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Abstract

The current paper presents work on identification and evaluation of a range of factors influencing accuracy and comparability of data generated by three laboratories carrying out stress-controlled thermo-mechanical fatigue crack growth tests. It addresses crack length measurements, heating methods and temperature measurement techniques. It also provides guidance for pre-cracking and use of different specimen geometries as well as Digital Image Correlation imaging for crack monitoring. The majority of the tests have been carried out on a coarse grain polycrystalline nickel-base superalloy using two phase angles, Out-of-Phase and In-Phase cycles with a triangular waveform and a temperature range of 400-750 oC.

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... Thus, to achieve accurate life predictions models within the crack growth phase, TMF crack propagation testing is needed. Several studies have been carried out to increase the knowledge and establish guidelines related to TMF crack growth testing [8][9][10][11]. The type of the TMF loading cycle used in testing generally depends on the target component being investigated. ...
... The model utilised associated flow rule with von Mises yield criteria. Ziegler's kinematic law plus a recall term for each of the back-stresses, , was used to describe the evolution law of the hardening model [53] = −̇p −̇p (11) where the total back-stress tensor is ...
Article
Exploring crack growth behaviour is needed to establish accurate fatigue life predictions. Cracked specimens were tested under strain-controlled out-of-phase thermomechanical fatigue conditions. The tests included dwell times and three different minimum temperatures. Higher minimum temperature gave faster crack growth rates while the additions of dwell times showed no effects. Crack closure was observed in all the tests where the addition of dwell times and change in minimum temperature displayed little to no effect on crack closure stresses. Finite element models with a sharp stationary crack and material parameters switching provided acceptable predictions for the maximum, minimum, and crack closure stresses.
... Nevertheless, TMF crack growth testing has emerged as an important topic when studying critical components. Thus, several researches have been focusing on investigating and laying guidelines for TMF crack propagation testing [4][5][6]. ...
Article
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Understanding of crack growth behaviour is necessary to predict accurate fatigue lives. Out-of-phase thermomechanical fatigue crack propagation tests were performed on FB2 steel used in high-temperature steam turbine sections. Testing results showed crack closure where the compressive part of the fatigue cycle affected crack growth rate. Crack closing stress was observed to be different, and had more influence on the growth rate, than crack opening stress. Crack growth rate was largely controlled by the minimum temperature of the cycle, which agreed with an isothermal crack propagation test. Finite element models with stationary sharp cracks captured the crack closure behaviour.
... 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
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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.
Article
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.
Chapter
Applying damage-tolerant design approaches requires a comprehensive understanding and reliable modeling of crack growth under service conditions. This in turn requires gathering the necessary database for model parametrization and validation from the observation of crack growth in component-like test piece geometries at service-like thermo-mechanical fatigue (TMF) conditions. A standard tool for gathering such information, employed in both research and industry, is the potential drop method. However, in combination with alternating temperature and power surges from heating devices, this method proves difficult. In recent years, the Institute for Materials Technology in Darmstadt, together with partners from the energy and aerospace industry, conducted various research projects on TMF crack growth in nickel alloys. One key element was the employment of the alternating current potential drop (ACPD) measurement technique to monitor crack growth during testing. This paper presents the measurement setup, the data assessment process, the validation results, and the most prominent findings from these projects. The data collection includes different standardized and individualized sample geometries made from multiple nickel alloys and heat-resistant steels and covers a temperature range from room temperature up to 1,100°C. The presentation of the experimental results is complemented by a theoretical evaluation of ACPD crack growth measurement using multiphysics finite element simulations. Through such modeling approaches, the effects of, for example, creep strain development or the di-electric material properties of different materials on crack growth measurement accuracy can be investigated. These approaches also help one gain a fundamental understanding of this still largely empirically based measuring technology.
Article
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.
Article
The crack growth behaviour of the alloy CM 247 LC is investigated for out-of-phase TMF and isothermal tests at the same temperature as the minimum temperature in the TMF tests. The results suggest that it is possible to characterise crack growth behaviour if experimental corrections for crack closure are accounted for. The replication of these experimental tests using a numerical FE-solver results in similar crack growth behaviour, suggesting that the main mechanism in place is plasticity-induced crack closure. A pragmatic analytical model to characterise crack closure including hold time effects is proposed. The comparison of the response from this model with the experimental and numerical results suggests that the proposed analytical model is capable to approximate crack closure effects for cases where substantial creep deformation is to be expected.
