NBSIR 73-129

Evaluation of GRP Rod and Rope Materials

and Associated End Fittings

Nixon Halsey, Richard A. Mitchell and Leonard Mordf in

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1972

Final Report

Prepared for Army Electronics Command,

Naval Facilities Engineering Command,

Rome Air Development Center and

United States Information Agency

NBSIR 73-129

EVALUATION OF GRP ROD AND ROPE MATERIALS

AND ASSOCIATED END FITTINGS

Nixon Halsey, Richard A. Mitchell and Leonard Mordf in

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1972

Final Report

Prepared for Army Electronics Command, Order Nos. 70-95358 and 71-90003

Naval Facilities Engineering Command, Work Unit Y-R009-03-03-914

Rome Air Development Center, MIPR No. F30602-70-X-0018

United States Information Agency, Contract I A— 14507— 23

NBS Lab No. 213.04/324

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

CONTENTS

Page

1. INTRODUCTION 1

2. ROD AND ROPE MATERIALS 3

2.1 Material A 3

2.2 Material E 3

2.3 Material G 4

2.4 Material N 4

3. EVALUATION OF COMMERCIAL END FITTINGS 4

3.1 Type P Fitting 4

3.2 Type F Fitting 6

3.3 Type F/A Fitting " 8

3.4 Type R/V Fitting 8

3.5 Type R/P Fitting 12

3.6 Summary 17

4. DEVELOPMENT OF IMPROVED END FITTINGS 19

4.1 Finite-Element Analysis 19

4.2 Analytical Parameter Study 21

4.3 Experimental Prototype Fittings 22

4.4 Experimental Stress Analysis 32

4.5 Tensile Tests with Mod 4 Fittings 32

5. DENSITY 36

5.1 Material A 36

5.2 Material E 37

5.3 Material G 37

5.4 Material N 39

5 . 5 Summary 41

6. TENSILE STRENGTH 41

6.1 Material A 42

6.2 Material E 42

6.3 Material G 42

6.4 Material N 46

6.5 Summary 48

7. MODULUS OF ELASTICITY 48

8. LOW-TEMPERATURE FLEXIBILITY 51

8.1 Material E 51

8.2 Material G 53

8.3 . Material N 53

8.4 Summary 54

ii
9. VIBRATION EFFECTS

10. STRESS-RUPTURE STRENGTH

10.1 Material G

10.2 Type R/P Fittings on Material G

10.3 Material N

10.4 Type R/V Fittings on Material N

10.5 Material A

10.6 Summary

11. LONG-TERM STORAGE

11.1 Material A

11.2 Material E

11.3 Material G

11.4 Material N

11.5 Summary

12. CONCLUSIONS

13. REFERENCES

APPENDIX

iii
EVALUATION OF GRP ROD AND ROPE MATERIALS

AND ASSOCIATED END FITTINGS

by

Nixon Halsey, Richard A. Mitchell and Leonard Mordfin

ABSTRACT

An extensive and varied test program was

carried out on four GRP rod and rope materials

to evaluate tensile strengths, moduli of elasticity,

flexibility at low temperatures, effects of simulated Aeolian vibration, and stress-rupture

properties at moderate elevated temperatures

both with and without high humidity. The effects

of elevated temperature on long-term storage

capabilities were investigated, and diametertemperature relationships were established for

avoiding buckling due to storage in a coiled

condition.

The performances of five commercially

available end fittings on these materials were

examined in terms of the breaking loads attained

in tensile tests. Using finite-element analyses,

an improved end fitting was developed. With this

end fitting the strength of the fitting-specimen

system approaches the true tensile strength of the

specimen for two of the GRP materials. An experimental stress analysis of the improved fitting was

performed.

Key Words: Aeolian vibration, simulated;

composite materials; end fittings for GRP rod

and rope; grips, guy; guys, antenna; humidity,

effects on GRP; mechanical properties of GRP;

pultruded rod; reinforced plastics, rod and

rope; rope, GRP; static fatigue of GRP.

1. INTRODUCTION

Glass-reinforced plastic (GRP) rod and rope products offer many

advantages relative to comparable steel products. GRP rod and rope are

strong, flexible, lightweight, non-conducting, corrosion resisting and

1
inexpensive. These products have been finding increased use in

applications which had previously utilized other materials. For

example, GRP rope has been used for tethering high-altitude balloons

[1]* and for deep-sea mooring of oceanographic buoys [2]. GRP rod has

been used as guy strain insulators [3], and both the rod [4] and the

rope have been used, instead of steel, for entire guy lines and other

elements of large communications towers and antenna arrays. The

investigation described here pertains primarily to the latter

application but many of the results obtained are relevant to the other

applications as well.

The purpose of this study was to evaluate some of the structural

properties of four commercially available GRP rod and rope materials.

This evaluation included

1. measurements of density, tensile strength and tensile

modulus of elasticity as a function of diameter,

2. determination of flexural moduli of elasticity at

sub-zero temperatures,

3. study of the effects of Aeolian vibration on tensile

breaking strength,

4. stress- rupture tests at moderate elevated temperatures

both with and without high humidity conditions, and

5. investigation of buckling failures resulting from

long-term storage in the coiled condition at moderate

elevated temperatures.

In addition, the influence of several commercially available end

fittings on the tensile breaking strengths of GRP rod and rope were

examined, and a new end fitting was developed.

This investigation was carried out in the Engineering Mechanics

Section of the National Bureau of Standards under the sponsorship and

with the financial assistance of the Army Electronics Command, the Naval

Facilities Engineering Command, Rome Air Development Center (USAF) , the

United States Information Agency, and NBS . The period of performance

was April 15, 1970 through July 14, 1972.

^Figures in brackets denote references cited on page 80.
In October, 1971 a new program was initiated at this laboratory,

under the sponsorship of the Air Force Materials Laboratory, to develop

an improved non-metallic antenna support material. A part of the

financial assistance which was provided was earmarked for an expansion

of the effort to develop improved end fittings for GRP rod and rope.

That portion of this effort which was completed prior to July 15, 1972

is included in this report. This effort is continuing.

The U.S.A. is a signatory to the General Conference of Weights and

Measures which gave official status to the metric SI system of units in

1960. However, for simplicity, only U.S. customary units have been used

in this report. Conversion factors for these units are given in the

Appendix.

2. ROD AND ROPE MATERIALS

Four, commercially available, GRP rod and rope materials were

tested in this program. All are fabricated with unidirectional, con- tinuous, glass fiber roving.

2.1 Material A

Material A is fabricated from S-glass and an epoxy resin matrix in

the form of small-diameter rods or strands up to 0.119 in in diameter.

The manufacturing process for this material has been patented [5].

According to the patent the individual fibers are 0.00036 in in diameter

(Type G) and the resin is a conventional epoxy formulation (i.e., based

on epichlorhydrin and bisphenol A) cured with boron trifluoride

monoethylamine complex. However, the manufacturer states that the resin

system actually used is different from that cited in the patent and was

selected for greater moisture resistance.

Material A is also available in a rope-type configuration

consisting of seven 0.119-in strands twisted together with a long

helical pitch. Both hand- and machine-wrapped products are made.

2.2 Material E

Material E is a rod product which is fabricated, by the pultrusion

process [3, 6, 7], from E-glass and a polyester resin matrix. Although

1/2-in diameter rod is available the manufacturer normally stocks only

13/16- and 1-in diameter products. The individual glass fibers are 0.00051 in in diameter (Type K) . As a consequence of the manufacturing

process an integral gel coating of the matrix material is formed on the

outer surface of the rod.

3
2.3 Material G

Material G is a rope product fabricated from E-glass and an epoxy

novolac resin matrix. The manufacturing process for this material has

been patented [8, 9]. According to the patent, the glass rovings are impregnated with the resin, several such impregnated rovings are compacted together to form a single strand, each strand is lubricated

with a silicone resin, then several strands are twisted together and

cured. In this process the rope assumes a permanently twisted

configuration although the lubricant permits the strands to slide

relative to each other. The rope is coated with a polyurethane jacket,

approximately 0.015 in thick. It is presently available in sizes from

1/8 to 1/2 in in diameter, which correspond to 1 x 7 and 7 x 19

configurations (no. of strands x rovings per strand), respectively.

2.4 Material N

Material N is a pultruded rod product fabricated from E-glass and a polyester resin matrix. It is normally available in sizes from 1/4 to

1-in in diameter. The individual glass fibers are 0.00051 in in

diameter (Type K) . The matrix resin system is reported by the

manufacturer to be an "epoxy-modif ied" polyester. The rod is coated

with epoxy that is heavily loaded with titanium dioxide for resistance

to ultraviolet radiation. Thickness of the coating is 0.005 to 0.015

in.

3. EVALUATION OF COMMERCIAL END FITTINGS

Five types of commercially available end fittings for GRP rod and

rope were evaluated. The evaluations were based entirely on tensile

tests which were carried out in universal testing machines at room temperature and a relative humidity between 45 and 50 percent. One size

of each of the four GRP materials was used, namely, Material A in the 7-

strand configuration, 1/2-in Material E, 7/16-in Material G and 1/2- in

Material N, but not every material was tested with every end fitting.

Except where otherwise specified, a crosshead speed of 0.75 in/min was

used.

3.1 Type P Fitting

The Type P fitting is a mechanical fitting of the dead-end type

which, according to its manufacturer, is intended primarily for use on Material N rod. In this program the fitting was used on Material A rope

and Material E rod in addition to Material N rod. The fitting, Figure

1, consists of a series of parallel, helically formed, galvanized steel

strands which are wrapped around the end of the specimen. The end of

4
Figure 1 - Type P end fitting.

the fitting is formed into a loop for attachment to thimble-eye type

hardware.

The fitting operates on the so-called "Chinese finger" principle;

as tensile load is applied the strands tend to grip the specimen more

tightly. The inner surfaces of the strands, where they contact the

specimen, are coated with an abrasive material to provide additional

gripping strength. Unfortunately, the strands also tend to twist, or unwind, under tensile load and this characteristic contributes to the

failure of the specimen.

The test results are given in Table 1. Failure of the Materials E

and N rods generally took place in the free lengths of the specimens but

was triggered by longitudinal splitting which apparently initiated in

the fitting. This splitting was quite obviously a shear phenomenon

induced, in part, by the twisting of the rods by the fittings. Test No.

31004 was an exception; the Material E rod pulled out of the fitting

prematurely and remained intact. The average maximum load for the other

two tests of Material E was 26000 lbf

Failure of the Material A rope took place at the end fitting with

the individual strands of the rope breaking in rapid succession. (This

mode of failure, which is discussed in several places in this report,

will hereafter be denoted by the phrase, "strandwise at the fitting".)

This failure also appeared to be induced, in part, by the twist imparted

to the rope by the fittings. The natural helical twist in the rope

material is small compared with the twist produced by the fittings.

The maximum loads obtained with Material A are in substantial

agreement with those of previous tests conducted in this laboratory. In

that work [10] three tests of 7-strand Material A with Type P fittings

gave maximum loads between 17000 and 18000 lbf. On this basis it

appears that the low strength value attained in Test No. 51065 is

atypical.

The maximum loads obtained with Material N are also in good agreement with those obtained previously [10]. In that work four tests of

1/2-in Material N with Type P fittings gave breaking loads ranging from

19300 to 21500 lbf.

Since the failures in all of these tests were induced, to some

extent, by the fittings it is felt that in no case was the true tensile

strength of the specimen material approached.

3.2 Type F Fitting

The Type F fitting, Figure 2, is a mechanical fitting of the deadend type which is very similar to the Type P fitting. According to its

6

Table 1 - Tension Tests with Type P Fittings

Test

No. Material

Gripped

length

Free

length

Maximum

load Failure

f n lbf

51064 A, 7-strand 30 35 17,200 (b)

51065 A, 7-strand 31 36 13,000 (b)

51066 A, 7-strand 31 37 17,000 (b)

1 S 700 flvprflpp

31004 E, 1/2-in dia 40 24 17,000 (c)

41037 E, 1/2-in dia 40 24 26,500 (d)

41038 E, 1/2-in dia 40 24 25,500 (d)

23,000 average

31003 N, 1/2-in dia 40 24 18,950 (d)

41041 N, 1/2-in dia 40 24 20,000 (d)

41042 N, 1/2-in dia 40 24 20,500 (d)

19,800 average

Each end.

Strandwise at the fitting.

Pullout.

