NBSIR 76-1084

Nondestructive Examination of

Glass-Reinforced-Plastic Rod

End Fittings

George F. Sushinsky

Leonard Mordfin

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washsington, D. C. 20234

April 1976

Final

Issued August 1976

Prepared for

U.S. Coast Guard

Washington, D. C. 20590

NBSIR 76-1084

NONDESTRUCTIVE EXAMINATION OF

GLASS-REINFORCED-PLASTIC ROD

END FITTINGS

George F. Sushinsky

Leonard Mordfin

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

April 1976

Final

Issued August 1 976

Prepared for

U. S. Coast Guard

Washington, D. C. 20590

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Acting Director

TABLE OF CONTENTS

page

1. INTRODUCTION 1

2. SPECIMENS 2

3. ULTRASONIC EXAMINATION 4

4. RADIOGRAPHIC EXAMINATION 10

4.1 X-Radiography 10

4.2 Neutron Radiography 16

5. CONCLUSIONS 23

6. REFERENCES 24

NONDESTRUCTIVE EXAMINATION OF

GLASS-REINFORCED-PLASTIC ROD END FITTINGS

George F. Sushinsky and Leonard Mordfin

ABSTRACT

An exploratory study was performed on the

feasibility of using nondestructive examination

techniques to detect voids in two types of potted

compression end fittings. Four methods of nondestructive examination were tried: conventional

pulse-echo ultrasonics, ultrasonic holography, x- radiography, and neutron radiography. The ultrasonic and x-ray methods that were tried proved

unsatisfactory as a nondestructive examination tool

for the two types of end fittings, and the neutron

radiogr>aphic procedures successfully displayed the

internal structure of only one of the specimen

types. The procedures and results from the different methods of examination are summarized.

Additional comments are given regarding the application of the methods to aluminum-jacketed

specimens, and regarding improved methods for the

inspection of steel-jacketed specimens.

Key Words: End fitting; glass-reinforced-plastic

rod end fitting; nondestructive examination; neutron

radiography; ultrasonics; x- radiography

.

1. INTRODUCTION

Glass fiber-reinforced-plastic (GRP) rods, manufactured by pultrusion

and related processes, are widely used as tension members in large, modern

antenna systems and arrays. In these applications the rods serve as structural guys and catenaries, or as insulators for guys and top-loading elements.

The tensile loads are introduced into the rods with end fittings (terminations) mounted on the rod ends and the load-carrying capability of a given

rod depends upon the efficiency of the end fittings used.

Various kinds of end fittings are used; several of these are described

in References 1 and 2. Some are more efficient than others, but 100 percent

efficiency is rarely achieved; tensile failures of GRP rods most commonly

initiate in or adjacent to an end fitting due to the discontinuities and

stress concentrations which are imposed by the fittings. Many of the end

fittings which are used belong to a class which might be described as potted compression end fittings. These consist of a hollow, conical metal

member, or basket, and related hardware such as yokes, bails and eyes. The

end of the rod is potted into the basket, usually with an epoxy formulation;
frequently, a wedge is driven into the rod end prior to potting in order

to give the end a roughly conical shape.

The potting compound is selected to provide good adhesion to the GRP

to enable the rod to resist pulling out under load. The basket is oriented

so that the cured, potted mass, or cone, is pulled more tightly into the

basket as the tensile load is increased. Some users treat the inside

surface of the basket with an epoxy-release agent, prior to potting. This

treatment facilitates this seating process. The conical shape of the

basket imposes radial compressive stresses upon the potted cone as load is

applied and these compressive stresses effectively increase the strength of

the bond between the potting compound and the rod.

If the potting process is not carried out properly, air bubbles or gas

pockets may be created in the potted cone which reduce its strength and the

efficiency of the connection. The result may be a premature tensile

failure in service. Since the GRP rods are frequently used in regions of

high electrical stress or electromagnetic radiation, voids in the end fittings may also contribute to failures of an electrical origin. Void discontinuities can conceivably foster increased corona activity and, if the

voids accumulate moisture due to rain, ocean spray or condensation, an

electrical path may be created leading to the development of arcing and

carbon tracking. The result, in either case, may be the failure of the rod

due to burning.

