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Microstructural strain localization and crack evolution in ductile iron
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-5635-8023
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
Jönköping University, School of Engineering, JTH. Research area Product Development - Simulation and Optimization.
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.ORCID iD: 0000-0002-0101-0062
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

Place, publisher, year, edition, pages
2015.
Series
JTH research report, ISSN 1404-0018
Keyword [en]
In-situ tensile test, digital image correlation, FE-Model, cohesive elements, micro-crack, ductile iron
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:hj:diva-27859OAI: oai:DiVA.org:hj-27859DiVA: diva2:851438
Funder
Knowledge Foundation
Available from: 2015-09-04 Created: 2015-09-04 Last updated: 2017-04-24Bibliographically approved
In thesis
1. Microstructure and deformation behaviour of ductile iron under tensile loading
Open this publication in new window or tab >>Microstructure and deformation behaviour of ductile iron under tensile loading
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
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:nbn:se:hj:diva-28335 (URN)978-91-87289-10-1 (ISBN)
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

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