Received: 27 October 2016

| Accepted: 23 March 2017

DOI: 10.1111/ijag.12272

ORIGINAL ARTICLE

Direct observation of crack propagation in a liquid crystal
display glass substrate during wheel scribing
Naoko Tomei1,2| Kumiko Murakami1 | Toshio Fukunishi1 | Satoshi Yoshida2 |Jun Matsuoka2
1
Research & Development Department,
Mitsuboshi Diamond Industrial Co., Ltd.,
Settsu, Japan2

Department of Materials Science, The
University of Shiga Prefecture, Hikone,Japan
Correspondence
Naoko Tomei
Email: ntomei@mitsuboshi-dia.co.jp

Abstract
In-situ observation of crack propagation in an LCD glass substrate was performed
during wheel scribing. It was found that a median crack initiated just beneath thewheel and stopped once, and that the crack re-propagated after passing throughthe wheel. In this study, the first pop-in median crack is defined as the first crack,and the re-grown median crack is defined as the second crack. From in-situ photoelastic measurements during a static indentation of the wheel, the maximum tensile stress was observed just beneath the wheel. The tensile stress beneath thewheel was also confirmed from FE analysis. It is considered that this tensile stressis the origin of the first crack nucleation. On the other hand, from post-observation of the cross-section of a scribing groove after scribing, it was found that themedian crack was closed in the plastic zone. It is considered that this closure
results from compressive stress in the plastic zone beneath the scribing groove,and that this compressive stress causes the first crack to be arrested.
KEYWORDS
FE analysis, in-situ photoelastic measurements, median crack propagation, wheel scribing

1| INTRODUCTION

Recently, thickness of LCD (Liquid Crystal Display) glass
substrate becomes thinner and thinner year by year due tohigh demand for thinner and lighter laptop or mobile device.
Therefore, it is of primary importance for cutting such a thinsheet glass with high efficiency. It is also known that edge
quality of glass substrate after machining affects strength of
the glass. In order to supply glass substrates with reliable
strength, it is important to get a deeper insight into mechanism of machining or cutting of the glass sheet.
In general, “scribing+breaking” technique is employed
for cutting LCD glass panels, because it is a high-speed
dry process without kerf loss and with less thermal damage. In the scribing process, a cutting tool called a scribing

wheel is used. During the scribing process, the scribing
wheel creates a groove on the glass substrate, and a mediancrack initiates at the bottom of the groove. In the breaking
process, bending stress causes the median crack to propagate further, and the glass substrate is separated into two.
If geometries of the scribing wheel and scribing conditions
are not suitable for the scribing and breaking processes fora given glass sheet, undesirable cracks, such as a lateralcrack or chipping may be generated. Such fault cutting
results in unfavorable low strength of the glass.1,2 However, there are less information available on effects ofscribing conditions and/or wheel geometries on the edge
quality of glass substrate.
On the contrary, there are many studies on crack initiation and propagation in glass during quasi-static indentation---------------------------------------------------------------------------------------------------------------------------------------------------------------------This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium,provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
© 2017 The Authors. International Journal of Applied Glass Science published by American Ceramics Society (ACERS) and Wiley Periodicals, Inc.
Int J Appl Glass Sci. 2017;1–9.

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using a diamond indenter. It is known that some characteristic cracks, such as a ring crack, a radial/median crack, and alateral crack are generated during an indentation cycle.3,4
The type of crack observed depends both on glass composition and on indenter geometries. In scribing process, on theother hand, the most important and “favorable” crack is themedian crack, because the crack propagates in the directionof thickness as described above. It has been reported thatthe median crack during quasi-static indentation nucleates
on loading.5-7 Indentation induces tensile stress under theplastic zone, which is the plastically deformed region.8,9Under a load of indentation, the median crack stays inside
the glass due to the compressive stress at surface. During
unloading, on the other hand, the tensile stress at the bottomof the plastic zone changes into compressive, and the stressat surface becomes tensile. This is the reason that the median crack grows into the radial crack at surface.4,10 In addition, the CME (Chiang, Marshall, and Evans) model11 also
explains various cracking sequences during the indentation
cycle. For example, the CME model confirms that the stresspertinent to radial cracking is surface localized, and that thestress increases during unloading half cycle. The CME
model also shows that the maximum stress for mediancracking is less than that for radial cracking. It also supports
an importance of radial cracking during quasi-static indentation cycle. On the other hand, shear band formation within
the plastic zone beneath an indenter has been reported.12 It
was suggested that hardening effect was presented fromobservation of void or crack formation at the intersection
points or along the flow lines, and that the shear band is theorigin of the median crack initiation.
As compared with the indentation-induced cracking, it isstill unclear where and when the median crack during scribing initiates and how it propagates. There is one report where
surface stress and internal stress of PMMA (poly-methylmethacrylate) and photoelastic sensitive material, CR-39(Ethylene glycol bis(3-butenoate)) are successfully visualized during a static contact with an aluminum wheel.13
Another report by the same authors14 shows polarized
images of soda-lime glass under a static indentation of a steel
wheel. Unfortunately, however, the obtained stress field is
not quantitative due to limited resolution of the polarized
images. There are other reports which show the relationshipbetween the median crack depth and the geometries of thewheel.14-19 The median crack depth decreases with anincrease in the tip angle or in the wheel diameter. In addition,
the contact mechanics of sharp “glass-cutting” rollers has
been analyzed. Cook showed that the residual constrained
deformation of the wheel groove by rolling was the driving
force for median cracking.19 Cook estimated the lengths of
median cracks generated by rolling contacts, and confirmedthat they were in good agreement with observations.
Although the relation reported is of primary importance for

