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IEEE Transactions on Dielectrics and Electrical Insula ion

Vol 8 No 2 April 2001

FRP Rods for Brittle Fracture Resistant

Composite Insulators

M. Kuhl

CeramTecAG

Wunsiedel, Germany

A B S T R A C T

Brittle fracture of fiberglas reinforced polymer FRP) rods can lead to mechanical failures of

composite insulators even at low mechanical lo ads durin g operational service. Although th is

fact has been known for

20

years, it may still be a problem in some designs of composite

insulators at the present time. In order to find counterm easures against brittle fracture,

a

study

was carried out in the early eighties. It turned ou t that brittle fracture is a problem

of

FR P

material and that material compositions exist, resistant to brittle fracture. A brittle fracture

resistant

FRP

rod introduced 1983 n on e particular design of comp osite insulators resulted i n

a

15

year excellent service performance. This st udy deals w ith details of brittle fracture of

F R P

rods. Test setups were established to induce brittle fracture artificially It was realized that

brittle fracture is some kin d of stress corrosion related to the composition of th e

FRP

material.

A broad variety of

FRP

materials was evaluated, showin g the influence of the components of

FRP material on the brittle fracture behavior of

FRP

rods as well as the effects of dif ferent

manufa cturing processes. The comp ositions of brittle fracture resistant FRP

rods

are disclosed.

Th e results from artificial testing are compared with b rittle fracture of FRP rods that occurred

in com posite insulators in operational service. Although no quantitative correlation could b e

established, th e trend concerning th e material behavior of

FRP

rods is similar.

1 INTRODUCTION

OMPOSITE

insulators consist of a glass fiber reinforced plastic rod

C FRP

rod), a shed housing made of polymeric material covering the

FRP rod a nd metal end fittings attached to the ends of the FRP rod. The

housing protects the FRP rod from weathering and supplies the nec-

essary creepage distance. This composite structure consists of several

interfaces which have to be designed and manufactured properly in or-

der to avoid ingress

of

moisture, and pollutants from the surrounding

environment into the interior of the composite structure. Laboratory

tests carried ou t more than two decades ago revealed some typical elec-

trical and mechanical failures of composite ins ulator s of early designs

[l].However, in the late seventies a new kind of mechanical failure oc-

curred on

FRP

rods of composite insulators installed in

HV

lines

[Z-51,

never seen before dur ing laboratory testing. This kind of mechanical

failure is called 'brittle fracture' of a

FRP

rod d ue to the unusual fracture

pattern of the fracture area

of

the concerned FRP rods. The failures oc-

curred a t very low nominal operational mechanical service loads

[3,4].

The outer features of the fracture areas ar e characterized by a razor cut

fracture surface running perpendicular to the axis of the

FRP

rod [6]. In

those days it was thought that brittle fracture was initiated by ingress of

chemicals such as dilute acids into the composite structure [Z

4/51,

The

improvements carried out on composite insulators in the last decades

led to elastomeric housing materials such as silicone rubber and ethy-

lene propylene diene monomer

(EPDM)

rubber an d to better interfaces

between the different materials of the composite components. New de-

veloped test standards

[7,8]

are the tools to check the integrity of the

composite structure.

No broadly accepted test exists

so

far to check the composite struc-

ture regarding brittle fracture of the

FRP

rod. This may be based on

the fact that brittle fractu re occurred on a small number of insula tors

under operational service conditions in comparison to the big numb er

of installed composite insulators . In spite of this fact, a mechanical

failure of an overhead transmission insulator may cause line dropp ing

which results in an outage of the transmission line. Aging of composite

insulators, and in particular, aging

of

the interfaces of the composite

structure, may be one

of

the reasons for the infrequent occurrence of

brittle fracture. M anufacturing defects on a small number of insulato rs

may contribute to the failures as well as specific environmen ts. These

conditions cannot be simulated in a design test performed

on

a small

number of test specimens. The only possibility to eliminate brittle frac-

ture of composite insulators at all was considered by using a brittle

fracture resistant

FRP

rod as member of the composite structure. The

result of this stud y has led to such a rod. It was introduced in the man-

ufacturing of composite insulators consisting of a housin g mad e of high

temperature vulcanized

(HTV)

silicone rubber in

1983.

Such insulators

1070-9878111 $3.00

001 IEEE

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183

have been manufactured in large quantities d uring the last 15 years.

The number of brittle fractures from those insulators is zero.

It is interesting to note that for several designs of other composite

insulator types brittle fracture is still a cur ren t problem [9-121. Most of

the concerned insulators failed in the USA because there is the largest

market for composite insulators [H I.

