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Time Load Testing of Non Ceramic Insulators

With Fiberglass Core Rod A 20 Year Summary

R. A. Bernstorf

Hubb ell Power Systems Ohio Brass

C o.

Abstract: Time-Load (static fatigue) testing of fiberglass rods used

in non-ceramic insulators (NCIs) has been underway for over 20

years. Data collected over that time frame is reported in this paper

for NCIs manufactured with 16 mm ( 5 / 8 ) diameter and 22 mm

(7/8 ) diameter epoxy fiberglass rods. All

of

the samples tested

utilized crimped steel end fittings for transference of the applied

tensile load to the fiberglass rod and from the rod to the support

structure. Samples with and without a polymer housing covering

the core rod

are

included.

The data includes time-load test results under constant load to

failure and residual strength evaluations after the removal of a long-

term time-load.

The results indicate a very flat time-load curve under nominal

conditions. The residual strength tests were unable to quantify any

significant loss in strength as a result of long-term overload

conditions.

Over the course of 20 years, the investigation looked into

look ed into the chara cteristics of 16 and 22 mm rod

insulators. In 1 998, the test program w as terminated, with

this paper reporting the final test results.

11. TIM E-LO AD TESTING

Time-Lo ad testing involves applying a constant load to an

insulator for an extended period of time (typically until it

fails). Unless otherwise noted, that is the means o f testing

employed for the included data.

Keywords: Non-ceramic Insulators, Time-load.

I

INTRODUCTION

With the introduction of no n-ceramic insulators in the late

60s and ea r ly 70s came a growing co n cern for the long term

strength characteristics of the fiberglass rods used as strength

mem bers, as well as the integrity of the end fitting attachment

methods to that rod. These concerns were a product of the

aerospace industry which had been testing composite

materials and their long -term creep characteristics.

Fig 1 Typical outdoor load racks 16

m m

samples.

In an effort to address those concern s, a series of long-term

loading tests (time-load) were undertaken. The goal of the

testing program was the development of an understanding of

the long-term load bearing characteristics of the non -ceramic

insulators.

The non-ceramic insulators evaluated in this test program

utilized either a 15.9 mm (nominal 5 8 inch) Or 22.3

(nominal 718 inch) diameter fiberglass rod. For the Purposes

of this paper, the

two

types will be referenced as 16 mm and

mm, respectively. The fiberglass rods were an epoxy

resin system with a E-type unidirectional glass fibers.

Fiberglass rod lengths were at least 20X the diameter in order

to minimize end effects.

End fittings were compo sed of forged steel. These fittings

were crimped to the fiberglass rod using a rolling crimp,

which proceeded from the innermost point of the end fitting

toward the end. The crimp lengths were 63.5 mm 2.5 in)

and 101.6 mm

(4

in) for the 1 6 mm and

22

mm diameter rod

insulators, respectively.

0-7803-5515-6/99/$10.00 1999

IEEE

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The minim um ultimate tensile strength load ratings for the

insulators were

89

kN for the 16 mm rod and 222 kN for the

22 mm rod. The recomm ended maximum loadings for these

insulators were 44 kN and 111 kN for the 16 mm and 22 mm

rod insulators, respectively.

Samples were tested in load frames located both indoors

Samples without weathersheds were only

nd outdoors.

tested indoors (see fig. 1)

The load frames used lever arms to apply a load to the

insulator. All loads were established by inserting a load cell

(originally hydraulic, later electronic) and m easuring the load

during three applications. The error listed for the data

represents the range. Pivot points within the load frames

were lubricated with a high pressure lubricant.

The date and time of loading was recorded exactly. For

newly loaded samples, conditions were checked daily. As

time went by, the samples were checked less frequently.

Sample failure was indicated by m echanical failure and noted

by the load bucket at the free end of the moment arm resting

on the floor. Time to failure was established as an average

time (between checks) and a range.

The data displayed large variations in time to failure as a

function of load. To mak e sense of the data, it was plotted as

a log-log graph (see fig. 2). The 16 mm rod diam eter data

was also analyzed using Chi-Squared analysis. The analysis

was performed in the statistical mode using a logarithmic

equation as shown on the face of the graph. Assumed errors

were 3% for all data.

16

R O D TIME L O A D D A T A

course of the test for the

5/8

samples. All of these tests were

performed with the insulator in a vertical position. Th e solid

line represents the equation derived using the Chi-squared

analysis. Since the samples which d id not fail are included,

the curve will be som ewhat pessimistic. Had those units been

permitted to remain on test until failure, the curve would

have dem onstrated less slope.

With the data for the samples which did not fail removed,

the graph shown in Fig. 3 results.

16 TIME-LOAD

FAILURES

o m 1

O w o o l

o w 0 1 owl 0 0 1 0 1

I

O

I W

,ME YEs Ln(Y)=-O OOor88'Ln(X)+4 414

ChiA2=0 00416

Fig.

3

Time-load data including

only

failures.