TOMEI ET AL.

high efficient cutting of a glass sheet, these papers15,16
focused only on the post observation of the crack accompanied with the scribing groove. Recently, micro-photoelastic
observations around the indentation imprint were reported
for strengthened glass20 and for silica and soda-lime
glasses21 after and/or during indentations. This micro-photoelastic technique has been applied to observation of a glass
substrate during wheel scribing.22 Although the authors22
suggest that the in-situ photoelastic measurement would be auseful tool for in-process management of the edge quality ofa glass substrate, the detail mechanism of crack propagation
during wheel scribing has not been clarified yet.
In this study, in-situ observation during wheel scribing
and post observation of the scribing groove were performed
in order to clarify the mechanism of crack propagation during wheel scribing. In addition, in-situ photoelastic measurement and FE analysis were conducted to estimate stress
distribution during wheel scribing.

2| EXPERIMENTAL PROCEDURE

Figure 1 shows a schematic illustration of a lab-madescribing equipment. A glass plate was placed on the sample
stage, which was stationary and equipped with a vacuum
chuck in order to fix the sample. As shown in Figure 1, a
scribing wheel can move in the direction of Y-axis fromleft to right during scribing. The scribing wheel(Figure 2A) with an axle was inserted into a wheel holder(Figure 2B), and the wheel holder was attached to a movable stage shown in Figure 1.
The scribing wheel and the axle were not locked to thewheel holder, thus both could rotate freely. The scribing
wheel used in this study had a diameter, D of 2.0 mm, aninside diameter, H of 0.8 mm, a thickness, T of 0.65 mm,
and a tip angle of the wheel, V of 130°, respectively. Also,
the scribing wheel was made of sintered Tungsten Carbide(WC) alloyed with 5.0 wt% of Co.

F I G U R E 1 Schematic illustration of a scribing equipment

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F I G U R E 2 Schematic illustrations of scribing components, (A) Wheel and (B) Wheel holder

The normal force during scribing was controlled by airpressure. In order to keep a constant normal force during
scribing, the wheel holder was allowed to move up and down
due to surface waviness of the glass sample. The scribing
speed was constant at 10 mm/s. The normal force during
scribing was monitored using a three-dimensional dynamometer (9119AA2; Kistler, Winterthur, Switzerland) which
was attached under the sample stage. The sampling rate of
the three force components was 2000 Hz. The scribing force,Fz, or the normal force during scribing, was determined as
the mean value of the forces in the direction of Z-axis during
the scribing of 50 mm. The scribing force was kept constantduring scribing, and increased stepwise from 3.13 N to
24.6 N. The standard deviation of the scribing force, Fz
varied from 3.46% for Fz=3.13 N to 0.52% for Fz=24.6 N.
The glass sample used in this study was non-alkaline(alkaline earth boro-aluminosilicate) glass (Eagle XG; Corning Inc., NY) with a thickness of 0.7 mm for an LCD glass
panel. The dimensions of the glass plate used were L (length)
50 mm9W (width) 100 mm9T (thickness) 0.7 mm for measurements of median crack depth.
Figure 3 shows a schematic illustration of in-situ observation system with a high speed camera (FASTCAM MH-4;
Photron, Tokyo, Japan). The high speed camera wasclamped on a tripod stand separated from the scribing equipment, therefore the in-situ observation images of crack propagation during scribing could be recorded at a fixed-point.
The observation was performed from the negative directionof X-axis with the transmitted light from the light source on
the opposite side of the camera. The optical pass length
through the glass sample was 20 mm. The shooting speedof high speed camera was 500 frames-per-second (fps).
In order to estimate the stress field of the glass sample
under a static wheel indentation, retardation, and fast axis
orientation were also measured with a polarization camera(FASTCAM MC2.1P; Photron). Figure 4 shows a schematic
illustration of in-situ observation system for the photoelastic
behavior. The photoelastic measurement was performed fromF I G U R E 3 Schematic illustration of in-situ observation systemwith a high speed camera

