2.Brittle Fracture Failure of Rods

8/6/2019 2.Brittle Fracture Failure of Rods

1/12

1> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1

Brittle Fracture Failure of Composite (Non-

Ceramic) Insulators

Maciej S. Kumosa

Abstract In this work, additional input is provided into the

ongoing discussion on the mechanisms, causes and remedies of

brittle fracture failures of composite (non-ceramic) insulators.

This has been done partially to complement the recently

published article on this topic entitled IEEE Task Force

Report: Brittle Fracture in Non-Ceramic Insulators [1]. At

first, the most important characteristics of the brittle process are

described in this study. Then, two important examples of

insulator failures by brittle fracture are shown and their causes

explained. Finally, several recommendations on how to avoid

brittle fracture in service are presented to supplement those

already given in [1]. This work should improve the current state

of knowledge on brittle fracture of composite insulators as well

as provide a potential means of avoiding it in-service.

Index TermsBrittle Fracture, Composite, Non-Ceramic,

Polymer Insulators, Failure Analysis, Prevention.

I. INTRODUCTION

A. Comp

OMPOSITE suspension insulators are used in

overhead transmission lines with line voltages in the

range of 69 kV to 735 kV. The insulators, also calledas either non-ceramic, polymer or polymeric insulators

rely on unidirectional glass reinforced polymer (GRP)

composite rods as the principle load-bearing component. The

rods are manufactured by pultrusion and the constituents are

either polyester, vinyl ester, or epoxy resins reinforced with

either E-glass or ECR-glass (also called boron-free E-glass)

fibers. In spite of many benefits, which the insulators can offer

in comparison with their porcelain counterparts (high

mechanical strength-to-weight ratio, improved damage

tolerance, flexibility, good impact resistance, and ease of

installation), they can fail mechanically in service by rod

fracture. One of the mechanical failure modes of the insulators

is a failure process called brittle fracture [1-26] which is

caused by the stress corrosion cracking (SCC) of GRP rods

[27-41,52]. Several attempts have been made over the years

[1-26] to understand the process and provide potential means

of avoiding it in-service.

osite (non-ceramic) insulators

Manuscript received March 1, 2005. This research has been supported

since 1992 by the Electric Power Research Institute (under five consecutive

contracts) and a consortium of electric utilities consisting of the Bonneville

Power Administration (under four contracts), Western Area Power

Administration (four contracts), Alabama Power Company (two contracts),

Pacific Gas and Electric (three contracts), National Rural Electric Cooperative

Association (one contract). It was also supported by Glasforms, Inc (one

contract) and NGK-Locke (one contract).

M. S. Kumosa is with the Center for Advanced Materials and Structures,

Department of Engineering, University of Denver, 2390 S. York St., Denver,

CO 80208 (phone: 303-871-3807; fax: 303-871-4450; e-mail:

[email protected]).

The reason why the brittle fracture problem has drawn so

much attention is not because of the number of brittle fracture

failures, which has certainly not been significant [1], but

because of its characteristic features, which are:

- the failure is catastrophic,

- the failure is unpredictable; can occur after a fewmonths or after a few years of being in service,

- there are no reliable monitoring techniques which

could be used to monitor composite insulators in

service for potential brittle fracture failures,

- the causes of failures, if one reads the available

literature on this topic, are uncertain due to a number of

confusing journal articles and conference presentations.

B. Brittle fracture research at OGI and DU

Mechanical failure mechanisms of composite high voltage

insulators have been extensively studied at the Oregon

Graduate Institute (OGI) [9-15,30, 44-46] and presently at the

University of Denver (DU) [16,17,19, 21, 24-26,31-43,47,48].

This has been done in addition to the other research activities

on this topic [1-8,18,20,22,23]. In the insulator research at

OGI and DU particular attention has been given to the

understanding of the brittle fracture process that can occur in

composite insulators in service. SCC, which causes brittle

fracture of unidirectional E-glass/polymer composites, is

caused by the combined action of mechanical tensile stresses

along the fibers and a corrosive acidic environment [27-41].

The brittle fracture research in our laboratory was initiated in

1992 as a result of numerous insulator failures on a 345 kV

line [9]. It was further expanded in 1996 after additional

insulator failures on a 500 kV line [14]. Since we had alreadysome past experience with SCC [27-29], we were selected to

perform this research.

The major accomplishments in this insulator research

regarding brittle fracture have been:

- explanation of the 345 kV and 500 kV brittle fracture

failures [9-16],

- identification of the type of acid responsible for brittle

fracture [17, 26],

- simulation of brittle fracture with and without high

voltage [10-12,15,19,24],

C


2/12


- identification of several critical conditions leading to

brittle fracture [9-17,19],

- providing a ranking of the commonly used GRP rod

materials for their resistance to stress corrosion

cracking in nitric acid and thus brittle fracture

[21,19,36-40],

- recommendations of several stress corrosion tests for

the initiation [21,30,36-40] and propagation [10-12,19,

24,31-35] of brittle fracture cracks in unidirectionalGRP composites.

II. CHARACTERISTIC FEATURES OF BRITTLE FRACTURE

A. Brittle fracture description

In brittle fracture large cracks are formed inside the GRP

rods and run perpendicular to the long axis of the insulators

(Fig. 1). Due to the complexity of the problem several

different analyses are required as part of the failure

investigation. Brittle fracture can only be determined if the

following three analyses of composite fracture surfaces are

performed: macroscopic, microscopic and chemical. Thesefundamental analyses must be performed, all of them, to be

absolutely sure that an insulator has failed by brittle fracture.

1) Macroscopic analysis: It has been shown in [9-16]

that brittle fracture can occur either inside or just outside of

the energized end fitting (see Fig. 1). There is a relationship

between the location of failure and the position of the grading

ring. When grading rings were present, a vast majority of

failures occurred in the rod just above the grading ring

[9,10,12,16,26]. Also the location of failure was related to the

size of the grading rings. For higher line voltages with larger

grading rings the location of failure was found to occur at

larger distances from the end fitting [9,10,14,16]. In the

absence of the grading rings the failure location was found tooccur usually inside the fitting [10,12]. In some sporadic cases

the failure was a combination of transverse cracks in the

composite rods inside and just outside the fittings linked by

long axial splits along the rod [10,12].

