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: hengly@ofsoptics.com (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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