the direction of the wheel edge (from the negative directionof Y-axis). The dimensions of the glass plate used were L(length) 20 mm9W (width) 20 mm9T (thickness) 0.7 mm
for in-situ observation and photoelastic measurement. The
retardation and the fast axis orientation at each pixel were
determined from four kinds of polarization images obtainedat different analyzer angles (0, 45, 90, 135 degrees). The
incident light was the circularly polarized light with a wavelength of 520 nm. The shutter speed of a polarization camera
was 500 ls, which includes the closure time of the shutter.
Thus, the actually exposure time was 498 ls. The retardation
and azimuth in the photoelastic observation system were
determined per imaging pixel. Therefore, the spatial resolution corresponds to the pixel size which is 2.492.4 lm.
After scribing, a scribing groove and cracks wereobserved using an optical microscope (VHX-1000; KEYENCE, Osaka, Japan), and a laser microscope (VK-9700;
KEYENCE). Magnified images of the cross-sectional plane
perpendicular to the scribing groove were obtained using
an SEM (S-3400N; Hitachi High-Technologies, Tokyo,
Japan). The cross-sectional plane was prepared using a

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F I G U R E 4 Schematic illustration of in-situ observation systemfor photoelastic measurements

Focused Ion Beam (FIB E-3500; Hitachi High-Technologies).
To discuss validity of the result of photoelastic experiments, the stress field under a scribing wheel was calculated with Finite Element Analysis (FEA). Table 1 shows
the parameters used for the FEA (ANSYS, Canonsburg,
PA). Figures 5A and B show the mesh patterns of the
FEA. Figure 5A shows quarter size of the model, and 5B
shows an enlarged view of the model. The FEA calculation
was performed for an elasto-plastic material, and determined the three-dimentional stress field. The minimum ofmesh-size around the contact point was 2 lm, the outermesh-size was 20 lm. The sample size in the FEA modelis 1 mm wide which is smaller than experimental size,
20 mm wide. However, the model size is large enough,because the stressed region (Figure 12) is limited within
about 30 lm from the center of the indentation.
Yield stress and a work-hardening coefficient shown in
Table 1 were determined from the load-depth curve for the
Berkovich indentation.23 At first, the load-depth curves
were measured by experimental indentation. The twoparameters were determined as fitting parameters of thecurves, assuming that stress-strain relationship could beapproximated by two straight-lines using “Isotropic Hardening rule24”. The bottom and side faces of the mesh pattern were constrained in vertical directions to each face.
The FEM simulations were quasi-static implicit. Loading
and unloading speed were 0.64 N/s, the maximum load
was 6.4 N, and holding time was 1 second. Rate-dependent
plasticity, viscoelasticity and creep behavior were excludedin the present simulation. In addition, permanent densification of glass was not considered in this calculation.

T A B L E 1 Parameters of FEA
Young’smodulus(GPa)

Poisson’s
ratio

Mass
density(kg/m3)Yield
stress(GPa)

Work-hardening coefficient
(GPa)