A model has been proposed [9,10,14-16,19,25,26], which

explains the location of failure in the insulators, e.g. failure

inside the fitting or above the grading ring. The model is

based on the formation of nitric acid either on the external

surface of the insulators due to water droplet coronas (failure

inside the fitting), or inside the insulators in the presence of

water and partial discharges (PD) (failure above the

hardware). To support the model, electric field calculations

were performed [10,13,19] in order to demonstrate thepossibility of PD that are necessary to create nitric acid in

service inside macroscopic delaminations and other large

cracks that are always associated with the brittle fracture

process. It has been shown in [10,13,19] that if the cracks are

partially filled with water, the field concentration is high

enough to initiate PD inside the transmission composite

insulators near their energized ends, and to produce

nitrogenous species (nitrates).

The fact that in brittle fracture transverse fracture surfaces

form in the GRP rod and run perpendicular to the rod axis

should not be immediately used as definite evidence of brittle

fracture. According to [47,48] a crimped composite insulator

can fail mechanically in-service by rod fracture caused not by

brittle fracture but overcrimping. If crimped composite

insulators are incorrectly designed and manufactured

(excessive crimping deformations, incorrect design of the

metal end fittings, excessive in-service mechanical loads, etc.)

failure of the rod can also occur very close to the end fitting

and the macroscopic fracture surface will be flat and almostperpendicular to the long axis of the rod (Fig. 2)[47]. In this

case, the macro-fracture features will be almost

indistinguishable from the fracture features caused by brittle

fracture. This type of failure can occur at either end fitting. If

crimping deformations are large, the loads at rod fracture

could be quite low [47,48]. Most likely some of the reports in

the past of brittle fracture failures at the cold ends must have

been related to the rod failure by a combination of

overcrimping and in-service mechanical tensile loads. The

only way to distinguish this type of failure from brittle

fracture is by performing a detailed microscopic analysis of

the rods fracture surface [47].

Fig. 1. Examples of brittle fracture failures of 500kV suspension insulators;

(a) failure inside the fitting and (b) failure above the hardware [14,26].


3/12


Fig. 2. Failure characteristics of overcrimped 115kV insulator; (a) macro-

damage zone, (b) composite fracture surface and (c) typical example of

fractured fiber from the surface in Fig. 2b [47].

Fig. 3. Mirror (*), mist (**) and hackle (***) zones on the fracture surface of

E-glass fiber exposed to nitric acid [47].

2) Microscopic analysis: The microscopic observations

of numerous field-failed insulators have shown that the brittlefracture surfaces in the GRP rods formed either inside or

above the hardware exhibit noticeably different micro-features

[9,10,12,14,15,19]. If failures occurred inside the fittings, the

surfaces were usually heavily contaminated by thick surface

deposits (see for example Fig. 5a). For the failures above the

hardware the amount of surface contamination was found to

be significantly lower. In addition, more resin decomposition

by PD was observed on the composite fracture surfaces above

the hardware than inside the fitting. These differences were

explained in [10-16,19] and the internal and external brittle

failure processes were simulated in small composite GRP

specimens under laboratory conditions [10-12,14,19].

The most characteristic feature of SCC of unidirectional E-glass/polymer composites and thus brittle fracture of non-

ceramic insulators is the formation of the mirror, mist and

hackle zones on the fracture surfaces of broken fibers. These

three zones can be easily observed for example in Fig. 3 of

this paper. The size of the mirror zones does not depend on

the chemical environment (usual misconception) but is related

to the magnitude of the mechanical stresses [27]. The fiber

fracture under stress corrosion is caused by the ion exchange

mechanism in which hydrogen ions from an acidic solution

replace metal ions (Al, Ca, Fe, Mg) in the fibers. This

weakens the fibers causing their fracture under low tensile

stresses [27].

According to the experimental observation presented in[49], cracks in brittle materials such as glasses suffer a

dynamic instability, which makes them unable to accelerate up

to high velocities predicted by classical theories of dynamic

fracture. Therefore, the transition from the mirror to hackle

zone is related to a critical crack tip velocity above which the

fracture process is highly unstable [34,49]. Therefore, it can

be shown that the size of the mirror zones depends primarily

on the applied stress. The entire fracture process, after the

formation of a C-type flaw, is very fast but not directly

related to an acid attack on glass fibers [27,34]; an acid is only


4/12


necessary to initiate a C-type flaw. If we consider the

average crack tip velocity across a glass fiber to be about 200

m/s [49] then the time for fracture of an average fiber with

14m diameter is approximately 70ns. From the moment of

the flaw initiation until the final failure of the fiber, the

process is dynamic not static (another usual misconception)

[34].

The zones shown in Fig. 3 are usually damaged during a

corrosion process. This process is called post failure damage

[10,12,27], occurring after the fracture of individual fibers,

and is caused by the chemical attack of a corrosive

environment on the newly formed fiber fracture surface. In

some cases, numerous surface cracks are formed in the mirror

zones due to the leaching of aluminum, calcium and other

metal components out of the glass fibers (see Fig. 4).

Fig. 4. Post failure damage on the fracture surface of E-glass fiber exposed to

oxalic acid [10].

The post failure damage is an important phenomenon very

useful in the failure analysis. Its characteristics can be used to

determine the cause of SCC for a composite structure. The

amount of post failure damage is related to the acid type, its

concentration and the time of exposure [10,12, 27,30]. For

example, nitric acid of pH 1 will not develop post failure

damage within a few weeks of exposure whereas other acids

of the same concentration (sulfuric, hydrochloric, oxalic) will

after a few hours or days [10,12,27,30]. In the case of oxalic

acid the post failure damage can be so extensive (Fig. 4) that

the entire fracture surface including the mirror, mist andhackle zones is essentially shattered.

Another very important issue related to the micro-fracture

features of composite insulators is the nature of the deposits

on their fracture surfaces, in particular, when they were

formed inside the fitting (see Fig. 5) [9,10,12,14,15]. Usually,

the deposits are so thick that the fracture surfaces of individual

broken fibers cannot be seen. Critical information about the

causes of failure by brittle fracture can be gained by

chemically analyzing the deposits [9,10,12,14,15,19]. For

example, metal ions such as iron (Fe) and zinc (Zn) from the

fittings can be traced along the insulators, which failed in-

service, by brittle fracture. By tracing the metal ions, the

extent of water/acid ingress inside the insulators can be

established. Also, the presence of other contaminates such as

sulfur, chlorine, sodium, potassium, etc., which could

contribute to the failure process, can be established by

chemically analyzing the deposits [9,10,12,14,15,19].

