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Development of a pattern making method for strain measurement on microstructural level in ferritic cast 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 Materials and manufacturing – Casting.ORCID iD: 0000-0002-0101-0062
Jönköping University, School of Engineering, JTH. Research area Materials and manufacturing – Casting.
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.

Place, publisher, year, edition, pages
2014.
Keyword [en]
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: urn:nbn:se:hj:diva-28326OAI: oai:DiVA.org:hj-28326DiVA: diva2:871745
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
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
2. On the deformation behavior and cracking of ductile iron; effect of microstructure
Open this publication in new window or tab >>On the deformation behavior and cracking of ductile iron; effect of microstructure
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the effect of microstructural variation on the mechanical properties and deformation behavior of ductile iron. To research and determine these effects, two grades of ductile iron, (i) GJS-500-7 and (ii) high silicon GJS-500-14, were cast in a geometry containing several plates with different section thicknesses in order to produce microstructural variation. Microstructural investigations as well as tensile and hardness tests were performed on the casting plates. The results revealed higher ferrite fraction, graphite particle count, and yield strength in the high silicon GJS-500-14 grade compared to the GJS-500-7 grade.

To study the relationship between the microstructural variation and tensile behavior on macroscale, tensile stress-strain response was characterized using the Ludwigson equation. The obtained tensile properties were modeled, based on the microstructural characteristics, using multiple linear regression and analysis of variance (ANOVA). The models showed that silicon content, graphite particle count, ferrite fraction, and fraction of porosity are the major contributing factors that influence tensile behavior. The models were entered into a casting process simulation software, and the simulated microstructure and tensile properties were validated using the experimental data. This enabled the opportunity to predict tensile properties of cast components with similar microstructural characteristics.

To investigate deformation behavior on micro-scale, a method was developed to quantitatively measure strain in the microstructure, utilizing the digital image correlation (DIC) technique together with in-situ tensile testing. In this method, a pit-etching procedure was developed to generate a random speckle pattern, enabling DIC strain measurement to be conducted in the matrix and the area between the graphite particles. The method was validated by benchmarking the measured yield strength with the material’s standard yield strength.

The microstructural deformation behavior under tensile loading was characterized. During elastic deformation, strain mapping revealed a heterogeneous strain distribution in the microstructure, as well as shear bands that formed between graphite particles. The crack was initiated at the stress ranges in which a kink occurred in the tensile curve, indicating the dissipation of energy during both plastic deformation and crack initiation. A large amount of strain localization was measured at the onset of the micro-cracks on the strain maps. The micro-cracks were initiated at local strain levels higher than 2%, suggesting a threshold level of strain required for micro-crack initiation.

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

In order to perform a micro-scale characterization of dynamic deformation behavior, local strain distribution on the microstructure was studied by performing in-situ cyclic tests using a scanning electron microscope (SEM). A novel method, based on the focused ion beam (FIB) milling, was developed to generate a speckle pattern on the microstructure of the ferritic ductile iron (GJS-500-14 grade) to enable quantitative DIC strain measurement to be performed. The results showed that the maximum strain concentration occurred in the vicinity of the micro-cracks, particularly ahead of the micro-crack tip.

Abstract [sv]

Denna avhandling fokuserar på effekten av variationer i mikrostrukturen på mekaniska egenskaper och deformationsbeteende hos segjärn. För att undersöka dessa effekter, två olika sorter av segjärn, (i) GJS-500-7 och (ii) högkisellegerad GJS-500-14, gjutits till plattor av olika tjocklekar för att generera mikrostrukturvariationen. Mikrostrukturundersökning, samt drag- och hårdhetsprov gjordes på de gjutna plattorna. Resultaten visade att en högre ferritfraktion, grafitpartikelantal och sträckgräns i den högkisellegerade GJS-500-14-sorten jämfört med GJS-500-7.

