Thermo-mechanical Fatigue Crack Growth Research Articles

Isothermal and thermomechanical fatigue crack growth behavior of 316LN stainless steel under load‐ and strain‐controlled modes

AbstractCrack growth tests of 316LN stainless steel under load‐controlled and strain‐controlled thermo‐mechanical fatigue (TMF) loadings at the temperature range of 350°C ~ 550°C were conducted. Moreover, the isothermal fatigue (IF) crack growth tests at the maximum temperature of 550°C were performed for comparison. The differences in IF and TMF crack propagation behavior under different loading conditions were thoroughly discussed, and the underlying mechanisms were revealed by a comprehensive characterization of the deformation state of the crack tip region using digital image correlation (DIC) and electron backscatter diffraction (EBSD) methods. Furthermore, it was found that the EBSD characterization was simply based on a single plane made it difficult to evaluate the complex crack growth behavior. In addition, the limitations of the classical parameter of stress intensity factor under TMF loading were presented.

Assessment of crack-tip fields of thermo-mechanical elastoplastic fatigue crack growth

The present paper computationally investigated and characterized thermo-mechanical crack-tip fields under elastoplastic fatigue crack growth. The ΔJ-integral is generally path-dependent and its far-field value is introduced to describe the singular crack tip field under considering thermo-mechanical loading influence. The influence of crack closure in fatigue crack growth is studied and quantified for thermo-mechanical fatigue crack growth. It confirms that the incorporation of temperature cycling does not affect the solution form of the crack-tip field dominated by ΔJeff, in analogy to an isothermal fatigue crack. However, the singular crack stress and the far-field effective ΔJefffor TMF are affected by the phase angle of the thermo-mechanical loads.

ProCrackPlast: a finite element tool to simulate 3D fatigue crack growth under large plastic deformations

Many structural components and devices in combustion and automotive engineering undergo highly intensive cyclic thermal and mechanical loading during their operation, which leads to low cycle (LCF) or thermomechanical (TMF) fatigue crack growth. This behavior is often characterized by large scale plastic deformations and creep around the crack, so that concepts of linear-elastic fracture mechanics fail. The finite element software ProCrackPlast has been developed at TU Bergakademie Freiberg for the automated simulation of fatigue crack growth in arbitrarily loaded three-dimensional components with large scale plastic deformations, in particular under cyclic thermomechanical loading. ProCrackPlast consists of a bundle of Python routines, which manage finite element pre-processing, crack analysis, and post-processing in combination with the commercial software Abaqus . ProCrackPlast is based on a crack growth procedure which adaptively updates the crack size in finite increments. Crack growth is controlled by the cyclic crack tip opening displacement varDelta CTOD, which is considered as the appropriate fracture-mechanical parameter in case of large scale yielding. The three-dimensional varDelta CTOD concept and its effective numerical calculation by means of special crack-tip elements are introduced at first. Next, the program structure, the underlying numerical algorithms and calculation schemes of ProCrackPlast are outlined in detail, which capture the plastic deformation history along with the moving crack. In all simulations, a viscoplastic cyclic material law is used within a large strain setting. The numerical performance of this software is studied for a single edge notch tension (SENT) specimen under isothermal cyclic loading and compared to common finite element techniques for fatigue crack simulation. The capability of this software is featured in two application examples showing crack growth under mixed-mode LCF and TMF in a typical austenite cast steel, Ni-Resist. In combination with a crack growth law identified in terms of varDelta CTOD for a specific material, the tool ProCrackPlast is able to predict the crack evolution in a 3D component for a given thermomechanical loading scenario.