70

0.203

2520

3.5

1

| RESULTS

Figures 6A and B show the cross-section plane perpendicular to the scribing groove and the cutting plane after scribing
at 24.6 N, respectively. It is found that the median crack
propagated in the thickness direction without branching (Figure 6A). In Figure 6B, on the other hand, two kinds ofarrest-lines of the median crack can be observed after scribing. These arrest-lines suggest that the median crack wasarrested once, and re-propagated again. In this paper, the first
pop-in median crack and the re-grown median crack aredefined as the first crack and the second crack, respectively.
Figure 7 shows a relationship between crack depth andscribing force, Fz. The crack depths were determined fromthe arrest lines of cutting plane (Figure 6B). The error bars
shown in Figure 7 represent1 SD, which was determinedfrom 10 scribing tests under an identical condition. However, the error bars of the first crack depth and Fz are omitted, because they are smaller than the symbols. The crack
depths of the first crack and the second crack increase withan increase in scribing force. It is found that the first crack
initiates at around 5 N. Meanwhile, the second crack starts
at around 21 N. In other words, both the 1st and the second
cracks show their different threshold forces for propagation.
In addition, the scribing force dependence of the depth of thesecond crack is much larger than that of the first crack. The
maximum depth of the second crack is more than 500 lm,
which is about 70% of the sample thickness, 0.7 mm.
In order to evaluate crack initiation behavior during
scribing, in-situ observation was performed using a highspeed camera. Figure 8 shows some snapshots of the glass
during scribing at 24.6 N. The white small arrows in eachsnapshot indicate the position of crack tip. The markings ofthe bottom edge are not related to the scribing groove on
the top surface. In order to obtain the cross-section of cutting plane, the reverse side of the scribing surface wasscribed and broken by hands. Therefore, the markings wereformed by scribing the reverse side. The crack depths in theframes of 0 second (Figure 8A) and 0.08 second (Figure 8B) are almost unchanged. It is considered that this corresponds to the arrest mark of the first crack as shown in
Figure 6B. It is found that the maximum depth of the first
crack is always located at just beneath the scribing wheel.
After the wheel passes through the photo frame as shown in
Figure 8C, the median crack propagates again, and grows
deeper and reaches the constant depth as shown in theframe of 1.0 second (Figure 8D). This final depth corresponds to the arrest mark of the second crack (Figure 6B).
It is also found in Figure 8D that the depth of the first crack
becomes unclear after the second crack propagation.
As shown in Figure 8, it is found that the first crack initiates just beneath the wheel, whereas that the second crack
propagates after the wheel passes by. In other words, the| 5

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F I G U R E 5 Mesh patterns of the FEA (A) Quarter size of the model, (B) Enlarged view of the modelF I G U R E 6 Microscope images of glass after wheel scribing at 24.6 N (A) Cross-section plane perpendicular to the scribe groove,(B) Cutting plane

The in-situ photoelastic image of the glass under a loadof 6.4 N is shown in Figure 9. The color bar indicates therange of retardation values from 0 nm (blue) to 130 nm(red). The maximum of retardation in the glass is found in
the point just beneath the wheel.

4

F I G U R E 7 Relationship between crack depth and scribing load

| DISCUSSION

Although the retardation shown in Figure 9 gives us information on the difference in principal stresses under a load,
it cannot indicate whether the stress at a given point is compressive or tensile. However, the stress field can be calculated using the retardation and the fast axis orientation,
assuming that the generated stress field is two-dimensional.
The relationship between the retardation and the principal
stress difference is defined as the following equation.25

Fz

first crack grows under a load, and the second crack grows
after unloading. In order to confirm it, in-situ photoelastic
measurements were performed under a static indentation of
the wheel.

R ¼ Ctðr11  r22 Þ(1)where R is the retardation, C is the photoelastic constant, t
is the optical path length, r11 and r22 are the principal
stresses.

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F I G U R E 8 Snapshots during wheel scribing of a glass under a normal force of 24.6 N (A) 0 s, (B) 0.08 s, (C) 0.2 s, (D) 1.0 s

F I G U R E 9 Retardation image during wheel indentation at 6.4 N

In order to estimate the stress field of the glass sample
beneath the wheel, the optical path length, t is required asshown in Equation 1. Therefore, the size of a residual
imprint was measured after wheel indentation. The profileof the indentation imprint is shown in Figure 10. Due tothe arc shape of the wheel, the stress field of the crosssectional plane perpendicular to the direction of light pass
(Y-axis) should be dependent of the Y-position. In order
to simplify the stress field, however, it is assumed thatthe two-dimensional stress field in X-Z plane is identicalat any Y-position of residual imprint whose length is
165 lm.
The retardation and the fast axis orientation (h0 ) were
determined from photoelastic experiments. From the difference in principal stresses and the slow axis orientation (h),which is obtained by subtraction 90° from h0 , the shear

F I G U R E 1 0 Top-view and cross-section profile of a residual
imprint after wheel indentation at 6.4 N

stress (sxy) in the X-Y plane can be obtained by the following equation.25
1
sxy ¼ ðr11  r22 Þsin2h2(2)Using the obtained shear stress (sxy) and the equilibrium
equations of two-dimensional stress field,25 the other stresscomponents are also obtained. x
Z
osxy
dx
(3)
rx ¼ ðrx Þ0 
oy
x0

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F I G U R E 1 1 Retardation image (A) and rx stress distribution (B) on the red line in (A) calculated from Equations 1-3

