Please citewedge spl

ARTICLE IN PRESS+ModelJECS-9809; No. of Pages 7Available online at www.sciencedirect.com

ScienceDirect

Journal of the European Ceramic Society xxx (2014) xxxxxx

Investigation of the impact of micro-cracks onproducts using wedge splitting test and d

Y. Belrhiti a,, O. Pop b, A. Germaneau c, P. DH. Harmuth d, M. Huger a,, T. Ch

a Sc 7315oges,

Curiunner

lle Amber

Abstract

For refracto ove tindustry. Their microstructure (micro-cracked) induces a non-linear mechanical behavior.

The present paper aims at applying digital image correlation (DIC) during wedge splitting test on magnesia spinel sample and comparing it topure magnesia sample so as to study the influence of spinel addition on fracture behavior. From strain measurements based on DIC, the initiationand the propagation of the macro-crack are investigated and the development of micro-cracks network in the case of magnesia spinel which isbehind crackThe experim 2014 Else

Keywords: Fr

1. Introdu

In refrashocks is oof the refracharacteriof specificmaterials. magnesia rthermo-meature. Due

Correspon Correspon

de Surface (SCedex, Franc

E-mail admarc.huger@

http://dx.doi.o0955-2219/ this article in press as: Belrhiti Y, et al. Investigation of the impact of micro-cracks on fracture behavior of magnesia products usingitting test and digital image correlation. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.09.001

branching phenomenon is analyzed. The evolution of the fracture energy required in crack propagation is studied for each material.ental results highlight also their mechanical properties dependent on their microstructure.vier Ltd. All rights reserved.

acture; MgO; Spinels; Refractories; Digital image correlation

ction

ctory industry, the ability to sustain severe thermalf prime importance in order to increase the lengthctory material life and to avoid their failure. Thesestics are generally obtained by the development

microstructure patterns on well-chosen refractoryAmong the materials used in refractory industryefractories are to be good candidates due to theirchanical properties at room and elevated temper-

to their properties, pure magnesia and magnesia

ding authors. Tel.: +33 5 87 50 25 60.ding author at: Science des Procds Cramiques et Traitements

PCTS UMR CNRS 7315), CEC, 12 rue Atlantis, 87068 Limogese. Tel.: +33 5 87 50 25 60.dresses: younes.belrhiti@etu.unilim.fr (Y. Belrhiti),unilim.fr (M. Huger), thierry.chotard@unilim.fr (T. Chotard).

spinel materials showed a great interest for commercialsand researchers.1,2 Pure magnesia bricks are known by theirresistance to corrosion and their relatively low thermal shockresistance. Besides, the incorporation of spinel improves thethermal shock resistance and can extend to three times longerservice life of cement rotary kiln3 compared to conventionalmagnesia chrome bricks. Indeed, spinel addition, leads to a bet-ter adaptability of the material to severe solicitations generatedduring high and quick temperature variations. Magnesia spinelrefractories still preferable to magnesia chromite ones whichare not recommended because of their toxicity due to chromitecontain. These two types of refractory have nearly the sameproperties such as a high thermal shock resistance.

Even if their process of processing can affect the economy,but they still preferable to magnesia chromite refractories whichhave nearly the same properties such as their high resistanceagainst thermal shock damage, erosion and corrosion but are notrecommended due to the toxicity of waste refractories containingchrome.

rg/10.1016/j.jeurceramsoc.2014.09.001 2014 Elsevier Ltd. All rights reserved.ience des Procds Cramiques et Traitements de Surface (SPCTS UMR CNRSb Groupe dEtude des Matriaux Htrognes (GEMH), Universit de Lim

c Institut Pprime, CNRS, Universit de Poitiers, 11 Boulevard Marie et Pierred Montanuniversitt Leoben, Department of Ceramics, Peter-T

e Institut Universitaire Technologique du limousin, Dpartement GMP, 2 AReceived 6 December 2013; received in revised form 1 Septe

ry materials, a large strain behavior before failure is desired to impr fracture behavior of magnesiaigital image correlationoumalin c, J.C. Dupr c,otard a,e,