Crack tip field analysis for thermo-mechanical fatigue loading

This study aimed to establish a computational strategy for the thermo-mechanical fatigue (TMF) simulations of crack tip fields considering both the in-phase (IP) and out-of-phase (OOP) loading histories. To this end, an algorithm for multi-physics numerical calculations was developed and implemented by incorporating Maxwell 3D, Fluent, and the transient structural modules of ANSYS 2021R1, to understand the mechanics of crack tip deformation under thermo-mechanical loading conditions. These computations are based on the coupled heat loss from magnetic field eddy currents and the forced convective air-cooling accounted for by turbulence model response, which provides the gradients of mechanical elastic–plastic deformations. The proposed algorithm was applied to replicate and analyse the crack growth test conditions of polycrystalline XH73M nickel-based alloys subjected to a triangular waveform of IP and OOP cycles and a temperature range of 400–650 °C. Further, multi-physics finite element (FE) modelling of the stress, strain, and displacement fields in uncracked and fractured single-edge-notch tension specimens was performed. The novelty of the results lies in the crack tip fields, which strongly influenced by the phase relation between mechanical loading and temperature, provided the electromagnetic and fluid dynamics characteristics and the elastic–plastic properties of the material are dependent on the current temperature and time across a particular deformation cycle. Notably, the complete three-dimensional multi-physics FE analysis presented herein is expected to contribute toward a better understanding of the different mechanisms driving TMF crack growth and also help address the related outstanding questions.

Waveform impact on thermo-mechanical fatigue crack growth of a non-crystallizing rubber: Experimental observation and numerical simulation

Compared with the sine waveform, the pulse waveform load is more in line with the real service condition of tire. It was investigated the effect of two pulse waveform loads on the thermo-mechanical fatigue crack growth rate (FCGR) of filled styrene-butadiene rubber and explained the underlying mechanism. The fatigue test and infrared thermal imaging test revealed that the shorter the pulse excitation time, the higher the heat build-up and the faster the FCGR. The underlying mechanism was explained by an established thermo-mechanical coupling method, and it was pointed out that the faster FCGR under high-frequency loading condition resulted from higher hysteresis energy rather than higher strain energy. The Parallel Rheological Framework model could not explain this phenomenon well.

Couple effects of temperature and fatigue, creep-fatigue interaction and thermo-mechanical loading conditions on crack growth rate of nickel-based alloy

The ambient and high-temperature fatigue crack growth behaviors in C(T) and SENT specimens of Ni-based superalloy for turbine disk application were studied in a wide interval of temperatures 25–750°C using a combination a electro- and servohydraulic test systems and fractographic investigations. The fatigue, creep-fatigue interaction and thermo-mechanical in-phase fatigue (TMF IP) crack growth tests are performed under isothermal and dynamic waveforms loading conditions. The interpretation of the experimental results is given in terms of the traditional stress intensity factors and C-integral as well as new normalized cyclic fracture diagrams. It is found that there are definite temperature-sensitive regions separate for harmonic fatigue and creep-fatigue interaction loading conditions in which the crack growth rate of Ni-based alloy increases sharply. Scanning electron microscopy in longitudinal sections containing cracks revealed the mechanisms responsible for fatigue crack initiation and growth. The couple effect of temperature ranging and isothermal and dynamic waveforms loading conditions on fatigue life was discussed.

Modelling of the transition from mode I to crystallographic crack growth in a single crystal gas turbine blade alloy under service-like loading conditions

In fatigue life prediction of single crystal gas turbine blades, the risk of rapid crystallographic crack growth along the close-packed planes poses a large uncertainty. A criterion is proposed to predict the transition from mode I to crystallographic crack growth, which is necessary for reliable prediction of the number of cycles from crack initiation to the onset of crystallographic crack growth. The proposed criterion is calibrated against tests performed under a wide range of conditions representative for a gas turbine blade, including isothermal fatigue crack growth tests and thermomechanical fatigue crack growth tests, some including hold times and pre-test aging.

Multi-physics FE-analysis and measurements for thermo-mechanical fatigue crack growth rate testing 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.