Failures due to voids inside end fittings are particularly insidious

because of the difficulty in detecting their presence. The investigation

described in this report was undertaken to explore the feasibility of detecting such voids by nondestructive examination (NDE) techniques. A

variety of inspection techniques was tried. These were ultrasonics (including both contact and immersion pulse-echo techniques and holography)

,

low energy x-radiography , and neutron radiography (using both thermal and

fast neutron sources).

This investigation was carried out at the National Bureau of Standards

under the sponsorship and with the financial assistance of the U. S. Coast

Guard. The technical assistance and loan of x-ray equipment by Michael

Kearney of Automation Industries are sincerely appreciated. The helpful

contributions of time and technical expertise by Martin Ganoczy and Harold

Berger of the NBS Reactor Division are gratefully acknowledged.

2. SPECIMENS

Several samples of two different kinds of rod-and-f itting assemblies

were furnished by the sponsor. Figures 1 and 2. All had seen service as

insulators in top-loading elements of Loran C towers. They had been

removed from service following several incidents of burning in some of

these samples as well as in other, nominally identical, units.

2
Figure 1 - Type 1 end fitting.
The Type 1 assembly, from the Loran station at Lampedusa, consisted of

17-mm (11/16-in) diameter, glass f iber-reinf orced-polyester rod, terminated

with potted compression end fittings. The Type 1 end fitting (Fig. 1) had

a conical steel basket which is normally fitted with a metal yoke and bail

(not shown in the figure) for attachment to thimble-and-eye hardware. A

rubber boot covered the narrow end of the basket and the first few centimeters of rod where it emerged from the basket. Prior to potting, the end

of the rod within the basket had been quartered and spread with a cruciform-shaped metal wedge similar to that shown in Figure 3. The basket was

approximately 5 mm (3/16 in) thick at its thickest point.

The Type 2 assembly, from the Loran station at Angissoq, consisted of

20-mm (25/32-in) diameter, glass f iber-reinforced-epoxy rod, terminated

with potted compression end fittings. The Type 2 end fitting (Fig. 2) is

also of steel, and is fabricated with integral clevises as shown in the

figure. Conical metal wedges were used to spread the quartered rod ends

prior to potting in these fittings. The wall of the basket part of the

fitting measures approximately 11 mm (7/16 in) at its thickest part and

10 mm (3/8 in) near the tip of the wedge, the suspected area of trouble.

See Figure 4.

In the Type 2 end fitting the potting compound completely surrounds

the rod end, separating it from the fitting. In the Type 1 end fitting

part of the spread rod appeared to be in line contact with the inner surface of the fitting. The nondestructive penetration of a minimum of three

interfaces, set at shallow angles to each other and to the entry surface,

is required for the examination of these parts. Penetration of three

materials (metal, epoxy and reinforced plastic) with vastly different

attenuation characteristics and acoustic impedances is also required for

the end-fitting inspection.

Due to the problems associated with the inspection of specimens with

complex geometry, numerous interfaces, and material mismatches, two simpler

but similar specimens (Fig. 5) were chosen for the initial feasibility

study. The first was an aluminum end-fitting with a potted rod and no

wedge. This end fitting was sectioned longitudinally and two short, transverse holes, 3.2 mm (1/8 in) in diameter, were drilled in the rod to serve

as artificial defects. The second sample specimen consisted of a potted

cone, containing a rod and wedge, removed from its metal end fitting.

3. ULTRASONIC EXAMINATION

Ultrasonic nondestructive examination techniques are commonly used to

detect and locate concealed flaws in a wide variety of inspection problems.

In ultrasonic NDE, high-frequency sound waves are emitted by a transducer

which is mechanically coupled into the test object, and are reflected from

material interfaces and other discontinuities (defects, voids, etc). In

4
Figure 3 - Cruciform wedge similar to those used in conjunction with

Type 1 end fittings. All dimensions in inches (1 in = 25.4 mm)

5
Figure 4 - Sectioned and unsectioned Type 2 end fittings from rods that

had experienced electrical burning damage. Note the void in

the rod at the tip of the conical wedge.