Article
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The crack driving mechanisms in a coarse grained nickel-base superalloy RR1000 when subjected to in- and out of phase thermo mechanical fatigue are investigated. It is found that the difference in fatigue crack growth rate between these two load conditions is accounted for by the different mechanical conditions at the crack tip region, rather than oxidation effects. This is based on digital image correlation and finite element analyses of the mechanical strain field at the crack tip, which demonstrate that in phase leads to larger crack tip deformation and crack opening. Notably, it is demonstrated that in- and out of phase crack growth rates coincide when correlated to the crack tip opening displacement.
Article
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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.
Article
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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.
Article
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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.
Conference Paper
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The high temperature fatigue characteristics of U720Li have been investigated over the temperature range 650°C to 725°C under imposed dwell times (at maximum load) of 1 and 20 seconds in vacuum and air conditions. The effect of varying grain size and coherent precipitate size under these conditions has been assessed. Testing in air resulted in oxidation dominated intergranular crack growth at all temperatures and dwell times with the slope (m-values) of the crack growth rate curves remaining constant. Increased crack growth rates are seen at the higher temperatures and at longer dwells, although no effect of dwell was observed at 650?C in the as-received fine grained variant. In vacuum crack growth rates were much lower than in air and a purely cyclic dependent regime was evident at 650°C. As temperature and dwell time at maximum load was increased, m-values increased and were accompanied by a change in crack growth mechanism from transgranular to intergranular cracking. This indicated that true, time-dependent, creep-fatigue processes were occurring. The large grain variant of the U720Li showed little advantage in crack growth rates within the cyclic dependent and creep-fatigue regime, but did show a significant increase in resistance to crack growth in the time dependent (oxidation-fatigue) regime. The effect of the large precipitate variant was to give similar or worse crack growth resistance than the baseline U720Li at temperatures up to 725°C (1 second dwell) but improved crack growth resistance when oxidation processes predominated at 725°C in air with an imposed 20 second dwell.
Conference Paper
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The microstructural evolution of γ′ has been investigated in a series of advanced nickel-base superalloys, produced via the powder metallurgy route. The size, morphology and distribution of γ′ within the microstructure has been found to be highly sensitive to the rate of cooling from the supersolvus solutioning temperature. Precipitate morphologies and the resulting γ′ particle size distributions, were correlated to the various cooling rates, ranging from 6 to 600°C/min. Cooling rates below about 60°C/min resulted in a bimodal distribution of γ′, while rates above 60°C/min were able to suppress the formation of tertiary γ′ and yield a unimodal distribution of γ′ in this set of alloys. Following the supersolvus heat treatment, the effect of ageing these various microstructures at 800°C was assessed. While characteristic precipitate coarsening was observed in microstructures containing a bimodal distribution of γ′, anomalous coarsening to a critical size followed by the splitting of the γ′ precipitates, was observed in specimens initially containing a unimodal distribution of γ′.
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A study was conducted to determine the processes which govern hold time crack growth behavior in the LSHR disk P/M superalloy. Nineteen different heat treatments of this alloy were evaluated by systematically controlling the cooling rate from the supersolvus solutioning step and applying various single and double step aging treatments. The resulting hold time crack growth rates varied by more than two orders of magnitude. It was shown that the associated stress relaxation behavior for these heat treatments was closely correlated with the crack growth behavior. As stress relaxation increased, the hold time crack growth resistance was also increased. The size of the tertiary γ' in the general microstructure was found to be the key microstructural variable controlling both the hold time crack growth behavior and stress relaxation. No relationship between the presence of grain boundary M 23 C 6 carbides and hold time crack growth was identified which further brings into question the importance of the grain boundary phases in determining hold time crack growth behavior.
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Compression precracking (CPC) has seen renewed interest as a possible alternative procedure for generating fatigue crack growth threshold data with minimal load history effects, but recent testing confirms results from the literature that compression precracking does induce load history effects through residual stresses that influence subsequent fatigue crack growth test data. Using the CPC method, specimens are precracked with both maximum and minimum compressive loads. Compressive yielding occurs at the crack-starter notch, resulting in a local tensile residual stress field through which the fatigue crack must propagate. Although the tensile residual stress field contributes to the driving force for precracking, it also introduces the possibility of history effects that may affect subsequent fatigue crack growth. The tensile residual stress field elevates the local driving force at the crack tip, promoting higher crack growth rates than would be expected from the applied loading. This paper presents three-dimensional finite element results and experimental data for compact tension specimens that characterize the load history effects induced by compression precracking. The analysis results indicate that for low tensile loading levels near the threshold region, the residual stresses cause the calculated crack tip driving force to increase from the applied driving force by 25 % or more. In addition, significant crack growth of about two times the estimated plastic zone size is needed to grow away from the residual stress field and reduce the calculated crack tip driving force to within 5 % of the applied driving force. Experimental results show that growth of about two to three times the estimated plastic zone size is necessary to establish steady growth rates under constant AK loading for the materials and loading levels evaluated. Constant AK testing following compression precracking will demonstrate when residual stress effects are no longer significant and will ensure consistent growth rates.
