a 6 hour dwell time significantly increased the crack growth rates for the IP-TMF cycle, a decrease in crack growth rates versus ΔKeff were actually seen for the OP-TMF cycle. The fracture behaviour during the different test conditions has been investigated using scanning electron microscopy.
The influence of microstructure on the dwell fatigue crack growth behaviour of an advanced nickel-based superalloy was investigated at a temperature of 700 °C. Microstructural variations were induced by heat treatment variables: different cooling rates of quenching from super-solvus solution heat treatment, 0.7 and 1.8 °C s−1, and an addition of a high temperature stabilisation heat treatment (2 h at 857 °C) between the solution treatment and the final ageing treatment. With a one hour dwell introduced at the peak load of the fatigue cycle, such different microstructural conditions can lead to a difference of up to two orders of magnitude in crack growth rates in air, when compared to those obtained under baseline fatigue loading. By performing such dwell fatigue and baseline fatigue tests in vacuum, it is confirmed that such increases in crack growth rates under dwell fatigue loading in air are mainly environmentally related. Transmission electron microscopy (TEM) was utilised to analyse both crack tip oxides and associated deformation mechanisms in the matrix. A novel mechanism taking into account competing interactions of crack tip oxidation (leading to increases in crack growth rates) and stress relaxation (leading to decreases in crack growth rates) is outlined.
This paper summarises Rolls-Royce's efforts in developing a production capable and industrially robust process for the production of dual-microstructure components. This has been possible through the use of process modeling, full-scale validation trials, detailed microstructural assessment through a variety of destructive and non-destructive methods and mechanical property evaluation. This paper reports the fundamental underlying relationships between thermal history and microstructural evolution and how these directly influence mechanical properties. This work has enabled a deeper understanding of the opportunities and difficulties faced in the application of this technique for producing dual microstructures, and how the technique can be scaled up from the research laboratory to full production capacity.
To study the effect of γ′ precipitate size on the deformation behaviour of a polycrystalline nickel-based superalloy, model microstructures with a unimodal γ′ size distribution were developed and subjected to loading experiments at 750 °C. Neutron diffraction measurements were carried out during loading to record the elastic lattice strain response of the γ and γ′ phase. A two-site elasto-plastic self-consistent model (EPSC) assisted in the interpretation of the elastic lattice strain response. In addition, the microstructures of the deformed specimens were analysed by (scanning) transmission electron microscopy (STEM). Excellent agreement was found between the EPSC and STEM results regarding a joint deformation of the γ and γ′ phase in the fine γ′ microstructures and for low plastic strains in the medium γ′ microstructures. With increasing γ′ size and increasing degree of plastic deformation, both experimental methodologies revealed a tendency of the two phases to deform independently.
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.
Understanding the relationship between deformation mechanisms and microstructure is essential if one wants to fully exploit the potential of advanced nickel base superalloys and develop future alloys. In the present work, the influence of the lattice misfit between gamma and gamma' has been studied by means of in-situ loading experiments using neutron diffraction in combination with crystal plasticity modelling on RR1000 and Alloy 720Li. Both alloys were processed to generate three simplified uni-modal gamma' microstructures to allow determination of gamma' responses and experiments were carried out at 750 degrees C. The results showed that a positive misfit strain increases the level of load partitioning from gamma to gamma' during plastic deformation introduced by uniaxial tensile loading.
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.
The influences of elevated Co and Ti levels on the mechanical properties of the Ni-base superalloy RR1000 have been investigated. Following heat treatment, the modified alloys had the typical γ-γ′ microstructure, with γ′ precipitate sizes comparable to similarly heat treated RR1000, but with a slightly higher volume fraction. The modified alloys exhibited a higher proof stress than RR1000 across the entire 20 to 800°C temperature range investigated. Superior creep rupture lives, when compared to RR1000, were observed in the modified alloys at 700°C, but not at 750°C, where extensive precipitation of topologically close packed σ phase occurred on the grain surfaces. The formation of this deleterious phase was linked to Cr and Mo enrichment of the c matrix, caused by the elevated Co and Ti additions.
The thermo-mechanical fatigue (TMF) behaviour of the Nimonic 90 Nickel base superalloy has been investigated within two laboratories. In-phase-tests (IP) where the maximum mechanical strain occurs at the maximum temperature (850°C), and 180°-out-of-phase-tests (180° OP) where the maximum mechanical strain coincides with the minimum temperature (400°C) have been applied. All tests were carried out at varying mechanical strain ranges with a constant strain ratio of Rε = - 1. A temperature rate of 5 K/s was used throughout the whole cycle without any additional cooling system during decreasing temperature. The fatigue life of 180° OP tests is longer compared to identical IP tests. The stress / mechanical strain hysteresis loops are completely different and some characteristic values are compared to each other. The fracture surfaces observed show that fatigue crack (or cracks) starts on the external surface and propagates inwards. The fractures of 180° OP tests are transgranular showing the presence of fatigue striations, while the fractures of IP tests are mixed transgranular and intergranular with no fatigue striations.
Within the gas turbine engine, the high transient thermal stresses resulting from throttle movement from idle to high settings give rise to the phenomenon of thermo-mechanical fatigue (TMF). These effects have been widely explored for turbine blade materials, typically single crystal nickel alloys. More recently however, a combination of thinner disc rims and further increases in turbine entry temperature has led to a situation where TMF in disc materials cannot be ignored. Turbine discs will usually be manufactured from polycrystalline nickel alloys, and as such it is now considered critical that TMF effects in this system of alloys is fully characterised. The current paper seeks to summarise the published work on TMF in polycrystalline nickel alloys for turbine disc applications, whilst introducing recent work at Swansea University. Previous research has concentrated particularly on the differences between in-phase (IP) and out-of-phase (OP) loading. The current work from Swansea indicates that this approach is overly simplistic and that for a complete evaluation of the alloy, intermediate phase angles should also be considered.
In-phase TMF crack growth testing with different lengths of the hold time at the maximum temperature of 550°C has been conducted on Inconel 718 specimens. Focus has been on establishing a method for TMF crack growth testing and investigating the effect of high temperature hold times on the TMF crack growth of the material. The tests are compared to isothermal crack propagation tests and show good correlation. It is concluded that the controlling effect of the crack growth is an embrittlement of the material. This embrittlement is related to the concept of a damaged zone active in front of the crack tip. The size of this damaged zone will control the crack propagation rate and therefore it does not matter if the load is cycled under isothermal or TMF conditions.
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.
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.
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 .
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