3) Chemical analysis: According to [1] the definitive

method of detection of brittle fracture consists of an elementalanalysis of the relative depletion of calcium and aluminum

ions in the fiberglass core rod. Scanning electron microscope

(SEM) and energy dispersive X-ray (EDX) analysis have been

used in studying the corrosive morphological damage and

elemental depletion analysis of fiberglass. Using these

techniques, the decreases in calcium and aluminum relative to

silicon can be determined. Other methods that are useful in

this detection include Auger spectroscopy to define the

constituents of the glass fiber.

Fig. 5. Examples of surface deposits on brittle fractures of composite

insulators [10,14,15].

Certainly, aluminum and calcium are depleted in the glass

fibers on the composite fracture surfaces in the field failed (by

brittle fracture) composite insulators [10]. Also, the depletion


5/12


of Al and Ca indicates SCC and thus brittle fracture as a cause

of failure as stated in [1]. However, EDX and SEM could not

be successfully used to determine the depletion in the field

failed E-glass fibers on the brittle fracture surfaces

[9,10,14,15]. Since the chemical composition of the deposit

shown in Fig. 5 consisted predominantly of Ca (with traces of

Fe, Zn, S, Cl, etc.), the application of the two techniques could

not determine the actual composition of the glass fibers on the

composite surfaces. This could only be accomplished usingAuger spectroscopy with depth profiling [10]. EDX showed

the chemistry of the deposit, not of the fibers.

Fig. 6. FTIR analysis of failed by brittle fracture suspension composite

insulator; (a) failure morphology with site locations for FTIR and (b) FTIR

spectra for the sites in Fig. 6a. Band at 1386.8cm-1 indicates presence of

nitrates [26].

Even if the depletion process in glass fibers in a field-failed

insulator is determined, this will still not establish the actual

cause of failure. In order to determine the actual composition

of a chemical environment responsible for brittle fracture a

detailed surface analysis must be performed, not by using

Auger, SEM and EDX, but for example by employing Fourier

Transform Infrared (FTIR) spectroscopy which can detect

nitrates [17, 26]. Using FTIR, nitrates were detected on the

composite surface in the brittle fracture zones of several

suspension insulators (Fig. 6) indicating that the failure was

caused by nitric acid [17,26]. It was also shown in [26] that

the highest concentration of nitric acid existed on the fracture

surface on both sides of a separation caused by water ingress

along the GRP rod/housing interface (Fig. 6, sites 2, 3 and 4).

This has finally proven that the model proposed in [9-16,19]

regarding the nitric acid formation process inside the

insulators is correct.

The most important information about the causes of brittlefracture failures is not in the E-glass fibers but in the surface

deposits (contamination). Since the surface chemistry is so

important, recommendations must be provided describing the

procedures for handling field-failed insulators. After failure,

fracture surfaces in composite insulators should be

immediately protected from any further contamination, for

example by placing both sides of a field failed insulator in

plastic bags. The composite fracture surfaces should not be

touched with fingers, sprayed with chemicals etc., which is

usually the case.

B. Results and frequency of occurrence

It is most unlikely that all the cases of brittle fracture

failures reported in past were fully analyzed macroscopically,

microscopically and chemically as recommended here and in

[1]. We also suspects that many failures reported in the past

were incorrectly identified as brittle fracture instead of as

overcrimped failures. Also, many brittle fracture failures have

not been reported at all. In our research we have identified

approximately five 115 kV, twenty 345 kV and four 500 kV

insulators [9,10,12,14,19], which failed by brittle fracture.

We must agree that the probability of insulator failure by

brittle fracture substantially increases with an increase in line

voltage [1]. Certainly, the higher field concentration will

create an environment more suitable for the initiation of brittlefracture [10,13,19]. Therefore, the only model of brittle

fracture, which could explain these observations, is a model,

which takes into consideration not only the presence of

moisture and mechanical stresses but also the effect of the

electric field [25]. However, even if our high voltage brittle

fracture model is accepted, it cannot be used to predict these

failures since all critical electrical, mechanical and chemical

conditions for brittle fracture are unknown at present, and

most likely, will never be known.

C. Examples and explanations of brittle fracture failures

The failures described in [1] as cases D and E were quite

widely publicized in the insulator community at different

IEEE meetings. These two cases, Cases D [9-13,15,19] and E

[14], were also extensively studied at OGI and DU. It is our

opinion that the two cases are of particular importance due to

the sheer number of failures in Case D and high voltage in

Case E.

345 kV failures

The failures occurred in 1990/1991 on a 345 kV line.

Fourteen brittle fracture failures occurred resulting in the drop

of the conductor (see Fig. 7) within two years of service. In


6/12


addition almost 200 insulators were found to be severely

damaged. We had full access to all the failed and damaged

units removed from the line [9]. The failure analysis of the

damaged units (not the failed ones) was found to be

particularly useful. The damage process was observed at

different stages of its development in the damaged insulators.

In order to explain the 345 kV failures numerous

experimental techniques were employed [10,12]. The field-

failed units were examined using optical and scanningmicroscopes on the macro and micro-levels [9,10,12,15,19].

Detailed chemical analyses of the fracture surfaces taken from

the failed units were performed. Unique new experimental

techniques were developed to simulate the brittle fracture

process under laboratory conditions with and without high

voltage fields. All of the above proved that the failures were

caused by brittle fracture. However, the exact causes of the

failures were much more difficult to determine. Finally, after

simulating numerically the mechanical performance of the

insulators with the epoxy cone end fittings using advanced

finite element techniques [15,19] (see Fig. 8), it was

established that a manufacturer must have applied excessivemechanical loads during proof testing [15,19]. The large loads

applied to the insulators during proof testing crushed the

epoxy cones inside the fittings, as stated in [1] and in [15,

19], allowing easy access of water to the GRP rod of the

insulators. In addition, water was also allowed to stay inside

the fittings near the triple point shown in Fig. 8a,b. It was also

established that all the units on the 345 kV line were not

protected against moisture ingress into their end fittings. No

silicone gel was applied at the fitting/rod interface to achieve

the moisture seal.

