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Studying Elastic Deformation Behaviour of Cast Irons by Acoustic Emission
Högskolan i Jönköping, JTH, Maskinteknik.
Högskolan i Jönköping, JTH, Maskinteknik.
2005 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, Vol. 18, no 4, 249-256 p.Article in journal (Refereed) Published
Abstract [en]

The deformation of metallic materials includes both an elastic and a plastic deformation. In the case of cast irons, the elastic region becomes less pronounced as the graphite changes from spheroidal to flake shaped, as observed in nodular and grey cast iron, respectively. The present study aims to correlate the shape of the graphite phase with the deformation behaviour, where the plastic deformation and other strain accommodating events are quantified by measurements of the acoustic emission events occurring in the interior of the material at loading. It also aims to explain how the appearance of cast iron stress–strain curves depends on the graphite morphology where, for instance, spheroidal graphite cast irons exhibit a seemingly linear elastic behaviour in contrast to flake graphite cast irons. The present study includes a series of pearlitic cast iron material grades with differences in nodularity and carbon equivalent, respectively. It is shown that as the roundness of the graphite phase increases, the ability to absorb energy increases. The measured acoustic emission indicates that plastic deformation occurs in the seemingly linear elastic region regardless of the cast iron grade, i.e. no cast iron grade exhibits perfect linear elasticity. The plastic deformation rate in the elastic region increases as the roundness of the graphite decreases and as the carbon equivalent increases. It is shown that the plastic deformation governs the resulting modulus of elasticity in all kind of cast irons, i.e. the modulus of elasticity decreases as the yielding of the material increases. The present study improves the understanding of the deformation behaviour in the elastic region of different cast irons. The survey shows that acoustic emission testing is a useful method when studying the deformation behaviour of cast irons.

Place, publisher, year, edition, pages
2005. Vol. 18, no 4, 249-256 p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-14418DOI: 10.1179/136404605225023117OAI: oai:DiVA.org:liu-14418DiVA: diva2:23470
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2013-11-28
In thesis
1. Influences of the Graphite Phase on Elastic and Plastic Deformation Behaviour of Cast Irons
Open this publication in new window or tab >>Influences of the Graphite Phase on Elastic and Plastic Deformation Behaviour of Cast Irons
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The amount and morphology of the graphite phase largely controls the resulting properties of cast iron. For instance, in flake graphite cast irons the mechanical properties are low while the thermal conductivity is high. This is in contrast with spheroidal graphite cast irons where the mechanical properties are high and the thermal conductivity is low. These differences are due to the different graphite morphologies and must be accounted for in the design work and material selection of cast iron components. In this work the influence of the graphite phase on the elastic and plastic deformation behaviour of cast irons has been studied.

The material grades studied originate from castings for marine diesel engine piston rings with different chemical analyses. Two groups of pearlitic cast iron materials were studied; one with differences in graphite morphology and one with grey irons that differed in graphite content. For these different material grades the mechanical properties were correlated to microstructural parameters. In addition to standard uniaxial tensile tests, acoustic emission measurements were used for the study of deformation.

When studying the modulus of elasticity of the cast iron it was found that the modulus of elasticity of the inherent graphite phase depends on the roundness of the graphite particles and is due to the strong anisotropy of the graphite phase. A linear correlation between nodularity and the modulus of elasticity of the graphite phase was derived. This correlation made it possible to account for the anisotropy of the graphite phase in the model used. By applying the linear function when modelling the effective modulus of elasticity, a high accuracy between experimental and theoretical values was achieved.

Another factor affecting the elastic response when subjecting a cast iron component to tensile load was found to be the plastic deformation that actually occurs at very low strains for all of the studied cast iron grades. It was observed that the plastic deformation in the low strain elastic region, quantified by using acoustic emission measurements, increased linearly with decreasing modulus of elasticity. These measurements showed that the amount of plastic deformation in the elastic region was largely controlled by the graphite morphology. It was concluded that as the roundness of the graphite particles increases, the plastic deformation activity in the elastic region decreases.

The plastic deformation activity continued linearly into the pronounced plastic region of the tensile tests. A decrease in roundness or increase in graphite fraction resulted in an increase of the amount of plastic deformation and the strain hardening exponent. A

dependence between strength coefficient and graphite fraction was observed. Models for the flow curves for pearlitic cast irons were developed and shown to accurately reproduce the observed experimental curves.

The surveys performed and conclusions from this thesis will be helpful in the design of new cast iron materials.

Place, publisher, year, edition, pages
Institutionen för ekonomisk och industriell utveckling, 2007
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1080
Keyword
Cast iron, Flake graphite, Compacted graphite, Spheroidal graphite, Elastic deformation, Plastic deformation, Modulus of elasticity, Graphite modulus of elasticity, Strength coefficient, Strain hardening exponent, Acoustic emission
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-8776 (URN)978-91-85715-52-7 (ISBN)
Public defence
2007-05-25, sal E1405, Ingenjörshögskolan i Jönköping, Jönköping, 10:00 (English)
Opponent
Supervisors
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2009-03-13
2. The influence of graphite morphology on the elastic behaviour of cast irons
Open this publication in new window or tab >>The influence of graphite morphology on the elastic behaviour of cast irons
2005 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The morphology of the graphite phase largely controls the resulting properties of different grades of cast iron. In flake graphite cast irons, the mechanical properties are low while the thermal conductivity is high. In contrast, with spheroidal graphite cast irons, the mechanical properties are high while the thermal conductivity is low. These differences must be accounted for in the design work of diesel engines when choosing the material grade of different components. In this work the influence of graphite morphology on the deformation behaviour of cast irons has been studied with an emphasis on the elastic region.

The studied material grades originated from castings for marine diesel engine piston rings with different chemical analyses. This gave rise to six material grades with different nodularities and three material grades with different carbon equivalents. For these different material grades the mechanical properties were determined and the deformation behaviour was examined by means of mechanical testing and acoustic emission measurements. These data were correlated to microstructural parameters.

When studying the modulus of elasticity it was found that the modulus of elasticity of the inherent graphite phase depends on the roundness of the graphite particles and is due to the different elasticity in the two main crystallographic directions of the graphite lattice. The graphite particles are structured in different ways in flake graphite and spheroidal graphite, which gives rise to different stiffness at loading. The modelling and experiments support the observation that the average modulus of elasticity of the graphite phase depends on the morphology. This improves the understanding of the differences in the modulus of elasticity of the different studied material grades. The correlation between the modulus of elasticity of the graphite phase and the bulk nodularity of the cast iron specimen was found to be linear. This linear function was used when modelling the effective modulus of elasticity of the different cast iron grades with very high accuracy between experimental and theoretical values.

Another factor affecting the elastic response when subjecting a cast iron component to tensile load was found to be the plastic deformation that actually occurs at very low strains for all of the studied cast iron grades. lt was found that the plastic deformation in the low strain, elastic region governs the modulus of elasticity of cast irons. The plastic deformation in the seemingly elastic region was quantified by using acoustic emission measurements. These measurements showed that the amount of plastic deformation in the elastic region was largely controlled by the graphite morphology. It was concluded that, as the roundness of the graphite particles increases, the plastic activity in the elastic region decreases and the energy absorption ability increases.

Publisher
25 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1140
Keyword
Cast iron, flake graphite, compacted graphite, spheroidal graphite, elastic behaviour, modulus of elasticity, graphite modulus of elasticity, nodularity, aspect ratio, plastic deformation, acoustic emission
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30053 (URN)15513 (Local ID)91-8529-707-0 (ISBN)15513 (Archive number)15513 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-28

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Sjögren, Torsten

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