1 4 IEEE Electrical Insulation Magazine

F E A T U R E A R T I C L EF E A T U R E A R T I C L EF E A T U R E A R T I C L EF E A T U R E A R T I C L EF E A T U R E A R T I C L E

In the first part of this article severalexamples of in-service insulatorfailures caused by brittle fracture arepresented in detail.

NIntroduction

onceramic insulators, also referred to as composite, poly-mer, or polymeric insulators, are used in overhead trans-

mission lines with line voltages in the range of 69 to 735 kV.Throughout this two-part article, we will refer to the insulatorsas nonceramic insulators. The insulators rely on unidirectionalglass reinforced polymer (GRP) composite rods as the principalload-bearing component (Fig. 1). The GRP rods are manufac-tured by pultrusion, and the constituents are polyester, vinyl es-ter, or epoxy resins reinforced with either E-glass or ECR-glass(boron-free glass) fibers. The rods are used in the insulators asthe mechanical load-bearing components. There are two metalend fittings attached to the GRP rod, one at each end, and thesurface of the rod is covered with a rubber housing with multipleweathersheds. The primary purpose of the rubber housing andthe weathersheds is to protect the GRP rods against the outsideenvironment (moisture, ozone, pollution, etc.) and to increasethe surface leakage distance between the conductor and the elec-trically grounded tower or pole. Essentially, all nonceramic insu-lators for both suspension and substation applications are basedon this design. However, they can differ with respect to the diam-eter and the length of the GRP rods, the type of the fittings, themethods of attaching the fittings to the rods, the way the rubberhousing is attached to the rod, the chemical compositions of therods, and the housing/weathershed materials. There are other lessimportant differences, which can also be observed in the designof the insulators made by different manufacturers.

Despite the many benefits that nonceramic insulators offer incomparison with their porcelain counterparts (high mechanicalstrength-to-weight ratio, improved damage tolerance, flexibility,good impact resistance, and ease of installation), they can fail

Failure Analyses of NonceramicInsulators Part 1: Brittle FractureCharacteristicsKey Words: Nonceramic, composite, polymer insulators, failure modes, brittle fracture,prevention, stress corrosion cracking

Maciej Kumosa, Lucas Kumosa, andDaniel ArmentroutCenter for Advanced Materials and Structures,Department of Engineering, University of Denver

mechanically in service by rod fracture. One of the mechanicalfailure modes of the insulators is a failure process called brittlefracture [1]–[31], which is caused by the stress corrosion crack-ing (SCC) of the GRP rods [32]–[51]. The process is catastrophicand unpredictable, leading to the drop of energized transmissionlines [13], [18]. Several attempts have been made over the years[1]–[31] to understand this process and provide potential rem-edies to avoid it in service. In Fig. 2(a–d), some examples arepresented of brittle fracture failures of failed nonceramic insula-tors operated at 115 kV (Fig. 2a, b), 345 kV (Fig. 2c), and 500kV (Fig. 2d). It should be noted that all insulators in Fig. 2 weremade by the same manufacturer and were based on the epoxy

May/June 2005 — Vol. 21, No. 3 15

of damaging the reputation of any insulator manufacturer byshowing examples of several real-life, field-failed units. The au-thors suspect that insulators of such design are no longer manu-factured and installed on transmission lines worldwide. The sec-ond part of the article will discuss a brittle fracture model andfailure prevention.

Brittle Fracture MorphologyBrittle fracture can occur usually either inside the fitting (Fig.

2a) or above the hardware (Fig. 2c, d). If the failure occurs insidethe fitting, the fracture surfaces are planar (Fig. 3a, b, and c) [13],[14], [16], [18], [19], [23], and there is usually no significantresin decomposition present between the glass fibers. The frac-ture surfaces of the fibers and the polymer matrix are almost onthe same plane. Another very characteristic feature of this typeof failure is the severe surface contaminations by surface depos-its. The relatively “clean” fracture surface shown in Fig. 3c canrarely be observed. Instead, the contamination of the compositerod surfaces can be so heavy that the individual fibers on thefracture surface cannot be observed (Fig. 3a, b). The deposits,however, can provide extremely important information about theorigin and progression of the failure process [13], [14], [16], [18],[19], [23], [49], [51].

The most characteristic micro-fracture feature of the failuresabove the hardware is the presence of severe resin decomposi-tion (Fig. 4a–c) [13], [14], [16], [18], [19], [23]. Similar levels ofresin decompositions were observed in small E-glass/polymercomposite rod specimens subjected to AC discharges and waterunder laboratory conditions in Oregon [14], [16], [19], [23], [29].It should be stated, however, that the micro-fracture features ofthe two types of fracture processes (inside and outside the fit-tings) are never so distinctly different. If, during the fracture in-side the fittings, the electric field gets diverted into the fitting atsome point during the process, the internal discharges will startplaying a significant role in the process [49]. These issues will befurther discussed later in this part and the second part of the ar-ticle.

The authors have noticed over the years that a vast majority offailures by brittle fracture of nonceramic suspension insulatorshave occurred above the hardware, near the energized end fit-tings [13], [14], [18]–[20], [30], [31], [51]. It has also been ob-served that the failures above the hardware always occurred abovethe grading rings, if present. Fig. 2d is a perfect example of thismode of brittle fracture failure. Not one single brittle fracturefailure was observed to take place between the fittings and thegrading rings. A few cases of failure illustrated in Fig. 2b werealso found that occurred in 115-kV suspension insulators installedwithout the rings. In our investigations, no insulator failure bybrittle fracture has been identified to occur near the un-energizedend fitting. A detailed explanation of the failures inside and out-side the fittings and the effect of the grading rings on the failureprocess will be discussed in the second part of the article.

SCC of GRP NonceramicsThe fact that brittle fracture is caused by the SCC of insulator

rod GRP nonceramics has been widely accepted [25]. SCC can

Figure 1. Simplified schematic of a composite insulator.

cone end fitting design [13]–[16], [19], [21], [23]. A detailedexplanation of the failures shown in Fig. 2(a–d) is provided inthe subsequent sections of this article.

