NBSIR 74-512

The Proof Testing of Porcelain

Insulators and the Application of

Acoustic Emission

A. G. Evans, S. M. Wiederhorn, M. Linzer and E. R. Fuller, Jr.

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

June 1974

Final Report

Prepared for Naval Civil Engineering Laboratory

Port Hueneme, California 93043
Evans, A. G., Wiederhorn, S. M., Linzer, M.,

Fuller, E. R., Jr., Proof testing of -

porcelain insulators and application

of acoustic emission. Am. Ceram. Soc . Bull.

54, No. 6, 576-581 (June 1975).

315
NBSIR 74-512

THE PROOF TESTING OF PORCELAIN

INSULATORS AND THE APPLICATION OF

ACOUSTIC EMISSION

A. G. Evans, S. M. Wiederhorn, M. Linzer and E. R. Fuller, Jr.

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

June 1974

Final Report

To be published in the Journal of The American Ceramic Society

Prepared for Naval Civil Engineering Laboratory

Port Hueneme, California 93043

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

ABSTRACT

An investigation of proof testing has been conducted on large cone and post porcelain insulators. The cone configuration was found

to be unsuitable for overload proof testing to the loads needed for

effective lifetime predictions. The only merit of subjecting this

configuration to a proof test is the assurance of no immediate

macrocracking during installation. This is achieved by testing to

a load marginally larger than the service load. Conversely, the

post configuration was found to exhibit the basic prerequisites for

effective proof testing; but detailed stress analysis of this system is

needed before specific proof testing procedures can be recommended.

Finally, acoustic emission measurements were found to be a major

asset for monitoring both macrocracking events and stress development

during the proof tests

.

1
1 . INTRODUCTION

Macrocrack formation has been observed by the Navy in a number of

the porcelain base insulators used to support large radio communications

towers. These cracks do not, per se, impose a severe structural limitation

because the primary stresses in the insulator are compressive. But,

in the presence of the high voltage R.F. fields, the heat generated

around the cracks may lead to structural degradation. Hence, towers

containing cracked insulators are shut down. The prevention of

macrocrack formation in porcelain base insulators is thus a primary

requirement, if continuous operation (of the radio towers) is to be

achieved.

One approach that can be used to prevent in-service macrocrack

formation in ceramic components is overload proof testing. This

is an evaluation procedure which subjects the insulator to a load

greater than the service load. Then, if the insulator survives the

proof test, it should satisfy engineering requirements of service

load and lifetime. [1] The proof test condition for components made

of porcelain must take into account the subcritical crack growth

that occurs in this material due to moisture in the environment.

[2] Subcritical crack growth leads to a time dependence of the strength,

and the time to failure is determined by the time necessary for a crack to grow from a subcritical to a critical size. [1] A mathematical

framework for the proof testing of components made of materials that

exhibit subcritical crack growth indicates that the important parameters

in proof testing are: [1,3,4] the proof test load, the test environment

and the loading cycle. In addition, for compressive proof testing,

a failure criterion must be specified, because crack size criticality

does not lead to complete fracture.

In this paper, the proof test approach is applied to two types

of porcelain base insulators commonly used to support radio towers . The

variables of the study are loading rate and proof test load. Particular

emphasis is placed on the use of acoustic emission to detect crack growth

and as an aid in the formulation of a failure criterion.

2
2 . EXPERIMENTAL PROCEDURE

The two types of insulators studied are shown in Figures 1 and 2.

the bell-shaped "cone" insulators (Figure 1) were approximately one meter

in height and one meter in diameter at the base. They were hollow, with

a wall thichness at the base of approximately 0.08 m. The ends of the

insulators were enclosed by steel end-caps, attached by Portland cement.

The bottom bearing surface and the side surfaces of the porcelain were

glazed, but the top bearing surface was not. The insulator was designed

for a maximum compressive force of 13.5 MN.

The "post" insulators (Fig. 2) were solid and cylindrical in shape,

with circumferential skirts for improved electrical insulation. They

too had steel caps at the top and bottom, attached by Portland cement.

These insulators were designed for a maximum compressive force of 4.5 MN.

Tests were conducted using the 50 MN NBS test facility. A

universal joint was placed at the top of each insulator to reduce loading

eccentricities, and strain gauges were placed around the circumference

to check on the uniformity. Loading and unloading times were varied from

several seconds to approximately 1 hour. Tests were conducted in air

(approximately 50 percent relative humidity) . A total of four insulators

were tested, two of each configuration.