A unified description of mechanical and actuation fatigue crack growth in shape memory alloys

The damage tolerance approaches to fatigue rely on fracture mechanics concepts to explicitly account for the fatigue crack growth process. These approaches typically adopt the Paris equation, which relates the crack growth per cycle to the range of variation of the stress intensity factor, ΔK, during cyclic loading by means of experimentally calibrated parameters. Conditions on the validity of a single parameter description of the near crack tip stress and strain fields restrict the applicability of ΔK to fatigue crack growth resistance in Shape Memory Alloys (SMAs) under general thermomechanical loading paths. These conditions are substantially relaxed for the cyclic ΔJ–integral approach, which can achieve similitude over a wider range of loading conditions and geometric configurations. Thus, ΔJ qualifies as a potential unified descriptor of thermomechanical fatigue crack growth resistance in SMAs, an assertion, which is tested in this work for data from both purely mechanical and actuation fatigue crack growth experiments of a high temperature SMA, Ni50.3Ti29.7Hf20. In the latter, compact tension specimens are subjected to thermal cycling under constant load between an upper and a lower cycle temperature that allows for the stable phase to alternate in each cycle. It is shown here that a Paris-type power-law crack growth expression based on the ΔJ-integral can fit fatigue crack growth rate data from both types of experiments with a single set of parameters. This new analysis method for both actuation and mechanical fatigue crack growth rates provides a unified description for fatigue crack growth in SMAs and can allow estimating actuation fatigue crack growth rates, laborious and challenging to measure, from easier to detect mechanical fatigue crack growth rates.

Modelling the crack growth behaviour of a single crystal nickel base superalloy under TMF loading with long dwell times

The influence of hold time on the crack growth behaviour of a single crystal nickel base superalloy under in-phase thermomechanical fatigue is investigated. Two da/dt models were calibrated using creep crack growth tests in the temperature range 750–950 °C: one based on K and the other on (Ct)avg. The models were applied, in combination with a cycle dependent model, to predict da/dN of in-phase thermomechanical fatigue crack growth tests with 1–6 h hold time. The predictions based on K were inaccurate and generally non-conservative, whereas the predictions based on (Ct)avg were accurate. da/dt vs (Ct)avg followed a single trendline for all temperatures.

Out-of-phase thermo-mechanical fatigue crack growth and the effect of the compressive minimum load level on crack closure at notches

Fatigue crack growth rate in Room Temperature and Out-of-Phase Thermo-Mechanical Fatigue notched specimen experiments on the nickel-base alloy IN792 is studied. It is shown that it is possible to explain the observed higher crack growth rate in OP TMF compared to RT testing for the same external load, if plasticity induced crack closure is considered. Modeling utilizes node-release finite element simulations with a temperature dependent yield stress, resulting in different yield stress in tension and compression. It is shown that a simple extension of the analytical Newman crack closure equation can describe the opening level in the performed experiments.

DevTMF – Towards code of practice for thermo-mechanical fatigue crack growth

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 °C.

On the mechanistic difference between in-phase and out-of-phase thermo-mechanical fatigue crack growth

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.

An analysis of thermo-mechanical fatigue crack growth in the titanium alloy Ti-6246

Titanium alloys are equipped with impressive high strength and low density, along with other notable mechanical properties. Often the choice for low to intermediate temperature mechanical applications, titanium alloys are well utilised within the aerospace industry, making up 40% of the aero-engine. Within the gas turbine engine, the high transient thermal stresses developed due to variations in power requirements during a typical flight cycle give rise to the phenomenon of thermo-mechanical fatigue (TMF). The lifing models utilised within this research focus on damage tolerance approaches. TMF crack growth test techniques have been developed for high performance titanium alloys, in which diverse phasing (φ) between mechanical loading and temperature have been investigated. In addition, factors affecting the TMF behaviour of Ti-6246, including peak temperature, minimum temperature and temperature range have also been explored.