Brittle fractures on

FRP

rods can be traced

to

stress corrosion of

E-

glass (electrical grade ) filaments. S tress corrosion involves an ion ex-

change mechanism. Sodium ions with a large ion radius are replaced

by hyd rogen ions with small radius, resulting in an increase of stress

in the g lass surface of the filaments [13]. E-glass filaments build up spi-

ral flaws on their surfaces after immersion in diluted acids [6,14,15].

The flaws in the filaments initiate the failure of the composite material

which can be described by the mechanism of fracture mechanics [16].

In order

to

improve the brittleness of

FRP

material, several measures

were proposed. Gel coats can be applie d on the surface. The correct

choice of the matrix resin as well as the use of an acid resistant glass

composition for the glass filaments may be successful [17,18].

2

EXPERIMENTAL

All HV insulators under operational service conditions ar e stressed

frequently by electrical surface discharges when the surfaces of the in-

sulators are polluted and humidity penetrates the pollution. Already

Cave ndish found in 1784 the generation of nitric oxides and nitric acid

by using electrical discharges in nitrogen and oxygen, while Birkeland

and Eyde fo und an industrial process proposed in 1905 for generating

nitric acid by using electrical discharges in hum id ai r [19]. It was very

likely to assume that the electrical discharges on

HV

devices may also

generate nitric oxides and derivatives such as nitric acid. In order to

show that nitric acid can be generated from power frequency voltage in

presence of air and humidity, a device according to Figure 1was built.

The electrical discharge was carried out at 25 kV,,, (50 Hz ). Distilled

water was fed through the filter paper wra pped around the energized

electrode at a flow rate of 10 cm3/h . After a time spa n of 1h the con-

tent of the water collector had a pH value of 2.9, and 2.52 mg nitrogen

was measured. More evidence for the presence of nitric acid and ni-

tric oxides and their derivatives were found on the surfaces of polluted

insulators from experiments and tests. 110 kV silicone composite insu-

lators showed 4 to 5 nitrogen in the pollution after 5 yr service. Salt

was taken from the surface of a silicone composite insulator after pass-

ing the 1000 h salt fog test described in [7]. The nitrogen content in

the salt amounted to 2.5%. A huge pollution content of nitrogen was

reported from dc insulators on the pacific coast of California [20]. The

autho rs of [20] considered that the nitrogen in the pollu tion could be

traced back to agricultural products and fertilizers.

Mechanical failures of test specimens from stress corrosion implies

the presence of two components, mechanical stress and simultaneous

application of an environmental medium such as nitric acid. To check

FRP

rods concerning their stress resistance corrosion, some have sug-

gested the use

of

bending stress. Experiments carried out by the author

under ben ding stress showed that after crack initiation, delamination

occurred lengthwise in

FRP

rods to different extents. The acid can run

out and th e stress cannot be held constant du e to the delamination in

the composite material. More reproducible results were obtained from

Water f l o w r a t e 10cm3/h

R e s u l t 2 52mg N/h

pH

2.9

Figure 1

Device for generating nitric acid at power frequency volt-

age 25

kV

50 Hz.

1: steel bar,

2:

supporter,

3:

insulated copper wire 1.5

mm diameter, 4: filter paper wra pped around, 5 : holder (acrylic glass),

6: PVC tube

OD

63 mm, ID 51 mm, 7: silver paint (ground electrode),

8: water supply (de-ionized water), 9: acid collector.

test specimens loaded under tension. The test arrangement for testing

FRP rods under tensile loads is shown in Figure 2. The arrangement

has the adv antage that assembled FRP rods can be evaluated. FRP rods

clamped into the interior of the end fittings may undergo higher me-

chanical stresses in the end fittings as can be expected for the free length

of the rods between the end fitti ngs. In this way the influence of the

stress in the end fittings can be estimated. Diluted nitric acid of 1 n

H N0 3 were chosen for simultaneous application of the env ironmental

medium

1

n is 63 g concentrated HN 03 added

to

937 g water).

I

i i

Figure

2

Test arrangement for the evaluation

of

brittleness

of

F R P

rods (tensile load an d 1

n HN03,

simultaneously). Procedure I End

fittings under acid. Procedure I1Infus ion of

HN03

rocedure

111

Acid

on the free rod length.