Longitudinal splitting in the fitting.

7
manufacturer it, too. Is intended primarily for Material N rod although

it was also used on Material A rope and Material E rod in this investigation. The most obvious difference between the Type P and Type F

fittings is in the loop. In the Type P fitting the strands in the loop are twisted together while in the Type F fitting they are not. Also,

for a given specimen diameter the appropriate Type F fitting is somewhat

shorter than the corresponding Type P fitting. The difference is small

except in the larger sizes.

The test results are given in Table 2. Failure of the Material A

rope took place in the same manner as it did with the Type P fittings,

i.e., strandwise at the fitting. Similarly, failure of the Material N

rod was in the same manner as with the Type P fittings, i.e., in the

free length following longitudinal splitting. With the Material E rod,

however, only one of the three specimens failed in this way; the other

two pulled out of the fitting. In no case did it appear as though the

true tensile strength of the specimen material had been approached.

3.3 Type F/A Fitting

The Type F/A fitting is a variation of the Type F fitting and is

intended, according to its manufacturer, for use with Material G rope.

See Figure 3. The fitting is in two parts, an armor jacket and a looped

fitting, the latter being similar to a large-diameter Type F fitting.

The jacket, which consists of helically formed steel wire strands, is

first wrapped around the rope and then the looped fitting is installed

over the jacket. The jacket and the looped fitting are wrapped in

opposite directions to reduce twisting under tensile load.

The test results are given in Table 3. Each of the three tests

resulted in a different mode of failure. In the first test one of the

fittings failed, in the second test the rope specimen failed in its free

length, and in the third test the rope specimen pulled out of one of the

fittings. Because of the free length failure, it is believed

that the true tensile strength of Material G was approached in these

tests

.

3.4 Type R/V Fitting

The Type R/V fitting is a basket-type mechanical end fitting for

GRP rod materials. See Figure 4, It grips the rod with a spring- loaded

jaw system contained within the conical basket. The jaws are,

essentially, wedges of a segmented cone. The basket is fitted with a yoke and bail for attachment to thimble-eye type hardware. The basket

and the yoke are aluminum alloy, the jaws and the bail are stainless

steel.

&
Table 2 - Tension Tests with Type F Fittings

Test

No. Material

Gripped

length U;

Free

length

Maximum

load Failure

in in lbf

51067 A, 7-strand 24 37 15 , 600 (b)

51068 A, 7-strand 24 24 15,300 (b)

15,400 average

31002 E, 1/2-in dia 34 24 26,000 (c)

41035 E, 1/2-in dia 34 24 26,000 (c)

41036 E, 1/2-in dia 34 24 26,500 (d)

26,200 average

31001 N, 1/2-in dia - 34 24 19,700 (d)

41039 N, 1/2-in dia 34 24 19,000 (d)

41040 N, 1/2-in dia 34 24 19,500 (d)

19,400 average

Each end.

Strandwise at the fitting.

Pullout

Longitudinal splitting in the fitting.

Table 3 - Tension Tests with Type F/A Fittings

Test t No.

' Material

Gripped

length

(

a ) Free

length

Maximum

load Failure

in in lbf

41043 G, 7/16-in dia 34 24 22,000 (b)

41044 G, 7/16-in dia 34 24 22,000 (c)

41045 G, 7/16-in dia 34 24 21,700 (d)

21,900 aVerage

(a)

(b)

(c)

(d)

Each end.

Failure of fitting.

Failure in free length,

Pullout.

9

Figure 3 - Type F/A end fitting

Figure 4 - Type R/V end fitting.

In this program the Type R/V fittings were used on Materials E and

N rods. An attempt to use these fittings on a rope material (Material

G) was unsuccessful; the rope slipped out of the fitting at a low

tensile load.

The test results are given in Table 4. Failure in all cases was by

a radial crushing of the rod, resulting in its being pinched off just

inside the fitting.

3.5 Type R/P Fitting

The Type R/P fitting, Figure 5, is a basket-type, potted, conical

compression fitting which is intended primarily for synthetic ropes such

as Material G. The outward appearance of this fitting is quite similar

to that of the Type R/V fitting. The conical basket, or potting head,

is fitted with a yoke and bail for attachment to thimble-eye type

hardware. The potting head and the yoke are aluminum alloy, the bail is

stainless steel.

To mount the Type R/P fitting on Material G the polyurethane jacket was first removed from the end of the rope to a length somewhat less

than that of the potting head. The strands of the rope were then

untwisted and separated into individual rovings prior to potting.

In this program Type R/P fittings were also used on Material E rod

specimens. In this application the end of the rod was slit into

quarters, lengthwise, to a distance of an inch or two, and a conical

aluminum- alloy wedge was driven axially into the center of the end to

separate the quarters prior to potting.

The inner surface of the potting head was treated with a commercially available epoxy-release agent before potting. This permits

the potted end to seat tightly into the conical potting head. This

seating process is believed to result in significant radial compressive

stresses being developed in the potted end. The epoxy-release agent

also facilitates removal of the potting material for reuse of the head.

Several potting compounds were used in these tests. Type GS is a two- component , high-bond strength, epoxy resin system which is com- mercially available and is one of several recommended by the

manufacturer of Material G for use with Type R/P fittings. It cures to

a rather hard state in seven days at room temperature and, according to

the resin manufacturer, exhibits the following mechanical properties:

Tensile shear strength, 3600 lbf/in

Compressive strength, 14000 lbf/in2

.

12
Table 4 - Tension Tests with Type R/V Fittings

Test

No. Material

Gripped

length

(

a ) Free

length

Maximum

load

<

b ) in in lbf

41032 E, i/2-in dia 6 24 20,700

41033 E, 1/2-in dia 6 24 18 ,500

19,600 average

41026 N, 1/2-in dia 6 24 21,000

41027 N, 1/2-in dia 6 24 20,200

20,600 average

Each end.

Failure by pinching off.

Table 5 - Mechanical Properties of CW Series Potting Compounds

Resin/activator ratio

2:3 1:1 3:2 2:1

Designation C2W3 C1W1 C3W2 C2W1

Bond strength^, lbf/in2 2450 3280

Compressive strength^a \ lbf/in2 32200 20700

(a) Elongation , percent 8.0 7.0

Elastic modulus (b)

, lbf/in2 x 10-5 0.51 2.75 3.16

(a) » if According to the manufacturer.

k^From compressive tests on 0.5-in diameter specimens performed in this

laboratory

.

13
Figure 5 - Type R/P end fitting.

A second potting compound, also available commercially, was selected on

the basis of previous work at this laboratory. It is a 100-percent-reactive, epoxy resin adhesive which requires an activator and a 2-h cure

at 165 °F. This material may be formulated with different

resin/activator ratios to provide different mechanical properties. The

designations and some of the properties for several mixture ratios are given in Table 5.

On the basis of analytical considerations to be discussed later, it

was felt that best results might be obtained through the use of a com- bination of two potting compounds in the potting head. Therefore,

several tests were performed with Material E rod in which half of the

head (the narrow end) was filled with a soft potting compound, C2W3, and

the balance was filled with a harder compound, C1W1. The idea was to

reduce the peak shear stress on the surface of the rod where it enters

the potting head and yet retain substantial bond strength between the

potting compound and the rod.

The test results are given in Table 6. The Material E specimens

failed by pulling out of the Type R/P fittings. This is a rather

complex process, involving various combinations of shear at the

interface between the rod and the potting compound, crushing of the

potting compound, and tensile failure of the rod in the vicinity of the

conical wedge.

Although only one specimen of Material E was tested with the C2W3

potting compound, the results suggest that this formulation provides

greater strength than is obtained with the combination of two potting

compounds. In none of these tests of Material E was the true tensile

strength of the rod approached.

Failure of Material G with the GS potting compound occurred by the

individual strands breaking in rapid succession immediately adjacent to

the end fitting. It appeared that the failures were the result of

combined tension and torsion. The rope has a tendency to twist as the

tensile load is applied, but this tendency was restrained by the end

fitting which was not free to rotate in the testing machine.

With the C1W1 potting compound the first Material G specimen tested

also failed strandwise at the end fitting. However, the next three

specimens failed by crushing, resulting in a pinching-off just inside

the end fitting.

With the C2W3 potting compound strandwise failures were again

experienced, but in one case this failure took place in the free length

of the specimen rather than at the fitting. It is believed, therefore,

that the maximum load in this test, 19800 lbf, is close to the true

tensile strength of the material.

15
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16
Of the three potting compounds tried with Material G, C2W3 gave the

highest breaking loads while GS gave the lowest.

Tests of 7-strand Material A and 1/2- in Material N were performed

in an earlier investigation [10] using Type R/P fittings and C1W1

potting compound. In four tests of Material A, one specimen pulled out

of the fitting prematurely and the other three showed an average

strength of 17300 lbf. Similarly, in three tests of Material N, one specimen pulled out prematurely and the other two showed an average

strength of 21200 lbf.

3.6 Summary

Table 7 summarizes the average breaking loads of each of the

specimen materials with each of the end fittings and potting compounds

with which it was tested. Data from the earlier investigation [10] are included. The load values are given to the nearest 1000 lbf. While the

number of tests performed is less than would be necessary for

statistical confidence, the table suggests the following tentative

conclusions

1. The two types of dead-end fittings, Type P and Type F,

appear to function about equally well.

2. The Type R/V fitting provides about the same tensile

strength with either of the two rod materials, Material E

and Material N.

3. The C2W3 potting compound appears to be the best of those

used with the Type R/P fitting.

4. The three types of fittings used on 7-strand Material A

(Type P, Type F, and Type R/P with C1W1 potting compound)

provide essentially equal strengths with this material.

5. The Type P fitting, the Type F fitting, and the Type R/P

fitting with C2W3 potting compound appear to provide

essentially equal strengths with Material E. The

Type R/V fitting is inferior with this material.

6. The Type F/A fitting provides the highest strengths

with Material G. The Type R/P fitting with C2W3

potting compound is nearly as good.

7. The four types of fittings used on Material N

(Type P, Type F, Type R/V, and Type R/P with

17
I

Table 7 - Summary of Average Breaking Loads (in lbf x 10" d

) with

Commercial End Fittings

Material

Potting E, 1/2-in G, 7/16-in N, 1/2-in

Fitting compound A, 7-strand dia dia dia

p - 17 26 - 20

F — 15 26 — 19

F/A 22

R/V 20 21

R/P GS 10

R/P C1W1 17 18 14 21

R/P C2W3 25 19

R/P C1W1 and

C2W3

21

18
C1W1 potting compound) provide essentially equal

strengths with this material.

4. DEVELOPMENT OF IMPROVED END FITTINGS

The specific objective of this task was to develop end fittings

that can attain or approach the true tensile strengths of available GRP

rod and rope materials. In order to accomplish this objective, an analytical parameter study of the problem and an experimental evaluation

of trial prototype fittings were pursued concurrently. The analytical

study involved the axisymmetric, finite- element stress analysis of a broad class of end connections consisting of a metal sleeve joined to

the GRP rod with a polymeric potting material. The experimental study

involved the fabrication and tensile testing of prototype, end fittings

employing several different gripping concepts . An important aspect of

this parallel analytical and experimental approach was the free transfer

of newly developed information from one study to the other. During the course of the work the orientation of each study was changed to a more

profitable direction based on information developed in the other study.

Some aspects of the analytical study were reported in greater detail

elsewhere [11].

4.1 Finite-Element Analysis

A new computer program was developed for the axisymmetric, finiteelement stress analysis of end connections. The basic finite-element

formulation used [12] was developed for the case of isotropic elastic

materials. That earlier formulation has been modified in the endfitting analysis in accordance with the assumption that the GRP rod is

homogeneous and transversely isotropic as defined by five independent

elastic constants. The materials of the metal sleeve and the potting

compound are assumed to be isotropic elastic.

In a finite-element analysis the continuum is subdivided into a network of elements that are connected to adjacent elements only at common nodal points. Elastic displacements within the individual

elements are defined by generalized functions that assure displacement

compatibility along common boundaries of adjacent elements. In the endfitting analysis elastic strains within the elements are assumed to be

uniform, thus assuring displacement compatibility along common element

boundaries

.

Figure 6 shows a representative finite-element analysis mesh for one general class of end connections. In this case, the metal sleeve is

conically tapered, and the outer end of the GRP rod is slit into

quarters and spread with a conical metal wedge. Where the rod is spread

by the wedge, the elastic constants for the region occupied by the

19
Figure 6 - Representative fi

general class of

20

nite-element mesh

end fittings.

for one
spread rod are recomputed according to the volume fractions of rod

material and potting compound.