), CEC, 12 rue Atlantis, 87068 Limoges Cedex, France 12 rue Atlantis, 87068 Limoges Cedex, Francee, 86962 Futuroscope Chasseneuil Cedex, France-Strasse 5, A-8700 Leoben, Austriandr Maurois, 87065 Limoges Cedex, France

2014; accepted 2 September 2014

heir thermal shock resistance during their service life in

Please cite micro-cracks on fracture behavior of magnesia products usingwedge spl

ARTICLE IN PRESS+ModelJECS-9809; No. of Pages 72 Y. Belrhiti et al. / Journal of the European Ceramic Society xxx (2014) xxxxxx

The behavior of magnesia spinel is related to their microstruc-ture and especially to the difference between the thermalexpansion of magnesia and spinel which is behind the devel-opment of micro-cracks around the spinel grains and canprevent crack propagation generated by thermal shock.47Moreover, their microstructure induces a significant non-linearstressstrain mechanical behavior allowing a high strain torupture, high fracture energy and as a consequence improvestheir thermal shock resistance. It has been reported that micro-cracking caused by the addition of spinel reduces both strengthand stiffness, and affect the fracture behavior.68 Both theknowledge of the mechanisms initiating such behavior and thepossibility to characterize those in the best accurate way areessential.

From thing to undethe non-linals and com

The frasplitting teity to prodThis meantest evaluat

To assessurface, diThis full-fiplacement now widelyimental medigital imadeformatiophenomenoin which crbranchesnomenon o

magnesia s

2. Materia

2.1. Mater

Pure mthis study of industri01 mm; 1

Magnesiamatrix

MicrocracksSpinel inclusion

Fig. 1. Microstructure of magnesia-spinel material.

spinel (13 mm) in the case of MSp. Spinel grains replaced thesame content of magnesia aggregates having the same amount.

ir pPa anrma

nsity to gnes

) andO4 =crac

appeteriam eaectioper or.

edge

ore

requthodst.1

h ceactorshowa we

V righef the

ly thce thed, t

Table 1Granulometry les.

Pure MgO MgO-15% spinel

Granulometry

0 1513.5 13.534.0 19.024.0 24.028.5 28.5

Properties 2.98 2.9517.5 15.3 this article in press as: Belrhiti Y, et al. Investigation of the impact of

ese considerations, this study is a part of a work aim-rstand the relationship between microstructure andear mechanical behavior of magnesia-spinel materi-pared to pure magnesia materials.

cture characterization is conducted through wedgest (WST) results. This test is well known for its abil-uce reliable data on stable crack propagation.915s that material characterization quality depends onion.s to strain measurements based on DIC on material

gital image correlation technique has been chosen.elds practical and effective tool for quantitative dis-and strain measurement of a planar sample surface is

accepted and commonly used in the field of exper-chanics. This non-contact optical method comparesges of the sample surface obtained before and duringn.1626 It allows highlighting the crack branchingn. It is a complex process of heterogeneous materialsack propagation induces the coalescence of existing

that correspond to existing micro-cracks. Such phe-ccurs due to the particularity of the microstructure ofpinel materials.

ls and methods

ials

agnesia and magnesia 15%-spinel (MSp) used inwere processed from different grain distribution

al magnesia with low iron content (fines < 0.1 mm;3 mm; 35 mm) and additional sub-stoichiometric

The140 M

Infotry, de

Duethe masionsMgAl2micro-cracksthe ma

Frocross-s

this pasimila

2.2. W

A mused fthe meting teresearc

of refrAs

using force Fmuch hmode oto app

Sinimprov

and properties of MgO-15% spinel samples compared to pure magnesia samp

Spinel 13 mm (%) MgO 35 mm (%) MgO 13 mm (%) MgO 01 mm (%) MgO fines (

Please citewedge spl

ARTICLE IN PRESS+ModelJECS-9809; No. of Pages 7Y. Belrhiti et al. / Journal of the European Ceramic Society xxx (2014) xxxxxx 3

try (a

the deformthe microst

Then, fdisplacemeas the meato propagaenergies cotic energy spinel intemechanism

GF = 1A

where A ishorizontal face fractuthe displacload.