Despite their relative structural simplicity, very complex,difficult-to-prevent, and predict failure modes can occur innonceramic insulators under in-service conditions [1]–[31], [51].This makes the evaluation of the effects of the manufacturingprocess, design, selection of materials, and in-service conditionson the mechanical and electrical integrity of the insulators verydifficult. The authors’ observations presented in this article arebased on a large multidisciplinary study of the failure analysis ofinsulators performed between 1992 and 2004 [13]–[21], [23],[26], [29]–[31], [37]–[61]. The three primary goals of the studyhave been 1) to identify mostly mechanical failure modes of theinsulators (including brittle fracture); 2) to evaluate the effect ofvarious manufacturing techniques, material selections, insulatordesigns, loading conditions, etc. on the structural integrity of theinsulators in service; and 3) to suggest feasible prevention meth-ods against mostly mechanical failures of the insulators.

In the first part of this article, several examples of in-serviceinsulator failures are presented in detail. They are presented herepurely for educational purposes. The authors have no intention

1 6 IEEE Electrical Insulation Magazine

occur in unidirectional E-glass/polymer nonceramics, if they areexposed to acidic environments and mechanical tensile stresses[14], [16], [23], [26], [32]–[48]. During this process, planar frac-ture surfaces are formed in the composite rods and run perpen-dicular to the fibers with very limited fiber pullouts anddebondings (Fig. 5a). If the same nonceramics are subjected toexcessive tensile mechanical stresses along the fiber directionsunder dry conditions without any acids, highly irregular fracturesurfaces are formed, consisting of multiple fiber fractures, fiber

pullouts, matrix cracking etc., with the macroscopic features ofthe fracture surfaces resembling a brush (Fig. 5b). This type offailure can easily be reproduced, for example, by bending or pull-ing a unidirectional glass fiber/polymer rod or a piece of wooduntil failure.

One of the most characteristic features of SCC and brittle frac-ture is the formation of mirror, mist, and hackle zones on thefracture surfaces of individual E-glass fibers as indicated in Fig.6a with (*), (**), and (***) for the three zones, respectively. A

Figure 2. Nonceramic insulators field failed by brittle fracture from (a, b) 115-kV, (c) 345-kV, and (d) 500-kV lines. All insulatorsare of the same design based on epoxy cone end fittings.

(a)

(b)

(c)

(d)

May/June 2005 — Vol. 21, No. 3 1 7

(a)

(b)

(c)

Figures 3. Examples of brittle fracture surfaces inside thefittings (a, b, c).

(a)

(b)

(c)Figure 4. Examples of brittle fracture surfaces above thefitting: (a) macro-fracture surface above the fitting and (b, c)micro-fracture features.

1 8 IEEE Electrical Insulation Magazine

detailed explanation of the process and reasons for the creationof such distinctly different fracture formations in the fibers canbe found in [41], [51], [62]. For comparison, the fracture surfacein an E-glass fiber caused by purely mechanical stresses can beseen in Fig. 6b. In this case, the three zones, so characteristic ofSCC, are not observed on the fracture surfaces of individual fi-bers [26], [33], [51], [56]. The presence of the three zones on thefracture surfaces of individual glass fibers in a field-failed,nonceramic insulator should be treated as the first indication thatthe insulator failed by brittle fracture.

It should be noted that, in some cases, when the insulators areovercrimped and overstressed, the failure will occur close to thefitting (either energized or unenergized), and the fracture sur-faces will be almost as flat as the classical brittle fracture sur-faces shown in this work [26], [51], [56], [57]. However, the frac-ture surface of the individual fibers will not exhibit the character-istic mirror, mist, and hackle zones (Fig. 6b) typical for SCC and

brittle fracture. Therefore, caution should be observed when clas-sifying a field-failed insulator as a brittle fracture failure just basedon the presence of large flat composite fracture surfaces. A de-tailed micro-fracture analysis of the broken fibers would be re-quired in this case to be absolutely certain [25], [51].

To initiate and propagate SCC in unidirectional E-glass/poly-mer nonceramics, two factors are critical, namely mechanicaltensile stresses and a corrosive environment containing free hy-drogen ions [33]. During the stress corrosion process, such metalions as Al, Ca, Fe, Mg, and Ti are leached out of the E-glassfibers and are replaced by hydrogen ions. This weakens the fi-bers, which can then easily fracture if low magnitude tensilestresses are applied along the fibers. The corrosion process canbe even initiated in individual E-glass fibers [14], [16], [37], [23]and their composites [26], [44] in the absence of externally ap-plied mechanical loads. However, the propagation of SCC acrossthe composite rods will not occur unless mechanical tensile

(a)

(b)

Figure 5. Fracture surfaces in unidirectional glass/polymercomposites generated under laboratory conditions: (a) causedby SCC in nitric acid and (b) by excessive mechanical tensileloads.

(a) (b)Figure 6. Fracture surface in E-glass fibers caused by (a) stress corrosion cracking and (b) purely mechanical stresses.

May/June 2005 — Vol. 21, No. 3 1 9

stresses are present whose magnitudes can be a fraction of theultimate tensile stress of the composites. The fact that suspen-sion nonceramic insulators are subjected, in service, to tensilemechanical stresses predominantly, which can be highly concen-trated in the rods inside the fittings, is rather obvious. However,the origin of an acidic environment is not apparent, if one readsthe available literature on this topic [1]–[31]. Also, it is not veryclear how the design of the fittings and the way they are attachedto the rods could influence the concentrations of mechanicalstresses in the GRP rods, either increasing or decreasing the prob-ability of SCC and brittle fracture.

