The deformation mechanisms of a single crystal nickel-base superalloy with and without Ru-doped have been investigated under out-of-phase thermomechanical fatigue. The Ru-doped alloy exhibits a thermomechanical fatigue life more than twice as high compared to the Ru-free alloy and a difference in thermomechanical fatigue behavior is also displayed. Microstructure studies by scanning electron microscopy and transmission electron microscopy revealed that the deformation mechanism of the Ru-free alloy in the initial stage is the movement of dislocations in the γ matrix. In the later stage of the thermomechanical fatigue test, large amounts of twins are formed in the material, and a large number of stacking faults and dislocations are sheared into the γ' precipitates. By comparing with the Ru-free alloy, the Ru-doped alloy has a higher matrix strength due to the solid solution strengthening effect of Ru, and is also prone to different deformation mechanisms. For example, the stacking faults are formed in the initial thermomechanical fatigue cycles and remain in the matrix throughout the entire thermomechanical fatigue process. The formation of twins, on the other hand, is suppressed by Ru addition. Such effects are believed to extend the thermomechanical fatigue life effectively.

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.

A new accelerated creep assessment method to evaluate the creep performance of metals and alloys from high-temperature tensile tests, i.e. slow-strain-rate testing (SSRT), is proposed and evaluated. The method consists of decomposing the inelastic strain into a plastic and creep component by adopting general assumptions on the inelastic strain behaviour of materials, formulated using a state variable formalism and verified by tensile tests with intermediate dwell times at constant stress. Either, the plastic and creep strain components are considered non-interacting and additive, as observed in the stainless steel AISI 316L at 600 °C. Or, as in the case of the ductile cast iron EN-GJS-SiMo5-1 at 500 °C and the nickel-base superalloy Hastelloy X at 800 °C, the components are considered unified, meaning that the effect of inelastic straining is the same irrespective of whether it is caused through creep at constant stress or by plastic deformation due to an instantaneous stress increase. Based on these assumptions, the proposed method is used to assess the creep strain from SSRT in good agreement with conventional creep test results.

Strain-controlled out-of-phase thermo-mechanical fatigue tests at 100–500 °C and various strain ranges were conducted on five cast iron grades, including one lamellar, three compacted and one spheroidal graphite iron. Investigations of graphite morphology and matrix characteristics were performed to associate parameters, such as geometrical features of graphite inclusions and the matrix microhardness, to the thermo-mechanical fatigue performance of each grade. From this, thermo-mechanical fatigue life as a function of maximum stress at half-life, is found to decrease consistently with increasing average graphite inclusion length irrespective of the graphite content. In contrast, no evident correlation between the fatigue life and the matrix microhardness is observed.

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.

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 degrees C.

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.

The complicated constitutive behaviour of cast iron, involving a non-linear elastic regime, tension-compression stress asymmetry, varying elastic modulus and an inflection in the tension-to-compression hardening curve, is investigated using a micromechanical modelling approach. In this way, it is demonstrated that the abnormalities observed in the constitutive behaviour are qualitatively and quantitatively explained by the interaction behaviour between the matrix and graphite constituents. In initial tension, the absence of linearity is rationalised by the successive loss in load-carrying capacity of the graphite phase due to debonding, which in subsequent cycling, results in the opening and re-contact of the matrix-graphite interface. This effect is demonstrated to result in tension-compression asymmetry in stress and elastic modulus, as well as the inflection in tension-to-compression loading. The given model of explanation is validated by comparison to the experimentally acquired microscopic strain field in EN-GJV-400 at locations where stress concentrations are generated due to the matrix-graphite debonding, using high-resolution digital image correlation of scanning electron images.

The development of fatigue life assessment models for vehicle components exposed to thermomechanical fatigue supports the establishing of adequate maintenance intervals that neither cause unnecessary vehicle downtime, nor jeopardize the function of the components. In modern automotive applications, braking is closely related to safety and is commonly performed with disc brakes. Failure here may result in structural damage or even breakdown and loss of lives. In the present work, the cyclic response of grey cast iron is analysed and the fatigue life of brake discs made from this material is studied by use of four different fatigue life assessment models: the Smith-Watson-Topper model, the Coffin-Manson model and two mechanism-based damage models. Results from isothermal and thermomechanical experiments on uniaxially loaded specimens are used for calibration of the models. Finally, the models are used to assess the life of a brake disc for a simulated brake dynamometer experiment. It is found that the fatigue model parameters that are calibrated using different sets of isothermal uniaxial test data show a substantial spread. A comparison with results from full-scale brake rig experiments shows that predictions by any of the models that have been calibrated using data from a well-designed thermomechanical test are in reasonable agreement with the estimated crack initiation phase for actual brake disc lives. It can be concluded that it is not sufficient to calibrate the studied fatigue life models using isothermal uniaxial tests for predictions of thermomechanical fatigue lives.

Due to increasing demands on sustainability exerted by end-costumers and policy makers, heavyvehicle manufacturers are urged to increase the engine efficiency in order to reduce the exhaust gas emission. However, increasing the efficiency is also associated with an elevated fatigue rate of the materials constituting the engine parts, which consequently reduces the engine service life. The aim of the present thesis is therefore to confront the expected increase by studying the fatigue behaviour and damage mechanisms of the materials typically employed in heavy-vehicle diesel engines. With this knowledge, this work seeks to guide the development of new heavy-vehicle engine materials, as well as to develop improved life estimation methods designated to assist the mechanical design of durable heavy-vehicle engines.

