Product Performance
Coating failure in the pull-out of a multiply-coated optical fiber
Heng Ly a,*, Mahmood Tabaddor a, Charles Aloisio a, Karofilis Konstadinidis a,
Tim Goddard a, Karl I. Jacob b
a Optical Fiber Cable Division, OFS, 2000 NE Expressway, Suite 2BO5, Norcross, GA 30071, USAb The School of Polymer, Textile and Fiber Engineering, Georgia Tech, Atlanta, GA 30332, USA
Received 22 July 2005; accepted 22 August 2005
Abstract
For fiber optic ribbon cables, stripping of the multiple coatings from the optical fiber glass is an important step in installation.
During this process, failures that lead to excess residue on the fiber create extra work that is costly. In this paper, we describe a
systematic experimental approach to understanding the failure mechanism observed during fiber optic ribbon stripping. The blades
in the ribbon-stripping tool create compressive stresses during the fiber pull-out process. These compressive stresses, in turn, may
lead to coating instability. The coating instability creates potential for stress risers and excessive wear of the innermost coating.
Reducing the length of the stripping sample has been found to significantly reduce the occurrence of this failure mode.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Pull-out; Coatings; Buckling; Interfacial strength; Debonding
1. Introduction
For optical fibers, protection from loading and
environments that degrade optical performance is
critical [1]. A system of multiple coatings has been
developed to protect the optical fiber. Typically two
polymeric coatings cover an individual optical fiber
leading to final dimensions typically near 0.25 mm.
However, for end-user purposes the fibers must be
bundled and delivered with additional protection. This
final form is the optical fiber cable. An intermediate
packaging step exists for one particular optical fiber
cable type known as ribbon cable. Ribbon cable
consists of a stack of ribbons. Each ribbon is a linear
array of the double-coated optical fibers as shown in
Fig. 1. Actually, there is a thin (of the order of microns)
0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymertesting.2005.08.007
* Corresponding author. Tel.: C1 770 838 5336; fax: C1 770 838
5032.
E-mail address: [email protected] (H. Ly).
third coating on each optical fiber, a color coating
expediting fiber identification. To form a ribbon, the
individual colored fibers are held together with another
polymeric material known as the matrix. Ribbons
generally contain anywhere from 2 to 24 optical fibers.
The ribbon design provides improved fiber density
within a cable and allows for more efficient fusion
splicing [2].
The splicing operation requires that a 30-mm
section of the coatings, ink and matrix covering be
removed from all fibers simultaneously. Special tools
have been developed for this operation. The success of
the ribbon stripping process is critical to the efficiency
and cost of optical fiber installation. In this paper, we
describe a systematic experimental approach to under-
standing the failure mechanism in stripping of optical
fiber ribbons. Before describing the test setup and
results, we present, in Section 2, a summary of
representative papers from the open literature on the
topic of fiber pull-out.
Polymer Testing 24 (2005) 953–962
www.elsevier.com/locate/polytest
Double-coated optical fiberMatrix
Fig. 1. Schematic of optical fiber ribbon.
H. Ly et al. / Polymer Testing 24 (2005) 953–962954
2. Literature review
Before an optical fiber ribbon can be spliced with
other optical fibers to form communication links, the
protective polymeric coatings, ink and matrix must be
removed from the glass fiber for a short length. If
the adhesive strength between the glass fiber and the
primary (innermost) coating is too high, stripping the
coating materials from the glass fiber becomes
extremely difficult [3]. If the adhesive strength between
the glass fiber and the primary coating is too low, it
leads to easy debonding and an ensuing incursion of
water, particularly upon exposure to high humidity,
which attacks the glass surface and reduces tensile
strength [4]. Hence, a moderate interfacial shear
strength between the glass fiber and inner (primary)
coating needs to be designed for most applications [3].
In addition, the primary coating and glass interface
must be weaker than all the other interfaces within the
ribbon composite structure: primary and secondary
coatings, secondary coating and ink, and ink and
matrix. The bulk of the research on the topic of
removing coatings from a fiber involves the pull-out,
push-out or microbond tests [3,5,6]. In all these works,
the main focus is on prediction of the interfacial
strength, maximum pull-out force and post-debond
frictional sliding behavior. Aside from the interfacial
strength, other parameters such as boundary conditions,
moduli of coating, and thickness of coatings have been
L
support
p
x
fiber
matrix
Fixed matrix sides
(a) (b)
Fig. 2. Boundary conditions for fiber pull-out (a) fixed matrix si
found to affect the load that initiates the initial debond
between coating and fiber and the post-debond
response.