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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.
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The complete fatigue process involving the growth of microstructurally small fatigue cracks prior to macrocrack initiation and the subsequent large crack propagation in notched compacted graphite iron, EN-GJV-400, specimens subjected to thermo-mechanical fatigue has been investigated. It is shown that microcracks are initiated at graphite tips within an extended volume at the notch which eventually leads to an abrupt microcrack coalescence event. As a macrocrack is generated in this way, the crack growth switches to conventional characteristics which is assessed in terms of elasto-plastic fracture mechanics parameters. Consequently, two important implications regarding lifetime assessment are identified; possible underestimation due to (i) how the stress is evaluated in view of the spacial distribution of microcracking and (ii) the crack retardation effect associated with the crack growth transition.
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The compression pre-crack method can be used to develop near-threshold fatigue crack growth rates much quicker than the ASTM standard load shed method. The compression pre-crack method is faster than the standard method and eliminates load history effects that are of concern when the standard method is used. However, crack growth developed using the compression pre-cracking approach is susceptible to different load history effects. A suite of tests using the compression pre-crack method that were performed on AISI 9310 steel are presented in this paper. An approach for identifying the critical crack growth increments required to develop valid data is presented, and the effects of three potential mechanisms are evaluated. For these tests, the residual stress field resulting from the initial compression field appears to be the more important effect on crack growth behavior. Crack growth increments measured from the notch tip that are 2.5 times larger than the size of the plastic zone resulting from the compression pre-cracking are required in order to develop valid data on the fatigue crack growth rate.
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 crack growth properties of various microstructures developed in one heat of Inconel 718 alloy were investigated at 650 °C under air and vacuum environments. The microstructures included fine-grained material (ASTM grain sizes 6–8), coarse-grained material (ASTM grain sizes 3–4) and material of a necklace structure (ASTM grain sizes 3–4 and 8–10). The effect of grain boundary β (Ni3Nb) phase precipitation was also studied. Continuous fatigue, creep and creep-fatigue conditions were examined. For continuous fatigue the influence of frequency was investigated over the range between 5 × 10−2 and 20 Hz. For creep-fatigue conditions, hold times of 10 and 300 s were superimposed on a 5 × 10−2 Hz triangular wave shape signal.It was shown that the grain boundary microstructure had a very strong effect when the fatigue crack propagation behaviour was essentially time dependent. This effect is associated with the occurrence of brittle intergranular fracture and dramatic increases in crack growth rate. The microstructure had no effect under vacuum testing.
Article
An assessment of the effects of microstructure and operating parameters on both crack initiation and propagation of short fatigue cracks is presented. The assessment was carried out on RR1000, U720Li and microstructural variants of U720Li. Fatigue tests were carried out at room temperature (20 Hz sinusoidal cycling) and at 650 °C (1–1–1–1 trapezoidal cycling). Comparisons of the performance of the different microstructures revealed that initiation occurred predominantly at pores at both temperatures. At room temperature, stage I crack growth predominated and the presence of large primary γ′ precipitates on the grain boundaries, larger grains and larger coherent γ′ sizes gave improved fatigue crack growth resistance, whereas at 650 °C, larger grains gave the most significant performance benefits.
Article
Small angle neutron scattering (SANS) has been used to evaluate the temporal evolution of the secondary and tertiary gamma' precipitates in the nickel-base superalloy, RR1000, in situ during an aging heat treatment at 1033 K (760 °C) following a supersolvus heat treatment and oil quench. The bimodal distribution of secondary and tertiary gamma' was analyzed using a specially developed polydispersive model capable of evaluating the scattering curves to obtain precipitate size distributions (PSDs) and volume fractions as a function of time. The model was designed to be suitable for high volume fractions of gamma' and takes into account the scattering interaction between precipitates. The results show an increase in the volume fraction and the mean particle size of both the secondary gamma' and tertiary gamma' during aging. The initial and final precipitate distributions have been characterized using transmission electron microscopy (TEM) and show satisfactory correlation with the SANS data across the scattering vector range.
The application of 3-dimensional finite element methods to fracture mechanics and fatigue life prediction
  • A C Pickard
Pickard AC. The application of 3-dimensional finite element methods to fracture mechanics and fatigue life prediction. Warley, England: Engineering Materials Advisory Services; 1986.