The horFV neglect

FH = F2 tanwhere is tFV reducesless elastic

AnotherWST is th

ess atrati.Fig. 2. Principle of wedge splitting test with samples geome

ation before cracking has to be higher by modifyingructure of the refractory.rom the experimental data in terms of load-

the strconcen

Eq. (3) this article in press as: Belrhiti Y, et al. Investigation of the impact of microitting test and digital image correlation. J Eur Ceram Soc (2014), http://dx.d

nt, the surface fracture energy GF (N m1) is definedn work per unit of projected fracture area requiredte a crack, and represented by the sum of distinctnsumed during the crack propagation process (elas-and dissipated one) due to the crack bridging, therlocking, shear in zigzag fracture planes and others. GF is calculated using Eq. (1).max

0

FH dH (1)

the area of the crack surface projection, FH is theforce and H is the horizontal deformation. This sur-re energy is calculated for values from zero up toement max corresponding to 10% of the maximum

izontal force FH is calculated from the vertical forceing any friction effects using the Eq. (2).

V(/2) (2)

he wedge angle. The fact that FH is much higher than the load applied on the machines frame and storing

energy in it and helping stable crack propagation. mechanical property of material estimated from thee nominal notch tensile strength NT. It represents

NT = 6

Where bis the distaFig. 2a.

In additi(1600 19the specklethanks to ding crack garound the

2.3. Digita

The damcrack advaobtained bthe analysiing a mechmeasuring the image cimage recowhich the pixel of theat the surfa) and image acquisition setup (b).

t the point of beginning of rupture when the stresson at the notch tip is neglected, is calculated by the-cracks on fracture behavior of magnesia products usingoi.org/10.1016/j.jeurceramsoc.2014.09.001

y FH,maxb h2 +

FH,max

b h (3)

(mm), h (mm) are samples dimensions and y (mm)nce between FH,max and the middle of h as shown in

on to the experimental setup, a 8-bits CMOS camera00 pixels2) used to record images is placed in front ofd surface in order to perform optical measurementsigital image correlation (Fig. 2b). This allows study-rowth during mechanical tests and the strain statecrack tip. The acquisition frequency was 1 image/s.

l image correlation technique

age level neighborhood to the crack tip and thencement are estimated from the strain cartographiesy DIC. This full fields optical method is based ons of successive digital images of a same sample dur-anical test. The displacement fields are obtained bythe degree of similarity of series of subsets betweenorresponding to an unloaded state and the deformedrded during the test. The zone of interest (ZOI), oncalculation is done, is represented in Fig. 2a. Eachse images stores a gray level value due to a patternce. To avoid ambiguities in the similarity process a

Please cite micro-cracks on fracture behavior of magnesia products usingwedge spl

ARTICLE IN PRESS+ModelJECS-9809; No. of Pages 74 Y. Belrhiti et al. / Journal of the European Ceramic Society xxx (2014) xxxxxx

random distribution of gray levels can be used called specklepattern. This pattern can be the natural texture of the speci-men surfacpaints. As aopaque paidried. Afteusing spray

The useon specificFrom theseciple of thisfor the calcstudy.29

3. Results

3.1. Mecha

Fig. 4 displacemematerials o

Horizonnon-linear post-peak rthe increasimum loadmechanicalow mechathis can be case of MSanisms occwake regiogation. As in the casethe thermalleads to the

In additthe peak, ain compari

In the twcharacterizbehavior, acoupled wthis phase and the las

Only thearly apparmicro-cracvisible. Thfields werethe presenc

Thanks strains aloncorrespondcorresponding to DIC32 32 pix

Table 2Nominal notched tensile strength and surface fracture energy values.