Of all these tests, the pull-out test is the most
comparable to the ribbon stripping procedure and most
studied, so we focus our review of the literature on fiber
pull-out papers. As the name implies, a force pulls the
fiber while some sort of restraining condition is applied
to the coating(s). To determine the initial debond load,
there are two distinct approaches to the theoretical
analysis of fiber pull-out. One approach is based on the
maximum shear stress criterion [8] and the other is
based on fracture mechanics [9]. However, consider-
ation must be given to the loading configuration in the
analysis, particularly on how the boundary conditions
are specified. Different boundary conditions would
induce different states and magnitudes of stress in the
constituents which determine the debond and frictional
pull-out force [6].
For the fiber pull-out test, three typical loadings
conditions have been studied. These are shown in
Fig. 2, namely the fixed matrix sides [5], fixed matrix
bottom [7], and the restrained matrix top [7]. In the
fixed matrix sides, the specimen is embedded into an
epoxy adhesive so that the external force is applied to
glass fiber at the end of the specimen and is equilibrated
by the shearing force distributed over the lateral surface
of the secondary coating [5]. In the fixed-matrix bottom
setup, the bottom of the specimen coating is supported
while the external force is applied on the fiber at the
opposite end. In the restrained-matrix top setup, the
coating near the fiber emergent end is supported with
the pulling force applied to the fiber emergent end. As
we will demonstrate later, the restrained-matrix top
model closely matches the ribbon stripping setup. For
all these cases, the interfacial shearing stress is known
to have a nonlinear distribution with the highest stress
occurring near the fiber emergent end [5].
p
x
L
Fixed matrix bottom
x
p
L
Restrained matrix top
(c)
des (b) fixed matrix bottom and (c) restrained matrix top.
Fig. 3. Ribbon stripping tool ready for stripping.
H. Ly et al. / Polymer Testing 24 (2005) 953–962 955
Specific expressions for interfacial shear stress and
maximum pull-out load have been developed from the
strength based pull-out model, which assumes a
maximum stress criterion for debonding at the glass
fiber-coating interface. The theoretical derivations for
the fixed-matrix sides, fixed-matrix bottom, and
restrained-matrix top are outlined in Refs. [5], [8],
and [10], respectively. All the materials were assumed
to be linearly elastic up to the failure.
Once the debonding process is complete then the
interfacial friction drives the remaining response
between the coating and the fiber. In most of the
work on fiber pull-out, the interfacial shear stress is
assumed to be governed by a Coulomb friction where
the friction of coefficient is constant. However, there
are instances where the classic Coulomb friction model
does not hold. A stick-slip type response has been
observed in some fiber pull-out experiments [11,12].
3. Experimental details
The ribbon stripping process is typically performed at
the cable installation site. It is a manual process.
However, the need for accurate measurements of strip
or pulling force and the displacement of the fiber
required sensors and special fixtures to hold
Fig. 4. Schematic detailing
the ribbon-stripping tool. Before describing the exper-
imental setup, we first describe the ribbon-stripping tool.
3.1. Ribbon stripping tool
A ribbon-stripping tool (Fig. 3) consists of a holder
grip and main body that is used to remove the various
layers that surround the optical fibers. The holder grip is
used for guiding the ribbon holder that maintains the
ribbon inside the main body. The main body consists of
two cutting blades (lower and upper) and a single
electrically heated plate located at the base of the body
(Fig. 4). The lower blade is attached to the heating plate
and the upper blade is attached to the lid of the main
body. Before starting the ribbon stripping operation, the
lid is closed to contain the heat for some waiting period
of the order of 30 s. The purpose of the heat is to reduce
the adhesive strength of the coating/fiber interface [12].