In essence, a large set of thermo-mechanical fatigue (TMF) and combined thermomechanical and high-cycle fatigue (TMF-HCF) tests is conducted at engine load conditions on laboratory specimens of lamellar, compacted and spheroidal graphite iron. In this way, the fatigue performance and associated damage mechanisms are investigated. In particular, a new fatigue property is identified, the TMF-HCF threshold, which quantifies how resistant a material is to superimposed high-cycle fatigue.

The damage mechanism at low temperatures (≲500°C) is confirmed to consist of the initiation, propagation and coalescence of numerous microcracks. Based on this, a successful fatigue life estimation model is formulated, allowing accurate estimations of TMF and TMF-HCF tests on smooth specimens, and TMF tests on notched specimens. In the latter case, the microcrack growth behaviour in non-uniform cyclic stress fields and its implications for life estimation are clarified. At elevated temperatures (≳500°C), surface oxidation is shown to govern the fatigue performance of cast iron grades intended for exhaust manifolds. It is observed that oxide intrusions are induced, from which surface fatigue cracks are initiated. Consequently, an optimal material at these conditions should have a low oxide growth rate and few casting defects at the surface, as these factors are found to stimulate the growth of intrusion.  

The damage mechanisms active in silicon-molybdenum cast irons, namely EN-GJS-SiMo5-1 and SiMo1000, under thermo-mechanical fatigue and combined thermo-mechanical and high-cycle fatigue conditions have been investigated. The studied load conditions are those experienced at critical locations in exhaust manifolds of heavy-vehicle diesel engines, namely a temperature cycle of 300–750 °C with varied total mechanical and high-cycle fatigue strain ranges. It is established that oxide intrusions are formed in the early life from which macroscopic fatigue cracks are initiated close to the end-of-life. However, when high-cycle fatigue loading is superimposed, small cracks are preferentially initiated at graphite nodules within the bulk. In addition, it is found that both the oxidation growth rate and casting defects located near the surface affect the intrusion growth.

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.

Thermomechanical fatigue properties of a compacted graphite iron in an out of phase configuration are investigated for different maximum temperatures and mechanical strain ranges. Furthermore, the stressï¿œstrain hysteresis loops are analysed, and, in particular, the unloading modulus, i.e. the elastic modulus measured during specimen unloading, is obtained from each cycle. This material parameter has earlier been explicitly related to the amount of microcracking in cast irons. The results show that the unloading modulus linearly declines with the numbers of cycles in all tests performed. In addition, the rate of change of the unloading modulus is closely related to the number of cycles to failure. Accordingly, it is concluded that microcracks are independently propagated by fatigue until a point of rapid crack linking resulting in ultimate failure. This is supported by microstructural analyses consisting of optical microscope images taken at different stages throughout the life of a specimen.

The eect of superimposing a high-cycle fatigue strain load on an out-ofphase thermo-mechanical fatigue test of a lamellar, compacted and spheroidal graphite iron, has been investigated. In particular, dierent total mechanical strain ranges, maximum temperatures and high-cycle fatigue strain ranges have been studied. From this, a new property has been identied, measured and compared, namely the thermo-mechanical and high-cycle fatigue threshold, dened as the high-cycle fatigue strain range at which the life is reduced to half. Using a model developed earlier, the lifetimes and the threshold have been successfully estimated for the lamellar and compacted graphite iron, however underestimated for the spheroidal graphite iron. Nevertheless, an expression of the threshold was deduced from the model, which possibly can estimate its value in other cast irons and its high-cycle fatigue frequency dependence.

The heavy-vehicle automotive industry is constantly subjected to higher demands. In particular, new European emission standards are formulated with the intention of improving the environmental friendliness of newly-produced vehicles through reduced exhaust emission. In one way or another, this implies a successive improvement of the engine efficiency, which in turn, inevitably will require a higher combustion pressure and temperature. This is a respectable challenge for future engine constructions, but also for the engineering materials used to embody them. As higher thermal and mechanical loads must be sustained, there is a higher rate of wear, and consequently, a negative effect on the extent of the engine lifetime.

The aim of the present thesis is to confront the expected increase in rate of wear, henceforth referred to as fatigue, by studying the effect on materials typically employed in heavy-vehicle engines, namely cast irons. Foremost, the intention has been to improve the understanding of the physical mechanisms of fatigue in these materials, in order to develop a lifetime estimation method designated to assist the mechanical design of heavy-vehicle engines.

In essence, a large set of thermo-mechanical fatigue (TMF) and combined thermo-mechanical and high-cycle fatigue (TMF-HCF) tests has been conducted at engine load conditions on laboratory specimens of lamellar, compacted and spheroidal graphite irons. In this way, these three different material groups have been experimentally compared and the associated fatigue mechanism has been studied. In particular, a new property related to TMF-HCF conditions has been identified and measured, . Regarding the fatigue mechanism, it has been affirmed to consist of the initiation, propagation and coalescence of numerous microcracks. Based on this, a successful lifetime assessment model was formulated, allowing good estimations of the fatigue life of laboratory specimens subjected to both TMF and TMF-HCF conditions.

The effect of adding a superimposed high-frequent strain load, denoted as a high-cycle fatigue strain component, upon a strain-controlled thermo-mechanical fatigue test has been studied on a compacted graphite iron EN-GJV-400 for different thermo-mechanical fatigue cycles and high-cycle fatigue strain ranges. It is demonstrated that the successive application of an high-cycle fatigue load has a consistent effect on the fatigue life, namely the existence of a constant high-cycle fatigue strain range threshold below which the fatigue life is unaffected but severely reduced when above. This effect on the fatigue life is predicted assuming that microstructurally small cracks are propagated and accelerated according to a Paris law incorporating an experimentally estimated crack opening level.