Simulation of Brittle FractureA. Initial Assumptions

Numerous brittle fracture failures on a 345 kV line in the be-ginning of the 1990s [13] caused the initiation of major researchefforts on the brittle fracture phenomenon in our laboratory. Theinitial goal of the project was to explain those failures [13] andreproduce the actual failure process under laboratory conditions.Obviously, to reproduce the process in a laboratory would re-quire a clear and unambiguous identification of the chemicalenvironment responsible for those failures. It quickly became clearthat the project was more difficult than was initially assumed andthat the identification of the chemistry of the process would re-quire major long-term efforts. Despite this, several assumptionswere made which, later on, turned out to be correct [21], [31]. Itwas assumed [13], [14], [16] that the failure process was trulycaused by SCC and that there was an acid (or acids) involved. Itwas also assumed, based on numerous stress corrosion experi-ments performed on unidirectional E-glass fiber/polymernonceramic insulators [14], [16] subjected to mechanical loadsand various acids (nitric, oxalic, sulfuric, and hydrochloric), thatthe most likely cause of those failures was the formation of nitricacid in service. There had also been other indications in the lit-erature that nitric acid could be responsible for the failure bybrittle fracture of nonceramic insulators [10], [12].

Only in nitric acid did the fracture surfaces in the nonceramicsclosely resemble the fracture surfaces inside the fittings, if thepresence of the heavy deposit is not considered in the field insu-lators [14]–[16], [19]. It should be stated that different acids ofdifferent concentrations can cause significantly different stresscorrosion effects in E-glass fibers [14], [16], [23], [37]. Theywill also create significantly different stress corrosion surfaces inthe composites [14], [16]. A skilled materials scientist can no-tice these effects and relate them to the micro-fracture features ina field-failed, nonceramic insulator.

B. Simulation of SCC Without High VoltageIf an E-glass/polymer nonceramic is subjected to mechanical

tensile stresses in the presence of an acidic environment, SCCwill occur either after a few seconds, a few hours, or a few days,depending on the acid type and concentration, the type ofnonceramic and its surface conditions, and the magnitudes of themechanical stress [4], [5], [10], [14], [16], [23], [24], [26]–[30],[32]–[50]. To simulate the corrosion process in an insulator com-posite under controlled laboratory conditions, numerous experi-

mental set-ups can be designed, each of them with its own sig-nificant advantages and disadvantages. In this research, the threeexperimental arrangements shown in Fig. 7 through 9 were foundto be particularly useful in the simulations of brittle fracture un-der laboratory conditions [14]–[16], [19], [23], [26], [29], [30],[38]–[51].

The setup shown in Fig. 7, based on the constant KI specimen

geometry, designed in the UK in the 1980s [32], [33]–[36], wasused to determine the effect of acid type on the propagation rateof SCC in a variety of insulator composites [14]–[16], [23], [26],[29], [38], [39], [41], [42]. Because the K

I composite specimens

can be made very thin (<1 mm thick), the crack location can beaccurately monitored optically using, for example, an opticalmicroscope. Also, the crack propagation rate can be very accu-rately monitored using acoustic emission (AE) using either reso-nance or wideband transducers [42]. Knowing the specimen thick-ness and the volume fraction of fibers in a composite, under cer-tain conditions, AE can be used to count quite accurately thenumber of fractured fibers under stress corrosion, which, in turn,can allow an estimation of the crack growth rate in the compos-ites [34], [35], [41]. The main advantage of the K

I approach is

that the fracture process in the specimen occurs under constantmechanical stress conditions (constant K

I), which is not the case

in the other two methods (Fig. 8 and 9).If one desires to investigate the initiation effects in the

nonceramic under stress corrosion, the KI setup should not be

used, as pre-cracks are always introduced into the specimen atthe root of the notch to initiate the failure process. A much betterapproach would be to employ the arrangement shown in Fig. 8[43], [45], [46], [50], [51] based on a relatively thin compositeplate subjected to four-point bending in the presence of an acidicsolution. Several examples of the application of the setup shown

Figure 7. Constant KI specimen setup with acoustic emission

monitoring.

20 IEEE Electrical Insulation Magazine

in Fig. 8 in the initiation study of brittle fracture will be pre-sented and discussed in the second part of this article.

The most realistic approach would be to test the entire rod,not small composite specimens, under the conditions that couldbe expected in service, as shown in Fig. 9. However, as in thecase of the other tests, the rod testing has its own limitations [40],[47]. This approach can be very useful as a simple screeningtest, as was done for example in [47]. However, for more sophis-ticated types of research, the size of the specimen, the large dis-tance between the AE sensors with respect to the location of fail-ure, an inability to localize the crack tip with time, in addition toseveral other problems, make this test unsuitable for both thestress corrosion initiation and propagation studies.

Other approaches can also be found in the literature to simu-late SCC in glass/polymer composites [10], [27], [48]. Any timean E-glass fiber/polymer composite is subjected to mechanicalloads (tensile, bending torsion, combined) in the presence of anacid, SCC will occur as long as there is a tensile stress compo-nent present in the portion of the composite affected by the acid.Therefore, numerous SCC testing arrangements can be envisagedfor this purpose. It should also be stated, however, that in highquality stress corrosion research, the effects of specimen geom-etry and loading conditions should be negligible. Such effectsshould be minimized and should have no significant influenceon the corrosion data for a given composite and acid.

C. Simulation of SCC Under High VoltageIn the corrosion tests presented in the previous sections,

nitric acid solutions were supplied to the set-ups shown in Fig. 7through 9. Therefore, one could ask whether these tests were truly

representative of the actual in-service conditions rich in mois-ture, ozone, and corona discharges. In fact, nitric acid can beeasily formed in the presence of corona discharges and water[24], [53]. There is a vast amount of literature on this topic spread-ing over the last two centuries [24], [63]. As a by-product of gasdischarges, nitrogen oxides are formed through chemical reac-tions between ozone and nitrogen. Then, nitrogen oxides, react-ing with water, form nitric acid. There are some indications inthe literature that the amount and type of pollution can signifi-cantly affect the rate of nitric acid generation through coronadischarges [63]. Actually, in the past, AC discharges in air overwater were used to manufacture industrial nitric acid. Therefore,the fact that nitric acid could be present on transmission systemsshould not be surprising. Thus, the possible explanation of thebrittle fracture process based on the formation of nitric acid inservice was logical and highly probable [14].