In addition to generating the usual line-printer output of computed

results, the computer program drives an electron-beam plotter which

produces contour plots of seven different stress components and the

angle of maximum principal stress, as well as a plot of the analysis

mesh. As an example, Figures 7, 8 and 9 give, respectively, plots of

the analysis mesh, the longitudinal normal stress and the longitudinal

shear stress for the Mod 4 end fitting which was studied in the

experimental stress analysis described later.

4.2 Analytical Parameter Study

The linear elastic finite-element computer model was used in a parameter study of the general class of end fittings represented

schematically in Figure 10. The numerals in the figure denote the eight

variable parameters studied. Parameters 4 and 5 are the elastic moduli

of the two potting compounds indicated.

It was assumed that the critical component of stress, with respect

to tensile strength, is the peak bond-shear stress at the interface

between the rod and the potting compound. This assumption was based on the fact that rod pullout was the most common failure mode observed in

tensile tests of this class of fitting.

The analyses were carried out for a 1/2-in diameter GRP rod. The

assumed properties of the metal sleeve and the conical wedge (not shown)

were those of 7075-T6 aluminum alloy.

The general indications for the eight parameters studied are:

1. End-fitting length. —Within the range of 6 to 16 in,

greater length results in significantly smaller peak bondshear stress, but the rate of change of peak stress

- decreases as length increases.

2. Potting thickness at inner end of fitting. —-Within the

range of 0.24 to 0.48 in, greater thickness results in

significantly smaller peak bond-shear stress, but the

rate of change of stress decreases as thickness increases.

3. Sleeve thickness at inner end of fitting. —Within the

range of 0.02 to 0.25 in, greater thickness results in

slightly greater peak bond-shear stress.

4. Elastic modulus of inner potting compound.—Within the

range of 40000 to 316000 lbf/in 2

, greater stiffness

21
of potting compound results in significantly greater

peak bond-shear stresses. This is true whether a single potting compound or a combination of two

different compounds is used.

5 and 6. Elastic modulus and length of outer potting

compound. —A combination of two potting compounds

having different elastic moduli produces two bondshear stress peaks (one at the inner end of each

potting compound). If parameters 4, 5 and 6 are proportioned so as to make the two stress peaks approximately equal, the peak stress is significantly

less than for the case of a single potting material.

7 and 8. Potting thickness and sleeve thickness at the outer

end of the fitting. —Parameters 7 and 8 have a relatively

small direct influence on the peak bond-shear stress.

4.3 Experimental Prototype Fittings

Two gripping configurations investigated early in the experimental

program gave disappointing results in tensile strength tests. One type

of experimental fitting consisted of a cylindrical metal sleeve that was

attached to the GRP rod with a thin layer of high-strength adhesive.

Another type of experimental fitting consisted of a thick metal sleeve

with a conical inside taper, potted to the GRP rod, and proportioned to

provide a relatively small thickness of potting material at the inner

end of the fitting. According to the finite-element analysis each of

these two concepts is characterized by a very high peak bond-shear

stress at the inner end of the fitting. Subsequent to these early

results, experimental fittings were designed to provide for a relatively

thick cushion of potting material at the inner end of the fitting.

A 6.5-in long experimental fitting (Mod 1), designed on the basis

of early results of the analytical study, is shown in Figure 11. The

highest strength attained with a Mod 1 fitting was 16240 lbf . At

failure in that test, one quarter of the 1/2-in Material N rod failed in

tension near the inner end of the conical wedge and the epoxy coating on

the rod sheared off as the rod pulled out of the fitting.

A 12-in long experimental fitting (Mod 2) , similar in concept to

the Mod 1 fitting, is shown in Figure 12. The highest tensile test load

applied to a Mod 2 fitting was 18150 lbf. In that test the Type R/P

fitting (with C1W1 potting compound) on the other end of the 1/2- in

Material E rod failed in the pullout mode. The Mod 2 fitting is

comparatively expensive to fabricate, and removal of the potted material

from the fitting, for reuse of the fitting, proved to be rather

22


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26
difficult due to the thin wall. Further evaluation of this fitting was,

therefore, deferred indefinitely.

An experimental fitting (Mod 3), consisting of a cylindrical metal

sleeve attached to the GRP rod with a thick layer of potting material,

is shown in Figures 13 and 14. This fitting consists of a 1 1/2-in

diameter rod of 7075-T6 aluminum alloy, 12 in long, which is drilled out

axially to a depth of 7 in with a 3/4- in drill. This hole is then

counterbored to a depth of 6 in, leaving a uniform 1/8-in wall

thickness. When used with GRP rod material the end of the rod, after

abrasive cleaning, is set into the 3/4- in hole in the base of the

cavity, and the rod is carefully aligned parallel and concentric with

the fitting while the remaining cavity is filled with the potting

compound. With GRP rope material the jacket, if any, is first removed

from the end of the rope to a length of about 11 in and the strands are untwisted and cleaned prior to potting. The 3/4- in hole at the base of

the cavity is not used with the rope materials.

The Mod 3 fitting was used for a series of twelve tests to

determine the relative bond-shear strengths of four different potting

compounds when used in this configuration with 1/2-in Material E rod.

The four potting compounds were made of the same two constituents mixed

in different ratios of resin to activator to give four different degrees

of stiffness (Table 5). The GRP rods were not slit or wedged and only

one potting compound was used in each test. All twelve specimens failed

by rod pullout and the results are given in Table 8. Although there is

great scatter in these data, the relatively high strengths of the

specimens with the most flexible potting compound are significant.

Finite- element analyses of the Mod 3 fittings, for the three potting

compound moduli given in Table 8, indicate . that a more flexible potting

compound produces a significantly lower peak bond-shear stress.

According to the analyses, the peak bond-shear stress with the most

flexible C2W3 potting compound is only 47 percent of the corresponding

stress with the stiff C3W2 compound. The strong dependence of peak

bond-shear stress on potting compound stiffness is apparently a major

factor in the relatively high maximum loads developed with the lowmodulus potting compound.

The Mod 3 concept was also evaluated in four tests using 7/16-in

GRP rope (Material G) and the low-modulus C2W3 potting compound. The

results of these tests are given in Table 9.

The greatest success thus far has been experienced using the

experimental Mod 4 end fitting shown in Figures 13 and 14. This end

fitting is essentially identical to the Mod 3 fitting except that the

potted length is increased to 13 in.

27
Figure 11 - Mod 1 experimental end fitting.

Figure 12 - Mod 2 experimental end fitting.

Figure 13 - Mod 4 (above) and Mod 3 experimental

end fittings.

oo

•HO

S-l o

00 HmO

S-i o

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00

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30

Table 8 - Tensile Tests with Mod 3 Fittings on 1/2-in Material E

Test

No.

Potting ,(a) compound v / Maximum

Designation Elastic modulus load

lbf/inz lbf

71169 C2W3 0.51 x 10 5 19850

71170 C2W3 0.51 15500

71171 C2W3 0.51 18650

Average: 18000

71166 C1W1 2.75 12200

71167 C1W1 2.75 8500

71168 C1W1 2. 75 19250

Average

13300

71172 C3W2 3.16 8000

71173 C3W2 3.16 14350

71174 C3W2 3.16 9400

Average: 10600

71175 C2W1 12350

71176 C2W1 17500

71177 C2.W1 18100

Average

16000

See Table 5.

Table 9 - Tension Tests with Mod 3 Fittings on 7/16-in Material G

Potting compound: G2W3

Test Maximum

No

load Failure

lbf

71185 15600 (a)

71187 16600 (b)

71188 17400 <"•) 71189 17300 (c)

(a) Pullout.

Inside fitting.

Strandwise at the fitting.

31
4.4 Experimental Stress Analysis

A specimen consisting of two Mod 4 end fittings attached to a 1/2-

in Material E rod was instrumented with twenty-six resistance strain

gages and subjected to six cycles of tensile loading. The loading

sequence was 0-2500-0-10000-0-10000-0-22000-0-23250-0-30100 lbf

(failure). Seventeen strain gages were located on the surface of one

end fitting and the remaining nine gages were mounted on the GRP rod

adjacent to that fitting. Twenty- two gages were oriented in the

longitudinal direction and nine of these were grouped in sets of three

and spaced 120° apart to detect eccentricity. Four gages were oriented

in the circumferential direction. The 7075-T6 aluminum alloy sleeve was

filled, to within 0.6 in of the inner end, with the low-modulus C2W3

potting compound.

Figures 15 and 16 are plots of longitudinal strain on the surface

of the instrumented fitting as predicted by the linear elastic finiteelement analysis and as measured by the strain gages. The individual

data points indicate the strains measured during the initial application

of the particular load indicated. The vertical lines in Figure 16

indicate the spread in strain data for subsequent loading cycles. The

major part of the spread is due to residual strain present in the

specimen at the beginning of Load Cycles 3 and 5. Those two cycles were

applied less than one hour after completing Load Cycles 2 and 4, (which

had maximum values of 10000 lbf and 22000 lbf, respectively) and the

strain gages had not been reset to zero. There is good agreement

between the linear elastic analysis and the experiment at 1000 lbf load

during the first cycle, but there is progressively poorer correlation at

higher loads.

The rate of change of surface strain with respect to distance along

the fitting is approximately proportional to the rate of transfer of

load in shear through the potting compound. That is, the slope of a smooth curve fitted to a set of strain data points in Figures 15 or 16

would be approximately proportional to the bond-shear stress acting at

the surface of the rod. This interpretation of the data suggests that,

at the higher loads and for the greater part of the potted length of the

rod, the bond-shear stress acting on the rod was roughly uniform. This

apparent degree of uniformity of bond-shear stress is believed to be a major factor contributing to the relatively high strength of the Mod 4

type fitting when used with the flexible C2W3 potting compound.

4.5 Tensile Tests with Mod 4 Fittings

The results of twenty-one tensile tests using Mod 4 fittings on three different rod and rope materials are given in Table 10. The lowmodulus C2W3 potting compound was used on all twenty-one specimens. The

average maximum load for the fifteen specimens of 1/2-in Material E was

32


Table 10 - Tension Tests with Mod 4 End Fittings

Potting compound: C2W3.

Gripped length: 12 in, each end.

Test Nominal Maximum

No. Material diameter load Failure

in lbf

7 1 1 Q O /XXoZ e.

1 /9 1/2 o i oca \&)

/ lloJ £

i In LI I jZo DU

711 /llO't ft A El i In LI I

9 c. "7 cr\ ZD/ DU

81242 E 1/2 27000 (a)

81243 E 1/2 28600 (a) oxqi 9AA Ei 1 /9 1/ Z

q 9 £ aa (.a; Q 1 9 /. G b

1 / 1 1/2 29600 (.a;

• Q 1 9 / A 1/9 1/2 zyouu va; 81247 E 1/2 31800 (a)

81248 E 1/2 32750 (a)

fl 1 9 / Q ph

i In LI I JOyDO (.a;

fti 9

Hi 1/9 1/2 Oil AH jiiUO (.a; R 1 9 ^ 1 T7Ei i In LI I

o i c aa (.a; 01 9£1 T?Ei Tin LI Z

9 9 9 E A Z JZDU va; 101315 E 1/2 30100 (a)

Average

:

29900

71190 G 7/16 19800 (b)

81239 G 7/16 20250 (c)

92271 G 7/16 21200 (b)

Average

:

20400

81240 N 1/2 23700 (a)

81241 N 1/2 17850 (a)

91263 N 1/2 18900 (a)

Average

:

20100

( a) Pullout.

( b ) Failure in the free length.

( c) strandwise in the fitting.

35
29900 lbf, corresponding to a nominal mean stress of 153000 lbf/in2

. One of these specimens failed at 32850 lbf in a tensile mode in its free

length, several inches from the fitting. This indicates that the true

tensile strength of the 1/2- in Material E is approached with the Mod 4

fittings. The other fourteen Material E specimens failed by pullout

with the failure located principally at the interface between the rod

and the potting compound.