The proof test load was established from a design diagram, developed

earlier from subcritical crack growth studies on porcelain specimens cut

from a similar cone insulator. [2] The design diagram (Fig. 3) gives a logarithmic plot of the minimiam expected time to failure, t . , versus mm

the service stress, a ^. Each straight line on this figure relates

to t . for a predetermined value of the proof ratio, i.e. the ratio of the min

proof stress (a ) to the service stress. Once a tentative determination

P

is made of both the service stress and the minimum acceptable time to

failure, the proof ratio can be determined form Fig. 3. For the cone insulators the maximum tensile stress at the service load (4.5 MN) , as

-2

determined by finite element stress analysis, [5] is 'V'30 MN m

* 5 1 MN = 2.4 X 10 Ibf.

3
Therefore, for a minimum service lifetime of 40 years, the requisite proof

ratio obtained from Fig. 3 is 2.5, giving a proof load of 11.3 MN.

As noted in the preceding section, insulator "failure" has essentially

occurred when macrocracks develop that are large enough to result in

excessive heat generation, at the crack, during service. Since these

cracks are mechanically stable and often do not appear at the external

surface, a nondestructive means of detecting their presence was required.

The two detection methods used in this study were ultrasonic pulse

echo and acoustic emission. Each of the insulators was inspected

* ultrasonically before and after each load cycle to detect the presence

of macrocracks. Acoustic emission transducers capable of detecting signals

emitted in a frequency range close to 180 kHz were attached to the insulators

and were monitored continuously during the test. Ultrasonic transducers

were also attached to some of the insulators during the tests and were periodically excited in an attempt to detect cracking, and hence, to relate

crack formation to the load level and the acoustic emission rate.

After testing, the insulators were sectioned to confirm predictions

made from acoustic emission observations and ultrasonic inspection,

and to determine the source of fracture.

3. EXPERIMENTAL RESULTS

3.1 Cone Insulator Tests

a) First Insulator

The first of two cone insulators was tested to three different

proof levels, 7, 11 and 16 MN, in order to assess the general features of

the structural and acoustic emission response. The initial loading rate was rapid (^^0.45 MN/min) up to approximately 60 percent of the proof

load, and then reduced to ^^^0.14 MN/min for the final load increment.

The insulator was unloaded at '^^0.9 MN/min. Strain gauges located at

All ultrasonic testing reported in this paper was performed by Naval

Research Laboratory personnel under the direction of S. Hart.

4
intervals of 120 degrees around the circumference at the mid-height

* indicated that the strains were within 3 percent of the average strain.

In all three cycles the acoustic emission rate increased during

loading, decayed during holding periods, but increased again as the

load was reduced. The increase on unloading was minimal for the 7 MN

test cycle, but rather substantial for the other two cycles, as depicted

in Fig. 4 for the 11 MN cycle. For this latter cycle (Fig. 4) , the

acoustic emission rate increased slowly between 11 MN and 1 MN (from

3 3 10 to 3 X 10 counts/s) ; but below 1 MN, the rate increased rapidly,

4

and reached a maximum of 3 x 10 counts/s at zero load. This order

of magnitude increase in the acoustic emission rate probably corresponded

to the growth of a large internal crack, which was identified after

the proof test by ultrasonic inspection, and later by sectioning.

Visual inspection after sectioning showed that the crack initiated

at the bearing surface between the porcelain and the metal end cap, and

propagated approximately 0.15 m below the lip of the end cap and approximately one-third of the way around the circumference.

Crack formation and acoustic emission were even more apparent during

the proof test cycle to 16 MN (which was 2.5 MN above the compressive

strength reported by the manufacturer) . As the load was increased above

13 MN peaks of continuous acoustic emission were obtained (i.e., an acoustic emission rate >10^ counts/s); similar, but more extensive,

peaks were noted when the insulator was unloaded. These emission peaks

probably corresponded to the growth of large cracks, which were identified

after the test by inspection. Visual inspection indicated that the

cracks initiated from the porcelain/cement bearing surfaces at the top

and bottom end caps. Cracks from the lower end cap frequently appeared

on the external surface.

* The strain gauge measurements and all of the mechanical testing instrumentation were performed by personnel from the Engineering Mechanics

Section at NBS.
The topology of the cracks was examined by ultrasonic and visual

* inspection after testing. Two principal types of crack were formed:

laminar cracks that grew from the top and bottom end caps roughly parallel

to the exterior surface, and orthogonal cracks that grew through the insulator

wall perpendicular to the exterior surface. An example of a laminar crack

that emerged at the exterior surface is shown in Fig. 5. By sectioning

the insulator we were able to show that this type of crack grew from the

sloped bearing surfaces at the base and top of the insulator (Fig. 6)

.