Fig. 7. 345 kV suspension insulator failed in-service by brittle fracture

[9,10,15,26].

Fig. 8. Finite element model of suspension insulator with the epoxy cone end fittings; (a) end fitting design (schematic) (b) deformed finite element mesh and (c)


7/12


mechanical stress distributions in the cones [15,19].

500 kV failures

The failures described as Case E of [1] were also studied in

this laboratory [14]. The failures were caused by spilt epoxy

on the hardware during manufacturing (see Fig. 9) allowing

easy access of water into the end fittings [1,14].

Fig. 9. Spilt epoxy on the upper surface of the hardware of 500kV suspension

composite insulator [14].

The research performed on the 500kV units [14] did not

provide any significant new information regarding the macro

and micro-features of brittle fracture. It showed however three

very important new things [14]:

1. The amount of water responsible for brittle fracture

must have been extremely small. All units from the

line, including the failed ones, passed the dye

penetration test.

2. Large amounts of water present on the surface of theGRP rods in the insulators did not lead immediately to

brittle fracture [14]. A couple of insulators from the

line were found with the rubber housing near the hot

end significantly damaged and the GRP rod completely

exposed to moisture, without brittle fracture (Fig. 10).

3. If water and PD are present at the rod/housing interface

above the hardware, cracks can be found in the rubber

housing initiating at the interface and, then, propagating

outwards across the rubber towards the external surface

of a composite insulator (see Fig. 11).

The formation of the cracks in the housing shown in Fig. 11

could explain the origin of the damage to the housing

presented in Fig. 10. It could also suggest that when such

cracks are present, large amounts of water will penetrate the

housing through those cracks, further accelerating the damage

process in the housing. However, if a large quantity of water

is present inside an insulator, nitric acid forming at the rubber

housing interface will not be concentrated high enough. As a

result, the critical chemical conditions for brittle fracture (acid

concentration) will not be satisfied [16]. It has been shown

that the critical pH for SCC in nitric acid for a polyester type

composite with E-glass fibers is somewhere between 3 and 3.5

and between 2.5 and 3 for an epoxy based composite [10,12].

Fig. 10. Damaged rubber housing in 500 kV composite insulator near its hot

end [14].

Fig. 11. Cracks in rubber housing at different stages of their formation; (a)

partial crack and (b) through crack [14].


8/12


D. Causes of brittle fracture

Obviously, if one knows the causes of brittle fracture one

can also provide remedies. Therefore, a few different models

of brittle fracture have been proposed over the years. The

most important ones ( Models I-III) are listed below.

Model I. According to [22] the failure of in-service NCI

in the brittle fracture mode can occur under the influence of

water and mechanical stresses, and the failure is more likely to

happen with water than with acids.

Model II. According to [18,23] the brittle fracture of the

FRP [Fiber Reinforced Polymer] rod of composite insulators

is associated with an acid, derived from the hardener, lodged

on the surface of the rod combined with the ingress of water at

the same location and a mechanical tension load applied to the

insulator.

Model III. Brittle fracture is caused by nitric acid being

formed in service due to electrical discharge, ozone and

moisture [6,8-12,14-17,19-21,24-26]. The acid can either be

formed by water droplet coronas on the insulator surface near

its energized end or by PD inside the insulator, close to its

end.The only agreement between Models I - III is that brittle

fracture is initiated by moisture ingress into a composite

insulator. The issue of the chemical causes of brittle fracture

has been recently discussed in [25]. In particular, the

credibility of the models I-III has been evaluated. It was

shown that the only model, which can explain all the aspects

of brittle fracture is Model III. This model also considers a

strong effect of the electric field on the brittle fracture process.

E. Manufacturing prevention

Design: For the prevention of brittle fracture of non-

ceramic insulators an excellent moisture seal and the use of

the proper sheath thickness are very important [1]. It has

already been recommended in [15] that a high quality sealant

should be used to protect the interface between the fitting and

the rod/housing. It has also been stated in [1] that the grading

ring is important since it will minimize corona cutting in the

rubber housing, which is certainly correct. However, none of

the field failed insulators described as Cases D and E of [1],

and other units from the same lines [9,10,14], showed any

evidence of any damage to the rubber housing from the

outside. Only in two insulators, which did not fail by brittle

fracture for Case E, numerous cracks were found in the

housing propagating from inside out (Fig. 11) [14]. Therefore,

it is doubtful that grading rings will prevent brittle fracture ifwater ingress is allowed through the fitting. Since the failure

above the hardware, which is the most common type, is

associated with the movement of moisture along the rod near

the rod/housing interface, the grading will only postpone the

process, but do not prevent it.

1) Materials: If the GRP rods can fail by SCC when

exposed to nitric acid and mechanical tensile loads, the easiest

way to prevent this process from occurring in-service is by the

chemical optimization of the rods making them immune to

brittle fracture. Unidirectional glass/polymer composites, used

in the GRP rods, can be designed for their very high resistance

to SCC in nitric acid [19,21,24,25,36-40], high resistance to

moisture absorption [21,42], high resistance to mechanical

surface damage [21], high resistance to the development of

leakage currents [21,43], and to ozone exposure [21]. The

final recommendations on how to prevent brittle fracture by

the proper chemical composition of the GRP rods were given

in [25,52]. Below, the most important conclusions are listed

from our rod design research performed on several GRP rodcomposites from a single supplier based on E-glass and ECR-

glass (with low and high seed counts) fibers embedded in the

modified polyester, epoxy and vinyl ester resins.

Resistance to SCC in Nitric Acid

The resistance to brittle fracture of insulator composites is

strongly affected by such factors as fiber and resin type,

surface fiber exposure, resin fracture toughness, moisture

absorption, and interfacial strength [19,21,24,25,27-41,52].