The temperature of the heating plate varies from 90 to
120 8C depending upon the tool manufacturer and the
ribbon product. With the closing of the lid, the blades
cut into the layers creating an initial flaw in the
coatings. After this initial wait period, the operator
pulls the two sections of the stripping tool apart. When
stripping is completed, the lid is opened to remove the
stripped portion of the coatings.
ribbon stripping tool.
Fig. 5. Experimental setup on Instron.
Table 1
Ribbon sample characteristics
Ribbon Coating
system
Thickness of primary coating,
mm
Ribbon
1 A 29.5 12-fiber
2 A 32.5 12-fiber
3 A 35.0 12-fiber
4 A 37.5 12-fiber
5 B 29.5 12-fiber
6 B 35.0 12-fiber
H. Ly et al. / Polymer Testing 24 (2005) 953–962956
3.2. Experimental fixture
Strip force, the force used to pull the ribbon out of
the stripping tool in order to remove the coatings
covering the optical fibers, is an important parameter
that must be measured. For this study, 12-fiber ribbons
were tested and so the strip force is the total force
necessary to remove the coatings from the 12 optical
fibers simultaneously. In order to measure the strip
force a special fixture was designed that would hold the
ribbon-stripping tool in a universal tensile testing
machine, an Instron model 4200 series (Fig. 5). The
bottom half of the fixture holds the main body of the
stripping tool while the top half holds the holder grip of
the stripping tool. Also, the bottom half of the fixture
has a displacement gauge to maintain a constant
displacement on the lid of the stripping tool during
ribbon stripping. Once the ribbon sample inside the
holder is placed into the main body and the lid is closed,
the heating time starts. The heating temperature was set
at 90 8C; heating time was set for 10 s; 10 representa-
tive samples were tested for each ribbon set; and the
ribbon-stripping length was varied from 10 to 30 mm.
The pulling test was performed with a commercial
ribbon-stripping tool (Sumitomo JR-4A), one of several
tools used by the craftsmen in the field, to closely
mimic the actual stripping process.
3.3. Ribbon samples
The sample ribbons contained 12 color-coded dual-
coated optical fibers. A parametric study on the
influence of geometric dimensions and elastic proper-
ties of the primary and secondary coatings was carried
out.
To study the effect of geometry, the outer diameter
of the primary coating and the inner diameter of the
secondary coating were varied. The outer diameter of
the optical fiber, the outer diameter of the secondary
coating, the outer diameter of the coloring ink, and the
ribbon thickness were maintained at constant values.
The thickness of the ribbons was 0.3 mm and the outer
diameter of the glass fiber was 0.125 mm.
Changes in material properties were also considered.
Four ribbons were made with coating system A with
primary thickness values of 29.5, 32.5, 35.0 and
37.5 mm. Two ribbons were made from coating system
B with the primary coating thickness values of 29.5 and
37.5 mm. All ribbons were fabricated under identical
process conditions (speed, cure levels, etc.). Tables 1
and 2 list the characteristics of the ribbons and
properties of coating materials, respectively.
3.4. Evaluation of samples
Successful ribbon stripping occurs when the stripped
composite of the protective materials is removed as one
piece (tube-off) and there is no breakage of the optical
fibers. When there is failure in achieving tube-off, the
protective materials crumble and create a powder-like
residue on the fiber. Figs. 6 and 7 show the difference
between successful and unsuccessful stripping pro-
cedures, respectively. The crumbling produces unde-
sired results because the crumbled materials leave
residue on the stripping tool requiring additional
cleaning effort before the subsequent strip. This extra
Table 2
Properties of materials measured at 90 8C for elastic modulus, E, and at 25 8C for Poisson’s ratio, n, and Tg
Coating system A Coating system B
Primary coating: EZ0.17 MPa, nZ0.50, TgZK288C Primary coating: EZ0.10 MPa, nZ0.50, Tg ZK178C
Secondary coating: EZ300 MPa, nZ0.33, TgZ558C Secondary coating: EZ900 MPa, nZ0.33, TgZ878C
H. Ly et al. / Polymer Testing 24 (2005) 953–962 957
effort reduces the efficiency of the stripping process. In
addition, the crumbled materials that are created inside
the tool during stripping could cause the fibers to move
closer to the blade of the stripping tool. Any contact
with the blade could result in a fiber break. Thus, design
of ribbon structures that have suitable stripping
characteristics requires an understanding of the inter-
actions among the various coating layers comprising
the developing ribbon [13].
