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Microstructure and deformation behaviour of ductile iron under tensile loading
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-5635-8023
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The current thesis focuses on the deformation behaviour and strain distribution in the microstructure of ductile iron during tensile loading. Utilizing Digital Image Correlation (DIC) and in-situ tensile test under optical microscope, a method was developed to measure high resolution strain in microstructural constitutes. In this method, a pit etching procedure was applied to generate a random speckle pattern for DIC measurement. The method was validated by benchmarking the measured properties with the material’s standard properties.

Using DIC, strain maps in the microstructure of the ductile iron were measured, which showed a high level of heterogeneity even during elastic deformation. The early micro-cracks were initiated around graphite particles, where the highest amount of local strain was detected. Local strain at the onset of the micro-cracks were measured. It was observed that the micro-cracks were initiated above a threshold strain level, but with a large variation in the overall strain.

A continuum Finite Element (FE) model containing a physical length scale was developed to predict strain on the microstructure of ductile iron. The materials parameters for this model were calculated by optimization, utilizing Ramberg-Osgood equation. For benchmarking, the predicted strain maps were compared to the strain maps measured by DIC, both qualitatively and quantitatively. The DIC and simulation strain maps conformed to a large extent resulting in the validation of the model in micro-scale level.

Furthermore, the results obtained from the in-situ tensile test were compared to a FE-model which compromised cohesive elements to enable cracking. The stress-strain curve prediction of the FE simulation showed a good agreement with the stress-strain curve that was measured from the experiment. The cohesive model was able to accurately capture the main trends of microscale deformation such as localized elastic and plastic deformation and micro-crack initiation and propagation.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering , 2015. , 50 p.
Series
JTH Dissertation Series, 9
Keyword [en]
Ductile iron, digital image correlation (DIC), in-situ tensile test, pit etching, Microscale deformation, micro-crack, finite elements analysis (FEA), cohesive elements
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-28335ISBN: 978-91-87289-10-1 (print)OAI: oai:DiVA.org:hj-28335DiVA: diva2:871909
Presentation
2015-10-09, E1405, Jönköping University, School of Engineering, Jönköping, 11:21 (English)
Opponent
Supervisors
Available from: 2015-11-17 Created: 2015-11-17 Last updated: 2017-04-21Bibliographically approved
List of papers
1. Development of a pattern making method for strain measurement on microstructural level in ferritic cast iron
Open this publication in new window or tab >>Development of a pattern making method for strain measurement on microstructural level in ferritic cast iron
2014 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The current paper focuses on development of a method for studying micro-scale strains on the microstructure of ferritic cast iron. For this purpose, in-situ tensile tests were done under the optical microscope combined with digital image correlation (DIC). Critical in this development was to be able to achieve a reliable high spatial resolution of strain around microstructural features, such as graphite particles. Measurement of local strain fi elds in cast iron materials have so far been relying on displacement of naturally occurring microstructure patterns such as graphite particles, which limits the spatial resolution of strain measurement. In order to increase the spatial resolution of the measured strain, a pit etching procedure was applied to generate a random speckle pattern on the ferritic matrix. Th e critical challenges of in-situ investigation of microstructural deformation were identifi ed as speckle pattern quality and accurate selection of subset size and strain window size. Th e traceability of this method was studied by benchmarking the measured elastic modulus with that obtained from full-scale tensile test. Th e elastic modulus calculated from average strains, measured by DIC, showed a good agreement with material’s elastic modulus. Th is validates the measured localized strain values and can be used as a validation for modeling of local deformation.