Out of three E-glass/polymer composites (based on modified

polyester, epoxy and vinyl ester resins), the resistance to the

initiation of SCC in nitric acid of E-glass/modified polyester

was found to be 10 times lower than for E-glass/epoxy, and

approximately 200 times lower than for E-glass/vinyl ester.The ECR-glass/polymer composites exhibited vast

improvement over the E-glass/polymer materials. The highest

resistance to the initiation of SCC damage in the ECR glass-

based composites was found for the low seed ECR-glass

fibers embedded in either epoxy or vinyl ester resins. The

ECR glass composites were shown to be equally resistant to

the propagation of SCC even under highly accelerated testing

conditions, which was not the case for the E-glass fiber based

composites.

Resistance to Moisture Absorption

The resistance to moisture absorption of insulator

composites affects their insulation properties, resistance to

SCC and brittle fracture, and their overall mechanicalproperties [19,21,42,43,52]. The E-glass-modified polyester

system exhibited by far the worst resistance to moisture

absorption with a very high rate of moisture diffusion and

high maximum moisture absorption in comparison with the

other two E-glass based materials. They had very similar rates

of moisture absorption, however the epoxy-based system did

take more moisture than the vinyl ester-based material. A

slight fiber effect was found to exist on the moisture

absorption properties of the composites based on the three

different resins. In general, composites based on the ECR

(high seed)-glass fibers took smaller amounts of moisture than

their E-glass and ECR-glass (low seed)-glass fiber-based

composites.Resistance to Interfacial Failure

Interfacial splitting along the glass/fiber interfaces

accelerates brittle fracture and therefore should be minimized

[19,52]. The resistance of the interfaces to failure caused by

shear in dry E-glass/polymer composites was found to be the

lowest in E-glass/modified polyester followed by E-glass

epoxy (approximately 50% higher) and E-glass/vinyl ester

(approximately 4% higher) than for the epoxy based

composite). Immediately after full saturation with distilled

water at 50C the resistance to interfacial splitting was only

slightly reduced. The quality of the interfaces of the ECR-


9/12


glass fiber composites has not been investigated. It can be

assumed however that their resistance to interfacial splitting

would be similar to the E-glass/polymer systems.

Resistance to Overcrimping Damage

If overcrimping can cause brittle fracture [1], it should be

minimized. It has been shown in our work that the resistance

to surface compression caused by crimping of the E-glass and

ECR-glass composites is scale dependent [21]. On a micro-

scale the vinyl ester based composites (with E-glass and ECR-glass fiber) exhibited the highest resistance to compressive

surface damage, which was attributed to a high fracture

toughness of their resin. The resistance of the vinyl ester

based systems was followed by the epoxy and modified

polyester based composites. On a meso-scale, the resistance to

surface compression strictly depends on the amount of glass

fibers left exposed on the composite surfaces during

pultrusion. The macro-resistance, on the other hand, depends

on the type of fibers, polymers, and the strength of their

interfaces.

Resistance to Leakage Currents

The resistance to leakage currents of insulator composites

[21,43] could be more important for their insulation propertiesthan for their resistance to brittle fracture. The fact is however

that the electric field plays a critical role in brittle fracture

[25]. Therefore, high leakage currents could accelerate the

nitric acid formation process inside the insulators. After

boiling in deioniozed water with 0.1% (by weight) of NaCl for

four days, E-glass/modified polyester developed significantly

higher leakage current that the E-glass/epoxy and E-

glass/vinyl ester systems, caused by its high water absorption.

For the same amounts of absorbed moisture by their resins, the

ECR (high seed)-glass fiber composites developed several

hundred times higher leakage currents than their E-glass

counterparts. The leakage currents in the composites with

recently developed ECR (low seed)-glass fibers were similarto the E-glass-based systems. The high leakage current for

high seed ECR-glass fiber composites subjected to moisture is

one of the major reasons why most manufacturers have not

used these materials until now.

Resistance to Ozone Damage

The three E-glass/polymer composites were exposed to

ozone for a period of nine days [21]. It was found that the

bonding environment in the modified polyester based

composite was severely compromised by ozone. The epoxy

based system showed similar evidence of damage, but to a

much smaller extent. The E-glass/vinyl ester material was

found to be the most resistant to ozone.

Ranking of Composites for Resistance to Brittle FractureWe have not yet identified all critical material related

factors affecting brittle fracture. In addition, such known

factors as the resistance of insulator polymers and their

interfaces with glass fibers to the decomposition by discharge,

acids, electric wind, etc., has not been systematically

investigated. Also, no systematic studies of the resistance to

moisture movement along E-glass and ECR-glass composites

have been performed. Considering however the thoroughly

examined material properties of several different composite

systems for high voltage insulation applications, E-

glass/modified polyester exhibits by far the lowest resistance

to brittle fracture with the ECR (low seed)- glass/vinyl ester

being the best. The ECR (low seed)/epoxy and E-glass/vinyl

ester must also be ranked very high [52].

2) Process Controls: The GRP rods should be free from

large voids, cracks or any other type of macroscopic damage,

as stated in [1] to minimize the probability of brittle fracture.

Certainly, such material imperfections could create problems

caused by the development of PD especially in the presence of

moisture. However, such imperfections can easily develop in-service if water is allowed into the insulators. [22]. Therefore,

the fact that voids and cracks are not present in the rods

during manufacturing does not mean that these imperfections

will not develop in service causing problems.

Some insulator manufacturers occasionally performed

sandblasting of their composite rods to increase adhesion

between the rods and the rubber housing [21,38,39,52].

During sandblasting the surface of the rods can be severely

damaged. This damage is usually created in the glass fibers

left on the composite surface. The initial assumption was that

sandblasting could have an extremely negative effect on the

resistance of the GRP rods to brittle fracture; therefore, this

effect was investigated in [21,38,39,52]. It was found howeverthat low/medium sandblasting could actually improve slightly

the resistance of the composites to SCC in nitric acid. This

was attributed to the relaxation of mechanical residual stresses

on the surface of the composites due to sandblasting. This

clearly indicates that not all types of damage to the composites

can negatively affect their resistance to brittle fracture.

It has been suggested in [1] that over-stressing the GRP rod

due to crimping can lead to brittle fracture. This is certainly an

important issue related to the insulator design and its

manufacturing. The fact is that extensive damage to the GRP

rods can be created by insulator manufacturers by applying

excessive crimping [44,47]. Therefore, this problem has been

investigated in great details by us in [19,21,44-48]. It has beenshown that overcrimping will certainly lead to the type of

fracture, which was described in Section II.B.1 of this work.