After each stripping run, the sample was observed to
determine whether it passes/fails based on the criterion
as shown in Figs. 6 and 7.
4. Experimental results
4.1. Trends in success and unsuccessful stripping
Fig. 8 shows a typical trend observed for the force
vs. displacement data for a successful ribbon stripping
operation. For an unsuccessful stripping operation, the
force vs. displacement plot is shown in Fig. 9. Both
show similarities early on but show some deviations
after the peak in the pull-out force. Using this data, the
ribbon stripping process can be categorized into three
phases. The first phase is a linear elastic response of
Fig. 6. Successful stripping of 12-fiber optical ribbon. Top: bare fiber
stripped of buffering. Bottom: Removed coatings, matrix and ink in
one piece (tube-off).
the coating to the external pulling of the fiber; the
second phase is a non-linear stable initiation/propa-
gation of a crack at the interface between the glass and
the coatings; and, the third phase is friction sliding of
the primary coating against the glass after complete
debonding.
To initiate a flaw at the interface between the fiber
and the primary coating, a normal force (or normal
displacement) is applied on the stripping tool after the
insertion of the ribbon but before pulling. This forces
the blade into the layers of coatings, ink and matrix.
This crack very quickly propagates perpendicular to the
fiber towards the interface upon the slightest amount of
pulling. At this point, very early on in the ribbon
stripping process, a flaw is created at the glass/primary
coating interface. It is observed that the strip force
displays a linear relation to displacement until a point
where the protective coatings debond from the glass at
the flaw site, indicated as a in the first phase (Fig. 8).
Once the crack is initiated along the interface, in the
second phase, the force displays a non-linearly
relationship with displacement characterizing the
crack propagation process. This process is in general
Fig. 7. Unsuccessful stripping of 12-fiber optical ribbon. Top: bare
fiber stripped of buffering. Bottom: Removed coatings, matrix and ink
crumble.
0
2
4
6
8
10
12
14
16
18
20
22
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
Displacement (mm)
Load
(N
) a
d
b
cc
I III II
Fig. 8. Strip force vs. displacement for a successful ribbon stripping operation.
H. Ly et al. / Polymer Testing 24 (2005) 953–962958
stable except for a very short final section where the
force is nearly independent of displacement. At point b,
the stability of the crack propagation process changes.
A sudden short drop from after point b indicates an
unstable propagation of the crack until complete
debonding is achieved. In the third phase, the primary
coating begins to slide along the surface of the glass
fiber where the stripping force decreases monotoni-
cally. This force may decrease if the dynamic force
required for sliding is lower than the available stripping
0
2
4
6
8
10
12
14
16
18
20
22
0 2 4 6 8 10 12 14 1
Displacem
Lo
ad (
N) a
b
c
II I
Fig. 9. Strip force vs. displacement for an un
force [12]. At point d, the layers of coatings and matrix
have been removed revealing 12 bare optical fibers.
For the unsuccessful ribbon stripping procedure, the
main dissimilarity appears in phase III, where instead of
a steep decline in the force there is a plateau. Clearly,
this suggests that the friction between the primary glass
and the coating has increased. Recall that the ribbon is
heated. Because the primary has a lower Tg than the
secondary coating, the primary coating will expand to a
greater extent than the secondary coating. This created
6 18 20 22 24 26 28 30 32
ent (mm)
d III
successful ribbon stripping operation.
Table 3
Performance of ribbon stripping with different primary thickness
Ribbon Coating
system
Thickness of primary coating,
mm
% Pass
stripping
1 A 29.5 100
2 A 32.5 100
3 A 35.0 40
4 A 37.5 0
5 B 29.5 100
6 B 35.0 90
H. Ly et al. / Polymer Testing 24 (2005) 953–962 959
a pressure build-up within primary coating, glass
surface, and secondary coating, thus resulted in an
increase of normal pressure between the glass and
coatings. However, it has been shown that this thermal
mismatch would result in a decrease in the normal
pressure [14]. Therefore, explanation must be sought
elsewhere. However, before examining the failure
mode in more detail, we summarize the results of the
parametric study.