Keyword
Ferritic cast iron, Digital image correlation (DIC), In-situ tensile test, Pit etching, Subset size, Strain window size
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28326 (URN)
Conference
23rd International Conference on Processing and Fabrication of Advanced Materials (PFAM-XXIII), Roorkee, India, December 5-7, 2014
Available from: 2015-11-16 Created: 2015-11-16 Last updated: 2017-08-14Bibliographically approved
2. Micro-Crack Initiation in High-Silicon Cast Iron during Tension Loading
Open this publication in new window or tab >>Micro-Crack Initiation in High-Silicon Cast Iron during Tension Loading
2015 (English)In: TMS2015 Supplemental Proceedings, The Minerals, Metals, and Materials Society, 2015, John Wiley & Sons, 2015, 947-953 p.Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
John Wiley & Sons, 2015
Keyword
Micro-crack, local strain, ferritic cast iron, digital image correlation (DIC), in-situ tensile testing
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28327 (URN)10.1002/9781119093466.ch115 (DOI)9781119082415 (ISBN)
Conference
144th Annual Meeting and Exhibition: Connecting the Global Minerals, Metals, and Materials Community, TMS 2015, Walt Disney World Orlando, United States, 15 March 2015 through 19 March 2015
Available from: 2015-11-16 Created: 2015-11-16 Last updated: 2017-08-14Bibliographically approved
3. Microstructural strain distribution in ductile iron: Comparison between finite element simulation and digital image correlation measurements
Open this publication in new window or tab >>Microstructural strain distribution in ductile iron: Comparison between finite element simulation and digital image correlation measurements
2016 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 655, 27-35 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a study on microstructural deformation of a ferritic–pearlitic ductile iron, utilizing in-situ tensile testing, digital image correlation (DIC) and finite element analysis (FEA). For this purpose, the in-situ tensile test and DIC were used to measure local strain fields in the deformed microstructure. Furthermore, a continuum finite element (FE) model was used to predict the strain maps in the microstructure. Ferrite and pearlite parameters for the FE-model were optimized based on the Ramberg–Osgood relation. The DIC and simulation strain maps were compared qualitatively and quantitatively. Similar strain patterns containing shear bands in identical locations were observed in both strain maps. The average and localized strain values of the DIC and simulation conformed to a large extent. It was found that the Ramberg–Osgood model can be used to capture the main trends of strain localization. The discrepancies between the simulated and DIC results were explained based on the; (i) subsurface effect of the microstructure; (ii) differences in the strain spatial resolutions of the DIC and simulation and (iii) abrupt changes in strain prediction of the continuum FE-model in the interface of the phases due to the sudden changes in the elastic modulus.

Keyword
Microstructural deformation; Ductile iron; Digital image correlation (DIC); In-situ tensile test; Finite elements analysis (FEA)
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-28330 (URN)
Available from: 2015-11-17 Created: 2015-11-17 Last updated: 2017-12-01Bibliographically approved
4. Microstructural strain localization and crack evolution in ductile iron
Open this publication in new window or tab >>Microstructural strain localization and crack evolution in ductile iron
2015 (English)Report (Other academic)
Abstract [en]

This paper focuses on the deformation and crack evolution in ductile iron under tension, investigated by coupled in-situ tensile test and finite element simulation. Micro-crack initiation and development were tracked at the microstructure level. The local strain around micro-cracks were measured by using Digital Image Correlation (DIC). The results obtained from the experiments were compared to a finite element  model including cohesive elements to enable crack propagation. The resulting local strains were analyzed in connection to the observed micro-crack incidents in both DIC and simulation. The predictions of the finite element model showed good agreement with those obtained from the experiment, in the case of early decohesion, the amplitude of the strain localization and macroscopic stress-strain behavior. The results revealed that decohesion was commonly initiated early around graphite surrounded by ferrite which was identified as high strain regions. By increasing the global deformation, micro-cracks initiated in these areas and propagated but were arrested within the ferrite zone due to strain hardening and stress shielding of pearlite. Both the DIC and the simulation revealed that irregular shaped graphite were more susceptible to strain localization and micro-crack initiation. It could be observed that the cohesive model was able to capture the main trends of localized plastic deformation and crack initiation

Series
JTH research report, ISSN 1404-0018
Keyword
In-situ tensile test, digital image correlation, FE-Model, cohesive elements, micro-crack, ductile iron
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-27859 (URN)
Funder
Knowledge Foundation
Available from: 2015-09-04 Created: 2015-09-04 Last updated: 2017-04-24Bibliographically approved

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