At the same time, not a single insulator failure by brittle

fracture initiated by overcrimping has been documented [1].

However, excessive and badly distributed crimping

deformations on the rod surface inside the fittings in

combination with high mechanical loads will significantly

increase the tensile axial stresses in the rods just outside the

fittings [44-48]. Then, if nitric acid is present on the surface of

the rod in this location, the probability of failure by brittle

fracture could increase due to the higher mechanical stresses.

The cracks caused by overcrimping [44,47], depending on

their location, size and orientation could reduce the resistance

of the insulators to brittle fracture. However, the critical

mechanical flaw size and type, and the loading conditions for

the initiation of SCC are unknown at present. Therefore, we

can also speculate which composite system (see the Materials

part of this section) will have the highest resistance to brittle

fracture if initiated by overcrimping. It can be safely

assumed, however, that the vinyl ester based composites will

have the highest resistance to brittle fracture due to its high

fracture toughness. Certainly more research is required to


10/12


understand the effect of overcrimping on the initiation of

brittle fracture

F. User Prevention

Composite insulators should not be mechanically damaged

at any stage; manufacturing, transportation, installation.

Certainly, damage caused by mishandling may result in a

brittle fracture or some other type of failure as stated in [1].

However, the evaluation of the effect of mechanical damage(of various types) on the electrical and mechanical in-service

performance of composite insulators is not a straightforward

problem. Numerous different types of failure caused by

mishandling can exist in non-ceramic insulators. At the same

time, we still do not know what is acceptable and what is not

regarding the mechanical damage and its effect on the short-

and long- term properties of the insulators and, in particular,

on brittle fracture.

The critical type of damage for the initiation of brittle

fracture in composite insulators subjected to the combined

effect of mechanical, electrical, and environmental stresses is

an issue which most likely is never going to be correctly

addressed. The best example of the complexity of this

problem is gun shot damage. In certain areas, the insulators

can be severely damaged by gunshots [51]. When damaged by

gunshots the insulators are entirely open to moisture ingress.

In addition, huge amounts of mechanical damage are created

to the GRP rods. It would be difficult to imagine any worse

type of mishandling of the insulators in-service than by

gunshots. Yet, not a single composite insulator has been

reported to fail by brittle fracture initiated by gunshots.

Therefore, the only credible recommendation, which can be

provided on how to avoid brittle fracture due to mishandling

is related to the moisture sealing conditions, which should

never be compromised during manufacturing, transportationand installation.

III. CONCLUSIONS

The brittle fracture process in composite non-ceramic high

voltage insulators has been discussed in this study. This was

done partially to supplement the information recently provided

in [1]. The most important failure characteristics, examples

and causes of failure, and possible prevention methods have

been presented in this work in an attempt to improve the

current state of knowledge on brittle fracture of the insulators.

It has been shown that despite the recent major advances in

the failure analysis of the insulators there are still severalcritical issues which need to be correctly addressed in the

future in order to fully understand this failure process and, in

particular, to be able to prevent it from occurring in-service.

ACKNOWLEDGMENTS

The author is especially grateful to Dr. J. Stringer and Dr.

A. Phillips of EPRI, Mr. C. Ek, D. Nicols and Mr. R. Stearns

of BPA, Mr. O. Perkins and Mr. F. Cook of WAPA, Mr. D.

Mitchell of APC, Mr. D. Shaffner of PG&E and Dr. T.S.

McQuarrie of Glasform, Inc. without whose technical and

financial support the insulator research at OGI and DU would

have not been possible. I am also very grateful to my

colleagues, Dr. P. K. Predecki, Dr. D. Armentrout and, in

particular, my son L. Kumosa for their help in the preparation

of this manuscript.

REFERENCES

[1] J. T. Burnham, T. Baker, A. Bernstorf, C. de Tourreil, J.-M. George, R.

Gorur, R. Hartings, B. Hill, A. Jagtiani, T. S. McQuarrie, D. Mitchell, D.Ruff, H. Schneider, D. Shaffner, J. Yu and J. Varner, IEEE Task Force

Report: Brittle Fracture in Non-Ceramic Insulators,IEEE Transactions

on Power Delivery, Vol. 17, No. 3, pp. 848-856, July 2002.

[2] H. D. Chandler, R. L. Jones and J. P. Reynders, Stress Corrosion of

Composite Long Rod Insulators, in Proceedings of the 4th

International Symposium on High Voltage Engineering, 1983, paper

23.09, Athens, Greece, 5-9 September, 1983.

[3] B. Noble, S. J. Harris and M. J. Owen, Stress Corrosion Cracking of

GRP Pultruded Rods in Acid Environments, J. Material Science, Vol.

18, pp. 1244-1254, 1983.

[4] H. Chandler and J. Reynders, Electro-Chemical Damage to Composite

Insulator, CIGRE 1984 Session, Paper 33-08, 1984.

[5] S. J. Harris, B. Noble and M. J. Owen, Metallographic Investigation of

the Damage Caused to GRP by the Combined Action of Electrical,

Mechanical, and Chemical Environments,J. Material Science, Vol. 19,

pp. 1596-1604, 1984.[6] A. Akhtar, J. S. Nadeau, J. Y. Wang, D. P. Romily and C. Taggart,

Brittle Fracture of Non-Ceramic Insulators, Report for the Canadian

Electrical Association (186 T 350), Prepared by the British Columbia

Hydro and Power Authority, September 1986.

[7] M. J. Owen, S. J. Harris and B. Noble, Failure of High Voltage

Electrical Insulators with Pultruded Glass Fiber-Reinforced Plastic

Cores, Composites, Vol. 17, pp. 217-226, 1986.

[8] A. Akhtar and J. Y. Wong, Failure Analysis of Brittle Fracture in

Nonceramic Insulators,J. Composite Technology and Research , Vol. 9,

pp. 95 100, 1987.

[9] M. Kumosa, Failure Analysis of Composite Insulators from the Craig

Bonanza Line, Final Report to the Western Area Power Administration,

Oregon Graduate Institute, Portland Oregon, 1994.