O

%

m/presvaluescale

ordgray

colo thansure

the ttake

reac

ne inone gnd thsia icenc

inah sa

repse fchan

ses wrameal is,. 4a a

the der

splim of

thap tha

MS the

rack

unded alsatio

growrocess, featuring many different physical mechanisms. Inmaterials, the damage is quite absent or confined in a

area. To underline its presence, different maps of strains and Y axis were obtained using digital image correlationrent loads (Figs. 5 and 6).ike pure magnesia, in addition to X-axis strains whichhe same direction as the crack opening, we notice the this article in press as: Belrhiti Y, et al. Investigation of the impact of

e or artificially made by spraying black and/or whiten example for samples surface preparation, a blacknt layer is deposited on the surface of sample thenr that, white speckles will be carefully projected on

paint.d DIC process consists in calculating displacements

points of several subsets which constitute a grid. displacements, strains will be calculated. The prin-

technique is represented in Fig. 3 and equations usedulation of displacements are detailed in the previous

and discussion

nical properties

represents the characteristic horizontal load vs.nt curves of pure magnesia and magnesia spinelbtained by wedge splitting test.tal load vs. displacement curve of MSp presents amechanical behavior up to the peak with a significantegion and residual strain when unloading. Moreover,e of spinel content seems mainly to lower the max-

and to enhance the pre-peak non-linearity of thel behavior. This material has a higher strain and anical resistance in comparison with pure magnesia;explained by the higher volume of micro-cracks in thep developed during cooling and toughening mech-urring around the crack, especially in the followingn, which may increase the resistance to crack propa-explained before, the higher volume of micro-cracks

of magnesia spinel is due to the difference between expansion coefficient of magnesia and spinel which

apparition of micro-cracks around the grain of spinel.ion to that, pure magnesia affords a lower strain atnd a thinner peak due to a smaller post-peak regionson with MSp.o cases, the mechanical behavior of these materials ised by three phases. One corresponding to the elastic

second one characterized by the elastic behaviorith damage increased by the addition of spinel andis usually referred as localization of macro-crack,t one corresponding to the crack growth.e main macro-crack can be easily observed, but theition of the macro-crack, as well as all the stressedks pre-existing around the middle plane, is not reallyat is why; strain fields calculated from displacement

chosen so as to analyze the crack propagation ande of damage process.to digital image correlation, the evolution of theg X-axis has been represented for different instantsing to different loading states (Fig. 4). These maps

to the zone of interest ZOI defined in Fig. 2a. Accord- principle, this area is subdivided using subsets ofels2 with a gap of 8 8 pixels2 and a scale factor of

Pure Mg

MgO-15spinel

0.067 mCrack strain color first in(from (blackhigherof mea

Forcrack beforeage zoThis zload amagnecoales

Nomfor eac(3) and

Thethe meincreais a pamateri

Figduringarea un

wedgethe suconfirmof MScase ofduring

3.2. C

To focusepropagcrack plex pbrittle small along Xat diffe

Unlhave titting test and digital image correlation. J Eur Ceram Soc (2014), http://dx.NT (MPa) GF (N m1) GF/NT (mm) 103

14.36 206.50 14.3914.38 182.00 12.66

6.65 361.00 54.256.75 396.00 58.63

pixels. Four maps were represented in Fig. 4a and b.ence and its opening correspond to the local highers (Fig. 4). To underline these latter, a specific strain

has been chosen. The choice of colors was doneer to identify the evolution of local higher strains

to red) and to eliminate measurement disturbancer). A presence of a crack corresponds to a threshold

0.001 (gray color) taking into account the accuracyment.wo materials, the initiation and the propagation of theplace after reaching critical load (FH,max). Besides,hing this critical load, there is a development of dam-

the neighborhood of the crack tip in the case of MSp.ets larger and moves with the increase of the appliede propagation of the crack unlike the case of pure

n which there is a lack of damage associated to thee of the micro-cracks.l notch tensile strength and surface fracture energymple have been calculated thanks to the Eqs. (1) andresented in Table 2.racture parameters introduced in Table 2 show thatical resistance decreases and the fracture resistanceith the increase of the spinel rate. The ratio GF/NTter of the brittleness of the material: the more brittle

the lower value of the ratio is.nd b represent also the evolution of the total energytest. The total energies are deduced from the totalhorizontal load vs. displacement curves obtained bytting test divided by final crack surface and represent

the elastic energy and dissipated one. The curvest during the test, the total energy is higher in the casen pure magnesia. The high dissipated energy in thep is due to the dense network of micro-cracks createdcrack initiation and propagation.