Simulation of anisotropic crack growth behavior of nickel base alloys under thermomechanical fatigue

Allvac®718Plus is a nickel base superalloy possessing outstanding abilities to withstand both mechanical and thermal loads, which make this material ideal for turbine disks and blades in aero engines. The thermomechanical fatigue (TMF) crack growth behavior depends on temperature, in-service load cycles and material orientation. In particular, the crack growth at high temperature dwell time shows significant influence of material anisotropy due to platelet-like precipitations, which leads to crack deflection from the maximum principal stress direction. These specific properties must be taken into account properly to ensure a safe design and the desired lifetime of components. The focus of this paper is to develop an appropriate fracture criterion to describe crack deflection for such kind of anisotropic material resistance. This criterion and the associated crack growth laws for thermomechanical fatigue are implemented in the FEM software Procrack for fully automated three-dimensional crack propagation simulation. The numerical simulation of experiments on CT-specimens with various material orientations provided evidence that the software can capture the actual fracture behavior. Moreover, a suitable set of material parameters could be identified. The performance of the presented approach to predict realistic three-dimensional TMF crack propagation is demonstrated by an application to a typical turbine component made from Allvac® 718Plus.

Toward a Better Understanding of Crack Growth in Nickel-Cast Alloys Under Creep-Fatigue and Thermo-Mechanical Fatigue Conditions

Abstract To ensure the feasibility of gas turbines, despite rising commodity prices and emission restrictions, an enhancement of both their efficiency and flexibility is necessary. The consequential higher loading of components at high temperature conditions calls for an increased use of damage tolerant design approaches. To still guarantee a safe operation, a sound understanding and reliable estimations for crack growth under service conditions are indispensable. In this paper, the results from several projects in this field conducted at the TU Darmstadt and involved partners are summarized to identify and describe the various influences on crack growth under creep-fatigue and thermo-mechanical fatigue (TMF) loading. The activation of damage mechanisms under TMF loading and interactions between them is dependent on the temperature cycle and the respective load phasing. Depending on the type of loading (force versus strain control), contrary influences of the phase shift on the TMF crack growth rates are found. This can partially be attributed to the differences in mean stress evolution. Crack initiation and propagation under creep-fatigue and TMF conditions are also often connected with significant scattering of initiation sites and crack growth rates. One reason for this nonuniform behavior is the interaction of geometric discontinuities with the microstructure. To investigate the role of the local grain structure for crack initiation and propagation, in situ observation techniques for crack tip movement and local strain fields were applied. Harsh gradients in the local deformation behavior were identified as origins of secondary crack initiation. To describe crack growth under creep-fatigue and TMF conditions, the linear accumulation model “O.C.F.” was developed. It is based on the contributions of fatigue, creep, and oxidation to crack growth per load cycle. This model is capable of reproducing the effects of time-dependent damage, different load ratios, TMF phase shifts, as well as component geometry. Substantial advantages of this method are its independence from empiric correction factors to assess changing load cycle forms and the possibility to give analytic estimations without the need of extensive data processing. The model is currently validated for three nickel cast alloys, also including single-crystalline and directionally solidified cast variants, different creep-fatigue and TMF loading scenarios, and crack geometries. The model's linear formulation allows assessing the dominant driver of crack growth at each stage of an experiment. These predictions are compared with fractographic investigations and in situ observations of crack paths to identify the mechanisms of crack growth under different TMF load cycle forms.

Thermo-Mechanical Fatigue Lifetime Assessment of Spheroidal Cast Iron at Different Thermal Constraint Levels

In previous work on the thermo-mechanical fatigue (TMF) of compacted graphite iron (CGI), lifetimes measured under total constraint were confirmed analytically by numerical integration of Paris’ crack-growth law. In current work, the results for CGI are further validated for spheroidal cast iron (SGI), while TMF tests at different constraint levels were additionally performed. The Paris crack-growth law is found to require a different CParis parameter value per distinct constraint level, indicating that Paris’ law does not capture all physical backgrounds of TMF crack growth, such as the effect of constraint level. An adapted version of Paris’ law is developed, designated as the local strain model. The new model considers cyclic plastic strains at the crack tip to control crack growth and is found to predict TMF lifetimes of SGI very well for all constraint levels with a single set of parameters. This includes not only full constraint but also over and partial constraint conditions, as encountered in diesel engine service conditions. The local strain model considers the crack tip to experience a distinct sharpening and blunting stage during each TMF cycle, with separate contributions to crack-tip plasticity, originating from cyclic bulk stresses in the sharpening stage and cyclic plastic bulk strains in the blunting stage.