Experiments with the three test arrangements shown in Figure 2

resulted in rejecting Procedure I because the acid attacked the metal

of the end fittings in such a way that the acidity suffered and led to

unreliable test results. The best reproductio n of test results could be

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Table 1 Epoxy resins and hardeners used to manufacture the test

Type

F

0164

X18

331

XlOO

MTHPA

HHPA

PSA mixt

1102

4,4 MDA

DDS

MNA

EH 640

specimens.

Trade name

Araldite F

Ruetapox 0164

Lekutherm X18

D. E. R. 331

Lekutherm XlOO

Ruetapox HX

Araldite 907

Araldite 905

Vers. Prod.1102

Araldite 972

Araldite 976

Araldite 906

VersamidEH 640

obtained with Procedure 111, because there is no metal involved in the

chemical stability of the diluted acid.

To manufacture

FRP

rods for this study, two manufacturing meth-

ods were used. For variation

of

different glass fibers a discontinuous

manufacturing pro cedure ha s been established. The glass fibers were

woun d up on a rotating wooden sheet forming loops of glass rovings

of a predetermined number. The loops were impregnated with heated

resin mixture in

a

tub and then pulled in steel tubes for crosslinking at

elevated temperatures. For the variation of different resin mixtures, a

continuous protrusion process was used. In this process it was simple

to

replace a resin mixture by another mixture. The glass content of the

FRP rods from both manufacturing procedures am ounted to 63 to 69% of

weight an d the mechanical properties of the rods from both procedures

were most equal when the same components were used. All FRP rods

were manufactured from an epoxy matrix resin because epoxy resin is

the best resin for FRP rods used in

HV

application d ue to their excellent

mechanical and electrical properties, although remarkable differences

exist within epoxies. The resin mixtures were prepared in ratios given

by the manufacturer of the resin mixtures. The curing state of the test

specimens concerned a curing state at 130C for

1 0

h, when no other

treatment is mentioned.

Table 1 ists the applied epoxy resins and hardeners. The resins F,

0164 and 331 are aromatic diglycidylether (bisphenol A base) with an

epoxy equivalent of

90. X18

is a distilled version of the epoxy resin

mentioned before (high purit y),

XlOO

is

a

cycloaliphatic diglycidylester

(HHPA base) with an epoxy equivalent of 185. The hardeners MTHPA,

PSA mixt, 1102 and MNA are liquid at room temperature. The harden-

ers HHPA, 4.4 MDA and

DDS

are solid at room temperature. Hardener

EH

640 is a 4.4 MDA diluted in 30% glycolene.

The glass fibers used consisted of either assembled or direct rovings

in 2400 or 4800 g/km. They were supplied f rom the companies Silenka,

PPG (Pittsburgh), OCF (Owens Corning), Bayer AG, Ahlstrom, Norsk

(Norsk Fiberglass), Vitrofil S.p.A., Gevetex (Stratifil), and were used as

delivered.

All test specimens were tested under constant static load as show n

in Figure 3. After loading diluted nitric acid of a concentration

of

1n

was applied immediately and the time to break was recorded by an

electrical clock connected via movement of the lever arm of the test

Kuhl:

FRP Rods

fo r Composite Insulators

setup indicating hours, minu tes and seconds. The tests were carried

out indoors at room temperature.

sample

Figure 3

Test setup to cause

artificial

brittle fractures on assembled

FRP

rods.

3

FEATURESOF BRITTLE

FRACTURE

The first brittle fracture on a 420 kV silicone rubber insulator oc-

curred in 1978 [ 5 ]on a 24 mm FRP rod assembled with en d fittings with

a wide cleavage (Figure 4). The sealing of the end fitting was opened,

so that chemicals could enter the interior of the end fitting.

Figure

4

Brittle fracture

of a

420 kV silicone rubber suspension in-

sulator after 3 yr of service.

The broken insulator was part of

a

double suspension insulator

string. The parallel insulator held the line and was brought down for

evaluation purposes. Mechanical tests carried out on this insulator re-

sulted in no reduction of the ultimate tensile load . Brittle fractu re can be

simulated by means of test Procedure I1 (Figure 2). Both fractures, the

natur al brittle fracture (Figure 4) as well as th e artificial brittle fracture

(Figure

5 )

show fracture surfaces arranged perpendicular to the axis

of the FRP rods and characteristic patterns of stress corrosion fractures

which cannot be simulated any other way.

Several tests performed with cyclic loads on test specimens without

simultaneous application of acid resulted in different fracture patterns.

Hence, they were obtained by much higher l oads and required longer

times to failure than necessary for stress corrosion failure.