Figure 5 - Left: Potted cone containing a wedged rod, removed from a compression end fitting. Center and right: Sectioned aluminum end fitting. Note the two small holes drilled into the

rod in the center view.

6
conventional pulse-echo ultrasonics, the reflected sound energy is received

by the sending transducer; this energy is then electronically processed and

displayed visually on a cathode-ray tube or by other means. The basic

ultrasonic NDE techniques are relatively fast, inexpensive, and accurate.

The ultrasonic examination of end fittings for GRP rod was performed

in the ultrasonic measurements laboratory of the Engineering Mechanics

Section at NBS. The laboratory arrangement is shown in Figure 6. It

consists of commercially available ultrasonic equipment and accessories

suitable for both contact and immersion ultrasonic testing. Ultrasonic

flaw detection equipment, with both tuned and untuned pulser-receiver

combinations, an immersion tank with a motorized scanning bridge and precision manipulator, and accessory equipment are available for ultrasonic

interrogation of test specimens. A detailed description of this measurement facility is given in Reference 3.

Problems were anticipated in the ultrasonic inspection of these specimens due to the complex geometry, the number of material interfaces, and

the curved and rough nature of the sound-beam entry surfaces and interfaces. Technique development began with the ultrasonic interrogation of the

simpler specimens (Fig. 5) using the broad-band pulser-receiver with 2.25

and 5.0 MHz (nominal center frequency) transducers. Using longitudinal

waves in a conventional pulse-echo mode, both contact and immersion techniques were tried. Technique development related to the detection and

identification of the various material interfaces and the known defects was

the primary concern in the inspection of these "simpler" problems. Little

success was achieved; only the return interface echo from the aluminum

jacket could be identified by its location on the time axis of the oscilloscope screen. The penetration of deeper material layers was not evident.

Despite this, the two types of steel end fittings were inspected. One

of the Type 2 fittings, which had been sectioned previously, contained a

gross defect at the tip of the wedge (Fig. 4). This 40 by 6 mm (1 1/2 by

1/4 in) void could not be detected by the ultrasonic methods that were

tried. The only echo that could be positively identified was the reflection from the inside wall of the steel jacket. Similar results were obtained with end fittings of Type 1.

Conventional ultrasonic methods do not appear to be a useful inspection tool for the solution of this problem due to specimen geometry and

material impedance mismatch at the various interfaces.

An ultrasonic hologram was made of the half of the split aluminum end

fitting that contained the artificial defects. The test conditions were

not necessarily optimized in this experiment and the result, as shown in

Figure 7, is not completely satisfactory. Part geometry, material impedance

mismatch, and inadequate system resolution were factors that limited the

quality of this hologram. It is believed that better results for this

7
8
/

Figure 7 - Ultrasonic hologram of the sectioned aluminum end fitting. The

end fitting is in the 9 o'clock position. The patterns in the

light hexagonal background represent "noise".

9
specimen could possibly have been obtained under optimum conditions using

the more advanced holographic and imaging systems that are currently

available.

4. RADIOGRAPHIC EXAMINATION

With the inability of conventional ultrasonic methods to detect

defects in the end-fitting samples, the feasibility of using radiographic

methods of nondestructive examination was examined. Radiography, like

ultrasonic methods, is relatively fast, inexpensive, and simple to perform.

In radiography, radiant energy in the form of neutrons, x-rays or gamma

rays is used to produce graphical film records of the relative soundness of

opaque objects under test. Both x- and neutron radiography were considered

to have advantages pertinent to the end-fitting problem.

4 . 1 X-Radiography

X-radiography is a widely used industrial technique for nondestructive

examination. Background material suitable for a basic understanding of the

experimental aspects of x-radiography is available in Reference 4.

