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The Transition from Micro- to Macrocrack Growth in Compacted Graphite Iron Subjected to Thermo-Mechanical Fatigue
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering. Scania CV AB, Materials Technology, Södertälje, Sweden. (Scania, unit 003, Södertälje)
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-8304-0221
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. no 186, p. 268-282
Keywords [en]
Cast iron, notches, delta J, crack tip opening displacement
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-143691DOI: 10.1016/j.engfracmech.2017.10.017ISI: 000418056700017OAI: oai:DiVA.org:liu-143691DiVA, id: diva2:1166160
Available from: 2017-12-14 Created: 2017-12-14 Last updated: 2018-05-04
In thesis
1. Fatigue of Heavy-Vehicle Engine Materials: Damage Mechanisms, Laboratory Experiments and Life Estimation
Open this publication in new window or tab >>Fatigue of Heavy-Vehicle Engine Materials: Damage Mechanisms, Laboratory Experiments and Life Estimation
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:nbn:se:liu:diva-145176 (URN)10.3384/diss.diva-145176 (DOI)9789176853900 (ISBN)
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

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The full text will be freely available from 2019-10-21 13:00
Available from 2019-10-21 13:00

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