The average maximum load for three specimens of 1/2-in Material N

with Mod 4 end fittings was only 20100 lbf, corresponding to a nominal

mean stress of 103000 lbf/in2

. Failure here was also by pullout but in

this case the failure was within the rods with several of the outer

layers of glass fibers being sheared off. However, in a test of 1/4-in

Material N, which will be discussed later, failure occurred in the free

length at a nominal mean stress of 154000 lbf/in2

. This suggests that a longer end fitting of the Mod 4 type would more nearly approach the true

tensile strength of 1/2-in Material N rod.

The average maximum load for three 7/16-in Material G rope

specimens tested with Mod 4 fittings was 20400 lbf.

A comparison of these results with those obtained with commercial

end fittings (Table 7) reveals that the Mod 4 end fitting is clearly

superior to the commercial types for Material E. For Material N the

performance of the Mod 4 fitting is equal to that of the commercial

types. For Material G the Mod 4 fitting is superior to all of the

commercial types except the Type F/A fitting. It thus appears that

there is no single type of end fitting which is best for all of the

materials. This is not an entirely unexpected finding since it is

reasonable to expect different behavior from rod and rope materials.

However, it also appears that different rod materials also require

different gripping techniques for optimum performance.

5. DENSITY

The percentage glass content in GRP rod and rope can vary not only

between manufacturers but, conceivably, between batches produced by a single manufacturer. For this reason it was considered desirable to

investigate the variations of tensile strength, modulus of elasticity

and density with diameter. The study of density is described here; the

studies of tensile strength and modulus of elasticity are presented in

subsequent sections of this report.

5.1 Material A

Small samples of single-strand Material A were carefully weighed in

air and in water. The densities and the specific gravities of the

36
samples were determined from these weighings and the known densities of

ambient air and water. The results of these measurements and

calculations are given in Table 11. The table also gives the calculated

weights per foot of the materials, based on the nominal diameters which,

for Material A, are close to the actual diameters.

As a check on the method, the density of the 0.119-in material was

also calculated from weight, length and diameter measurements made

earlier [10]. The average density so determined agrees exactly with the

value obtained in the present work.

The nominal specific gravity of Material A reported by its manufacturer is 1.93 ± 0.17. All of the values listed in Table 11 fall

within this range except that for the smallest size. However, the

weight-per-f oot values given in the table are all within the ranges

specified by the manufacturer for the various sizes.

The data do not show any consistent variation of density with

diameter.

5.2 Material E

The specific gravity and density of two samples of 1/2-in Material

E were calculated from weight, length and diameter measurements. The

two determinations agreed within one percent and showed a specific

gravity of 1.99 and a density of 0.0719 lb/in3

. The calculated weight

of 1/ 2- in Material E rod, based on the actual diameter, is 0.192 lb/ft.

Measurements made earlier [10] on 13/16-in Material E show a specific gravity of 1.88 and a density of 0.0677 lb/in .

5.3 Material G

Density measurements were made on three types of Material G

samples: unjacketed rope, single strand, and single roving. The method

of measurement was the same as that described above for Material A. The

results are given in Table 12,

The results show that

1. there is variability in the densities of nominally

identical specimens, suggesting a certain amount of

nonunif ormity of composition,

2. the differences in density as measured on rovings,

strands and rope are comparable to those observed on nominally identical specimens, suggesting that there

is no systematic difference in the densities of the

three forms , and

37
Table 11 - Density of Material A

Test Nominal Specific Weight

No. diameter gravity Density per ft

in lb/in 3 lb /ft

62120 0.031 1.72 0.0621 0.000562

62121 0.043 1.94 0.0701 0.00122

62122 0.063 1.77 0.0640 0.00241

62123 0.084 1.77 0.0640 0.00426

62124 0.119 1.92 0.0694 0.00926

Average

1.82 0.0657

Table 12 - Density of Material G

Test Nominal Specimen Specific Weight

No

diameter type gravity Density per ft^ _

a) lb /in 3 lb/ft

72191 5/16 single roving 1.85 0.0668

72192 5/16 single roving 2.05 0.0741

72193 5/16 single roving 1.81 0.0654

Average

1.90 0.0688 0.0634

72194 5/16 single strand 1.97 0.0711

72195 5/16 single strand 2.04 0.0737

72196 5/16 single strand 2.06 0.0744

Average

2.02 0.0731 0.0674

7219 7 7/16 single roving 1.94 0.0701

72198 7/16 single roving 2.07 0.0748

72199 7/16 single roving 1.89 0.0683

Average

1.97 0.0711 0.128

72200 7/16 single strand 1.93 0.0697

72201 7/16 single strand 1.92 0.0694

72202 7/16 single strand 1.91 0.0690

Average

1.92 0.0694 0.125

72203 5/8 bare rope 2.06 0.0744

72204 5/8 bare rope 2.03 0.0733

72205 5/8 bare rope 2.08 0.0752

Average

2.06 0.0743 0.274

( a'Bare rope.

38
3. there is no consistent variation in density with the

diameter of the rope.

The measured values of specific gravity ranged from 1.81 to 2.08,

which bracket the nominal value of 2.0 reported by the manufacturer.

The weights of the full-sized ropes, minus the jackets, were

calculated from the averages of each series of density measurements.

See Table 13. For this purpose the volume of the bare rope was

calculated from the nominal diameter.

The weights of the jacketed ropes were also calculated (Table 13)

by adding the weights of the jackets to the weights of the bare rope.

Jacket weights were calculated from the nominal density (0.0451 lb/in3

) and thickness (0.015 in) of the jacket as reported by the manufacturer.

In addition, the actual weight-per-f oot values were calculated from

direct measurements of the weight and length of long samples of Material

G. These two determinations of the weight of Material G are compared in

Table 13. Also given in this table are the rated values according to

the manufacturer.

Comparison of the calculated and actual weights shows excellent

agreement in the two smaller sizes but a substantial discrepancy in the

largest size. This suggests that in the two smaller sizes the nominal

diameter provides a good estimate of the actual cross sectional area of

the bare rope, while in the 5/8-in size the actual cross sectional area

is substantially smaller than this estimate.

Comparison of the actual weights of Material G with the manufacturer's rated values (Table 13) shows that the rated values are low

in every case, particularly in the smaller sizes where the discrepancies

are about 20 percent.

5.4 Material N

The densities of several sizes of Material N were calculated from

weight, length and diameter measurements which were made on the coated

material. The results are given in Table 14. The actual diameters of

the coated rods were used to calculate the weights per foot.

The specific gravity of Material N, according to its manufacturer,

is between 1.85 and 2.05. The values in Table 14 fall within this range

except for the 1/4-in size. Earlier measurements of the 1/2- in size

[10] gave average values of specific gravity, density and weight which

agree with the present values within less than one percent.

39
Table 13 - Weight of Material G

Weight per foot

Calculated Actual

Nominal

diameter

Bare

rope Jacket

Jacketed

rope

j acketed

rope Rated**

3' in lb /ft lb /ft lb /ft Vol ft lb/ft

5/16 0.0654<b > 0.0080 0.0734 0.0732 0.061

7/16 0.126 (b > 0.0112 0.137 0.140 0.116

5/8 0.274 0.0159 0.290 0.243 0.233

According to the manufacturer,

(k) Average from measurements on strand and roving.

Table 14 - Density of Material N

Test Nominal Specific Weight per foot

No. diameter gravity Density Calculated Rated

in lb/in 3 lb /ft lb /ft

62108 1/4 1.70 0.0614

62109 1/4 1.72 0.0621

62110 1/4 1.72 0.0621

Average

1.71 0.0619 0.053 0.057

62111 3/8 1.97 0.0712

62112 3/8 1.98 0.0715

62113 3/8 1.96 0.0708

Average: 1.97 0.0712 0.124 0.10

62114 1/2 1.99 0.0719

62115 1/2 1.99 0.0719

62116 1/2 1.99 0.0719

Average

1.99 0.0719 0.190 0.17

62117 5/8 2.03 0.0733

62118 5/8 2.02 0.0729

62119 5/8 2.02 0.0729

Average

2.02 0.0730 0.328 0.27

40
The specific gravity data show a high degree of consistency within

each size, suggesting considerable uniformity of composition. While

there is a trend toward increasing density with diameter, the variation

is negligible except for the smallest size.

Comparison of the calculated weights per foot with the

manufacturer's rated values (Table 14) shows large differences except in

the smallest size. In the other three sizes the rated values are all

low, by as much as 20 percent.

5.5 Summary

The densities of all of the materials tested varied from 0.061 to

G.075 lb/in . However, there was almost as much variation between sizes

and specimens of individual materials. Thus, there does not appear to

be any justification for choosing among the materials on the basis of

density. Rather, a nominal density of about 0.07 lb /in3 would appear to

be a reasonable value to use in design considerations involving any of

the materials.

No significant, consistent variation of density with diameter was observed for any of the materials.

The nominal diameter of Material G, which approximates the diameter

of the bare rope, was found to be a poor index of the cross sectional

area of this material.

The actual weights of Materials G and N, in most sizes, were found

to be significantly greater than the rated values reported by their

respective manufacturers.

6. TENSILE STRENGTH

In order to investigate the variation of tensile strength with

diameter for the rod and rope materials, it is necessary to use end

fittings which are capable of generating the true tensile strengths of

these materials. Although significant progress has been made in the

development of such end fittings, the state of the art has not yet

progressed to the point where the true tensile strengths of all of the

materials can be developed consistently.

Tensile tests were performed on various sizes of the materials.

Except where otherwise specified, the test, conditions were the same as those used previously to evaluate commercial end fittings.

41
6.1 Material A

Tests were performed on four sizes of single-strand Material A

using Type R/P fittings. The specimens were each 18 to 20 in long with

a gripped length of 2 in at each end. The test results are given in

Table 15. The first two tests listed were performed at a constant

loading rate of 125 lbf/min. All of the other tests were performed at a constant crosshead speed of 0.2 in/min. The maximum tensile stresses

were calculated on the basis of the nominal diameters which, as pointed

out earlier, are close to the actual diameters for this material.

All but three of the tests were terminated by the specimen pulling

out of the end fitting, intact. The three specimens which fractured

were the only ones which attained the manufacturer's rated breaking

loads. The maximum stresses in these three cases were in the vicinity

of 290000 lbf/in2

.

Better results were obtained in an earlier investigation using

aluminum block end fittings [10]. In that work, two specimens of

0.0865-in Material A were fractured in the free length at an average

breaking load of 1820 Ibf, which corresponds to a tensile strength of

308000 lbf/in2

.

As mentioned earlier, 7-strand Material A rope is composed of seven 0.119-in strands. The highest breaking load ever achieved in this

laboratory with 7-strand Material A is 20200 lbf [10]. This specimen

was tested with aluminum block end fittings and failed by crushing in

the fittings. The calculated maximum stress for this case is 259000

lbf/in .

6.2 Material E

Only 1/2-in diameter specimens of Material E were tested in this

investigation. Of these, only one failed in the free length (Test No.

71183, Table 10). The maximum load attained, 32850 lbf, is believed to

be indicative of the true strength of this material and corresponds to a maximum stress of 168000 lbf/in2

.

Several tests of Material E in the 13/16-in diameter size were

performed in an earlier study [10]. None of these specimens failed in

the free length but the highest maximum load attained was 45850 lbf.

This exceeds the manufacturer's rated breaking load, 35000 lbf, but

corresponds to a maximum tensile stress of only 88000 lbf/in2

.

6.3 Material G

A series of tests were performed on three sizes of Material G.

Following one of the manufacturer's recommendations, these tests were

42
Table 15 - Tensile Tests of Material A (Single Strand)

End fitting: Type R/P

Test

No.

Nominal

diameter

Potting

compound

Rated

load

Maximum

load

Maximum

stress Failure

in lbf lbf lbf /mz

82252 0.031 C2W3 220 106 141000 (a)

82253 0.043 C2W3 440 363 250000 (a)

92274 0.043 C1W1 440 365 252000 (a)

92275 0.063 C1W1 800 560 178000 (a)

92276 0.063 C1W1 800 210 67000 (a)

92277 0.084 C1W1 1500 1590 287000 (b)

92278 0.084 C1W1 1500 1000 181000 (a)

102326 0.119 C2W3 3000 3300 297000 (b)

102327 0.119 C2W3 3000 3210 288000 (b)

( a)Pullout.

(b) Fracture adjacent to fitting.

43
carried out with Type R/P end fittings and GS potting compound. All of

the specimens failed strandwise at the fitting at comparatively low

loads. These data are not reported here.

Better results were obtained with Mod 4 end fittings and C2W3

potting compound. See Table 16. The specimens were 48 in or more in

length and were tested at a crosshead speed of 0.2 in/min.