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IEEE Transactions on Dielectrics and Electrical Insulation Vol 8 No 2 April 2001

I85

Composition

Glass

B 2 0 3 YO

Silenka

6.7

Silenka

5.1

PPG

4.9

Bayer 4.8

Vitrofil

4.6

OCF8 9

4 3

Ahlstrom

4 0

OCF424 3.3

Norsk

2.6

Norsk ECR 18720

Figure 5 Brittle fracture

of

a test specimen according

to

test Proce-

dure

11.

The experience with brittle fracture of silicone insulators showed

that in every case failure of the sealing between the housing and at

least one of the end fittings was invo lved. Brittle fractures on the free

length of the

F R P

rod had never been found for this particular insulator

design. Tests carried out on test specimen with and without silicone

sheath according to Figure 2, test Procedure 111, found that artificial

brittle fractures of the

FRP

rods can be obtained only from naked rods.

4

ARTIFICIAL BRITTLE

FRACTURES

4.1 INFLUENCE OF NOMINAL

TENSILE

STRESS

The results shown in this study refer to nominal tensile stresses.

They are defined as those tensile stresses calculated from the applied

static tensile load divide d by the unloa ded cross section of the

FRP

rod.

This also is applicable for the e nd fittings; however, it is a stress indica-

tion only and does not specify any real stresses quantitatively.

The general characteristics of brittle fracture of FRP rods obtained

from nomin al tensile stress and simultaneous application of

1

n nitric

acid is shown in Figure 6.

These results were obtained from

24

mm rods made of E-glass in

postcured condition (180C for 16 h). In Figure 6curve1envelops the

most resistant E-glass composition (glass type OCF 859/resin type

BAY

XlOO/HHPA) and cu rve 2 envelo ps one of the most susceptible E-glass

composition (glass type Silenka/resin type BAK/M THP A). oth curves

represent artifici al brittl e fractures obtained from the free length of FRP

rods. Curve 3 envelops failures of FRP rods within end fittings designed

as wide cleavage of the cone wedge type. It can be seen that the stress

caused by end fittings can lead to a drastic reduction of the time to

failure

(FRP

composition from curve 2). The section between curves

1

and 2 also represents the load time characteristics of all evaluated FRP

rods from 16 to 37 mm m ade of E-glass. It can be seen that considerable

differences exist regard ing the brittleness of FRP rods mad e of E-glass

caused by stress corrosion. For all

FRP

rods ma de of E-glass,a load level

exists indicating that at loads below this level brittle fracture does not

occur. For the free length of the rods, the level may exist at 60 MPa and

for the rod ends at 15 MPa under the above described test parameters.

10

1

1000

io000 i ooooa

Breaking

Time

[minutes]

Figure

6

Load time characteristics of brittle fractures of F R P rods

24 mm

diameter. Curves 1 and

2:

Test Procedure 111, Curve

3:

Test

Procedure

11,

wide cleavage.

Table 2 Variation

of

glass fibers. Breaking time (min)

of

24 m m FR P

rods at nominal stress

of 77

MPa, test Procedure

11,

end fittings

with

wide cleavage.

[ydrolysis

:ant

3

61

34

51

114

>18720

ptible

U

Cured

617

40

1

~

5821

'ostcurec

508

180

2982

levels can be assumed at operational service. From this point of view

it is most likely that brittle fracture may occur on composite insulators

under operational service conditions if the FRP rod is manufactured

from E-glass.

4.2

BRITTLE FRACTURE

RESISTANT FRP

RODS

The brittleness of

FRP

rods caused by stress corrosion can be influ-

enced by the glass composition as well as by the epoxy resin matrix and

the interface between glass fibers and matrix.

Table 2shows several parameters influencing the brittleness of

FRP

rods. As can be seen, the boron content of the glass fibers is of great

importance. Boron free glasses

B203

content 0.15%) result in brittle

fracture resistant FRP rods. A part from the boron free glass type Norsk

ECR

mentioned inTable

2

there exist some more boron-free glass fibers

as Stratifil (Gevetex), NT712 (PPG) and S2 (OCF). 24 mm FRP rods made

of

these glass fibers show the same or even better brittle fracture resis-

tance as found from Norsk ECR. Although boron free glass fibers used

in

FRP

rods result in brittle fractu re resistant

FRP

ods, the role of boro n

oxide in the glass composition is still unclear.