The rough surfaces of the laminar cracks served as nucleating sites for

the longitudinal through cracks, shown in Fig. 7.

b) Second Insulator

The proof test of the second cone insulator was designed in an attempt to establish a specific proof test procedure that would provide an effective lifetime assurance. Two load cycles were used, differing only

in the rate of load application; the maximum load in both cases being

7 MN. In the first cycle, the loading and unloading rates were 0.14 MN/min.

In the second cycle, the loading rate was 0.9 MN/min and the average

unloading rate was approximately 7 MN/min. Ultrasonic transducers were

attached to the external surface of the insulator to detect the growth

of cracks during the test. Strain gauges located 120 degrees apart

around the circumference at the mid height were within 10 percent of

the average strain.

* Other crack forms were observed in isolated cases; visual observation

identified one circumferential crack orthogonal to the applied load, and

ultrasonic inspection indicated the presence of many small internal

cracks after rapid unloading.

6
The acoustic emission rate slowly increased as the insulator was

loaded and further increased as the insulator was unloaded (Fig. 8)

.

A large acoustic emission peak was observed at 9 MN during unloading.

This peak was shown to correspond to macrocrack formation by an ultrasonic

inspection during the proof test, in which the back surface reflection

under one of the transducers slowly disappeared and was replaced by a reflection approximately one-half the distance between the interior and

exterior surfaces. Additional ultrasonic inspection after the proof

test confirmed the existence of this crack, which extended one-third

of the way around the circumference and approximately 0.15 m below the

upper end cap. The second proof test cycle resulted in saturation level

5 acoustic emission rates (>10 counts/s) when the specimen was unloaded.

Ultrasonic inspection of the cone insulator after testing detected no additional large cracks, but many small reflections were detected suggesting

the nucleation and growth of many small cracks throughout the body of

the insulator.

3.2 Post Insulator Tests

a) First Insulator

The first of the two post insulators was tested to several loads,

3, 4.5, 5.4, 5.3 and 8 MN in order to assess the basic structural and

acoustic emission response of the configuration. Axial strains measured

by four strain gauges placed 90 degrees apart around the circumference

at the mid height of the insulator were within 7 percent of the average

load at 3 MN. In the first cycle, the insulator was loaded to 3 MN at a rate of 0.14 MN/min and unloaded at a rate of 0.16 MN/min. A gradual

increase in the acoustic emission rate accompanied the load increase

(Fig. 9) and a decrease was observed when held at the proof load. But,

the acoustic emission rate decreased during unloading (Fig. 9), a behavior which differs from that observed on the cone insulators.

Ultrasonic examination after the proof test gave no indication of macrocrack formation.

7
The acoustic emission behavior at proof test loads of 4.5 and

5.4 MN was in general similar to that observed for the test at

3 MN. The acoustic emission level increased as the load increased, but

decreased both during the holding and unloading periods . During the proof

test to 6.3 MN, acoustic emission peaks were observed during loading and

unloading, and subsequent visual inspection showed that some spalling

occurred at the lower insulator skirts. Finally, when the insulator

was loaded to 8 MN extensive acoustic emission was observed. But, gross

fracture did not occur until it was unloaded; then, at a load of 7 MN, the

skirts completely spalled off and longitudinal cracks running the length

of the insulator were formed, as shown in Fig. 10 (resulting in an abrupt load decrease from 7 MN to 6 MN) . The formation of these massive

cracks was preceded by a rapid increase in acoustic emission, for a period of about 5 seconds before the observable event,

b) Second Insulator

The second post insulator was subjected to a single load cycle,

up to 3 MN, in an attempt to establish an effective proof test. The

axial strains from the four strain gauges were within 13 percent of

the average strain at 3 MN. This insulator was loaded at a rate of

0.18 MN/min to 3 MN; the initial unloading (to 2 MN) was conducted

rapidly, at an average rate of 3 MN/min, and then continued at a substantially reduced rate (0.18 MN/min) to zero load.

The acoustic emission from the second post insulator is also

shown in Fig. 10. Again the acoustic emission rate increased as the load

increased, and decreased during unloading; but the decrease was marginal

in the rapid unloading regime (from 3 to 2 MN) , which contrasts with the

continuous decrease observed during slow unloading in the first post

insulator. An untrasonic inspection of the post insulator after testing

gave no indication of macrocrack formation.