A portable x-ray unit was used to produce the radiographs of the

specimens. The two operating parameters of the x-ray unit were continuously variable from 35 to 160 kV and 2 to 5 ma.

Two types of industrial x-ray film (ASTM Class I and ASTM Class II)

were used. Class I films are medium speed, extra fine grain, very high

contrast films which are used primarily for the critical radiography of

light metals and alloys. Class II films are high speed, fine grain, high

contrast films which are used for the radiography of light metals at low

voltages (less than 250 kV) and thin steel sections (less than 50 mm

(2 in)) at voltages less than 1000 kV. Both of the film types were compatible with the specimen dimensions and the possible x-ray unit energy

radiation levels. Commercially available film packets containing one of

each film type were used. This packaging feature is useful in defining the

exposure settings needed to produce good radiographs, relative to the x-ray

current and voltage, for each particular specimen.

For this investigation, only elementary x-ray techniques were employed.

The film packet was placed on a concrete floor* with the specimens placed

*It has since been learned that

backstop may produce film and detail

use is, in general, not recommended.

the use of a concrete floor as a fogging due to backscatter and its

10
directly on the packet. The x-ray beam port was located perpendicular to

the floor, approximately 750 mm (30 in) from the film plane. Radiographs

were made of the joined halves of the sectioned aluminum end fitting (Fig.

5), one half of the sectioned Type 2 fitting (Fig. 4), and Type 1 and Type

2 end fittings as received (Figs. 1 and 2).

As many specimens as possible were radiographed simultaneously. Reasonably good results were achieved on the assembled sections of the aluminum

specimen as shown in Figure 8. Fair results on the half of the sectioned

Type 2 fitting (Fig. 9) and poor results on the Type 1 end fittings (Fig.

10) were produced.* A complete Type 2 end fitting was not penetrated.

These results are considered to be of limited usefulness since aluminum end

fittings were outside the intended scope of this investigation and the

results on the Type 1 and Type 2 end fittings were unsatisfactory.

A summary of quantitative results for the better radiographs produced,

based on x-ray tube voltage, exposure time, and film class is presented in

Table 1. This listing indicates the range of variables over which the

radiographs were made and also provides subjective ratings of the quality

of the radiographs within each end-fitting group.

It is evident from the results obtained to date that the simple experiments performed are inadequate for all but the aluminum end fitting. It is

probable that more advanced x-ray techniques would lead to better results

on aluminum end fittings or on thin-walled steel end fittings similar to

Type 1. Obvious improvement would result from the use of specimen shielding

to reduce radiation scatter and from the development of compensation techniques, related to part geometry and thickness, to provide uniform radiation penetration. The use of higher energy radiation, the optimization of

x-ray film, specimen, and radiation parameters, and image enhancement

techniques could also be beneficial. These variables are briefly discussed

in Reference 4.

A 350-kV x-ray facility will soon be operational in the Applied Radiation Division of NBS . The higher energy radiation available at this facility

plus the expertise of that division's personnel may be expected to provide

the solution to some of these radiographic problems.

^Figures 8, 9, 10 are prints or radiographs that were developed using

normal photographic procedures. These figures do not present the detail

that is evident in viewing the original radiographs using a diffused light

source. Image enhancement methods, such as densitometry, may provide even

greater detail of the original radiographic films.

11

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14
Table 1 - Subjective Evaluations of X-ray Radiograph Quality

as a Function of X-ray Parameters .

ASTM

film

class

X-ray

tube

voltage

Exposure

time

Radiograph

rating^"''

GrouD

rating

K.V s

Half of Type 2 fitting

I 160 75 2

II 160 20 KFig. 9)

Type 1 fitting

Group A

I 160 60 4

I 160 90 2

I 160 120 KFig. 10)

T

1 J.OU 240 3

Group B 2

TTJ. J. 1 fin 15 3

II 160 30 2

II 160 60 1

Aluminum fitting

Group A 2

I 160 15 2

I 160 20 1

1 ±dU 30 3

Group B 3

I 60 180 1

1

fin 300 2

Group C 1

II 60 120 2

II 60 150 KFig. 8)

II 60 300 3

All radiographs made with an x-ray tube current of 5 ma and a source-to-film distance of approximately 750 mm (30 in).