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186

Resin Hardener Break time (min)

type cured postcured

F HHPA

333 2 5 3

0164 HHPA

368

118

X18 HHPA 416 106

F PSAmix 661 220

0164 PSAmix

551

231

F 1102 217 79

F

HHPA 333 263

F THPA 374 309

F 4,4MDA 459

F

EH640

>1300*

F DDS

67

17

Kuhl:FRP

Rods for

Composite Insulators

Resin i

characteristic

bisph.Amixture

bi5ph.A regular

bisph.Adistilled

9

6

8

10

15

30

50

glass fibers itself. For the curing state of the matrix and the interface,

some effects can be assumed. Thus, some evaluations

on

the postcuring

state of epoxy matrix systems were carried out. Test specimens were

molded of various epoxy matrix systems and treated with various cur-

ing states. The test specimens were tested for bending strength and

deflection [21], tensile strength and elongation [22], Youngs modulus

[22] and density [23]. The curing state was determined by measuring

the softening temperature of the resin according to Martens [24],called

T temperature (Martins temperature).

3 pt

bending Def lect ion

Tensile

Pa

O L

O 100

120

Figure

7 Properties of an epoxy matrix depending on the softening

temperature T . System DOW331/MNA.

Figure 7shows the results obtained from the System 331/MNA be-

cause this system exhibits the post curing effect most impressively

As

known from cast resin, the mechanical prope rties of epoxy resin

sys-

tems improve with increasing curing state. This is also the case for

the system 331/MNA, One can assume that the increasing curing state

means an increasing density of crosslinking what results in a higher

stiffness of the material a nd in a more dense material,

Figure 7shows that these assumptions a re incorrect, It was found

that all evaluated epoxy systems showed a decreasing Youngs Modu-

lus and a decreasing density w ith increasing curing state to a different

extent. Curing and postcuring of epoxy matrixes mean that the matrix

duri ng crosslinking is subjected to a swelling effect [25,26]. In case of

reinforced epoxy

it

can be assumed that a compression force acts an

the interface between the

glass

fibers and the epoxy matrix. Postcur.

ing means also tha t the finish layer on the surface of the gla ss fibers

is hardening . Both effects result in propagation of cracks due to stress

corrosion that is not stopped at the interface, and that time to failure

due to stress corrosion can be expected earlier than in

the

case of

non.

postcured systems.

Figure

8

shows the scattering of time to fililure of the most brittle PRP

rod found d uring this stud y It was also found that the more brittle a PRP

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IEEE Transactions

on

Dielectrics and Electrical Insulation

Vol.8

No

2 April 2001

187

1 2

3

4 5

6

7

8

9 1 0 1 1

Rod No.

Figure 8 Scattering of time

to

failure.

FRP

rod

37

mm ystem DOW

331/MNA/PPG

E-glass, test Procedure I1 (wide cleavage) nominal

stress77

MPa.

rod system is, the more is the scattering of the test results. Apar t from

that, the results from the end fittings (test Procedure 11) scatter more

than from free length of

FRP

rods ( test Procedure 111). Test Procedure 111

may be

a

tool to check the brittleness of

FRP

rods in general. The test

is easy to perform in the test ar rangement shown in Figure

3

with test

specimens show n in Figure 2 (Procedure 111). The acid container made

of polyethylene should have such a size that the FRP rod is surrounded

by liquid thickness not

3

1cm and a liquid level of

> 4

cm. The lower

end of the acid container ha s to be attached and sealed to the surface of

the FR P rod in ord er to prevent the acid from coming in contact with the

lower end fitting of the FRP rod. The acid container should be covered

to prevent evaporation of the liquid >5 of its volume du ring the test

period,

The test specimen prepared this way can be loaded in the test setup

shown in Figure 3with a load high enough to cause a tensile stress of

340 MPa with in the cross section of the free length of the FRP rod and

maintain this stress for

a

time span of 96 h. Immediately after app lying

the load,

a

nitric acid of a concentration of

1

n

HN03

can be poured

into the acid container. The acid must not come into contact with the

end fittings of the test specimen. Brittle fracture resistant

FRP

rods can

be realized, if no fracture of the

FRP

rod occurs during the test period

of

96 h [14], see also Figure 6.

Brittle fracture

of FR P

rods made of E-glass can be initiated by any

other diluted acid with pH values t 4 . Tests performed with strong

organic acids (checked were formic acid, chlorinated acetic acid and ox-

alic acid) showed that these acids also can attack FRP rods. On the other

hand, FRP rods made of boron-free

glass

fibers withstood all diluted

acids. Water did not affect any of the eva luated

FRP

rod systems.