_ An acoustic emission peak was noted at 4 MN. However, this cracking

did not represent the general behavior of this insulator configuration

because visual inspection showed that the cracking initiated from a portion of one of the skirts that had been damaged before testing.

8
4 . DISCUSSION OF RESULTS

The proof tests described in the preceding section have generated

important information about two aspects of the insulator proof test

procedure; (a) the use of acoustic emission to monitor crack

propagation, and (b) the lifetime assurances that can be provided byproof testing. These features of the proof test are discussed for both

insulator configurations.

4.1 Acoustic Emission

Acoustic emission during proof testing can result from:

(a) plastic deformation in the steel end caps, (b) frictional effects

between the porcelain and cement, or (c) microcrack formation and

macrocrack growth in the porcelain (or cement) . It is clearly very

important that an appreciation of the emission expected from each of

these sources be developed before attempting to analyze the acoustic

emission data.

4.1.1 Plastic Deformation

Plastic deformation results in intense acoustic emission

if the dislocations move in discrete glide "packets". [6] This occurs only during the initial loading period when new dislocation sources are being generated, and the acoustic emission due to a deforming part will

typically exhibit the features shown in Fig. 11. Defdonation of the

steel end caps might, therefore, contribute to the emission during

initial loading, but is unlikely to be a significant contribution

during the constant load and unloading portions of a proof test cycle.

4.1.2 Frictional Effects

During proof testing slippage may occur at the porcelain/

metal/cement interfaces. Slippage will be particularly severe for cone insulators because the bearing surfaces are not orthogonal to

the applied load. Similar slippage behavior might be expected during

loading and unloading, although the amplitude will be somewhat reduced

during unloading if the slippage is partially irreversible. Thus, some of the acoustic emission observed in proof tests of cone insulators

can almost certainly be attributed to frictional effects during slippage.

By contrast, we shall see later (Section 4.1.4) that frictional effects

are unlikely to be the major source of acoustic emission in tests conducted

on the post insulators q
4.1.3 Macrocrack Growth

Acoustic emission during macrocrack growth in porcelain

has been investigated by Evans and Linzer. [2] They showed that the

acoustic emission rate is approximately proportional to the crack velocity,

(Fig. 12) . A direct comparison between their acoustic emission data

and the data in this paper is not possible, due to differences in specimen

shape and size, the transducer location etc.; but their data illustrate

the important trends needed for the qualitative analysis of macrocrack

growth. A quantitative treatment can be performed and some objective

conclusion reached with a modest amount of additional information (see

Appendix)

.

* Cracks that initiate in the tensile zones of the cone insulator

(type A zones) arrest in the structure, because they invariably propagate

* into adjacent compressive zones (type B zones) . The tensile stresses

that aid crack growth are greatest at the top and bottom bearing interfaces, decrease with distance from the interface, and finally become

compressive approximately 0.15 m away. The crack growth rate is thus

expected to exhibit an initial increase in zone A, reach a maximum

value (see Appendix) , and then decrease in zone B essentially to zero. As the crack velocity increases from zero to a maximum, the

acoustic emission rate must also go through a maximiam. Hence, a peak

in the acoustic emission rate is expected when macrocrack growth occurs. Peaks of this sort were observed in the cone insulators (Figs. 4

and 8) and correlated with macrocrack formation. No significant peaks

were observed during the simulation proof test in the post insulators.

* The terms tensile zone and compressive zone are used here to describe

the regions in the insulator prior to macrocrack development in which

the largest normal stresses orthogonal to the observed crack propagation

planes are, respectively, tensile and compressive; hereafter, these

regions are referred to as zones A and B, respectively.

10
An estimate of the proportionality constant relating the acoustic

emission rate and crack velocity can be determined for the cone insulators

from the maximum acoustic emission rate and the time taken for macrocrack

growth. The maximiam acoustic emission rate during the peaks was approxi5 mately 10 counts/s; the duration of the peaks was approximately 50 s and the crack growth was approximately 0.2 m, giving an average crack

-3

velocity of 4 x 10 m/s . If we assume that the average velocity is

not very different from the maximum velocity, the proportionality is

7

2 X 10 counts/m (see the Appendix for a more detailed illustrative

analysis) ; whereas the data of Evans and Linzer obtained on small specimens

5 gave 10 counts/m.

4.1.4 Microcrack Formation

It has been shown [2] that microfracture of quartz

particles in porcelain is a profuse source of acoustic emission; and

that the rate of acoustic emission depends on the rate at which the

quartz particles break. The dependence of acoustic emission rate on

stress and time has been quantified for several loading conditions.