Using a modified version of the test shown in Fig. 7 to make itmore realistic with nitric acid being formed independently dur-ing the fracture experiment, the brittle fracture process was suc-cessfully reproduced in Oregon by subjecting E-glass/polyestercomposite specimens to an AC discharge in the presence of smallamounts of water and mechanical tensile stresses (Fig. 10a, b)[14], [15], [19], [23], [29]. The discharge was created at the tipof an energized needle at 12 kV with small amounts of waterplaced between the needle and the specimen. Major differencesin the fracture characteristics caused either by nitric acid or ACdischarge with water were observed. Under AC discharge, themacroscopic fracture surfaces were also planar (Fig. 10b). It wasnoticed, however, that the polyester resin between the fibers dis-integrated under high voltage. Later on, this phenomenon was

Figure 8. Four point bend fixture used in SCC research.

May/June 2005 — Vol. 21, No. 3 21

Figure 9. GRP rod subjected to tension with acoustic emissionmonitoring.

(a) (b)

Figure 10. High voltage brittle fracture experiment: (a) setup and (b) composite fracture surface.

further investigated by subjecting the composite to a strong nitricacid solution, hoping to achieve the same effect [44]. However,after more than a month of acid exposure, no noticeable effect ofthe acid on the polymer was detected. Therefore, it has to be con-

cluded that the resin between the fibers on the composite surfacewould only disintegrate (and the disintegration process can bevery fast; within a few hours in straight polyester) if the compos-ite was subjected to an AC discharge. In an epoxy-based compos-ite, the rate of resin disintegration was found to be much slower[14]. A detailed description of the high voltage experiments andtheir limitations can be found in [14], [29].

Causes of 345 and 500-kV Insulator FailuresAs indicated in [25], several design- and manufacturing-re-

lated effects can influence the initiation of brittle fracture innonceramic insulators. Two of them are briefly elaborated withrespect to two series of brittle fracture failures on 345-and 500-kV lines.

A. 345-kV Insulator FailuresFourteen catastrophic brittle fracture failures were experienced

on a 345-kV line in the early 1990s with at least two hundredother insulators damaged to various degrees [13]. All fourteenfailures occurred above the grading ring, very close to the firstweathershed [13]. The field-failed unit in Fig. 2b is a typical ex-ample of those failures. It was also found that none of those insu-lators were protected against moisture ingress into their end fit-tings [13], [14], [16], [19], [23]. There was no sealant present onthe hardware to protect the rubber/fitting interface against mois-ture ingress, which can be clearly seen in Fig. 2b and in otherinsulators of the same design (Fig. 2a, c, d). In addition, the leastsuitable composite material was used, both in terms of the type ofglass fiber and the type of polymer, with the lowest resistance toSCC and, thus, brittle fracture [14], [16]. This will be further elabo-rated upon in the second part of this article.

Moreover, the insulator design (Fig. 11a) was inappropriate[19], [23]. It encouraged water to stay inside the fittings, espe-cially under tensile mechanical loads, as water pockets were al-lowed to exist inside the fittings adjacent to the rod (Fig. 11b). Tomake the situation worse, excessive tensile loads were applied to

22 IEEE Electrical Insulation Magazine

the insulators during proof testing, crushing the epoxy cones thatwere used to mechanically attach the GRP rods to the fittings(Fig. 11c) [13], [14], [16], [19], [23], [25]. This allowed waternot only to penetrate the fitting easily and stay there, but also toreach the surface of the GRP rod easily. All of these problems

became quite apparent after this particular design was numeri-cally simulated using advanced finite element techniques [19],[23], [51]. The numerical displacement and stress distributionsshown in Fig. 11(b and c) were found to be extremely useful inthe exact identification of the actual causes of the failures. The

Figure 11. 345-kV insulator design: (a) schematic, (b) numerically simulated internal gaps, and (c) numerically predictedcrushing stresses in epoxy cones.

Figure 12. Brittle fracture failures of 500-kV insulators based on the metal wedge design: (a) inside the fitting and (b) above thehardware.

(a) (b)

May/June 2005 — Vol. 21, No. 3 23

original explanation of the 345-kV failures was independentlyconfirmed by the insulator community in [25].

B. 500-kV Brittle Fracture FailuresIn 1995, we were approached with a request to help with an-

other series of brittle fracture failures on a 500-kV line [18]. Three500-kV nonceramic insulators failed by brittle fracture; two ofthem are shown in Fig. 12 [18]. Two catastrophic brittle fracturefailures occurred just above the grading ring and one deep insidethe fitting. A few others were found to be seriously damaged, atdifferent stages of their structural disintegrations (Fig. 13). Thedesign of the 500-kV insulators was entirely different from thedesign of the 345-kV insulators [25]. The insulators were basedon the metal wedge design, whereas the 345-kV insulators had

the epoxy cone design. The failures on the 500-kV line were es-sentially caused by a problem related to quality control [18], [25],[51].

The overall quality of the 500-kV insulators, as far as theirresistance to moisture ingress, was much better in comparisonwith the 345-kV units [18]. Three of them did fail by brittle frac-ture caused by water ingress into their energized end fittings;however, the amounts of water responsible for those failures werevery small. Actually, all of the insulators from the line, as testedin [18], passed the dye penetration test. However, small amountsof water still managed to penetrate the fittings because of thepresence of spilled epoxy on the top of the hardware (Fig. 13),making the sealant ineffective. Although large amounts of waterwere involved in the 345-kV insulator failures, the 500-kV fail-ures were caused by such small quantities of water that it tooksignificant research efforts before the actual path of water move-ment in the insulators could be established [18]. The explanationof the 500-kV failures provided in this research has been inde-pendently confirmed by the insulator community [25].