Radiographs are numerically ranked according to quality within

each group. Groups are numerically ranked within end-fitting

type. Increasing numbers denote decreasing quality.

15
4.2 Neutron Radiography

Materials containing hydrogen (e.g., plastics) are somewhat opaque to

neutrons while most metals are relatively transparent. Neutron radiographs

were made of Type 1 and Type 2 end fittings with thermal neutrons (energy

level of 0.25 eV) using the facilities of the 10-MW research reactor at

NBS. Fast neutrons (energy level of 800 keV) were then used in an attempt

to improve the radiograph quality for the thicker Type 2 end fitting. The

fast neutrons were produced using the 3.0-MeV Van de Graaf generator at NBS

through a proton-to-neutron collision reaction (Li(p,n)).

To produce the radiographs using the thermal neutron source, two

neutron radiography film types were used: high-speed Type TLX film and

high-resolution Type AA film. Two different neutron beam port apertures,

9.5 mm (3/8 in) and 25 mm (1 in), were used. A 2-m (80- in) source-to-film

distance was maintained and the specimens were positioned as closely as

possible to the film plane.

The use of the smaller aperture produced insufficient radiation

levels for radiographs on the end fittings. Only radiographs using the 25-

mm (1-in) aperture and Type AA film produced satisfactory results with a

Type 1 end fitting. These results were significantly better than those

obtained with x-ray techniques. For example, the tapered cruciform wedge

of the Type 1 end fitting (Fig. 11) was well defined after a 10-minute

exposure to thermal neutron radiation. But the conical wedge of the Type 2

end fitting (Fig. 12) was still not evident after a 15-minute exposure. At

this exposure the Type 1 end fitting was slightly overexposed. Table 2

summarizes the conditions used to produce the thermal neutron radiographs.

Figure 13 shows the results of a fast neutron radiograph of one-half

of the sectioned Type 2 end fitting using Blue Brand film and a TL-2 film

screen. These results are better than those obtained using x-rays, but

results on half of a specimen are of little practical use. Figure 14 is a

fast neutron radiograph of a complete Type 2 end fitting using RP/R type

film and a TL-2 film screen. Only a faint definition of the interface

between the steel fitting and the potting compound is visible. The conditions used to produce these radiographs are listed in Table 3.

Based on these results a satisfactory method for the nondestructive

inspection of Type 1 end fittings appears to be feasible through neutron

radiography. If the imaging problems associated with radiation scattering

and geometric distortion (problems similar to those encountered with x-ray

methods) could be minimized, neutron radiography would probably be the best

method for solving this complex inspection problem. Portable neutron

sources, which would be needed for in-service inspection procedures, have

already been effectively demonstrated in field tests (Ref . 5). The prognosis for Type 2 end fittings is not nearly as promising.

16
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Table 2 - Test Conditions Used to Produce Thermal Neutron Radiographs

Exposure Film, . Figure

End fitting time type Film screen No

s

Type 1 600 AA vapor-deposited gadolinium 11

Type 2 900 AA 0.025-mm (0.001-in) thick 12

gadolinium conversion

screen

Type 1 30 TLX gadolinium oxy-sulfide - fluorescent screen

Type 2 40 TLX gadolinium oxy-sulfide

fluorescent screen

All radiographs listed were made with 25-mm (1-in) aperture diameter and

a source-to-film distance of approximately 2 m (80 in)

Commercial film types are identified in order to adequately specify

the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does

it imply that the films are necessarily the best available for the

purpose.

19
Figure 13 - Print of a fast neutron radiograph of one half of the Type 2

end fitting. The gross defect at the tip of the wedge is

faintly visible.

20
Figure 14 - Print of a fast neutron radiograph of a Type 2 end fitting

showing poor definition.

21
Table 3 - Test Conditions Used to Produce Fast Neutron

Radiographs (a,b)

End Fitting Film type Figure No.