Glass monofilaments may contain tiny capillaries related to gase-

ous

bubbles in the molten glass during manufacturing of the monofila-

ments. This was discovered in the seventies for E-glass. Some interest-

ing effects concerning the electrical performance of

FRP

rods containing

capillaries were foun d by the author. Test standards were developed in

order

to

quan tify capillarity fo r the selection of

FRP

rods suitable for

HV

applications. A link between capillarity and bri ttle fracture of

FRP

rods

could not be estab lished , Today capillarity of FRP rods is not

a

problem

anymore. Both E-glass as well as boron-free glass exist that are nearly

fre e from capillaries.

4.3

CONCLUSIONS FROM

ARTIFICIAL BRITTLE

FRACTURE

Brittle fracture of

FRP

rods made of E-glass can be simulated by ap-

plying tensile stress and simultaneous application of diluted acids. The

fracture patte rns of artificial brittle fractures are very close to the frac-

ture patterns of brittle fractures of broken composite insulators out of

service. Tests conducted with test specimens under static and dynamic

loads without application of acid led to fracture patterns not compara-

ble

to

brittle fracture patterns. Brittle fracture of FRP rods occurred in

service

at

load levels far lower than the ultimate failing load of com-

posite insulators. This is consistent with load levels found for artificial

brittle fractures simulated with tensile stress and simultaneous applica-

tion of dilute d acids. These facts have led to the conclusion that brittle

fractures of composite insulators in service are the result of stress cor-

rosion, initiated by diluted acid at the surface of E-glass fibers under

tensile stress.

Brittle fracture of composite in sulato rs can be avoided by using FRP

rods made of boron-free glass fibers. This class

of

glass fibers is re-

sistant to acids and stress corrosion at load levels known from service

experience [14].

In general brittle fracture failure of

FR P

rods made of E-glass can

vary in a wide span of time. The fractures are subjected to a load time

characteristic depending on the applied tensile stress and the p H value

of the corrosive medium. The nominal stress can be enhanced by the

design of the end fittings at the end s of the FRP rods. Other general

factors influencing the brittle fracture failures are the boron content of

the glass fibers, the toughness of the resin matrix and the curing state

of the resin matrix.

These results obtained from artificia l testing of FRP rods made of E-

glass showed tha t the choice of the m aterial components (glass, resin,

hardener) can also lead to an unpredictable brittle fracture behavior,

In the frame of this stud y 62 variations of FRP rods have been evalu-

ated. Although some general trend s resulting from the

FRP

material

components could be found, there were some results which could not

be explained, such as shown in Table 2for the glass OCF 859 in combi-

nation with two different resins.

5 NATURAL BRITTLE

5 1 FRP RODS EXPOSED

FRACTURES

OUTDOORS

It is common practice for manufacturers of composite insulators to

evaluate the load time characteristic

of

FRP

rods under static loads

as

described in

[7].

In most cases it takes several years

to

obtain a load

time curve for a particular assembled

FRP

rod, Up to now, nothing is

specified about the condition of test specimens for the realization of the

load time curves. Knowing the impact of postcuring on brittle fractures,

24 mm

F R P

rods were exposed outdoors under static loads in a test

device similar to Figure 3.Rods with and without a sheath of silicone

elastomer in postcured condition (180C for 16 h) were tested.

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t

Y

3.5

6

9

14

Kuhl: FRP Rods for Composite Insulators

Observation

cracks and

delamination on

FRP

rod surfaces.

less surface erosion

brittle fracture on

free

F R P

rod length

surface erosion

F R P

rod surface

strongly eroded

1crack near end

fitting. minor

erosion

During the first three years of o utdoor testing

no

difference could

be observed between test specimens with and witho ut silicone sheath.

The obtained load time curves were similar. However, after three years

of lo ading visual in spection of th e naked FRP rods revea led some differ-

ences between

F R P

rods of differen t compositions as well

as

differences

between naked and sheathed rods. While the sheathed rods showed

minor influence of w eathering , all naked rods made of aromatic epoxy

systems showed strong discolo ration and stro ng erosion on their sur-

faces. A layer of loose gla ss fibers covered th e surfaces of the rods

due to ero sion of t he arom atic epoxy matrix; the more, the longer the

time of exposure . The rods compo sed from cycloaliphatic epoxy matrix

showed far less surface erosion for the same exposu re time.

Table 4shows the most interesting results from24 mm rods exposed

outdoor s under static loads in the plant location of CeramTec AG. The

environm ent of this plant ca n be characte rized by a relatively clean at-

mosphere,

M

500 m above sea level.