C. Other Brittle Fracture FailuresNot only can suspension nonceramic insulators with epoxy

cones [13], [14], [16], [19], [23] or metal wedges [18] fail inservice by brittle fracture, crimped nonceramic suspension insu-lators can also fail in this manner. Two failures of this type werebriefly investigated in [31]. Brittle fracture can also affectnonceramic guides as shown in Fig. 14 [31]. Several other brittlefracture failures of nonceramic guides and crimped suspensioninsulators have been reported to the authors over the years. Itcan also be suspected that, under certain conditions, line postinsulators with large diameter GRP rods could also fail by brittlefracture. None of them, however, has yet been formally reportedwith the exception of one rather questionable case shown in Fig.15.

A field-failed, 115-kV line post insulator submitted to theproject for analysis is shown on the left side of Fig. 15 [26]. Thefield-failed unit is compared in this figure with another line postinsulator with a similar design (right side), which was mechani-cally broken by excessive bending loads applied in a laboratory[52]. It can be observed that, on the compressive side of the rods(on their left side in Fig. 15), the compressive fracture zones ofthe two insulators are very similar. However, on their tensile side,the field-failed unit contains a well-established large transversecrack, whereas the other one exhibits the classical tensile failure(brush-like). Therefore, at first glance, the failure on the left sideof Fig. 15 could be classified as brittle fracture.

The in-service failure occurred at the unenergized end of theinsulator. Traces of iron rust were found deep inside the fitting,indicating water ingress into its fitting. However, no traces ofnitric acid (or any other acidic environment) were detected onthe fracture surfaces [26]. Initially, it was speculated in [26] thatthe failure could have been caused by the formation of iron ox-ides inside the fitting in the presence of mechanical tensile stresses.(The formation of iron oxides lowers the pH and could causeSCC of the E-glass fibers [26].) Also, very few fibers were de-tected on the surface of the transverse crack exhibiting the classi-

Figure 13. Electromechanical damage to a 500-kV insulator.

Figure 14. Brittle fracture failure of a non-ceramic guide.

24 IEEE Electrical Insulation Magazine

cal mirror and hackle zones (Fig. 6a), characteristic of brittle frac-ture. At the same time, a vast majority of fibers exhibited on theirfracture surfaces the features shown in Fig. 6b, indicating thepresence of excessive mechanical stresses instead of stress cor-rosion.

Considering all of the available mechanical and chemical data[26], it was concluded that the failure could be a combination ofSCC and mechanical failure, with the mechanical element beingthe dominant one. Therefore, this failure does not fully qualify tobe recognized as the typical brittle fracture failure. It should bementioned that the failure of the 115-kV line post insulator shownin Fig. 15 is the only case known to the authors of a large diam-eter rod composite insulator failing through this mechanism. Sofar, it appears that brittle fracture has been a failure mode affect-ing small diameter rod insulators used only for transmission ap-plications. The failure of the line post insulator is shown in thiswork to further indicate that field-failed units cannot always beimmediately classified as failures by brittle fracture based on justvisual observations. Despite the fact that such failures can ex-hibit several characteristic macro-fracture features (such as thepresence of large transverse cracks in their GRP rods), more in-vestigation is always required to be absolutely certain that aninsulator truly failed in service by brittle fracture [25], [49], [51].

SummaryThe most important characteristics of the brittle fracture pro-

cess, which can occasionally affect high voltage nonceramic trans-mission line insulators, leading to their catastrophic in-servicefailures, have been presented in this article. In addition, severalexperimental techniques were suggested for the simulation ofbrittle fracture under laboratory conditions. Only the most im-portant aspects of the brittle fracture process have been discussedhere. For more information, with a much more systematic andthorough evaluation of brittle fracture, the reader is invited toexamine more closely some of the references presented in thiswork. In particular, the recently published works on brittle frac-

ture [29]–[31] would be a good place to start to obtain a muchdeeper understanding of the brittle fracture phenomenon.

It should be emphasized that the main purpose of this articleis to make the reader familiar with brittle fracture failure and toprovide the reader with enough background information to beable to identify it. A more in-depth discussion on the brittle frac-ture problem will continue in the second part of this article [49]and will focus, in particular, on the selection of the most suitablebrittle fracture model and feasible prevention methods.

AcknowledgmentsThis research has been supported since 1992 by the Electric

Power Research Institute and a consortium of electric utilitiesconsisting of the Bonneville Power Administration, Western AreaPower Administration, Alabama Power Company, Pacific Gas andElectric, National Rural Electric Cooperative Association. It wasalso supported by Glasforms, Inc. and by NGK-Locke.

The authors are especially grateful to Dr. J. Stringer and Dr.A. Phillips of EPRI; Mr. C. Ek, Mr. D. Nickols, Mr. D. Ruff, andMr. 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 Glasforms, Inc. without whose technical and fi-nancial support the insulator research at OGI and DU would havenot been possible. We are also grateful to our distinguished col-leagues, namely Prof. P. K. Predecki, Dr. A. Bansal, Dr. Q. Qiu,Prof. S. H. Carpenter, Prof. D. Smith, Prof. A. Chughtai, Prof. T.Stephen, and many others who have also made significant con-tributions to our insulator research over the last 10 years.

References[1] CIGRE Working Group 10 of Study Committee 22, “Suspension

and tension composite insulators for overhead lines,” CIGRE 1980Session, Paper 22-80.

[2] H. Weihe, R. E. Macey, and J. P. Reynders, “Field experience andtesting of new insulators in South Africa,” CIGRE 1980 Session,Paper 22-03.

Figure 15. Non-ceramic 115-kV line post insulators: in-service failure (left) and purely mechanical laboratory failure byexcessive mechanical bending loads (right) shown for comparison.

May/June 2005 — Vol. 21, No. 3 25

[3] E. Bauer, K. H. Muller, H. Karner, and P. Verma, “Service experi-ence with the German composite long rod insulator with silicone-rubber sheds since 1967,” CIGRE 1980 Session, Paper 22-11.