All of the 5/16- in diameter specimens failed in the free length.

The maximum loads are thus believed to be indicative of the true

strength of the material and come within four percent of the manufacturer's rated breaking load.

Two of the three 7/16-in diameter specimens also failed in the free

length. The third specimen failed strandwise in the end fitting at a comparable load level. The maximum loads exceed the manufacturer's

rated breaking load for this size and compare favorably with the results

of other tests which were believed to have approached the true tensile

strength of 7/16-in Material G. (See Table 3.)

The first 5/8-in specimen tested also failed in the free length

with a maximum load close to the manufacturer's rated breaking load.

The second 5/8-in specimen pulled out of the end fitting at a com- paratively low load.

The maximum tensile stresses listed in Table 16 were calculated on

the basis of the nominal diameter. Considering the specimens which

failed in the free length it appears that, on a stress basis, the 5/8-in

size has considerably lower strength than the other two sizes. However,

it was shown from density measurements that the nominal diameter of

Material G is not, in fact, a good index of the actual cross sectional

area of this material. A better parameter to use in comparing the

strengths of different sizes is the strength/weight ratio. This is

demonstrated in Table 16 which shows that, for those specimens which

failed in the free length, the strength/weight ratio is a more

consistent indicator of strength, for various sizes, than is stress.

The strength/weight ratio shows no correlation with density (Table 12)

and no consistent variation with diameter. For all of the specimens

which failed in the free length the strength/weight ratio is 140000 lbfft/lb, within 10 percent.

Tests conducted elsewhere on 3/16-in Material G showed a somewhat

higher strength/weight ratio [13]. However, it is apparent that these

44
Table 16 - Tensile Tests of Material G

End fitting: Mod 4

Potting compound: C2W3

Test

No.

Nominal

diameter

Rated

load

Maximum

load

Maximum

stress^

Strength/

weight Failure

in lbf lbf lbf/in2 lbf- ft /lb

92268 5/16 10000 9980 130000 i rt r\ rt 136000 (c)

92269 5/16 10000 9650 126000 132000 (c)

92270 5/16 10000 9940 129000 136000 (c)

71190 7/16 17500 19800 132000 141000 (c)

81239 7/16 17500 20250 135000 145000 (d)

92271 7/16 17500 21200 141000 151000 (c)

92272 5/8 32000 31100 101000 128000 (c)

92273 5/8 32000 21700 71000 89000 (e)

( a) Based on nominal rope diameter.

(^Maximum load divided by actual weight per foot from Table 13.

( c) Failure in free length.

( d) strandwise in end fitting.

( e) Pullout.

45
ratios were calculated using the rated, rather than actual, weight of

Material G.

6.4 Material N

In the evaluation of commercial end fittings it was found (Table 7)

that all of the fittings tested perform equally well with Material N.

Accordingly, the Type R/V fitting, which is the easiest to use, was

initially selected for this study of the effects of size on the tensile

strength of this material. All of the specimens failed by pinching off

at the end fitting although the manufacturer's rated breaking loads were

attained in several instances. These data are not reported here.

The test series was repeated using Mod 4 fittings and C2W3 potting

compound, and somewhat better results were obtained. These results are given in Table 17. The free length of all of the specimens was 16 in or

more. Test Nos. 91262, 81240 and 81241 were performed at a crosshead

speed of 0.75 in/min. The other tests were performed in a different

testing machine at 0.2 in/min. The maximum stresses were calculated on

the basis of the nominal diameters, which approximate the actual

diameters of the rods minus the coatings.

All but one of the specimens failed by pulling out of the Mod 4

fitting. The exception, in Test No. 92265, failed in the free length.

This specimen exhibited the highest maximum stress, 154000 lbf/in 2

, and

failed at a load within three percent of the manufacturer's rated

breaking load. One specimen in each of the other sizes exceeded the

rated breaking load although free length failures were not obtained.

The Mod 4 fitting used for the test of the 5/8-in specimen was

specially designed with a thicker wall to avoid the possibility of a failure in the fitting.

46
Table 17 - Tensile Tests of Material N

End fitting: Mod 4

Potting compound: C2W3

Test Nominal Rated Maximum Maximum

XT-, JNO • diameter ioad load stress Failure

in lbf lbf lbf/ in2

92264 1/4 7800 3000 61000 (a)

Q 0 0 C R 1/ 4

"7 OA A 7o00 "7 r r A 7550 T C /. A A A

1541)00 (b;

91262 3/8 12000 13100 119000 (a)

92266 3/8 12000 9800 89000 (a)

81240 1/2 20000 23700 121000 (a)

81241 1/2 20000 17850 91000 (a)

91263 1/2 20000 18900 96000 (a)

92267 5/8 30000 38000 124000 (a)

( a )Pullout.

(b) Failure in the free length.

47
6.5 Summary

The various end fittings which were used in this study were

generally inadequate to generate the true tensile strengths of the rod

and rope materials. There were several exceptions, however, notably

with Material G, in which failures in the free lengths of the specimens

were obtained. In all of these cases the measured breaking loads of the

materials were close to, or in excess of, the manufacturers' rated

loads. In several cases where free length failures were not obtained

the maximum loads nevertheless exceeded the rated values. It appears,

therefore, that while some of the end fittings used are capable of

generating the rated loads of the rod and rope materials, the full

potential of these materials is not being realized. There is an indicated need for evaluation of other types of commercial end fittings

and for further development of improved fittings.

The limited data available do not show any consistent variation of

true tensile strength with either density or diameter. Thus, it appears

that the maximum stress for failure in the free length does not depend

on size for Materials A, E and N. Unfortunately, stress is not a useful

parameter for Material G since the cross sectional area of this material

cannot be calculated accurately from the nominal diameter. For this

material the strength/weight ratio is a more consistent and meaningful

parameter than the maximum stress. Similarly, strength comparisons of

Material G with other materials should be made on the basis of the

strength/weight ratio rather than the maximum stress or the breaking

load for a given nominal diameter. Note, however, that these

considerations apply to the true tensile strengths of the rod and rope

materials, not to the actual breaking loads achieved with commercial end

fittings. The breaking stresses of these materials with commercial end

fittings do, in fact, tend to increase as the diameter of the rod or rope is reduced. Thus, for example, the rated breaking load for two

parallel rods or ropes of 1/4-in diameter is greater than that of a single 3/8-in member, although the combined weight or cross sectional

area of the former is less.

Although the true tensile strengths of the specimen materials could

not be obtained consistently, the data show quite conclusively that

Material A is significantly stronger than Materials E, G and N. This is

in agreement with the manufacturers' rated breaking strengths and

results, no doubt, from the fact that Material A is reinforced with Sglass while the other materials use E-glass reinforcement.

7. MODULUS OF ELASTICITY

A series of tests was performed to determine the tensile moduli of

elasticity of the rod and rope materials. These tests were performed in

48
a universal testing machine at a crosshead speed of 0.2 in/min. The

strain measuring system was calibrated against a micrometer-screw

extensometer calibrator having a resolution of 10 microinches.

The selection of end fittings for these tests was not critical

since the specimens did not have to be loaded to failure. The tests

were, in fact, discontinued prior to failure. The load-strain curves were linear in every case. The results are given in Table 18. The

moduli of elasticity were calculated, from the slopes of the curves, on

the basis of the nominal diameters of the specimen materials. As

pointed out earlier, for single-strand Material A and for Material E the

nominal diameter is close to the actual diameter. For Materials G and N

the nominal diameter approximates the average diameter of the unjacketed

rope and the uncoated rod, respectively.

The results show that for Materials A and N the modulus of

elasticity does not vary in any consistent manner with diameter and, in

fact, there were variations in modulus even within a given size and

material. This observation is consistent with earlier findings [10] and

would suggest that the compositions of these materials are not uniform

even within a single size. However, the density measurements reported

earlier tend to refute this. For these two materials all of the

measured modulus values exceed the rated values reported by the

manufacturers (7.0 x 10

6 lbf/in 2 for Material A, 6.34 x 10° lbf/in2 for

Material N)

.

On the basis of the measurements made, a nominal modulus of 7.5 x

10 6 lbf/in 2 would appear to be a reasonably typical value for design

purposes involving Materials A, E and N.

The moduli of elasticity for Material G, as listed in the table, are obviously inaccurate since they were calculated on the basis of the

nominal diameters which, it has been shown, do not provide good

estimates of the actual cross sectional areas. If the listed moduli are corrected by multiplying them by the ratio of calculated weight to

actual weight, the values in Table 19 are obtained. It is seen that,

although there is a trend toward lower moduli as diameter increases, the

variation is comparatively small. However, each of the modulus values

is significantly less than the manufacturer's rated value, 6.0 x 10

lbf/in 2

.

Several tests were performed to determine the moduli of uncoated

1/2-in Material N rod and unjacketed 7/16- in Material G rope. The

results show that the coating and the jacket contribute little to the

stiffness of the products in tension.

49
Table 18 - Tensile Modulus of Elasticity

Test Nominal Modulus of

No. Material diameter e las ti ci ty

in lbf /in2

92292 A 0.043 7. 7 x 10 6 92293 A 0.063 7.1

92294 A 0.063 7.4

92295 A 0.084 7.9

92296 A 0.084 7.3

102328 A 0 119 7 8

102329 A 0 . 119 7.8

92297 E 1/2 7.4

92285 G 5/16 5.2

92286 G 5/16 5.0

92287 G 7/16 4.7

92288 G 7/16 4.7

92290 G 5/8 3.9

92291 G 5/8 3.6

92279 N 1/4 8.6

101321 N 3/8 8.5

92282 N 1/2 6.9

92283 N 1/2 7.4

92284 N 5/8 8.2

(^Calculated on the basis of the nominal diameter.

Table 19 - Corrected Modulus of Elasticity of Material G

Test

No.

Nominal

diameter

Modulus of elasticity

Uncorrected Corrected*^

in lbf/inz lbf/in2

92285 5/16 5.2 x 10 6 5.2 x 10 6 92286 5/16 5.0 5.0

92287 7/16 4.7 4.6

92288 7/16 4.7 4.6

92290 5/8 3.9 4.7

92291 5/8 3.6 4.3

(a^From Table 18.

(^Multiplied by ratio of calculated to actual weights of jacketed

rope, Table 13.

50
8. LOW-TEMPERATURE FLEXIBILITY

Three-point bending tests were performed on Materials E, G and N in

order to evaluate their flexural moduli of elasticity at low temperatures. The specimens were simply supported on 2-in diameter metal

rods, over fixed spans, and loaded transversely at the center through

another 2-in diameter rod. The test fixtures were completely enclosed

in a low- temperature test chamber which was mounted between the

crossheads of a universal testing machine. The test temperature was

achieved with a fan which circulated air over a supply of dry ice and

the specimen. Thermocouples were used to measure temperature during the

initial exploratory experiments but a low- temperature thermometer was

found to be adequate for the tests. The specimens were exposed to the

test temperature for about one hour prior to loading. Load was then

applied at a crosshead speed of about 1 in/min and an extension rod was

used to transfer the center displacements of the specimens to the

outside of the chamber where they were measured with a dial gage.

8.1 Material E

Low- temperature flexibility tests were performed on specimens of

1/2-in Material E which were 25 in long and supported on a 20-in span.

The load-deflection curves were linear up to high levels, after which

the slopes decreased slightly. From elementary strength-of-materials

considerations, the flexural modulus E^ was calculated from the slope

S of the linear portion of each curve using the expression

E. = = 54320 S

where D is the nominal diameter of the rod and L is the span. The

results are given in Table 20.

These results show that the flexural modulus of elasticity is

essentially unaffected by temperature down to -81 °F. It is also seen

that the flexural modulus is larger than the tensile modulus of

elasticity, which was found to be 7.4 x 10 6 lbf/in 2 for Material E

(Table 18) . This implies that the modulus of elasticity for Material E

is substantially greater in compression than in tension.

All of the tests were discontinued without failure of the specimens

since the test setup limited the permissible deflections. It is of

interest to compare the maximum strains reached in the bending tests

with those obtained in tension tests. Assuming linear elastic behavior

it can be shown that the maximum strain corresponding to a deflection of

3.9 in is 0.029. The actual maximum strain, due to the nonlinear

51
Table 20 - Low-Temperature Flexibility of Material E

Flexural Maximum

Temperature modulus deflection C a)

°F lbf/in2 in

95298 77 8.2 x 10 6 3.87

95299 77 8.2 3.96

95300 77 8.5 3.81

95301 -20 8.4 3.93

95302 -38 8.3 3.84

95303 -58 8.3 3.80

95304 -63 8.5 3.78

35008 -65 8.3 3.96

95305 -74 8.3 3.90

95306 -81 8.6 2.37

Tests discontinued without failure.