[2] These results are expanded in a companion paper [7] to give the

acoustic emission rate, dN/dt, for a constant stress rate, d , constant

Li stress, a^, and constant unloading rate, d^. For a constant stress

rate , d the acoustic emission rate (dN/dt) due to particle fracture

for a homogeneous stress o, except for stresses close to zero, is:

[7]

log (dN/dt) = { [m(n+l)/(n-2]-l}log a+ { 1- [m/ (n-2) ] } log a + (1)

where m is the shape parameter for the quartz particle fracture stress

distribution, c() is a constant, and n is the slow crack growth exponent

(see Appendix) . For a constant stress, a , (following loading)

3. the acoustic emission rate, except for very short times is: [7]

11
log (dN/dt) = { [m/(n-2) ]-l} log t + log (dN/dt) + 4)' (2)

where (|) ' is a constant and (dN/dt) is the acoustic emission rate immediately after loading is completed. For unloading ^ at a constant

rate, the acoustic emission rate except for stresses close to is: [7]

log (dN/dt) = n log {a/a ) -r (n-m-2) / (n-2) ] log f l+(j)" [1- (a/a )"'^''"]}

P P

+ loa (dN/dt) (3)

P

where ^" is a constant that depends on the loading, holding and unloading

times; and (dN/dt) is the acoustic emission rate at the proof stress

P

immediately prior to unloading. These three relations can be used in

combination to predict the acoustic emission expected during a proof

test cycle, (Fig. 13), when the acoustic emission is due onlv

to microcrack formation. Alternatively, this source of acoustic emission

can be used as an approximate monitor of stress variations that may

occur in a component, due to unidentified or extraneous effects.

Some similarity between the predicted and observed emission rates

(Figs. 13 and 9) is apparent for the post insulators, suggesting that

acoustic emission in the post insulators is due mainly to microcrack

formation (occurring in the porcelain and/or the cement) . A more

quantitative comparison can be made by using the acoustic emission test

data to calculate m and n of Eqs . (1) through (3) . The slopes of

predicted curves in Fig. 13 are given by , x = [m (n+1) / (n-2) ] -1 , for the

constant loading rate portion of the curve and y = [m/(n-2)]-l for the

constant stress portion. By substituting for x and y (the measured

values of these slopes ), we find: m - 5 and n - 10. Similar values

were obtained in the laboratory on flexural specimens cut from an equivalent insulator [2] (m = 3 to 4; n = 10 to 12) , thus adding

additional credence to our suggestion that microcracking (occurring

mostly in the porcelain) is the primary source of acoustic emission

in these insulators

.

The values of m and n can now be used to compute the acoustic

emission rate, due to microcracking, for a reversible, elastic system

12
during unloading. [7] The results for the post insulator, shown in

Fig. 14, indicate an acoustic emission rate exceeding that expected

for a reversible system. A similar calculation for the cone insulators

would indicate a much larger discrepancy. This additional acoustic

emission is almost certainly due to residual stress development (resulting from some irreversible slip at the porcelain/cement interface)

,

which causes additional microcracking; and, for the cone insulators,

leads to macrocrack formation. A mmeasure of the severity of the residual

stress can be computed by substituting the observed acoustic emission

rate on unloading into Eq. (3) . [7] The result (Fig. 15) indicates

a small, but significant, residual stress that persists to zero load.

4.2 Lifetime Assurance After Proof Testing

4.2.1 Cone Insulator

The observations of macrocracking and profuse acoustic

emission during unloading of the cone insulators show that extensive

irreverisble slippage occurs in this configuration at the porcelain/cement

interfaces, which leads to substantial residual stress development

during load release. The stresses that develop during unloading apparently

exceed the stresses at the proof load. Hence, no realistic lifetime assurances after proof testing can be provided.

In systems that exhibit substantial irreversibility during proof

testing the only merit of proof testing is the assurance that there

will be no instantaneous failure during installation. This can be

ensured by proof loading to a load just in excess of the service load

(by say 5 percent) . This load should then be maintained on the component

until installation, using a clamping fixture, to avoid macrocracking

in the period between proof testing and installation.

Finally, it is noted that although this proof test procedure gives

no guarantee that all surviving insulators will resist in-service

macrocracking, a proportion of the insulators will give a satisfactory

in-service performance. The proportion that gives good performance

13
depends on the statistical nature of the flaw size distribution in the

critical parts of the insulators, [1.4] i.e., near the top and bottom

interfaces. The distribution function is difficult to obtain and is

notoriously dependent on fabrication variables, [8] but approximate

values could be obtained from a survey of the performance of insulators

already in service.