[10] M. Kumosa, Q. Qiu, M. Ziomek-Moroz, A. Moroz and J. M. Braun,

Micro-Fracture Mechanisms in Glass/Polymer Insulator Materials

under Combined Effects of Electrical, Mechanical and EnvironmentalStresses, Final Report to the Bonneville Power Administration, Electric

Power Research Institute and the Western Area Power Administration,

Oregon Graduate Institute, Portland, Oregon, July 1994 (under contract

DE-AC79-92BP61873).

[11] M. Kumosa, Q. Qiu, E. Bennett, C. Ek, T. S. McQuarrie and J. M.

Braun, Brittle Fracture of Non-Ceramic Insulators, in the Proceed.

Fracture Mechanics for Hydroelectric Power Systems Symposium'94,

Canadian Committee for Research on the Strength and Fracture of

Materials (CSFM), BC Hydro, Sept. 1, pp. 235-254, 1994.

[12] Q. Qiu, Brittle Fracture Mechanisms of Glass Fiber Reinforced

Polymer Insulators, Ph.D. Thesis, Oregon Graduate Institute of Science

& Technology, Portland, Oregon, October 1995 (with Dr. M. Kumosa as

the thesis advisor)

[13] N. Fujimoto, J. M. Braun, M. Kumosa and C. Ek, Critical Fields in

Composite Insulators: Effects of Voids and Contaminations, Ninth

International Symposium on High Voltage Engineering, Graz, Austria,

August 28-September 1, 1995.

[14] M. Kumosa and Q. Qiu, Failure Investigation of 500 kV Non-ceramic

Insulators Final Report to the Pacific Gas and Electric Company,

Department of Engineering, University of Denver, May 1996.

[15] M. Kumosa, H. Shankara Narayan, Q. Qiu and A. Bansal, Brittle

Fracture of Non-Ceramic Suspension Insulators with Epoxy Cone End-

Fittings, Composites Science and Technology, Vol. 57, pp. 739-751,

1997.

[16] Interview with Maciej Kumosa, Research of Brittle Fractures in

Composite Insulators, Insulator News & Market Report, pp. 46-51,

July/August, 1997.


11/12


[17] A. R. Chughtai, D. M. Smith and M. Kumosa, Chemical Analysis of a

Field-Failed Composite Suspension Insulator, Composite Science and

Technology, Vol. 58, pp. 1641-1647, 1998.

[18] C. de Tourreil, L. Pargamin, G. Thevenet and S. Prat, Brittle Fractures

of Composite Insulators: Why and How they Occur, Power

Engineering Society Summer meeting 2000, Vol. 4, pp. 2569-2574,

2000.

[19] M. Kumosa, Fracture Analysis of Composite Insulators, EPRI, Palo

Alto, CA: 2001. 1006293.

[20] M. Kuhl, FRP Rods for Brittle Fracture Resistant Composite

Insulators,IEEE Transactions on Dielectrics and Electrical Insulation,Vol. 8, No. 2, pp. 182-190, 2001.

[21] M. Kumosa, Failure Analysis of Composite High Voltage Insulators,

EPRI, Palo Alto, CA: 2002.1007464.

[22] J. Montesinos, R. S. Gorur, B. Mobasher and D. Kingsbury, Mechanism

of Brittle Fracture in Non-Ceramic Insulators, IEEE Transactions on

Dielectrics And Electrical Insulation, Vol. 9, No. 2, April 2002.

[23] F. Schmuck and C. de Tourreil, Brittle Fractures of Composite

Insulators. An Investigation of their Occurrence and Failure Mechanisms

and Risk Assessment, for CIGRE WG 22-03

(http://www.corocam.com/papers.htm).

[24] M. Kumosa, L. Kumosa and D. Armentrout, Can Water Cause Brittle

Fracture Failures of Composite Non-Ceramic Insulators in the Absence

of Electric Fields?, IEEE Transactions on Dielectrics and Electrical

Insulation, Vol.. 11, No. 3, pp. 523-533, 2004.

[25] M. Kumosa, L. Kumosa and D. Armentrout, Causes and Potential

Remedies of Brittle Fracture Failures of Composite (Non-Ceramic)

Insulators,IEEE Transactions on Dielectrics and Electrical Insulation,

Vol. 11, No. 6, pp.1037-1048.

[26] A. R. Chughtai, D. M. Smith, L. S. Kumosa and M. Kumosa, FTIR

Analysis of Non-Ceramic Composite Insulators, IEEE Transactions on

Dielectrics and Electrical Insulation, Vol. 11, No. 4, pp. 585-596, 2004

[27] D. Hull, M. Kumosa, J. N. Price, Stress Corrosion of Aligned Glass

Fiber-Polyester Composite Material, Materials Science and

Technology, Vol. 1, pp. 177 182, 1985.

[28] M. Kumosa, D. Hull, J. N. Price, Acoustic Emission from Stress

Corrosion Cracks in Aligned GRP, J. of Materials Science, Vol. 22, pp.

331-336, 1987.

[29] M. Kumosa, Acoustic Emission Monitoring of Stress Corrosion Cracks

in Aligned GRP,J. Phys. D: Appl. Phys. Vol. 20, pp. 69-74, 1987.

[30] Q. Qiu and M. Kumosa, Corrosion of E-Glass Fibers in Acidic

Environments, Composites Science and Technology, Vol. 57, pp. 497-

507, 1997.

[31] D. Armentrout, T. Ely, S. Carpenter and M. Kumosa, An Investigationof the Brittle Fracture in Composite Materials used for High Voltage

Insulators, J. Acoustic Emission, Vol. 16, No. 1-4, pp. S10-S18, 1998.

[32] M. Kumosa et al., Micro-Fracture Mechanisms in Glass/Polymer

Insulator Materials under the Combined Effect of Mechanical, Electrical

and Environmental Stresses, Final report to BPA, APA, PG&E, WAPA

and NRECA, University of Denver, Denver, Colorado, December 1998.

[33] S. H. Carpenter and M. Kumosa, An Investigation of Brittle Fracture of

Composite Insulator Rods in an Acidic Environment with Static or

Cyclic Loading, Journal of Materials Science, Vol. 35, Issue 17, pp.

4465-4476, 2000.

[34] T. Ely and M. Kumosa, The Stress Corrosion Experiments on an E-

glass/Epoxy Unidirectional Composite, J. Composite Materials, Vol.

34, pp. 841-878, 2000.