Table 21 - Low-Temperature Flexibility of Material N

Test

No. Temperature

Flexural

modulus

Maximum

deflection^ 3) °F lbf/in2 in

95307 77 7.4 x 10 6 3.51

95308 77 7.4 3.51

95309 77 7.5 3.65

95310 -40 7.6 2.87

35007 -60 7.4 2.96

350O6 -67 7.8 2.56(b)

95311 -76 7.8 2.97

( a) Def lection at failure, except as noted.

(b)Test discontinued prior to failure.

52
behavior, was undoubtedly larger than this. The tensile strain

corresponding to the true tensile strength of Material E (Test No.

71183, Table 10) is only 0.023. It thus appears that this material is

capable of withstanding larger strains in bending than in tension.

8.2 Material G

Bending tests of 7/16-in Material G proved to be difficult to

accomplish due to the extreme flexibility of this material. It was necessary to reduce the span to 4.75 in in order to increase the

required loads to values which were significantly larger than the weight

of the specimen itself.

Three tests were performed at temperatures down to -67 °F. The

load-deflection curves were nonlinear right from the start due to

slippage in the rope construction. No meaningful values of the flexural

modulus could be obtained, but a comparison of the load-deflection

curves for the three tests showed that the flexibility was unaffected by

the temperature.

8.3 Material N

Three-point bending tests on 1/2-in Material N were performed in

the same manner as those described for Material E. With the exception

of one test which was terminated prematurely, all of the tests

culminated in failure due to crushing of the matrix material on either

the tensile or compressive sides of the rod specimens. The results of

the tests are given in Table 21.

The flexural modulus of elasticity at room temperature is seen to

be within the range of values obtained for the tensile modulus (Table

18). All of these values exceed the manufacturer's rated flexural

modulus, 6.39 x 10 6 lbf/ in . The results also show an insignificant

increase in the flexural modulus as temperature is reduced to -76 °F.

Assuming linear elastic behavior, it can be shown that the maximum

bending strain for a deflection of 3.5 in is 0.026. The actual maximum

bending strain at room temperature was undoubtedly larger than this due

to the observed nonlinearity in the load-deflection curves at large

deflections. The maximum tensile strain for this material (Test No.

92265, Table 17) is only 0.022. Thus, in keeping with the findings for

Material E, this material is also capable of withstanding larger strains

in bending than in tension.

The manufacturer of Material N reports a flexural strength of

125000 lbf/ in 2. If this stress is divided by the rated flexural

modulus, a maximum bending strain of 0.0196 is obtained, which is

53
substantially less than the values obtained at room temperature in the

present tests.

8. A Summary

The flexibility of Materials E, G and N is essentially unaffected

by temperature down to about -70 °F. However, the rope product,

Material G, is far more flexible than either of the rod products due to

normal slippage in the rope construction under bending deformations.

The load-deflection relationships for the two rod products are linear elastic up to large deflections. The strain at fracture for both

of these materials is greater in bending than in tension. However, with

Material N the bending strain at fracture is substantially less at -40

°F than it is at room temperature.

The flexural modulus of elasticity of Material E is noticeably

greater in bending than in tension.

For Material N the flexural modulus of elasticity and the bending

strain at fracture both exceed the manufacturer's rated values at room

temperature.

9. VIBRATION EFFECTS

Simulated Aeolian vibration tests were performed on 7/16-in

Material G rope and 1/2-in Material N rod. In these tests the specimen

was pretensioned to a predetermined load level, allowed to relax under

load for 24 h, and then vibrated transversely while the original load

level was maintained essentially constant. The transverse vibration was

excited with an elect rodynamic shaker which was mechanically coupled to

the specimen immediately adjacent to the first nodal point. The shaker

was tuned to the resonant frequency of the specimen. The maximum double

amplitude of vibration of the specimen was measured with a filar

microscope. All of the tests were performed under the ambient

laboratory conditions of temperature and humidity.

The first tests were carried out in an apparatus in which the

tensile loads are applied by dead weights through a 10:1 lever system.

The tensile load capacity of this apparatus is 5500 lbf and the required

specimen length is 18 ft. When it was found that failures of the

specimen materials could not be obtained with this arrangement a larger

facility was assembled. The new facility employs a screw-powered

testing machine to apply the tensile loads and a more powerful

elect rodynamic shaker for larger amplitudes of vibration. Specimen

length for this facility is 23 ft.

54
The test results are given in Table 22. GRP guy-lines in field

installations have been reported to vibrate at frequencies within the

range used in these tests, but the amplitudes of vibration observed in

the field are apparently smaller than those used in these tests. It

should be noted that, for a given amplitude of vibration, considerably

more power was required for Material G than for Material N. This

implies that Material G, due its rope construction, has a greater

damping capacity than Material N rod.

The results show that two end fitting failures were obtained with

Material G specimens. In neither case did this cause failure of the

rope itself. Two specimens of Material N failed at the point where the

shaker was attached to the rod. These failures were due to fretting and

would not have occurred under actual wind-induced vibration. One

Material N rod failed at 16000 lbf before the transverse vibration was

applied

.

In no case did a rod or rope specimen fail under conditions which

could be attributed to the Aeolian vibration itself. It appears,

therefore, that Aeolian vibration is not a matter of serious concern

with these materials, at least under conditions comparable to those used

for the tests. It has been suggested, however, that when a long guyline is assembled from two different materials in series, with a heavy

connector, then Aeolian vibration can induce a whipping action with

substantially greater amplitudes than those examined here.

The test results also give some indication of the long-term

strengths of the specimen materials under ambient conditions. It

appears that 7/16- in Material G can sustain 9000 lbf for extended

periods, while 16000 lbf may be an upper limit for 1/2- in Material N

with Type R/V fittings.

10. STRESS -RUPTURE STRENGTH

Sustained load tests, in tension, were carried out on Materials G

and N in order to study the long-term strengths of these materials under severe environments. The tests were performed at elevated temperatures,

both with and without high humidity conditions.

The specimens were mounted such that the central portion of the rod

or rope material passed concentrically through a cylindrical

temperature-humidity test chamber while the ends remained exposed to the

ambient laboratory environment. The specimens were sufficiently long so

that the end fittings were unaffected by the temperature and humidity in

the chamber.

55
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56
With a specimen in place, the chamber was heated to the desired

test temperature in about 45 min. For tests conducted under high

humidity conditions, the humidity in the chamber was also raised to the

desired level during this interval. The specimen remained under these

conditions for two to three hours prior to testing. The test loads were

then applied and thereafter maintained constant until failure resulted

or the test was terminated.

10.1 Material G

Stress-rupture tests of 7/16-in Material G were performed at 200

°F, under both ambient and high humidity conditions. In addition, one

test was accomplished at room temperature. Two types of end fittings

were used; Type R/P with C1W1 potting compound and Mod 4 with C2W3

potting compound. Several failures were obtained at the Type R/P

fittings; none at the Mod 4 fittings.

Table 23 presents the results of those tests which resulted in

failure of the specimen within the chamber or which were discontinued

without failure. The tests which resulted in failure at the end fitting

will be discussed later.

The short-time tensile strength of 7/16-in Material G at room temperature is approximately 20500 Ibf (Table 16). Table 23 shows that

with a load of only 16000 Ibf failure occurs in less than a half hour at room temperature. Raising the temperature to 200 °F causes further

reductions in the load-carrying capability of this material. The shorttime tensile strength at 200 "F is only 13700 Ibf and the long-term

strength is less than 9000 Ibf. The diminution of strength with time

under load has also been observed at room temperature for Material G

[13].

Although elevated temperature severely degrades the strength of

Material G, humidity appears to have little effect. This is, perhaps,

attributable to the polyurethane jacket on Material G, which remained

intact even after failure of the rope strands. In fact, the test

results at 200 °F, both with and without high humidity, appear to be

part of a single statistical population, as shown in Figure 17. The

"best-fit" straight line drawn through the plotted points was determined

by the method of least squares.

10.2 Type R/P Fittings on Material G

As mentioned above, two types of end fittings were used for the

long-term tests of 7/16-in Material G: Type R/P with C1W1 potting com- pound, and Mod 4 with C2W3 potting compound. No failures were obtained

at the Mod 4 fittings. The results of those tests which culminated in

failure at the Type R/P fittings are given in Table 24. Also included

57
Table 23 - Stress-Rupture of 7/16-ln Material G

Test

No. Temperature Humidity Load

Rupture

time

83255

53070

53073

53078

53079

53080

83256

103325

103324

103322

77

200

200

200

200

200

200

200

200

200

pet rh

45-50

<25

<25

<25

<25

<25

>95

>95

>95

>95

lbf

16000

13700

12000

9500

9000

8500

13125

11000

10000

9000

0.45

0.00

0.35

0.6

17.3

>140. (a)

0.00

0.1

3.4

27.1

(a) Test discontinued prior to failure.

Table 24 - Stress Rupture of Type R/P End Fittings

with 7/16-in Material G

Potting compound: C1W1

Test Rupture

No. Load time

lbf h

53071 13000 0.1

72313 13000 0.2

53072 12500 1.5

53074 11000 0.05

53075 10500 6.5

53076 10000 1.1

53077 9500 8. (a)

83254 9000 12. (a)

53080 8500 >140.<b >

(a)± 7 h, exact time not determined.

Test discontinued prior to failure.

58
59
is one test which was discontinued prior to failure. Temperature and

humidity data are not given in the table since the fittings were all

exposed to the ambient laboratory atmosphere, approximately 77 °F and 45

to 50 percent relative humidity.

The results are plotted in Figure 18 with the "best-fit" straight

line. These data may be compared with the short-time tensile strength

of the Type R/P fittings with C1W1 potting compound on 7/16-in Material

G, which was found to be approximately 13600 Ibf (Table 6). Comparison

of Figures 17 and 18 shows that for short times the strength of the

fitting at room temperature exceeds that of Material G at 200 °F, but

the trend of the data suggest that for long times the reverse is true.

It is clear that this combination of end fitting, potting compound and

rope material is not conducive to long-term strength even under ambient

temperature and humidity conditions.

10.3 Material N

Stress-rupture tests of 1/2-in Material N were performed at 150,

175 and 200 °F both with and without high humidity conditions. Type R/V

end fittings were used. Most of the specimens failed in the test

chamber, some failed at the end fitting, and some tests were discontinued prior to failure. In addition, three specimens which were tested

at 150 °F failed in the free length just outside of the test chamber.

The temperature and humidity conditions at the point of failure in these

specimens are not known accurately but are obviously less than 150 °F

(the test temperature) and less than 45 percent rh (the ambient

humidity), respectively.

Table 25 presents the results of all tests except those which

resulted in failure at the end fittings. These results are plotted in

Figure 19, which shows that the stress-rupture properties of Material N

are unusual in several respects. The stress-rupture strength of this

material appears to be essentially unaffected by temperature and

humidity in a range of temperatures extending from some point below 150

°F up to at least 200 °F. (This, of course, explains why failures in

the free length just outside of the test chamber were apt to occur.) It

is clear that the lower limit of this temperature range does not extend

down to room temperature since, as Table 22 shows, this material can sustain higher loads at room temperature.

The "best-fit" straight line is seen to be nearly horizontal. Examination of the figure suggests that a straight line may not actually

represent the data properly; except for one anomalous point at 16000 lbf

and 48.4 h the data suggest a stress- rupture curve which is asymptotic

to the horizontal at about 14000 lbf. It thus appears that the stressrupture behavior of 1/2-in Material N is characterized by a threshold

load above which the time to rupture is relatively short. Loads below

60
61
Table 25 - Stress Rupture of 1/2-in Material N

Test

No. Temperature Humidity Load

Rupture

time

63125

63126

63127

63129

63128

63130

63131

63134

63135

63136

°F

200

200

200

200

200

200

200

175

175

175

pet rh

<25

<25

<25

<25

<25

<25

<25

<25

<25

<25

lbf

16000

15500

15250

15000

15000

14000

14000

14250

14000

13000

48.4

0.05

0.1

0.05

0.2

6.7

>286.