[4] H. D. Chandler, R. L. Jones, and J. P. Reynders, “Stress corrosion ofcomposite long rod insulators,” in Proc. 4th Int. Symp. High Volt-age Eng., (1983 ISH), Athens, Greece, September 5–9, 1983, Pa-per 23.09.

[5] B. Noble, S. J. Harris, and M. J. Owen, “Stress corrosion cracking ofGRP pultruded rods in acid environments,” J. Mater. Sci., vol. 18,pp. 1244–1254, 1983.

[6] C. de Tourreil, “Influence of acidic solution on the fracture mode ofnonceramic insulator GRP rods,” Trans. Eng. and Operation Divi-sion, (Canadian Electrical Association) vol. 22, pt. 4, 1983.

[7] H. Chandler and J. Reynders, “Electro-chemical damage to compos-ite insulator,” CIGRE 1984 Session, Paper 33-08.

[8] S. J. Harris, B. Noble, and M. J. Owen, “Metallographic investiga-tion of the damage caused to GRP by the combined action of elec-trical, mechanical, and chemical environments,” J. Mater. Sci., vol.19, pp. 1596–1604, 1984.

[9] M. Ezrin and J. Gartner, “Test method for evaluation of the resis-tance of fiberglass rods to combined mechanical and chemicalstress,” IEEE Trans. Power App. Sys., vol. PAS-103, pp. 2741, 1984.

[10] A. Akhtar, J. S. Nadeau, J. Y. Wang, D. P. Romily, and C. Taggart,“Brittle fracture of nonceramic insulators,” Can. Elect. Assoc. Rep.186 T 350. Prepared by the British Columbia Hydro and PowerAuthority, Sep. 1986.

[11] M. J. Owen, S. J. Harris, and B. Noble, “Failure of high voltageelectrical insulators with pultruded glass fiber-reinforced plasticcores,” Composites, vol. 17, pp. 217226, 1986.

[12] A. Akhtar and J. Y. Wong, “Failure analysis of brittle fracture innonceramic insulators”, J. Composite Technol. Res., vol. 9, pp. 95–100, 1987.

[13] M. Kumosa, “Failure analysis of composite insulators from theCraig Bonanza line,” Final report to the Western Area Power Ad-ministration, Oregon Graduate Institute, Portland, OR, 1994.

[14] M. Kumosa, Q. Qiu, M. Ziomek-Moroz, A. Moroz, and J. M. Braun,“Micro-fracture mechanisms in glass/polymer insulator materialsunder combined effects of electrical, mechanical and environmen-tal stresses,” Final Report to the Bonneville Power Administration,Electric Power Research Institute and the Western Area PowerAdministration, Oregon Graduate Institute, Portland, OR, Jul. 1994(under contract DE-AC79-92BP61873).

[15] M. Kumosa, Q. Qiu, E. Bennett, C. Ek, T. S. McQuarrie, and J. M.Braun, “Brittle fracture of nonceramic insulators,” in Proc. Frac-ture Mechanics for Hydroelectric Power Systems Symposium ’94,Canadian Committee for Research on the Strength and Fracture ofMaterials (CSFM), BC Hydro, Sep. 1, 1994, pp. 235–254.

[16] Q. Qiu, “Brittle fracture mechanisms of glass fiber reinforced poly-mer insulators,” Ph.D. Diss., Oregon Graduate Inst. Sci. Technol.,Portland, OR, Oct. 1995.

[17] N. Fujimoto, J. M. Braun, M. Kumosa, and C. Ek, “Critical fieldsin composite insulators: Effects of voids and contaminations,” in9th Int. Symp. High Voltage Eng. (1995 ISH), Graz, Austria, Aug.28–Sep. 1, 1995.

[18] M. Kumosa and Q. Qiu, “Failure analysis of composite insulators(failure investigation of 500 kV nonceramic insulators for PacificGas & Electric Company),” Final Report to the Pacific Gas andElectric Company, May 1996, Dep. Eng., Univ. Denver.

[19] M. Kumosa, H. Shankara Narayan, Q. Qiu, and A. Bansal, “Brittlefracture of nonceramic suspension insulators with epoxy cone end-fittings,” Composites Sci. Technol., vol. 57, pp. 739–751, 1997.

[20] Interview with M. Kumosa, “Research of brittle fractures in com-posite insulators,” Insulator News & Market Rep., pp. 46–51, Jul./

Aug. 1997.[21] A. R. Chughtai, D. M. Smith, and M. Kumosa, “Chemical analysis

of a field-failed composite suspension insulator,” Composite Sci.Technol., vol. 58, pp. 1641–1647, 1998.

[22] C. de Tourreil, L. Pargamin, G. Thevenet, and S. Prat, “‘Brittlefracture’ of composite insulators: Why and how they occur,” IEEEPower Eng. Soc. Summer Mtg. 2000, vol. 4, pp. 2569–2574, 2000.

[23] M. Kumosa, “Fracture analysis of composite insulators,” EPRIRep. 1006293, Palo Alto, CA, 2001.

[24] M. Kuhl, “FRP rods for brittle fracture resistant composite insula-tors,” IEEE Trans. Dielect. Elect. Insul., vol. 8, no. 2, pp. 182–190, 2001.

[25] 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 nonceramic insulators,”IEEE Trans. Power Delivery, vol. 17, no. 3, pp. 848–856, Jul. 2002.

[26] M. Kumosa, “Failure analysis of composite high voltage insula-tors,” EPRI Rep. 1007464, Palo Alto, CA, 2002.

[27] J. Montesinos, R. S. Gorur, B. Mobasher, and D. Kingsbury, “Mecha-nism of brittle fracture in nonceramic insulators,” IEEE Trans.Dielect. Elect. Insul., vol. 9, no. 2, pp. 236–243, Apr. 2002.

[28] F. Schmuck and C. de Tourreil, “Brittle fractures of composite in-sulators. An investigation of their occurrence and failure mecha-nisms and risk assessment,” CIGRE WG 22-03 (www.corocam.com/papers.htm).