4.2.2 Post Insulator

Residual stress development also occurs in the post

insulators during unloading, but the stresses are comparatively modest,

and not large enough to generate macrocracks . This configuration thus

exhibits the basic features needed for effective lifetime assurances

after proof testing.

Quantitative lifetime predictions after proof testing require

values for the maximum stress during the proof test, a^, and the

maximum stress during service, a . [1] The ratio a /a then gives ^ 'a p a

^ the minimum failure time from Fig. 3, if the stress distributions

are comparable. [1,4] For a system that exhibits reversible behavior,

this ratio is given directly by the ratio of the proof load to the

service load . But, for a system that exhibits some irreversibility,

this load ratio does not suffice. Instead, a detailed stress analysis

(for the complete proof cycle and the service condition) is needed to

obtain values for the maximum stress in the proof test, and its

distribution, and the maximum in-service stress . This analysis has not

been performed, and hence, we cannot yet determine whether an effective

proof test procedure for this system is a practical feasibility. We

suggest that urgent attention is devoted to this stress analysis

(both analytically using stress calculations and empirically using

acoustic emission) to enable this issue to be resolved.

5. CONCLUSIONS AND RECOMMENDATIONS

Proof test have been conducted on cone and post porcelain insulators.

At the proof loads needed to guarantee approximately 40 hrs . lifetime

14
without macrocracking, extensive irreversible interface slippage

occurs in the cone configuration, and this leads to macrocracking

while unloading. This specific configuration is thus not capable of

effective overload proof testing. It is recommended, therefore, that

the insulators are only proof tested to a load just above the in-service

load, if an assurance of no immediate "failures" during installation

is required (and that nondestructive inspection is used to identify the

proof test failures) . No significant lifetime predictions can be provided

for this insulator.

The post insulator can be subjected to the loads needed for

effective overload proof testing without excessive slippage and without

gross macrocracking. This configuration thus has the basic prerequisites

for lifetime prediction after proof testing. However, the detailed

prediction requires information about the stress distribution in these

insulators, both during proof testing and in service. It is hoped that

this information can be provided by stress analysis and acoustic emission

measurements. Then it will be possible for proof test procedures to

be developed.

Acoustic emission was monitored throughout the proof tests and was found to generate information about the two most crucial problems

in insulator proof testing: internal stress development and macrocracking

(the latter constituting a proof test failure in compression proof testing)

.

Analyses have been presented which form the basis for the quantitative determination of residual stress development and for the detection of macrocracking events using acoustic emission. Acoustic emission measurements

during proof testing thus permit a substantial extension of the proof

testing technique to quantitative lifetime predictions, although more

extensive calculations of acoustic emission rate are required to explore

the full potential of the technique in this application. Finally, for

future proof test studies, we recommend that a large acoustic emission

system, employing several correlated transducers, is utilized. This

would add an extra dimension to the study, by permitting spatial resolution

of the acoustic emission sources.

15
APPENDIX

ACOUSTIC EMISSION DUE TO MACROCRACKING

Firstly, we need to obtain the relation between the stress intensity

factor, , and the crack length for the component. This is determined

for a symmetrically loaded central through crack from the relation: [9] a a dx (Al)

, 2 2.1/2

(a -X )

where a is the stress orthogonal to the crack plane and 2a is the crack

y

length; this relation also applies, to a good approximation, for an edge

crack of length a. For illustration purposes we shall now consider an edge crack

propagating in a linearly decreasing stress field (Figl.l5a):

a = a y t

1 - ^

(A2)

where is the maximum tension in zone A and a* is the size of zone A,

Substituting in Eq. (Al) and integrating gives:

= a^vTra 1 - 2a

TTa'' (A3)

or, if we define K as the stress intensity factor for initial flaw

o propagation (K^ = a^/rra^, where a^ is the initial flaw size)

then:

= Koyjri^ TTa*j (A4)

Next, we shall differentiate to determine the maximum stress intensity

factor, (K^) ; this gives

I max

16
K

dK =

_2v/a

1^'

* = 0

so that a = TTa /6 and:

2K

(^I^max

{A5)

Also, note that K^=0, when a=TTa /2. Hence, we can construct relatively

simple (Kj , a) diagrams for this system, as depicted in Fig. 16b.