A

lot of forests and agriculture

domina te this area of m oderate clima te. The most interesting observa-

tion made was the fact that one brittle fracture occurred on one aromatic

epoxy system and rods from other systems showed some cracks run-

ning perp endicular to the ro d axis on its free length like a start of brittle

fracture.

In Table 4are listed also three epoxy systems and their time to break

from artificial brittle fractur e testing with nitric acid. There might be

a correlatio n between artificial brittle fracture testing and those obser-

vations made in outdoor exposure of

FRP

rods. Questions arose why

the aromatic F system from Table 4did not brea k. Microscopic evalua-

tion of the sur face of those r ods revealed s ome differences to the 0164

system. U nder the microscope the same cracks could be seen

on

the F

system

as

seen macroscopically on t he 0164 system; however, the de-

gree of matrix erosion due to weatherin g on the F system exceeded by

far the erosion seen on the 0164 system. It can be a ssumed th at crack

propag ation du e to brittle fracture caused by stress corrosion of th e

glass fibers and the speed of matrix erosion caused by wea thering are

in competition w ith each other. The high s peed of the matrix erosio n of

the F system inhibits crack propagation d ue to stress corrosi on.

The reason for brittle fractures observed on naked F R P rods exposed

outdoor s (without voltage, without acid) are still unknown. It can be ar-

gued th at diluted acid can exist in every moderate climatic atmosph ere

as

can be found in Germany. On the other hand, the evaluated epoxy

systems are mostly crosslinked with acidic hardeners. Hydrol ytic ef-

fects on the matrix system may play a role. It can also be assum ed that

the ultraviolet

UV)

part of the solar radiation may cause nitric acid for

crack initiation on the surface of FRP rods. The FRP rods covered with

silicone elastomer did not show any signs of brittle fra cture. For the

naked rods it can be concluded fromTable4that the observations made

on them follow the same trend

as

found for the results from artificial

testing.

5.2 FRACTURES ON SILICONE

COMPOSITE

NS

U LATO

R

S

Table 5lists the brittle fractu res

of

FRP

rods ma de of E-glass used in

silicone composite insulato rs from o perationa l service.

As

shown, the

majority of the broken in sulators were installed at harsh environm ental

condition s at high service voltages.

Table 4. Observations made on 24 mm F R P rods exposed outdoors

under static load. t b is the time to break under test procedure

2,

t

the exposure time under

a

tensile stress St

Matrix

0164

MTHPA

F PSA mixture

X100 HHPA

Art. test

t

MPa

6 1

102

252

217

In all cases the sealing between housi ng and the end fitting opened

and, except in one case, the live end fitting sides of the insulato rs were

involved . Brittle fracture on the free length

of

the

FRP

rods did no t occur

in contrast to what is reported in

[ 2 ]

and [ll] A corr elation between

the expected service tensile stresses and the time to f ailure could not

be found . However, 13 brittle fractur es out of 19 occurred on tension

insulators . The numb er of tension insulators in a

HV

transmissi on line

is small compared to the num ber of suspensi on insulators. In spite of

this fact brittle fracture occurred preferably on tension insulator s which

are usually more highly loaded than suspension insulators. This may

be an indication that tensile stress from service loading plays

a

role

concerning the statistical occurrence of brittle fr actures.

Table

5

pre sent s the result s of artificial testing of

FRP

rods from test

Procedure

11.

The compariso n between the time to failure from artificial

testing and from operationa l service leads to

a

similar trend concern-

ing the evaluated material composition. The resin matrix 331/MNA

combined with th e E-glass type PPG 712 was foun d to be the most

sus-

ceptible system concerning brittle fractu re during artificial testing, see

also Figure 8.

This system was also most susceptible under operational service

conditio ns. The systems 0164/MTHPA/Silenka and F/PSA mix t/OCF

859 resulted in sm all differences from artificial testing.

Under operation al service the performance of these two systems can

be considered as equal. The average time

to

failure of both systems

under operational service conditions results in 9,l years (one can also

assume that the time until ope ning of th e sealing between housing and

the end fitting is eq ual statistically). The system XlOO/HHPA/OCF859

exhibited

a

remarkable resistance

o

the occurrence of brittle fractur e in

artificial testing

as

well

as

under operational service. Although some

thousand silicone composite insulators still exist utilizing this type

of

FRP

rod in lin es up to 245 kV with end fittings of wide cleavage,

only one brittle fracture under operatio nal service conditions occurred

within a service time of more than 25 yr. Finally,

a

vast number of sili-

cone rubber insulators have been installed since 1983 utilizing

FRP

rods

containin g boron free acid resista nt glass fibers from which is k nown

the rate of brittle frac tures is zero. This fact is also consi stent with th e

resu lts of artificial testi ng.