branching phenomenon

rline the presence of damage process, this studyo on the strains along X and Y axis created by the

n of the crack during wedge splitting test. Indeed,th in heterogeneous materials is often a very com-doi.org/10.1016/j.jeurceramsoc.2014.09.001

Please citewedge spl

ARTICLE IN PRESS+ModelJECS-9809; No. of Pages 7Y. Belrhiti et al. / Journal of the European Ceramic Society xxx (2014) xxxxxx 5

Fig. 3. Principle of digital image correlation method.

presence of strains along Y-axis perpendicular to the crack open-ing direction. However, their intensity stills lower than X-axisstrains. Y-axis strains are related to the creation of micro-cracksaround grains of spinel during mechanical test. This is due tothe difference of the expansion coefficient of MgO and spinel.

In the case of MSp, thanks to xx strains obtained by digitalimage correlation results, we notice the complexity of crackgrowth in hmechanism

many interacting micro-cracks or micro-branches (coalescenceof micro-cracks). Besides, the branches evolve from the micro-cracks which are initially parallel to the main crack, but deviatesmoothly from the original crack orientation.

As perspectives, it is worth mentioning that experimentalresults obtained by digital image correlation can be coupledwith numerical results by finite element simulation in order

e ac

ram

eterogeneous materials which feature many physicals such as crack branching phenomenon. It results of

to havture pa this article in press as: Belrhiti Y, et al. Investigation of the impact of microitting test and digital image correlation. J Eur Ceram Soc (2014), http://dx.d

Fig. 4. Horizontal load vs. displacement and energy curves of pure magnesia (a) andcess to nonlinear stressstrain laws. Besides, frac-eters could be determined also using digital image-cracks on fracture behavior of magnesia products usingoi.org/10.1016/j.jeurceramsoc.2014.09.001

magnesia-15% spinel (b) obtained thanks to WST.

Please cite this article in press as: Belrhiti Y, et al. Investigation of the impact of micro-cracks on fracture behavior of magnesia products usingwedge splitting test and digital image correlation. J Eur Ceram Soc (2014), http://dx.doi.org/10.1016/j.jeurceramsoc.2014.09.001

ARTICLE IN PRESS+ModelJECS-9809; No. of Pages 76 Y. Belrhiti et al. / Journal of the European Ceramic Society xxx (2014) xxxxxx

Fig. 5. Maps of longitudinal strains along X axis (a) and Y axis (b) for pure magnesia sample.

Fig. 6. Maps of longitudinal strains along X axis (a) and Y axis (b) for magnesia spinel sample.

Please cite microwedge spl

ARTICLE IN PRESS+ModelJECS-9809; No. of Pages 7Y. Belrhiti et al. / Journal of the European Ceramic Society xxx (2014) xxxxxx 7

correlation using energetic methods such as the integral invariantmethod.

4. Conclusion

This study was dedicated for a better understanding of therelationshipmicrostrucinvestigateference beis due to tthe case ofthe thermagrains. To influence ohas been asIndeed, by inal notch the measurof spinel ition of spinnon-linearihigh level oto digital iallows an is considerresistance.

Acknowled

This wothrough c(Austria) anetwork. Tmaterials.

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magnesia-spinel materials. This is a consequence ofl expansion mismatch between magnesia and spinelstudy the mechanisms of micro-cracking and thef damage on the non-linearity, wedge splitting testsociated (instrumented) to digital image correlation.the calculation of mechanical properties such as nom-tensile strength and fracture energy in addition toement of strain fields, the influence of the additions clarified. In fact, it was confirmed that the addi-el influences clearly mechanical properties and the

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Investigation of the impact of micro-cracks on fracture behavior of magnesia products using wedge splitting test and digit...1 Introduction2 Materials and methods2.1 Materials2.2 Wedge splitting test2.3 Digital image correlation technique

3 Results and discussion3.1 Mechanical properties3.2 Crack branching phenomenon

4 ConclusionAcknowledgementsReferences