Thermomechanical fatigue crack growth in a single crystal nickel base superalloy

Thermomechanical fatigue crack growth in a single crystal nickel base superalloy was studied. Tests were performed on single edge notched specimens, using in phase and out of phase thermomechanical fatigue cycling with temperature ranges of 100–750 °C and 100–850 °C and hold times at maximum temperature ranging from 10 s to 6 h. Isothermal testing at 100 °C, 750 °C and 850 °C was also performed using the same test setup. A compliance-based method is proposed to experimentally evaluate the crack opening stress and thereby estimate the effective stress intensity factor range ΔKeff for both isothermal and nonisothermal conditions. For in phase thermomechanical fatigue, the crack growth rate is increased if a hold time is applied at the maximum temperature. By using the compliance-based crack opening evaluation, this increase in crack growth rate was explained by an increase in the effective stress intensity factor range which accelerated the cycle dependent crack growth. No significant difference in crack growth rate vs ΔKeff was observed between in phase thermomechanical fatigue tests and isothermal tests at the maximum temperature. For out of phase thermomechanical fatigue, the crack growth rate was insensitive to the maximum temperature and also to the length of hold time at maximum temperature. The crack growth rate vs ΔKeff during out of phase thermomechanical fatigue was significantly higher than during isothermal fatigue at the minimum temperature, even though the advancement of the crack presumably occurs at the same temperature. Dissolution of γ′ precipitates and recrystallization at the crack tip during out of phase thermomechanical fatigue is suggested as a likely explanation for this difference in crack growth rate.

Prediction of texture evolution and crack growth rate of nickel based superalloys in thermo-mechanical fatigue by ultrasonic technique

In this research, texture evolution and crack growth rates were investigated by ultrasonic technique in Hastelloy X during thermo mechanical fatigue (TMF) and also in Inconel 718 during isothermal fatigue. For this purpose, TMF tests were performed at various modes, i.e. constant temperature, in-phase (IP) and out of phase (OP) in addition to application of linear elastic fracture mechanics (LEFM) and elastic-plastic fracture mechanics (EPFM). The existing ultrasonic model for predicting the texture evolution was modified and a model on the base of the relationships between ultrasonic wave velocity, elastic constants and texture parameters was developed. For predicting the TMF crack growth rate, an integrated ultrasonic model was proposed on the base of dislocation density and cyclic J-integral for ultrasonic wave velocity. The validation tests for texture evolution and TMF crack growth rates show a good correlation between experimental results and those predicted by ultrasonic method. For predicting the initiation and growth of unstable cracks up to failure of the specimen during TMF, a new parameter in the form of critical ultrasonic wave velocity has also been introduced in this research.

Estimation of fatigue crack growth under complex loading using an accumulative approach

A model procedure to obtain estimates for thermo-mechanical fatigue crack growth rates based on isothermal test results is presented. This ‘O.C.F.-model’ consists of a linear accumulation of temperature independent fatigue crack growth and two temperature dependent crack growth parts. Temperature dependent crack growth is constituted from creep crack and oxidation damage. For the oxidative damage part, the formation of a γ′-free zone at the crack tip and sub-sequent softening is considered to be determining. The possibility to assess creep crack growth in alloys with very limited creep ductility using linear-elastic fracture mechanics assumptions is investigated and compared to visco-plastic approaches. The isothermal data presented in this paper allows for an estimation of crack growth under both isothermal and anisothermal conditions with and without dwell times. Furthermore it allows assessing the fractions of crack growth caused by the different sources.