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Fatigue of Heavy-Vehicle Engine Materials: Damage Mechanisms, Laboratory Experiments and Life Estimation
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.  

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. , p. 49
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1894
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-145176DOI: 10.3384/diss.diva-145176ISBN: 9789176853900 (print)OAI: oai:DiVA.org:liu-145176DiVA, id: diva2:1182415
Public defence
2018-03-16, ACAS, Hus A, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Funder
VINNOVASwedish Foundation for Strategic Research Available from: 2018-02-13 Created: 2018-02-13 Last updated: 2018-02-13Bibliographically approved
List of papers
1. Damage evolution in compacted graphite iron during thermomechanical fatigue testing
Open this publication in new window or tab >>Damage evolution in compacted graphite iron during thermomechanical fatigue testing
2016 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, E-ISSN 1743-1336, Vol. 29, no 1-2, p. 26-33Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Taylor & Francis, 2016
Keywords
Thermomechanical fatigue, Out-of-phase loading, Compacted graphite iron, Microcracking
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-121028 (URN)10.1179/1743133615Y.0000000019 (DOI)000377468800005 ()
Funder
VINNOVA, FFI-2012-03625Swedish Foundation for Strategic Research , SM12-0014
Note

Funding agencies: Scania CV AB; Strategic Faculty Grant AFM (SFO-MAT-LiU) at Linkoping University [2009-00971]

Available from: 2015-09-03 Created: 2015-09-03 Last updated: 2018-02-13Bibliographically approved
2. Thermo-mechanical and superimposed high-cycle fatigue interactions in compacted graphite iron
Open this publication in new window or tab >>Thermo-mechanical and superimposed high-cycle fatigue interactions in compacted graphite iron
2015 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 80, p. 381-390Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Elsevier, 2015
Keywords
Cast iron, Thermo-mechanical fatigue, High-cycle fatigue, Fatigue crack growth, Life prediction
National Category
Other Engineering and Technologies Mechanical Engineering
Identifiers
urn:nbn:se:liu:diva-121029 (URN)10.1016/j.ijfatigue.2015.06.005 (DOI)000360596500040 ()
Available from: 2015-09-03 Created: 2015-09-03 Last updated: 2018-02-13Bibliographically approved
3. The effect of superimposed high-cycle fatigue on thermo-mechanical fatigue in cast iron
Open this publication in new window or tab >>The effect of superimposed high-cycle fatigue on thermo-mechanical fatigue in cast iron
2016 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 88, p. 121-131Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Cast iron, Thermo-mechanical fatigue, High-cycle fatigue, Fatigue crack growth, Life prediction
National Category
Mechanical Engineering Other Engineering and Technologies
Identifiers
urn:nbn:se:liu:diva-121030 (URN)10.1016/j.ijfatigue.2016.03.020 (DOI)000375817000013 ()
Note

Funding agencies: Scania CV AB; Swedish Governmental Agency for Innovation Systems [FFI - 2012 - 03625]; Swedish Foundation for Strategic Research [SM12 - 0014]; Linkoping University [2009 - 00971]

Vid tiden för disputation förelåg publikationen endast som manuskript

Available from: 2015-09-03 Created: 2015-09-03 Last updated: 2018-02-13Bibliographically approved
4. Damage Mechanisms in Silicon-Molybdenum Cast Irons Subjected to Thermo-mechanical Fatigue
Open this publication in new window or tab >>Damage Mechanisms in Silicon-Molybdenum Cast Irons Subjected to Thermo-mechanical Fatigue
2017 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 99, no 2, p. 258-265Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Cast iron, Thermo-mechanical fatigue, High-cycle fatigue, Enviromental assisted fatigue, Fatigue crack growth
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-137287 (URN)10.1016/j.ijfatigue.2017.01.014 (DOI)000400718300007 ()2-s2.0-85017123996 (Scopus ID)
Note

Funding agencies: Scania CV AB; Swedish Governmental Agency for Innovation Systems [FFI-2012-03625]; Swedish Foundation for Strategic Research [SM12-0014]; Strategic Faculty Grant AFM (SFO-MAT-LiU) at Linkoping University [2009-00971]

Available from: 2017-05-10 Created: 2017-05-10 Last updated: 2018-02-13Bibliographically approved
5. The Transition from Micro- to Macrocrack Growth in Compacted Graphite Iron Subjected to Thermo-Mechanical Fatigue
Open this publication in new window or tab >>The Transition from Micro- to Macrocrack Growth in Compacted Graphite Iron Subjected to Thermo-Mechanical Fatigue
2017 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, no 186, p. 268-282Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Cast iron, notches, delta J, crack tip opening displacement
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-143691 (URN)10.1016/j.engfracmech.2017.10.017 (DOI)000418056700017 ()
Available from: 2017-12-14 Created: 2017-12-14 Last updated: 2018-05-04

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