(

a>

>115. (a)

>112.( a) >lll.( a)

63137

63139

63142

150

150

150

<25

<25

<25

15000

14500

14125

0.3

144.6

32.05

63138

63140

63141

<150

<150

<150

<45

<45

<45

14750

14250

14250

0.1

1.2

5.1

73208

73209

200

200

>95

>95

14250

14125

68.5

99.5

73210 175 >95 14250 0.4

(a) Test discontinued prior to failure.

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the threshold can be sustained for longer periods. The threshold load,

14000 lbf, corresponds to a tensile stress of about 70000 lbf/in2 and is

70 percent of the manufacturer's rated load.

10.4 Type R/V Fittings on Material N

Table 26 presents the results of those stress-rupture tests of

Material N which culminated in failure at the Type R/V end fitting.

Also included in the table are those tests which were discontinued prior

to failure. Temperature and humidity data are not given because the

fittings were exposed to the ambient laboratory atmosphere. These

results may be compared with the short-time breaking strength of Type

R/V fittings on 1/2-in Material N, which is approximately 20600 lbf

(Table 4).

The stress-rupture data are plotted in Figure 20. A meaningful

straight line which characterizes these data cannot be obtained in this case since all but one of the failure points are bunched into less than

one time decade. In this situation the least-squares method is unduly

influenced by the single point at 16000 lbf and 0.1 h. Nevertheless,

the general trend of the data is clear; it is similar to Figure 19 in

that it indicates a threshold at about 14000 lbf. Since the Type R/V

fitting is a metallic, mechanical fitting its strength is virtually

time- independent in the temperature range of interest. Its long-term

strength when gripping Material N rod is determined by the properties of

the rod which, as pointed out, are lower at 150 °F than at room

temperature. It may be concluded that the performance of Type R/V

fittings on Material N would be less than adequate for long-term service

at moderately elevated temperatures.

10.5 Material A

Stress-rupture tests of single-strand Material A, which were

conducted elsewhere [14], show that the load-carrying capability of this

material, like Material G, decreases continuously with time under load.

Furthermore, these data also indicate no detrimental effect of high

humidity on the long-term strength at room temperature.

10.6 Summary

The long-term strength of Material N is somewhat less at 150 °F

than at room temperature. In the range 150 to 200 °F the stress-rupture

properties are characterized by a threshold at about 70000 lbf/in 2

; stresses above this value tend to produce failure in relatively short

times while below this threshold the rupture time can be substantial.

64
Table 26 - Stress Rupture of Type R/V End Fittings

with 1/2- in Material N

Test Rupture

No. Load time

lbf h

74216 16000 0.1

63133 14500 2.85

73207 14500 5.65

63132 14500 19.6

63134 14250 >115.( a) 73211 14125 9.8

73212 14125 16.5

63135 14000 >112.

(

a> 63136 13000 >111.

(

a)

< a>Test discontinued prior to failure.

Table 27 - Effects of Storage on Tensile Breaking Loads

of Material A (7-strand)

Coil diameter: 2.5 ft, unsupported

End fitting: Type F

Gripped length: 24 in, each end

Test

No.

Storage

temperature

Free

length

Maximum

load( a) °F in lbf

56086 125 24 15800

56087 150 24 15600

56088 175 22 15400

56089 200 24 15000

( a) All specimens failed strandwise at the end fitting.

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The long-term strength of Material G is considerably less at 200 °F

than at room temperature. The load-carrying capabilities of Material G

and Material A decrease continuously with time under load.

Quite surprisingly, there is no evidence of a detrimental effect of

high humidity on the stress-rupture strengths of these materials. It is

possible that such an effect, if it exists, manifests itself only after

long times.

11. LONG-TERM STORAGE

One of the principal advantages of the rope products over the rod

products is their flexibility, which permits them to be coiled to

relatively small coil diameters for convenience in shipment and storage.

In sizes up to about 5/8-in diameter the rod products may also be

coiled, albeit to relatively large coil diameters. It has occasionally

been observed, however, that coiled rods exhibit a tendency to buckle

under moderate elevated temperatures such as might be encountered in

certain storage situations.

The buckling failures are quite localized. See Figure 21. The

mechanism by which they form is not fully understood. The glass fibers

near the inside surface of the coil apparently buckle radially inward

and fracture. This process is undoubtedly facilitated by the reduced

stiffness of the matrix material at the elevated temperature.

A series of tests was conducted to examine the combinations of coil

diameter and temperature at which buckling failures occur. For this

purpose a large, walk-in type, elevated-temperature test chamber was designed and constructed to accomodate coils up to 10 ft in diameter,

which is about the largest practical shipping size. The chamber is 12 x

12 x 8 ft high and is fabricated from aluminum honeycomb sandwich panels

with a window for visual inspection of the specimens during test. The

temperature in the chamber can be raised to 200 °F in one hour. Heat is

supplied by a distributed bank of small radiant heaters having a com- bined capacity of 11 kW. Power to each heater is adjustable with a variable autotrans former and the temperature is maintained by a temperature controller. An electric fan provides continuous air circulation for better temperature uniformity.

Each specimen consisted of three full turns of the rod or rope

material with the ends fastened to prevent uncoiling. Most of the

specimens were unsupported, that is, coiled x^ithout a drum, but several

were coiled inside or outside a sheet metal drum. The coils were

suspended, by fine wires, from the ceiling of the chamber is order to

permit free air circulation around them. Four thermocouples, mounted at

90° intervals around each coil, were used to monitor temperature. The

67

Figure 21 - Typical buckles in 1/2-irt Material E, resulting

from coiled storage at elevated temperature.

temperature distribution on each coil was uniform within 5 °F at each

test temperature.

Tests were performed at 125, 150, 175 and 200 °F. The duration of

each test was 30 days (720 h) . Following each test the specimens were

uncoiled, examined for permanent set, and cut into convenient lengths

for tensile testing. The tensile test conditions were the same as those

described earlier.

11.1 Material A

Material A in the 7-strand configuration was coiled to a diameter

of 2.5 ft and subjected to storage tests at 125, 150, 175 and 200 °F.

The maximum elastic bending stress and strain due to coiling are approximately 31000 lbf/in2 and 0.0040, respectively. No buckles were

observed after 720 h at each test temperature.

After the storage tests the coils were unwound. There was no evidence of any significant amount of permanent set in the material as a result of the storage testing. A length of material was cut from each

coil and tested in tension. The results of these tests are given in

Table 27. All of the specimens failed strandwise at the end fitting.

By comparison with the results given in Table 2 it is seen that the

storage conditions had essentially no effect on the breaking load of the

material.

11.2 Material E

Storage tests of 1/2- in Material E were performed on 7- and 10-ft

diameter coils. The maximum elastic bending stresses and strains due to

coiling are approximately 50000 lbf/in2 and 0.0060, respectively, for

the 7-ft coil and 35000 lbf/in2 and 0.0042, respectively, for the 10-ft

coil. Tests were conducted on unsupported coils, on coils wrapped

inside a drum, and on coils wrapped outside a drum. The approximate

times to buckling are given in Table 28. The table shows that at 125 °F

both ,coil sizes survived the 720-h test without buckling. The 7-ft

coils tested at 150 °F all buckled within three days. There is some evidence that the support offered by a drum delays buckling, and that

wrapping outside a drum is preferable to wrapping inside a drum. A 10-

ft unsupported coil buckled at 175 °F. Supported coils of 10-ft

diameter, which were intended for test at 200 °F, buckled at 180 °F

during heat-up to the test temperature.

The storage specimens were uncoiled after the storage tests. No

appreciable amount of permanent set was evident in the material from

coils which had not buckled. Material which had buckled was permanently

bent in the vicinity of the buckles.

69
Table 28 - Storage Tests of 1/2-in Material E

Test Coil Coil Storage Approximate

No. diameter support temperature time to buckling

ft °F h

36010 7 T] >72f)

36011 10 u 125 >720.

36016 7 u 150 0.5

46058 7 ID 150 24.

46058A 7 0D 150 72.

36017 10 U 150 >720.

36022 10 u 175 0.25

66147 10 ID 180 < b ) 0.0

66148 10 OD 180 0.0

( a)u = unsupported; ID = wrapped inside drum; OD = wrapped outside

drum.

(b

> Buckled at 180 ° F during heat-up to 200 °F.

70
The material was cut into lengths convenient for tensile testing.

The tensile breaking loads of fifteen specimens which contained buckles

were between 3800 and 12000 lbf, depending upon the severity of the

buckles. All of the tensile failures initiated at buckles.

The results of tensile tests on specimens which did not contain

buckles are given in Table 29. The first specimen listed was cut from a 7-ft coil which had been stored at 125 °F and had not buckled. The

other specimens were cut from 7-ft coils which had been stored at 150 °F

and which had buckled, although the tensile specimens themselves did not

contain buckles.

The two specimens which were tested with Type R/V fittings failed

by splitting inside the fitting. This is a different mode of failure

than had been obtained on Material E which had not been stored at

elevated temperature. In those tests the specimens failed by pinching

off at the fitting. The maximum loads, however, are essentially the same (see Table 4)

The specimens which were tested with Type R/P fittings exhibited

two different modes of failure. In two of the tests failure was by

pullout. In the other tests failure was by pinching off the specimen

just inside the fitting. The maximum loads are not directly comparable

with those obtained previously with virgin Material E (Table 6) since

different potting compounds were used. On the basis of the different

failure mode obtained with the Type R/V fittings and the low load

achieved in Test No. 76229, it appears that some degradation of the

strength of Material E may have resulted from the elevated temperature

exposure.

11.3 Material G

Storage tests of 1.5-ft diameter coils of 7/16- in Material G were

performed at 125, 150, 175 and 200 °F. All of the coils were

unsupported and all sustained the 720-h tests without buckling or permanent set.

Tensile tests were performed on specimens which had been cut from

these coils after the storage tests. The tests were conducted with Type

R/P fittings and C1W1 potting compound. All of the specimens failed by

pinching off just inside the fitting. The results of the tests are given in Table 30. These results may be compared with those from tests on Material G which had not been stored at elevated temperature (Table

6). This comparison shows that the elevated temperature storage appears

to have produced a small improvement in the breaking loads, which is

probably attributable to a beneficial postcuring effect of the elevated

temperature exposure.

71
Table 29 - Effects of Storage on Tensile Breaking Loads of Unbuckled

1/2- in Material E

Coil diameter: 7 ft

Free length of tensile specimens: 21 in.

Test Storage Coil End Potting Maximum

No. temperature support ( a) fitting compound load Failure

°F lbf

56097 125 U R/V 19200 (b)

56098 150 U R/V 19400 (b)

66158 150 ID R/P C1W1 17300 (c)

61159 150 ID R/P C1W1 22200 (d)

76229 150 ID R/P C1W1 13000 (d)

76230 150 ID R/P C1W1 24300 (c)

76231 150 ID R/P C1W1 17500 (c)

76232 150 ID R/P C1W1 20700 (c)

66156 150 OD R/P C1W1 18200 (c)

66157 150 OD R/P C1W1 17950 (c)

( a) u = unsupported; ID = wrapped inside drum; 0D = wrapped outside drum.

( b ) Longitudinal splitting in fitting.

(°)pinching off.

( d)Pullout.

72
Table 30 - Effects of Storage on Tensile Breaking Loads of

7/16-in Material G

Coil diameter: 1.5 ft, unsupported

End fitting: Type R/P

Potting compound: C1W1

Gripped length: 7 in, each end

Test Storage Free Maximum

No. temperature length load< a) °F in lbr

56093 125 20 16400

56094 150 20 15600

56095 175 21 16400

56096 200 20 14800

(a)

F

aiiure by pinching off at the end fitting.

Table 31 - Storage Tests of 1/2-in Material N

Test Coil Coil Storage Approximate

No. diameter support^ 3) temperature time to buckling

ft °F h

36013 7 U 125 >720.

36014 10 U 125 >720.

36019 7 U 150 >720.

46059 7 ID . 150 >720.

46059A 7 OD 150 >720.

36820 10 U 150 >720.

36024 7 u 175 0.75

76220 7 ID 175 24.

76221 7 OD 175 >720.

36025 10 U 175 >720.

46056 10 U 200 >720.

76218 10 ID 200 >720.

76219 10 OD 200 >720.

(a)u - unsupported; ID = wrapped inside drum; OD = wrapped outside

drum.