Half of Type 2 Blue Brand 13

Complete Type 2 RP/R 14

A TL-2 film screen, a source-to-film distance of approximately

380 mm (5 in), and an exposure of 10^ neutrons/cm^ (16 x 10^

neutrons /in^) were used.

A commercial film screen and film types are identified in order

to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the

National Bureau of Standards, nor does it imply that the products identified are necessarily the best available for the

purpose.

22
5. CONCLUSIONS

An exploratory study was performed to investigate the feasibility of

detecting voids and defects in potted compression end fittings mounted on

GRP rods. Four techniques were tried: conventional pulse-echo ultrasonics, ultrasonic holography, x- radiography , and neutron radiography.

Ultrasonic methods proved unsatisfactory due to geometric and material

considerations. X-ray methods yielded promising results with thin-walled

aluminum end fittings and it is suspected that improved x-ray techniques

could provide satisfactory results with a thin-walled steel end fitting

(Type 1) . This end fitting was successfully penetrated by thermal neutron

radiation, thereby demonstrating a potentially good nondestructive inspection technique for steel end fittings of this general size (less than 5 mm

(3/16 in) thick). None of the techniques as tried was capable of detecting

voids inside a thick-walled steel end fitting (Type 2).

23
6. REFERENCES

1. Halsey, N., Mitchell, R. A. and Mordfin, L., Evaluation of GRP

Rod and Rope Materials and Associated End Fittings, NBSIR 73-129

(Dec. 1972).

2. Mitchell, R. A., Woolley, R. M. and Halsey, N., High-Strength End

Fittings for FRP Rod and Rope, J. Engineering Mechanics Div. ASCE

100 , EM4, pp. 687-706 (Aug. 1974).

3. Eitzen, D. G. , Sushinsky, G. F., Chwirut, D. J., Bechtoldt, C. J.

and Ruff, A. W. , Improved Ultrasonic Standard Reference Blocks,

NBSIR 75-685 (April 1975).

4. McMaster, R. C., editor. Nondestructive Testing Handbook, Vol. 1

(Ronald Press Co., N. Y., 1959).

5. Anon., Beyond X-rays—Seeing Beneath the Surface with Neutron

Photography, Dimensions/NBS 59, No. 5, pp. 106-107 and 118 (1975).

24
NBS-114A (REV. 7-73)

BIBLIOGRAPHIC DATA

SHEET

i. PUBLICATION OR Rr.FORT NO.

NBSIR 76-1084

£. Oov t Accession

No.

i. Kecipicnc s Accession No.

4. TITLE AND SUBTITLE

NONDESTRUCTIVE EXAMINATION OF fIT A^lS-RFTNEORrPTlPLASTIC ROD END FITTINGS

5. Publication Date

April 1976

6. Performing Organization Code

7. AUTHOR(S)

George F. Sushinsky and Leonard Mordfin

8. Performing Organ. Report No.

NBSIR 76-1084

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 20234

10. Project/Taslc/Work Unit No. 2130143 & 2130445

11. Contract/Grant No.

MIPR //Z-70099-5-51020

IZ. sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

U.S. Coast Guard

Washington, D. C. 20590

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less tactual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

An exploratory study was performed on the feasibility of using nondestructive

examination techniques to detect voids in two types of potted compression end fittings.

Four methods of nondestructive examination were tried: conventional pulse-echo ultrasonics, ultrasonic holography, x-radiography , and neutron radiography. The ultrasonic

and x-ray methods that were tried proved unsatisfactory as a nondestructive examination

tool for the two types of end fittings, and the neutron radiographic procedures successfully displayed the internal structure of only one of the specimen types. The procedures and results from the different methods of examination are summarized. Additional

comments are given regarding the application of the methods to aluminum-jacketed specimens, and regarding improved methods for the inspection of steel-jacketed specimens.

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)

End fitting; glass-reinf orced-plastic rod end fitting; nondestructive examination;

neutron radiography; ultrasonics; x-radiography.

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