F I G U R E 1 2 rx stress maps by FEA (A) during wheel indentation at 6.4 N and (B) after the indentation. The blue and red regions represent
compressive and tensile stress, respectivelyZy
ry ¼ ðry Þ0 
y0

osxy
dy
ox

(4)

where x0 and y0 are the integral starting point, (rx)0 and(ry)0 are the normal stresses of the integral starting point,respectively.
Figure 11B shows the normal stress, rx, distribution
calculated from the photoelastic image as a function of the
distance, x, on the red line shown in Figure 11A. The
width of red line in Figure 11A is about 7 lm, and thecenter of the line is located about 11 lm from the surface.
As shown in Figure 11B, it is found that the maximum
tensile stress which is about 2.3 GPa, is observed justbeneath the wheel. It is considered that this tensile stresscauses the first crack initiation.
Figure 12 shows the FEA stress maps of rx in the X-Z
plane during and after a static indentation of wheel. The
maximum indentation load is 6.4 N. Although the threedimentional FEA calculation was performed to determine
the stress field, the two-dimensional (X-Y plane) stressmaps at the center of the wheel are shown in Figure 12. In
Figure 12, the blue and red regions represent compressive

F I G U R E 1 3 SEM image of the surface layer of glass after
wheel scribing at 9.9 N

and tensile stresses, respectively. On loading (Figure 12A),
the maximum tensile stress is 1.09 GPa. It is consideredthat FEA calculation can reproduce qualitatively the tensilestress which is also obtained from photoelastic image in
Figure 11B. A considerable difference in the value of the
maximum tensile stress between FEA and photoelasticity

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F I G U R E 1 4 Schematic illustrations of the 1st and the 2nd

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The stress fields estimated from FEA (Figure 12) andthe model of the 1st and the second crack propagation(Figure 14) are supported by the CME model,11 which
explain that the tensile stress at the bottom of the plastic
zone remains even after complete unload. However, theother authors have reported that the stress under the plastic
zone changes from tensile on loading into compressive
after unload.4,10 Although the issue of the residual stress ofglass after indentation or scribing is still controversial, further experimental and modeling works will be required inorder to obtain more precise and quantitative stress fieldaround the indentation or the scribing groove. It is
expected that more information on glass plasticity could
improve more efficient machining of a glass sheet using ascribing wheel.

cracks propagation during wheel scribing

probably may result from an assumption of two-dimensional stress field for photoelasticity and/or from the estimated values of parameters for plastic deformation of glass
for FEA. After unloading (Figure 12B), the compressive
stress still remains beneath the imprint of wheel, and thetensile stress also exists below this compressive region.
Figure 13 shows an SEM image of the cross-sectional
plane perpendicular to the scribing groove. In Figure 13,
there exists the region where the median crack is closed.
Below the region of the crack closure, the crack-openingdistance becomes wider. It is considered that this crack closure results from the compressive stress which can be
reproduced by FEA calculation as shown in Figure 12B.
The compressive stress should exist in the plastic zone,
because the compressive stress balances the tensile stresswhich can be estimated from the expanding cavity model
as shown below.
Figure 14 shows schematic illustrations of cracking
sequences during wheel scribing. Figures 14A and B represent the model of the first crack initiation during loading.
Figures 14C and D show the model of the second crack
propagation during unloading. During loading, the plastic
zone is created beneath the wheel as shown in Figure 14A.
The compressive stress in the plastic zone is balanced withthe tensile stress outside the plastic zone (Figure 14A). The
tensile stress causes the first crack to initiate (Figure 14B).
During unloading process, on the other hand, a decrease incompressive stress beneath the wheel enables the crack togrow to the surface, or to the indenter (Figure 14C). Furthermore, because the tensile stress still remains at the bottom of plastic zone, the crack re-propagates as the second
crack. The wider crack-opening-distance below the regionof crack closure (Figure 13) also confirms an expansion of
the plastic zone, which would be an origin of the tensilestress outside the zone and the balanced compressive stressinside the zone.

5| CONCLUSION

Through in-situ observation of cracking in glass during
wheel scribing, it was clarified that the first crack initiates
beneath the wheel and the second crack re-grows after
passing through the wheel. In addition, the results of in-situ
photoelastic measurement and FEA represented that the
maximum tensile stress exists just beneath the wheel duringloading. From an observation of the cross-section of ascribing groove, it was found that a part of the mediancrack was closed in the plastic zone. It is considered thatthe tensile stress at the bottom of the plastic zone causes
the origin of the first crack initiation, and that the compressive stress in the plastic zone is the origin of the closure of
the median crack.
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How to cite this article: Tomei N, Murakami K,
Fukunishi T, Yoshida S, Matsuoka J. Direct
observation of crack propagation in a liquid crystal
display glass substrate during wheel scribing. Int J
Appl Glass Sci. 2017;00:1-9. https://doi.org/10.1111/
ijag.12272