För att studera förhållandet mellan mikrostrukturell variation och spännings-töjningsbeteendet på makroskala, modellerades detta med hjälp av Ludwigson-ekvationen. De erhållna spännings-töjningsegenskaperna modellerades baserat på mikrostrukturell karaktäristika genom multipel linjärregression och variansanalys (ANOVA). Modellerna visade att kiselhalt, grafitpartikelantal, ferritfraktion och porfraktion var de viktigaste bidragande faktorerna. Modellerna implementerades i ett simuleringsprogram för gjutningsprocessen. Resultatet från simuleringen validerades med hjälp av experimentella data som inte ingick i underlaget för regressionsanalysen. Detta möjliggjorde att prediktera spännings-töjningsbeteendet och dess variation hos gjutna segjärns komponenter med liknande sammansättning och gjutna tjocklekar som användes i denna studie.

För att kunna undersöka deformationsbeteendet på mikroskala utvecklades en metod för kvantitativ mätning av töjning i mikrostrukturen, genom DIC-tekniken (digital image correlation) tillsammans med in-situ dragprovning. I denna metod utvecklades en grop-etsningsprocess för att generera ett slumpvis prickmönster, vilket möjliggjorde DIC-töjningsmätning i matrisen och i området mellan grafitpartiklarna med tillräcklig upplösning. Metoden validerades genom benchmarking av den uppmätta sträckgränsen mot materialets makroskopiska sträckgräns mätt med konventionell dragprovning.

Det mikrostrukturella deformationsbeteendet under dragbelastning karakteriserades. Under elastisk deformation avslöjade töjningsmönstret en heterogen töjningsfördelning i mikrostrukturen, och bildandet av skjuvband mellan grafitpartiklar. Sprickbildning initierades vid låg spänning och redan vid de spänningsnivåer som ligger vis ”knät” på dragprovningskurvan, vilket indikerar energidissipering genom både begynnande plastisk deformation och sprickbildning. Den lokala töjningen vis sprickinitiering skedde då den lokala töjningen översteg 2%, vilket indikerar att detta skulle kunna vara en tröskelnivå för den töjning som erfordras för initiering av mikro-sprickor.

En kontinuum Finita Element (FE) modell utvecklades för att prediktera töjningen hos ett segjärn och dess fördelning i segjärns mikrostruktur. Materialparametrarna för denna modell optimerades genom att anpassa parametrarna i Ramberg-Osgood ekvationen. De predikterade töjningsfördelningarna jämfördes med de experimentell uppmätta töjningsmönstren uppmätta med DIC, både kvalitativt och kvantitativt. Töjningsmönstren överensstämde i stor utsträckning, vilket resulterade i att modellerna kunde anses vara validerade på mikronivå.

För att kunna mäta töjningsmönster under dynamiska förlopp på mikronivå utvecklades en metod för att skapa prickmönster och att utföra in-situ CT provning i ett svepeletronmikroskop (SEM). Prickmönstret skapades genom avverkning med en fokuserad jonstråle (FIB), och provades på det ferritiska segjärnet (GJS-500-14 grad). Resultaten visade att maximal töjningskoncentration fanns i närheten av mikrosprickorna, framförallt framför sprickspetsen.

Place, publisher, year, edition, pages
Jönköping: Jönköping University, School of Engineering, 2017. 75 p.
Series
JTH Dissertation Series, 27
Keyword
Spherical graphite iron, component casting, high silicon ductile iron, digital image correlation (DIC), in-situ tensile testing, in-situ cyclic testing, DIC pattern generation, pit etching, micro-scale deformation, micro-crack, finite element analysis (FEA), focused ion beam (FIB) milling, segjärn, komponentgjutning, högkisellegerat segjärn, digital image correlation (DIC), insitu dragprovning, in-situ cyklisk provning, DIC-mönstergenerering, grop-etsning, mikroskalig deformation, mikrosprickor, finite element analys (FEA), fokuserad jonstråle (FIB) avverkning
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:hj:diva-36852 (URN)978-91-87289-28-6 (ISBN)
Public defence
2017-08-08, E1405 (Gjuterisalen), Tekniska Högskolan, Jönköping, 10:00
Opponent
Supervisors
Available from: 2017-08-14 Created: 2017-08-14 Last updated: 2017-08-29Bibliographically approved

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