4.2. Parametric study
First we examine the effect of changing the thickness
of the primary coating. Since we have constrained the
outer diameter of the secondary coating, the thickness
of the secondary coating is changing. An increase in the
primary coating thickness entails a decrease in
the secondary coating thickness. Table 3 summarizes
the results of changing the primary coating thickness.
0
2
4
6
8
10
12
14
16
18
20
22
0 2 4 6 8 10 12 14Displacem
For
ce (
N)
Fig. 10. Strip force versus displacement for ribbons with varying thickness of
thickness values of 29.5, 32.5, 35.0, and 37.5 mm.
The % pass stripping denotes the percentage of ribbons
passing based on the visual examination of the 10
samples. Failure typically occurs when the passing rate
is below 80%. If the removed coatings and matrix are
crumbled and the fiber has significant residue then the
stripping process is deemed a failure. All strip lengths
are 30 mm. The difference in the pulling force versus
displacement plots for the four different primary
diameters for coating system A is shown in Fig. 10.
The numbers clearly show that as the thickness of the
primary coating increases, the percentage of passing
ribbons decreases. We see that the plateau of the post-
debond region is a good indicator of stripping failure
matching the visual observations.
The effect of coating moduli can be seen in Table 3. It
appears that coating system A, with the softer coatings,
degraded at a faster rate than coating system B. Table 4
lists properties of the different phases.
4.3. Buckling of coatings
To determine the cause of ribbon stripping, we look
at a picture of one sample, which has failed (Fig. 11).
The coatings and matrix clearly display an undulating
behavior suggestive of buckling. The one major
difference between the restrained top matrix boundary
condition and all the others is that there is a zone of
compressive stresses that develop within the coatings
near the fiber emergent end. For the ribbon stripping
process, the coatings come up against the blades as
16 18 20 22 24 26 28 30ent (mm)
29.532.535.037.5
primary coating (coating system B). Four cases are shown for primary
Table 4
Properties of ribbon stripping in different phases
Ribbon Max force
(N)
Slope in
phase I
(N/m)
Slope in
phase II
(N/m)
Slope in
phase III
(N/m)
1 18.24 13.75 0.62 K0.73
2 18.19 12.77 0.58 K0.59
3 18.85 12.48 0.64 K0.36
4 20.83 13.63 0.91 K0.21
5 17.8
6 18.1
H. Ly et al. / Polymer Testing 24 (2005) 953–962960
the fiber is pulled. These compressive stresses, if large
enough, can buckle the coatings and the matrix.
Because of the order of magnitude difference in the
elastic moduli of the primary coating versus the
secondary coating and the matrix, it is likely that,
possibly before debond occurs, that the secondary
coating and the matrix have buckled. Upon debonding,
the buckling of the primary coating occurs along the
debond length increasing the pressure on the fiber.
Once the entire interface is debonded, the frictional
sliding force increases due to this increase in normal
pressure.
By crudely approximating the secondary coating and
matrix materials (ignoring the very thin ink layer) as a
single beam sitting on an elastic foundation where the
spring constant is determined by the properties of the
primary coating, one can estimate the critical buckling
load. For a beam on an elastic foundation, the critical
load is estimated to be
Pcr zffiffiffiffiffiffiffiffiffiffiffiffikpEsIs
q(1)
where Pcr is the critical load, kp is the stiffness of the
primary coating that acts as a spring, Es is the Young’s
Fig. 11. Matrix/coatings undulation seen during an unsuccessful
ribbon stripping procedure.
modulus of the secondary coating and matrix, and Is is
the least moment of inertia of the secondary coating and
matrix [15]. Also we can assume kp zEp
t, where Ep is
the Young’s modulus of the primary coating and t is its
thickness [16]. This expression shows that the stiffness
of the beam is known to increase with increasing
diameter of secondary coating and decrease with
increasing diameter of primary coating. The results
for the varying thickness of the primary coating show
up in two ways in Eq. (1). First the stiffness of the
foundation decreases as the thickness of the primary
coating increases. However, as the secondary coating
outer diameter is fixed, the thickness of the secondary
coating is decreasing, decreasing the moment of inertia.