73
11.4 Material N

Storage tests of 1/2-in Material N were performed on 7- and 10-ft

diameter coils. The maximum elastic bending stresses and strains due to

coiling were approximately 44000 lbf/in2 and 0.0060, respectively, for

the 7-ft coil, and 31000 lbf/in2 and 0.0042, respectively, for the 10-ft

coil. The results of the storage tests are given in Table 31. It was

found that a 7-ft unsupported coil buckled at 175 °F and that the time

to buckling was delayed by wrapping inside a drum. When wrapped outside

a drum no buckling occurred during the 720-h duration of the tests. The

10-ft coils did not buckle at temperatures up to 200 °F.

After the storage tests the specimens were uncoiled and an interesting phenomenon was observed. While the coils which had been

exposed in the unsupported condition did not exhibit any appreciable

permanent set, those coils which had been wrapped inside or outside of a drum tended to remain in a circular configuration with radii of

curvature between 20 and 26 ft. The reason for this phenomenon is not

known

Tensile specimens of Material N were cut from the coils. The free

lengths of all of the specimens were 20 in or more. None of the

specimens contained buckles. The results of tensile tests on these

specimens are given in Table 32.

A variety of failure modes was observed. In the tests of specimens

which had been exposed to 150 °F, the lowest maximum loads were obtained

on specimens which failed by longitudinal splitting inside the Type R/P

fitting. Because of the curvature in the specimens it appeared that the

splitting was induced, to some extent, by the tendency for the specimens

to straighten under the tensile loads. However, an attempt to correlate

the splitting failures with the radii of curvature of the specimens was

unsuccessful.

A splitting failure was also observed in the specimen which had

been -exposed to 175 °F and which had been tested with Type R/V fittings.

Virgin Material N which had been tested with Type R/V fittings had not

failed this way (Table 4) . On the basis of this observation plus

similar ones with Material E it is concluded that the elevated

temperature exposure produces some subtle change in the rod materials

which increases their susceptibility to splitting failures.

The maximum loads obtained with Type R/V and Type F fittings are

only slightly less than those obtained with virgin material (Tables 4

and 2, respectively).

74
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11.5 Summary

The rope products. Materials A and G, showed no tendency to buckle

due to long-term exposure to elevated temperatures in a coiled

condition. A 2.5-ft diameter coil of 7-strand Material A and a 1.5-ft

diameter coil of 7/16-in Material G survived 30 days at 200 °F without

evidence of buckling, permanent set, or reduction in breaking strength.

The rod products, Materials E and N, were both susceptible to

buckling when exposed to elevated temperatures in a coiled condition.

Tests on 1/2-in diameter rod showed that with a coil-diameter/roddiameter ratio of 168, Material E buckled at 150 °F and Material N

buckled at 175 °F. With a diameter ratio of 240, Material E buckled at

175 °F while Material N survived 30 days at 200 °F without buckling.

The stress and strain levels were only a fraction of those imposed in

the tension, bending and stress- rupture tests.

Buckling, when it occurred, invariably took place within the first

72 h of elevated-temperature exposure. Rods which did not buckle within

3 days also survived the entire 30-day exposures.

The support provided by wrapping coils on a drum delays or resists

buckling compared with unsupported coils. Coils wrapped outside a drum

are more resistant to buckling than those wrapped inside a drum.

However, with Material N, exposure to elevated temperatures while

wrapped on a drum caused some permanent set to develop in the material.

The tensile strength of buckled material is severely reduced.

However, there are indications that the breaking strength of material

apart from the buckles is only slightly impaired.

12. CONCLUSIONS

An extensive and varied test program was carried out on four, com- mercially available, GRP rod and rope materials. Materials A and G are

rope products, Materials E and N are rod products. The results of these

tests warrant the following conclusions:

1. A nominal density of 0.07 lb/in 3 is a reasonable value

for use in design considerations involving these materials,

regardless of diameter. The actual weights of Materials G

and N were found to be significantly greater than the

rated values reported by their respective manufacturers.

2. Material A, which is fabricated with S-glass, is significantly stronger than the other three materials, which use

E-glass reinforcement. The true tensile strengths of

76
Materials A, E and N do not appear to vary with diameter.

Since the cross-sectional area of Material G cannot be

calculated from the nominal diameter, the strength/

weight ratio is a more consistent and meaningful parameter

for this material than tensile strength.

3. A tensile modulus of elasticity of 7.5 x 106 lbf/in2 is a reasonably typical value for design purposes involving Materials A, E and N, regardless of diameter.

, This exceeds the manufacturers' rated values. The

modulus of Material G, appropriately corrected for

the cross sectional area, is approximately 5 x 106 lbf/in .

4. The flexibility of Materials E, G and N is essentially

unaffected by temperature down to about -70 °F.

However, the rope product, Material G, is far more

flexible than the rod products. The strain at

fracture for the rod products is greater in bending

than in tension but, for Material N, the bending

strain at fracture is substantially less at -40 °F

than at room temperature.

5. Aeolian vibration does not appear to be a serious

problem in GRP guy lines, at least under conditions

similar to those which were simulated in the laboratory. This conclusion may not apply to guy lines

which, because of their design, are subject to

whipping action. Material G, due to its rope con- struction, has a greater damping capacity than

Material N.

6. The stress-rupture strength of Material N, in the

range 150 to 200 °F, is characterized by a threshold

at about 70000 lbf/in2 . Stresses above this value tend

to produce failure in relatively short times while below

this threshold the rupture time is comparatively long.

By contrast, the load-carrying capabilities of Materials

A and G decrease continuously with time under load.

There is no evidence of a detrimental effect of high

humidity on the stress-rupture strength of these

materials. If such an effect does exist it becomes

apparent only after long exposures.

7. The rod products are susceptible to buckling when

exposed to elevated temperatures in a coiled condition

involving relatively low bending stresses. The

resistance to buckling is increased by wrapping the

coils on a drum but this tends to produce some

77
permanent set in Material N. Diameter-temperature

relationships for avoiding buckling were established.

The rope materials are not susceptible to buckling.

The tensile breaking strengths of the materials are not seriously degraded by the coiled exposures except

where buckling has occurred.

The performance of the GRP materials was evaluated with five,

commercially available end fittings. These include three dead-end typ

(P, F and F/A) , a mechanical compression type (R/V) , and a potted

compression type (R/P) . It was found that:

1. Not all of the fittings are suitable for all of the

GRP materials but, with judicious selection, the

manufacturers' rated breaking loads of the GRP materials

can be approached with the commercial fittings. With

only one combination (Type F/A fitting on Material G)

,

however, could the true tensile strength of the GRP

material be attained.

2. Although the true tensile strengths of the materials

show no appreciable variation with diameter the actual

breaking stresses, with commercial end fittings,

tend to increase as diameter is reduced.

3. The performance of the Type R/P fitting is strongly

dependent on the potting compound which is used.

4. Certain of the commercial end fittings were found to

be inadequate either for conditions involving Aeolian

vibration or for long-term service at moderately

elevated temperatures.

A new, experimental, shear- type potted end fitting was developed.

This fitting, the NBS Mod 4, is capable of attaining the true tensile

strengths of Materials E and G, and is at least as good as the

commercial fittings on Material N. No failures of the Mod 4 fitting

were experienced in simulated Aeolian vibration tests or in stressrupture tests.

78
* * *

The valuable contributions of Mrs. Ruth M. Woolley and Messrs

Donald E. Marlowe, Oscar 0. Owens and Raymond G. Russell, of the

Engineering Mechanics Section, to the successful conduct of this program

are gratefully acknowledged.

79
13. REFERENCES

1. McKee, R. B., Reinforced Fiberglas as a Balloon Tether, Proc AFCRL

Tethered Balloon Workshop 1967 , Report No. AFCRL- 68-009 7, ed. T. W.

Kelly, pp. 163-176 (Air Force Cambridge Research Laboratories,

Bedford, Mass., March 1968).

2. Morey, R. L. and Berteaux, H. 0., Alleviation of Corrosion Problems

in Deep Sea Moorings, presented at Third International Congress on Marine Corrosion and Fouling, NBS , Oct 1972.

3. Connors, L. P., Use of Pultruded Reinforced Plastic Structurals

Grows in the Electric Industry, Transmission & Distribution , June 1970, pp. 46-48.

4. Dong, Y. H. , World's Largest Log-Periodic Antenna, J. Structural

Division ASCE 97, ST9 , pp. 2371-2381 (Sept 1971).

5. Shulver, W. and Blake, D. L. (inventors), Tensile Members,

Apparatus and Process, U.S. Patent No. 3,619,317 (Nov 9, 1971).

6. Anon., Pultrusion Breakthrough Opens New Markets, Plastics World , March 1971.

7. Morrison, R. S., Pultrusion, Modern Plastics Encyclopedia 1972-1973 , pp. 642-644, McGraw-Hill, N. Y. (1972).

8. Costello, R. B. and Graham, G. B. (inventors), Glass Plastic Rope,

U.S. Patent No. 3,371,476 (March 5, 1968).

9. Costello, R. B. and Graham, G. B. (inventors), Method of Making

a Glass Textile Strand Product, U.S. Patent No. 3,496,717 (Feb 24,

1970).

10. Kalsey, N. , Properties of Glass Fiber-Reinforced Polymers for

Application to Guy-Insulator Systems of Communication Towers,

"Phase 2, Test Methods and Evaluations of Glass Fiber-Reinforced

Polymer Rods for Use in Guy-Insulator Systems of Loran C Towers,

NBS Report 10 261 (June 1970).

11. Mitchell, R. A., Woolley, R. M. and Halsey, N., Development of

Tensile End Fittings for FRP Rod and Rope, presented at ASCE

Specialty Conference on Composite Materials, Pittsburgh, Nov 1972.

(To be published)

12. Mitchell, R. A., Woolley, R. M. and Fisher, C. R. , Formulation and

Experimental Verification of an Axisymmetric Finite-Element Structural

Analysis, J. Res. NBS 75C3&4-322, pp. 155-163 (1971).

80
13. Hanna, G. L., Glass-Epoxy Composite Cables for Tethered Balloons,

Proc AFCRL Tethered Balloon Workshop 1967 , loc. cit ., pp 129-135.

14. Kane, J. L., Paper No. XII, Proc AFCRL Tethered Balloon Workshop

1967 , loc. cit. . pp 115-122.

81
APPENDIX

Factors for converting U.S. customary units to the International

System of Units (SI) may be found in ASTM Standard Metric Practice Guide

(ASTM Designation E380-70) . Copies are available from the American

Society for Testing and Materials, 1916 Race Street, Philadelphia,

Pennsylvania 19103. Conversion factors for units used in this paper are given in the following table:

82
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83

orm NBS-1UA (i.7i) "

U.S. dept. OF comm. 1 . PUBLICATION OR REPORT NO.

BIBLIOGRAPHIC DATA

SHEET NBSIR 73-129

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Evaluation of GRP Rod and Rope Materials and Associated

End Fittings

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

Nixon Halsev. Richard A. Mitchell and Leonard Mordfin

8. Performing Organization

NRSTR ,73-129

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task7Work Unit No. 2130451, -2, -3, -4

2130145

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Army Electronics Command, Fort Monmouth, New Jersey

Naval Facilities Engineering Command, Washington, D. C.

Rome Air Development Center, Griffiss Air Force Base, N. Y.

United States Information Aeencv, Washington. D. C.

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

An extensive and varied test program was carried out on four GRP rod and rope

materials to evaluate tensile strengths, moduli of elasticity, flexibility at low

temperatures, effects of simulated Aeolian vibration, and stress-rupture properties

at moderate elevated temperatures both with and without high humidity. The effects

of elevated temperature on long-term storage capabilities were investigated, and

diameter-temperature relationships were established for avoiding buckling due to

storage in a coiled condition.

The performances of five commercially available end fittings on these materials

were examined in terms of the breaking loads attained in tensile tests. Using

finite-element analyses, an improved end fitting was developed which is capable of

approaching the true tensile strengths of two of the GRP materials. An experimental

stress analysis of the improved fitting was performed.

.7. KEY WORDS

Aeolian vibration, simulated; composite materials; end fittings for GRP rod and rope;

grips, guy; guys, antenna; humidity, effects on GRP; mechanical properties of GRP;

pultruded rod; reinforced plastics, rod and rope; rope, GRP; static fatigue of GRP.

• KEY WORDS (Alphabetical order, separated by semicolons)

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