[29] M. Kumosa, L. Kumosa, and D. Armentrout, “Can water causebrittle fracture failures of composite (nonceramic) insulators in theabsence of electric fields?”, IEEE Trans. Dielect. Elect. Insul., vol.11, no. 3, pp. 523–533, 2004.

[30] M. Kumosa, L. Kumosa, and D. Armentrout, “Causes and potentialremedies of brittle fracture failures of composite (nonceramic) in-sulators,” IEEE Trans. Dielect. Elect. Insul., vol. 11, no. 6, pp.1037–1048, 2004.

[31] A. R. Chughtai, D. M. Smith, L. S. Kumosa, and M. Kumosa, “FTIRanalysis of nonceramic composite insulators,” IEEE Trans. Dielect.Elect. Insul., vol. 11, no. 4, pp. 585–596, 2004.

[32] P. J. Hogg, and D. Hull, “Corrosion and environmental deteriora-tion of GRP,” in Developments in GRP Technology – Volume 1, B.Harris, ed. Elsevier Publisher Ltd., 1983.

[33] D. Hull, M. Kumosa, and J. N. Price, “Stress corrosion of alignedglass fiber-polyester composite material,” Mater. Sci. Technol., vol.1, pp. 177–182, 1985.

[34] M. Kumosa, D. Hull, and J. N. Price, “Acoustic emission fromstress corrosion cracks in aligned GRP,” J. Mater. Sci., vol. 22, pp.331–336, 1987.

[35] M. Kumosa, “Acoustic Emission Monitoring of Stress CorrosionCracks in Aligned GRP”, J. Phys. D: Appl. Phys., vol. 20, pp. 69-74, 1987.

[36] J. N. Price, “Stress corrosion cracking in glass reinforced compos-ites,” in Fractography and Failure Mechanisms of Polymers andComposites, A. C. Roulin-Moloney, ed. Elsevier Science PublisherLtd., 1989, p. 495.

[37] Q. Qiu and M. Kumosa, “Corrosion of E-glass fibers in acidic en-vironments,” Composites Sci. Technol., vol. 57, pp. 497–507, 1997.

[38] D. Armentrout, T. Ely, S. Carpenter, and M. Kumosa, “An investi-gation of the brittle fracture in composite materials used for highvoltage insulators,” J. Acoust. Emission, vol. 16, no. 1–4, pp. s10–s18, 1998.

[39] M. Kumosa, et al., “Micro-fracture mechanisms in glass/polymerinsulator materials under the combined effect of mechanical, elec-trical and environmental stresses,” Final report to BPA, APA, PG&E,WAPA, and NRECA, Univ. Denver, CO, Dec. 1998.

26 IEEE Electrical Insulation Magazine

[40] S. H. Carpenter and M. Kumosa, “An investigation of brittle frac-ture of composite insulator rods in an acidic environment with staticor cyclic loading,” J. Mater. Sci., vol. 35, no. 17, pp. 4465–4476,2000.

[41] T. Ely and M. Kumosa, “The stress corrosion experiments on an E-glass/epoxy unidirectional composite,” J. Composite Mater., vol.34, pp. 841–878, 2000.

[42] T. Ely, D. Armentrout, and M. Kumosa, “Evaluation of stress cor-rosion properties of pultruded glass fiber/polymer composite ma-terials,” J. Composite Mater., vol. 35, pp. 751–773, 2001.

[43] M. Megel, L. Kumosa, T. Ely, D. Armentrout, and M. Kumosa,“Initiation of stress corrosion cracking in unidirectional glass/poly-mer composite materials,” Composites Sci. Technol., vol. 61, pp.231–246, 2001.

[44] L. Kumosa, D. Armentrout, and M. Kumosa, “An evaluation of thecritical conditions for the initiation of stress corrosion cracking inunidirectional E-glass/polymer composites,” Composites Sci.Technol., vol. 61, pp. 615–623, 2001.

[45] L. Kumosa, D. Armentrout, and M. Kumosa, “The effect of sand-blasting on the initiation of stress corrosion cracking in unidirec-tional E-glass/polymer composites used in high voltage composite(nonceramic) insulators,” Composites Sci. Technol., vol. 62, no.15, pp. 1999–2015, 2002.

[46] L. Kumosa, M. Kumosa, and D. Armentrout, “Resistance to stresscorrosion cracking of unidirectional glass/polymer composites basedon low and high seed ECR-glass fibers for high voltage compositeinsulator applications,” Composites Part A. Appl. Sci. Manufact.,vol. 34, no. 1, pp. 1–15, 2003.

[47] D. Armentrout, M. Kumosa, and T. S. McQuarrie, “Boron free fi-bers for prevention of acid induced brittle fracture of compositeinsulator GRP rods,” IEEE Trans. Power Delivery, vol. 18, no. 3,pp. 684–693, 2003.

[48] 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,” Com-posites Technol. Res., vol. 25, no. 4, pp. 202–218, 2003.

[49] M. Kumosa, L. Kumosa, and D. Armentrout, “Failure analyses ofnonceramic insulators: Part II—The brittle fracture model and fail-ure prevention,” IEEE Elect. Insul. Mag., to be published in Jul./Aug. 2005.

[50] L. Kumosa, M. Kumosa, and D. Armentrout, “Resistance to brittlefracture of glass reinforced polymer composites used in composite(nonceramic) insulators,” IEEE Trans. Power Delivery, (in press),2005.

[51] M. Kumosa, “Brittle fracture failure of composite (nonceramic)insulators,” IEEE Trans. Power Delivery, submitted in 2005.

[52] M. Kumosa, “Analytical and experimental studies of substationNCIs,” Final Rep. the Bonneville Power Administration, OregonGraduate Inst. Sci. Technol., Portland, OR, 1994.

[53] A. Bansal, A. Schubert, M. V. Balakrishnan, and M. Kumosa, “Fi-nite element analysis of composite substation insulators,” Compos-ites Sci. Technol., vol. 55, pp. 375–389, 1995.