[35] T. Ely, D. Armentrout and M. Kumosa, Evaluation of Stress Corrosion

Properties of Pultruded Glass Fiber/Polymer Composite Materials, J.

Composite Materials, 35, pp. 751-773, 2001.[36] M. Megel, L. Kumosa, T. Ely, D. Armentrout and M. Kumosa,

Initiation of Stress Corrosion Cracking in Unidirectional Glass/Polymer

Composite Materials, Composites Science and Technology, Vol. 61, pp.

231-246, 2001.

[37] L. Kumosa, D. Armentrout, M. Kumosa, An Evaluation of the Critical

Conditions for the Initiation of Stress Corrosion Cracking in

Unidirectional E-glass/Polymer Composites, Composites Science and

Technology, 61, pp. 615-623, 2001.

[38] L. Kumosa, D. Armentrout and M. Kumosa, The Effect of Sandblasting

on the Initiation of Stress Corrosion Cracking in Unidirectional E-

glass/Polymer Composites Used in High Voltage Composite (Non-

Ceramic) Insulators, Composites Science and Technology, Vol. 62, No.

15, 2002, pp. 1999-2015, 2002.

[39] L. Kumosa, M. Kumosa and D. Armentrout, Resistance to Stress

Corrosion Cracking of Unidirectional Glass/Polymer Composites Based

on Low and High Seed ECR-glass Fibers for High Voltage Composite

Insulator Applications, Composites Part A, Vol. 34, No. 1, pp. 1-15,

2003.

[40] D. Armentrout, M. Kumosa and T. S. McQuarrie, Boron Free Fibers for

Prevention of Acid Induced Brittle Fracture of Composite Insulator GRP

Rods,IEEE Transactions on Power Delivery, Vol. 18, No. 3, pp. 684-

693, 2003.

[41] D. Armentrout, M. Gentz, L. Kumosa, B. Benedikt and M. Kumosa,

Stress Corrosion Cracking in a Unidirectional E-glass/PolyesterComposite Subjected to Static and Cyclic Loading Conditions,

Composites Technology & Research, Vol. 25, No. 4, pp. 202-218, 2003.

[42] L. Kumosa, B. Benedikt, D. Armentrout and M. Kumosa, Moistures

Absorption Properties of Unidirectional Glass/Polymer Composites

Used in Non-Ceramic Insulators, Composites Part A, Vol. 35, pp.

1049-1063, 2004.

[43] D. Armentrout, M. Kumosa and L. Kumosa, Water Diffusion into and

Electrical Testing of Composite Insulator GRP Rods, IEEE

Transactions on Dielectrics and Electrical Insulation, in press, Vol. 11,

No. 3, pp. 506-522, 2004.

[44] M. Kumosa, Analytical and Experimental Studies of Substation NCIs,

Final Report to the Bonneville Power Administration, Oregon Graduate

Institute of Science & Technology, Portland, Oregon, 1994.

[45] A. Bansal, A. Schubert, M. V. Balakrishnan and M. Kumosa, Finite

Element Analysis of Substation Composite Insulators, Composites

Science and Technology, Vol. 55, pp. 375-389, 1995.

[46] A. Bansal and M. Kumosa, Mechanical Evaluation of Axially Loaded

Composite Insulators with Crimped End-Fittings,Journal of Composite

Materials, Vol. 31, No 20, pp. 2074-2104, 1997.

[47] M. Kumosa, Y. Han and L. Kumosa, Analyses of Composite Insulators

with Crimped End-Fittings: Part I Non-linear Finite Element

Computations, Composites Science and Technology, Vol. 62, pp. 1191-

1207, 2002.

[48] M. Kumosa, D. Armentrout, L. Kumosa, Y. Han and S.H. Carpenter,

Analyses of Composite Insulators with Crimped End-Fittings: Part II

Suitable Crimping Conditions, Composites Science and Technology,

Vol. 62, pp. 1209-1221, 2002.

[49] M. Marder and J. Fineberg, How Things Break, Physics Today,

September 1996, pp. 24-29.

[50] T. S. McQuarrie, Improved Dielectric & Brittle Fracture Resistant Core

Rods for Non-Ceramic Insulators, Insulator 2000 World Congress on

Insulator Technologies for the Year 2000 & Beyond, Barcelona, Spain

November 14-17, 1999.[51] J. T. Burnham and R. J. Waidelich, Gunshot Damage to Ceramic and

Nonceramic Insulators,IEEE Transactions on Power Delivery, Vol. 12,

No. 4, pp. 1651-1656, 1997.

[52] L. Kumosa, M. Kumosa and D. Armentrout, Resistance to Brittle

Fracture of Glass Reinforced Polymer Composites Used in Composite

(Non-Ceramic) Insulators, IEEE Transactions on Power Delivery, in

press, 2005.

Maciej S. Kumosa received his Masters and

Ph.D. degrees in Applied Mechanics and

Materials Science in 1978 and 1982 from the

Technical University of Wroclaw in Poland.

He is currently a Professor of Mechanical

Engineering and the Director of the Center for

Advanced Materials and Structures at the

University of Denver. In the past, Dr. Kumosa

worked six years at the University ofCambridge in England. Between 1990 and

1997 he was an Associate Professor of

Materials Science and Electrical Engineering

at the Oregon Graduate Institute in Portland,

Oregon. Dr. Kumosas research interests include the experimental and

numerical fracture analyses of advanced composite systems for electrical and

aerospace applications. He has performed research for a variety of funding

agencies in the US including the National Science Foundation, Air Force

Office of Scientific Research, NASA Glenn, Electric Power Research Institute

and a consortium of US electric utilities and insulator manufacturers

consisting of the Bonneville Power Administration, Western Area Power

Administration, Alabama Power Company, Pacific Gas and Electric Company

and National Rural Electric Cooperative Association, Glasforms, Inc. and


12/12


NGK-Locke. Dr. Kumosa has published over 85 publications in numerous

composites, materials science, applied physics, applied mechanics, general

science and IEEE international journals, and 45 publications in conference

proceedings. Dr. Kumosa is on the Editorial Board of the Journal of

Composites Science and Technology (http://myprofile.cos.com/mkumosa).

2.Brittle Fracture Failure of Rods

Documents

Transcript of 2.Brittle Fracture Failure of Rods