Now, the (K^,a) relation can be combined with the (K^,V) relation to

obtain velocity, time diagrams, as follows;

Using, V = AK^", for the slow crack growth relation [1] (where n and A are separate quantities for each region of crack growth) , and

combining with Eq. (A4) gives;

n/2

t = AK /

* -T [l-2a/TTa ] n/2

da (A6)

where t is the crack propagation time. Expansion and integration

then gives, for s.^;

'2a^ \ , , > / 2a,

n/2 ^ t = Ak" a, ^-2/2

o 1

1 n-2 (n-4) \7Ta*

n(n+l)

2(n-6) \Tra

(A7)

For 2a^/Tra <n, this becomes

2a. 2a,

t =

2 (K . ) II

AK^. (n-2)

V^(n-2)

Tta^ V^(n-2)

(A8)

where is the crack velocity when a = a^ . The total duration of the

macrocracking event is obtained by putting a.. a and V, = V , o 1 o But,

a more important quantity for the acoustic emission analysis is the

duration which gives an acoustic emission rate in excess of the background,

This is obtained by putting equal to the velocity, V, where the

acoustic emission due to macrocracking exceeds the background emission;

and a^ becomes the corresponding crack length.

17
References

1. A. G. Evans and S. M. Wiedernorn, NBSIR 73-147 (March 1973), Intl.

Jnl. Frac, in press.

2. A. G. Evans and M. Linzer, Jnl. Amer. Ceram. Soc. 56_, 575 (1973).

3. A. G. Evans and E. R. Fuller, Mater. Sci . Eng., to be published.

4. S. M. Wiederhorn, Ceramics for High Performance Applications, in press.

5. A. H. Heuer, private communication.

6. A. S. Tetelman and A. G. Evans, Fracture Mechanics of Ceramics, 2,

895, (1974).

7. A. G. Evans, J. Am. Ceram. Soc, to be published.

8. A. G. Evans and T. G. Langdon, Progress in Materials Science, to be

published

.

9. S. Vaidyanathan and I. Finnie, Jnl. of Basic Engineering (ASME)

,

June 1971, p. 242.

18
Figure Captions

Fig. 1. A typical cone insulator used in this study.

Fig. 2. A typical post insulator used in this study.

2 Fig. 3. A proof test diagram for porcelain in humid environments , giving the minimum service life time as a function of the

service stress, o , at various proof stress levels, a . a ^ P

Fig. 4. The relation between acoustic emission rate and load obtained

on a cone insulator loaded to 11 MN, and unloaded rapidly.

The large rate increase during unloading (just prior to zero

load) can be attributed to macrocrack formation.

Fig. 5. An example of a laminar crack formed in a cone insulator.

Fig. 6. A schematic of the laminar and orthogonal cracks that form

in cone insulators.

Fig. 7. An example of a longitudinal crack in a cone insulator.

Fig. 8. The acoustic emission rate obtained on a second cone insulator

loaded to 11 MN and unloaded slowly- An acoustic emission peak

appears during unloading at 9 MN, and has been attributed to

macrocracking

.

Fig. 9. The acoustic emission rates obtained on the post insulators

during loading to 3 MN and unloading

.

Fig. 10. A post insulator after testing to 7 MN and unloaded to 6 MN.

Fig. 11. The acoustic emission expected due to plastic deformation of

the steel end caps.

Fig. 12- The relation between acoustic emission rate and crack growth

rate during slow crack growth in porcelain.

Fig. 13. The predicted acoustic emission rate from microcracking in an elastic, reversible system.

Fig. 14. A comparison of the acoustic emission rates measured during

unloading with the rates predicted for a reversible, elastic

system.

Fig. 15. The relative residual stress in the post insulators during

unloading as predicted from the excess acoustic emission;

is the residual stress and is the stress at the proof load.

Fig. 16. (a) A schematic of a linearly decreasing stress field defining

the parameters used in the text.

(b) The relation between stress intensity factor, , and crack

length, a, for the stress field in Fig. 16 (a)

.