[54] A. Bansal, “Finite element simulation and mechanical character-ization of composite insulators,” Ph.D. Thesis, Oregon GraduateInst., Portland, OR, 1995.

[55] A. Bansal and M. Kumosa, “Finite element simulation of compos-ite insulators with crimped end-fittings,” J. Composite Mater., vol.31, pp. 2074–2104, 1997.

[56] M. Kumosa, Y. Han, and L. Kumosa, “Analyses of composite insu-lators with crimped end-fittings: Part I – Non-linear finite elementcomputations,” Composites Sci. Technol., vol. 62, pp. 1191–1207,2002.

[57] M. Kumosa, D. Armentrout, L. Kumosa, Y. Han, and S. H. Carpen-ter, “Analyses of composite insulators with crimped end-fittings:Part II – Suitable crimping conditions,” Composites Sci. Technol.,vol. 62, pp. 1209–1221, 2002.

[58] D. Armentrout, M. Kumosa, and L. Kumosa, “Water diffusion intoand electrical testing of composite insulator GRP rods,” IEEE Trans.Dielect. Elect. Insul., vol. 11, no. 3, pp. 506–522, 2004.

[59] L. Kumosa, B. Benedikt, D. Armentrout, and M. Kumosa, “Mois-ture absorption properties of unidirectional glass/polymer compos-ites used in composite (nonceramic) insulators,” Composites PartA: Appl. Sci. Manufact., vol. 35, pp. 1049–1063, 2004.

[60] L. Kumosa, D. Armentrout, B. Benedikt, and M. Kumosa, “Aninvestigation of moisture and leakage currents in GRP compositehollow cylinders,” IEEE Trans. Dielect. Elect. Insul., (in press),2005.

[61] L. Kumosa, D. Armentrout, and M. Kumosa, “New procedure forthe evaluation of moisture and leakage currents in GRP compos-ites,” IEEE Trans. Dielect. Elect. Insul., submitted for publicationin 2005.

[62] M. Marder and J. Fineberg, “How things break,” Phys. Today, pp.24–29, Sep. 1996.

[63] A. Goldman, M. Goldman, R. S. Sigmond, and T. Sigmond, “Analy-sis of corona products by means of their reactions in water,” inProc. 9th Int. Symp. Plasma Chem. – Vol. 2, R. d’Agostino, ed.International Union of Pure and Applied Chemistry, 1989. p. 1654.

Maciej S. Kumosa received hisMaster’s and Ph.D. degrees in AppliedMechanics and Materials Science in1978 and 1982, respectively, from theTechnical University of Wroclaw inPoland. He is currently Professor ofMechanical Engineering and Direc-tor of the Center for Advanced Mate-rials and Structures at the Universityof Denver. In the past, Dr. Kumosaworked several years at the Univer-sity of Cambridge in England. Be-

tween 1990 and 1997, he was Associate Professor of MaterialsScience and Electrical Engineering at the Oregon Graduate In-stitute in Portland, Oregon. Dr. Kumosa’s research interests in-clude the experimental and numerical fracture analyses of ad-vanced composite systems for electrical and aerospace applica-tions. He has performed sponsored research for a variety of pri-vate and federal funding agencies in the US, including the Na-tional Science Foundation, Air Force Office of Scientific Research,NASA, Electric Power Research Institute, and a consortium ofUS electric utilities and insulator manufacturers. Dr. Kumosa haspublished over 85 publications in composites, materials science,applied physics and applied mechanics, general science, in IEEEinternational journals and 45 publications in international con-ference proceedings. Dr. Kumosa is on the Editorial Board of theJournal of Composites Science and Technology (http://myprofile.cos.com/mkumosa).

May/June 2005 — Vol. 21, No. 3 2 7

Lucas S. Kumosa graduated in 2001with a Bachelor of Science degree in Bio-logical Sciences from the University ofDenver. Already as an undergraduate stu-dent, he was doing engineering researchin the Center for Advanced Materials andStructures at the University of Denver.Lucas Kumosa has been the main authorof five journal publications and co-authorof approximately 18 other articles pub-lished in international composite and IEEE

journals and 7 papers in conference proceedings. His main areaof interest over the past five years has been the initiation of stresscorrosion cracking and moisture absorption in glass/polymercomposites used for high voltage insulator applications.

Daniel L. Armentrout graduated witha bachelor’s degree in physics and philoso-phy from Drake University, Des Moines,IA, in 1985 and a Ph.D. in Physics fromthe University of Denver in 1991. Cur-rently, Dr. Armentrout is Associate Re-search Professor and Lecturer in the De-partment of Engineering at the Universityof Denver. His employment experienceincludes Rocky Flats Environmental Tech-

nology Site (1990–1996), where he worked with the SEM onpolymer encapsulation of radioactive waste. A large portion ofhis research support over the last six years has been from theElectric Power Research Institute and several U.S. utilities andinsulator manufacturers for investigating electrical and mechani-cal failure mechanisms in high voltage composite insulators. Dr.Armentrout has published over 35 articles in international jour-nals and conference proceedings.

FREE SUBSCRIPTION TO ELECTRICAL INSULATION MAGAZINEPostal regulations require that a request to receive/continue receiving a free subscription to IEEE EI

Magazine must be signed every year. Please fill in the following information and return it to the addressbelow or send an e-mail to j.densley@ieee.org. DEIS members already receive a copy as part of theirmembership fee, therefore there is no need to fill in this form.

Name: ____________________________________________________________

Company: ____________________________________________________________

Address: ____________________________________________________________

City: __________________________State:_____________ZipCode:_________

Country: ____________________________________________________________

E.Mail: ____________________________________________________________

Signature: ____________________________Date:____________________________

Please send to: John Densley4008 Rolling Valley DriveMississauga, Ontario, Canada L5L 2K8

Or send an e-mail to j.densley@ieee.org. To satisfy postal regulations please enclose one piece ofpersonal information such as place of birth, mother’s maiden name, etc.