So the critical load decreases at a fast rate as these two
terms decrease, most likely in a nonlinear fashion.
Referring back to the experimental results, ribbons
made from coating system B tend to strip more
successfully than ribbons made from coating system
A for the same dimensionally structured ribbon. For
example, Ribbon 5 with coating system B and a
primary thickness of 35 mm had 90% strip yield and a
maximum strip force of 18.1 N. Ribbon 3 made from
coating system A with a primary thickness of 35 mm
had a stripping success rate of only 40% and a
maximum stripping force of 18.8 N. These two ribbons
have relatively the same maximum strip force;
however, the product of the moduli (of the primary
and secondary coatings) for coatings system A is lower
than coating system B leading to a lower critical
buckling load and an increased propensity for buckling.
So as the coatings and matrix buckle, significant
deformation of the primary coating near the surface of
the fiber is a likely precursor to the final failure mode,
and may cause the retention of residue from primary
coating to remain on the bare optical fiber.
4.4. Effect of strip length
Using this buckling model, one potential modifi-
cation to the stripping process that might reduce the
propensity for buckling is a reduction in the length of
the stripped sample. So now, instead of stripping 30-
mm in a single step, a 2-step process will be needed.
The logic here is that the shorter the length of the
column (secondary coating and matrix), the greater the
resistance to buckling.
Fig. 12 shows the results of the stripping force versus
fiber displacement for the case of a 20-mm strip length.
From the sliding friction region, the data suggests that
this sample passes and indeed it does. Similar results
were obtained for 15- and 10-mm length strip lengths.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Displacement (mm)
For
ce (
N)
29.5
35.0
37.5
Fig. 12. Strip force versus displacement of ribbons (coating system A) with a 20 mm strip length.
H. Ly et al. / Polymer Testing 24 (2005) 953–962 961
Actually the passing rate was 100% for both coating
systems and all primary thickness values except for the
thickest case were the passing rate was an acceptable
80%.
An additional advantage of the shorter strip length is
that the overall force is lower. With the reduction in
force and the increase in the critical buckling load, the
success rate for stripping is greatly enhanced. Fig. 13
shows the dramatic difference when the coatings
display no surface undulations and the stripping process
is successful.
Fig. 13. Matrix/coatings during a successful ribbon stripping
procedure.
5. Summary
In this paper, we presented results from a systematic
experimental effort to understand the failure mechan-
ism responsible for poor ribbon stripping. The fiber
optic ribbon is a complex composite structure consist-
ing of a linear array of triple-coated glass optical fibers.
The first two coatings surrounding the optical fiber are
typically UV-curable polymeric materials. The third
coating is a thin coating of ink. Finally, all these coated
fibers are held together in a single ribbon structure by a
matrix material, which is typically a UV-curable
polymer.
Failure in ribbon stripping consists of residue and
debris covering the bare optical fiber. This debris
requires additional cleaning and handling of the optical
fibers possibly leading to fiber breaks. For sake of cost
and installation efficiency, failures must be eliminated.
In this study, we found that the main precursor to
failure is an obvious buckling of the secondary coating
and matrix. This instability then leads to significant
pressure on the fiber increasing the frictional sliding
portion of the stripping process. The undulating shape
of the primary coating against the glass then leads to
increased deformations and wear. The wear particles
show up as residue on the bare optical fiber.
Eliminating buckling is the key preventive step.
Our parametric study supports this model as we
found that increasing the thickness of the softer primary
H. Ly et al. / Polymer Testing 24 (2005) 953–962962
coating increased the likelihood of stripping failure.
Furthermore, a coating system, where the product of the
primary and secondary coatings modulus were reduced,
was found to exhibit a lower pass rate.
Finally, we found that by reducing the strip length
from 30-mm to any value less than or equal to 20-mm
dramatically improved the stripping success rate. The
effect of reducing the strip length is two-fold: one, the
shorter column of coatings and matrix is stiffer and
second, the maximum strip load applied to the coatings
is lower. Therefore, by a simple change in the ribbon
stripping process, from one step to 2 steps, the range of
ribbon products that can strip successfully increases.
Acknowledgements
The first author would like to express his gratitude for
the support of his educational endeavors by the manage-
ment of OFS from which this work has come about.
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