19
Fig. 1. A typical cone insulator used in this study.

20
Fig. 2. A typical post insulator used in this study.

21
S 10^ -

LJ

a:_J

2

3 10"=' -

0.1 I 10

STRESS (MN m"2)

Fig. 3. A proof test diagram for porcelain in humid environments , giving the minimum service life time as a function of the

service stress, a , at various proof stress levels, a . a -t

22
(s jad s;uno3)

3iVa N0ISSIIAI3 Olisnoov

<u 0

c u

•H . d)

'T3 O

0 -H 4J cu

Ti a u

fd M o

O -H

(U a^ . "13 TJ C

C rtl +J o

(0 O W -H H d -P C -n (0H

M T3 &i O

13 ^ * rO U

g O to

0)

4-1

C

0

•HW

W

•H

(D rH o o

•H +J

+J

c u

d o

tn u

_ . d O

13 (U Tl -P O

O (tJ (U

fO 0 W <U

IS -P U -H P (0 -H -P

C

o

•H

-p 3 a; (13 c fd 0) •H M X)

0) 0) c

C cn n3 r^ 0 M o

<u u («

(1) (U ft3 CJ ^ O

O H

•H

23

UPPER END CAP

(STEEL)

Fig. 6. A schematic of the laminar and orthogonal cracks that form

in cone insulators.

25

5-1 O

P P^ 0

(0 4J iH G

0 -0 m •H (U c CO 4J •H is bu

Q) E •H

(U »^ 0

u OT at

•P C CO C

0 <u o 0 o

u x>

CO (d cn

td An ha c0 nd

(d

0) H

•H lo

-p CO 0 ed P

Q) T) (d P nj

nJ 0 0> M nl UT

C!0 (d •H 73 0

CO C iH

CO (0 C

•H d

em ng

i

u r-l •H

iH

+J d ^

CO 0 O

0 +J (d 0 CO >^ u -d u u

(0 OJ (d 0

T) u

(S a o

0

E-t iH (0 e

(s Jdd s;uno3)

3iva N0ISSI1AI3 oiisnoov

•00

tn •H

27
28

dt

FAILURE

STRESS

Fig. 11. The acoustic emission expected due to plastic deformation of

the steel end caps.

30
Fig. 12. The relation between acoustic emission rate and crack growth

rate during slow crack growth in porcelain.

2

31
^

CO CSI

CD 5 CD O

3iVy N0ISSI!Ai3 3liSn03V

32
o

<:o

(s J8d s;uno3)

31Va N0ISSI1AI3 3liSn03V

•H tn

.3 ^

0) 0)

>

O iw •H

U (U •H M P

WO

U

"3 to

(U

-P o xi

4J

15C

•H . ft T) S

E ft) S

O O 4-> ^ >

33
<

/^

cr t) o C rH •H ,^ u M-l 0 0

•H o

cn

u

w cu

m •H u g (1) 0 a) x; P +j

(0 oH 4J

+J nJ

w OJ

j3w

•Hw

u (U 4J (0 }^ W 4J 0 0) to D<

<D <u <U u r; ^ X 4J

-P <u

to

CJ Q) •H •H

tom

g

Q) 0

-P U-l n

13 to iH (U CO

(0 4J d) o >^ •d •H -P •H ^3 CO

CO (U

0) iH

>-) ft nJ

0) ;3 'd > to •H

•H CO P &^ (U

n3 c: tH •H

0) 10 0)

nJ0 -p

0) rH

c: CO 3 •H

oo CO

CD

in

0»
Fig. 16. (a) A schematic of a linearly decreasing stress field defining

the parameters used in the text.

(b) The relation between stress intensity factor, K^, and crack

length, a, for the stress field in Fig. 16 (a).

35
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NBS.114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 74-512

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

The Proof Testing of Porcelain Insulators and the

Application of Acoustic Emission

5. Publication Date June 1974

6. Performing Organization Code

7. AUTHOR(S)

A. G. Evans, S. M. Wiederhorn, M. Linzer, and E. R. Fuller, Jr.

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9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 20234

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1 1. ^-oniract/Orant No. PO-3-0072

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Naval Civil Engineering Laboratory

Port Hueneme, California . 93043

13. Type of Report & Period

Covered

Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

To be published in the Journal of The American Ceramic Society

16. ABSTRACT (A 200-word or less factual summary ol most sigriificant information. If document includes a significant

bibliography or literature survey, mention it here.)

An investigation of proof testing has been conducted on large cone and post porcelain

insulators. The cone configuration was found to be unsuitable for overload proof

testing to the loads needed for effective lifetime predictions. The only merit of

subjecting this configuration to a proof test is the assurance of no immediate macrocracking during installation. This is achieved by testing to a load marginally larger

than the service load. Conversely, the post configuration was found to exhibit the

basic prerequisites for effective proof testing; but detailed stress analysis of this

system is needed before specific proof testing procedures can be recommended. Finally,

acoustic emission measurements were found to be a major asset for monitoring both

macrocracking events and stress development during the proof tests.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) ,

' Acoustic emission; macrocracking; porcelain insulators; proof testing; residual strain.

18. AVAILABILITY [

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