It should be noted that more than

95

of the samples

which failed during the test sustained a rod failure. The

remainder sustained crimp slips

(the

crimped fitting

slides

from the rod) or hardware failures (the metallic coupling

zone fails).

1

3

2

s

9

U1

Y

4

1

1 1 1

0001 1

0 1 1 10 1

EUPSED TIME. YEARS

WO= 000839*Ln(X)+43948

Ch1 2= 00213

Fig.

4

Outdoor load racks for

22

m m rod samples.

Fig.

2

Time-load data including samples which did

not

fail.

This graph includes all of the data obtained, including the

time to removal for samples which did not fail during the

The test program for the 22 mm insulators was performed

in a similar manner. In this instance, all of the insulators

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were tested in frames which placed the insulators in a

horizontal position (see fig. 4). All of the tests were

performed outdoors, requiring the use of full insulators.

The data for the 22 mm samples was not run through the

Chi-squared a nalysis. How ever, the data is plotted in fig. 5.

22 mm

Rod Time Load Data

10

OOWOl o w 0 1

ow

001 0 1 1 10

Elapsed

T i m . Y e a n

Fig.

5

Time-load data including samp les which did not fail.

111. ULTIM ATE A ND RESIDU AL ST RENGTH

The time-load results indicate the ability of the insulator to

sustain a fixed load for a period of time. But that load is

typically well above the loads which would be expected in

the field. The primary concern of most users involves the

residual strength in the design resulting from the insulator's

service history.

Before the time-load tests were begun, control samples

were prepared. These insulators were tested in tension to

determine the ultimate tensile strength. Wh en the test

program was established, every third insulator manufactured

was tested to ascertain the characteristic tensile strength of

the batch. Since no test protocol existed, the load was

applied at a relatively uniform rate until failure occurred. A

typical example of the loading profile (see fig. 6) and the

initial data collected for all

of

the samples is shown in Fig. 7.

After the termination of the time-load tests, the unfailed

samples which were removed from the loading racks were

subjected to the sam e ultimate tensile strength test. Those

results are shown in Fig. 8for the 16 mm test samples.

1m

im

8

6

I

40

2

1998 UTS Time Load

Sample

5 4 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5

El.pr.d T h u .

Fig.

6

-Typic al Ultimate Tensile Strength load profile.

16mm

-CONTROL SAMPLES UTS

1E-07

O.OOWO1

O.OOW1 0.0001 0.001 0.01 0.1 1

EIAPSED

TIME.

YEARS

Fig. 7 Control Samples UTS data.

The data was analyzed using a student's t-test [l]. For the

analysis, a two-tailed distribution was assumed. Details

follow:

1977: Averag e(l9)

=

124.25

1998: Averag e(l1)

=

120.95

P(T,n) = 0.837

=

9.63

=

7.48

2

1

-P)

=

.326

The analysis indicates a 32.6% probability that the two

groups of data are from the same general population. That

probability is substantial enough to make doubtful any

supposition that the insulators sustained a measurable loss in

strength as a result of the time under load.

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16mm

RODS

UTS

SAMPLES BEFORE AND AFTER

V. CONCLUSIONS

The data collected over 20 years indicates that non-ceramic

insulators exhibit very stable time-load characteristics.

p ; ~

xtrapolations based upon the e xisting data indicate that the

2-to-1 safety factor applied to this type of insulator, if

properly followed, provides sufficient margin to assure

appropriate long term service.

.SZ

A comparison of ultimate strength data collected at the

start of the test program w ith that collected after 20 years of

ODOOOOOI ooowo, O o w o l oow1 ow1

001 0 1

ELIPSEDIIHE,IEIR~

.

977

DATi

ONTROLS testing indicates no significant reduction in strength.

SERIES2 1998

DATA POST LOAD

Fig. 8 Control samples and post test UTS data

VI. REFERENCES

IV. DISCUSSION

The data collected over 20 years of testing indicates that

the time-load strength redu ction for n on-ceramic insulators is

minimal. The 2-to-I safety margin normally used for these

insulators is sufficient to permit the insulator to be operated

without fear of failure for any reasonable service life.

The ultimate strength data collected before and after the

time-load test indicates no substantial loss of strength

resulting

from

the loading. How ever, since insulators do fail

during time-load testing, there must be strength loss. The

strength

loss

may be

so

gradual that it cannot be precisely

determined or it may be non-linear. In either case, the data

implies that evaluations of tensile strength for insulators

removed from service may not be an indicator of their

condition.

[ l ] S

L.

Meyer, Data Analysis for Scientists and

Engineers John Wiley and Sons, 1975, pp. 279-

282.

VII. BIOGRAPHIES

R. Allen Bernstorf graduated with a B.A. from Gettysburg

College and

an

M. S. in physics from the University of Akron .

In his current position as principal engineer, insulators, he is

responsible for the testing and application of insulator

products. He is a member

of

the IEEE and is active

in

the

IEEEPES, the ANSI C-29 Committee, the CSA C411

Committee and NEMA HVITC.

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