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IEEE Transactionson Dielectrics and Electrical Insulation

Vol.

8 No 2 April 2001

Line

roltage

kV

420

420

420

420

420

420

420

123

123

420

230

420

420

420

420

123

420

420

420

15

89

String

config.

triple tension

dto

inv.V

suspens

double

tension

dto

dto

dto

double

suspens

triple tension

double

tension

double

suspens

dto

invertv

double

tension

spacer

suspens

spacer

double

tension

tension

Resin

system

11

MNA

164

MTHPA

PSA

mixt

100 HHPA

t

nounted

Y

0.08

0.08

0.4

1.3

10

10

1

6

15

12

2

g

a

ss

type

PG 712

jilenka

m

CF 859

~

Pos.

Reason

fracture fracture

life end bad sealing

groundend dto

lifeend dto

dto dto

dto dto

dto dto

dto dto

dto dto

dto dto

dto dto

dtoto

Table 5 Brittle fractures experienced on silicone composite insulators.

4

6

7

12

7

20

19

29

RP

rod

diam.

mm

37

37

37

37

24

24

24

24

24

24

24

24

24

37

37

24

24

24

37

dto dto

dto bad sealing

dto dto

dto bad sealing

dto dto

dto

dto

dto wide cleavage

range

t b

iostcurec

min

21

121

220

2982

anuf

year

979

1979

1979

1979

1981

1981

1981

1979

1979

1979

1981

~

1975

1975

1975

1975

1977

1975

1975

1976

1968

~

Env.

very severe, coast

dto

dto

dto

coastal severe

dto

dto

medium

medium

medium

rural

dto

coastal severe

severe

Alps

very severe

severe

very severe

very severe

railway

6 CONCLUSIONS

OMPOSITE

insulators installed outdoors in HV lines can suffer from

C

atastrophic mechanical failures at tensile loads far below their ul-

timate tensile strength. This fact, together with the unusual fracture

pattern of the

F R P

rods, leads to the assumption that brittle fracture of

FRP

rods is initiated by stres s corrosion. Nitric acid can be formed by

electrical discharge in hum id air and m ay hav e access to the surface of

FRP

rods.

FRP

rods have been tested artificially under tensile stress and simul-

taneous application of 1n nitric acid. The results from this testing lead

to the conclusions that brittle fracture of FRP rods

1. is

a

matter of stress corrosion,

2. follows the rules

of

fracture mechanics,

3. can be initiated by diluted acids and simultaneous application of tensile

4. is a matter

of

stress corrosion of

glass

fibers containing boron oxide

5 . can be prevented by using boron-free glass fibers.

A

broad variety of

FRP

rods made of different material components

were evaluated. It was sho wn that all E-glass fibers used in F R P rods

lead to rods susceptible to stress corrosion to different extent. Consid-

ering the para meter s of influence found, the occurrence of catastrophic

mechanical failures of composite insulators installed outdoors on H V

lines can be related to incidents and reasons:

1. Defect of the sealing between end fitting and FR P rod. Moisture pen-

etration into the inside of the end fitting with generation of an acidic

stress,

solution due

to

electrical activity (corona) in the area near the end fit-

ting. Acid attack upon non-brittle fracture resistant FRP

rods

which are

permanently under mechanical load.

2. Glass fibres used for

FRP

rods contain 8203

3. Epoxy matrix is not suitable. The main parameters that influence the

brittle fracture resistance are the curing state, the

toughness

and the

swelling characteristic of the matrix.

4. Interface between matrix and fibres is weak. Moisture in the interface,

missing coupling agents and bad sizing can lead to a weak fibre-matrix

bonding. FR P rods with weak fibre-matrix bondings are expected to be

more sensitive to brittle fracture.

It is most likely that more than one of these parameters determines

the time to failure of a particular composite insulator installed on a HV

line, as long as FRP rods are used made of

E-glass.

The brittleness of

FRP

rods can be checked simply by means of ten-

sile stress in the simultaneous presence of di luted acids ( test Proce-

dure 111). The test r esults obtained from this testing are consistent

qual-

itatively with the experience obtained from operational service of com-

posite insulators installed in outdoor

HV

lines. The use of boron-free

glass fibers in

FRP

rods have led

to

a new generation of silicone com-

posite insulators free

of

brittle fracture in 1983. This is proven by the

vast number of suc h insulators manufactured during the last

15

yr. The

number of brittle fractures from those insulators is zero.

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