NBSIR 73-233

Non-Metallic Antenna-Support Materials

First Annual Interim Report

Donald E. Marlowe, Nixon Halsey, Richard A. Mitchell, and Leonard Mordfin

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

April 1973

Interim Report for Period October 16, 1971 - October 15, 1972

Prepared for Air Force Materials Laboratory

Wright Patterson Air Force Base

Ohio 45433

NBSIR 73-233

NON-METALLIC ANTENNA-SUPPORT MATERIALS

First Annual Interim Report

Donald E. Marlowe, Nixon Halsey, Richard A. Mitchell, and Leonard Mordfin

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

April 1973

Interim Report for Period October 16, 1971 - October 15, 1972

Prepared for Air Force Materials Laboratory

Wright Patterson Air Force Base

Ohio 45433

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

CONTENTS

Page

1. INTRODUCTION 2

2. SCOPE 3

3. DEVELOPMENT OF IMPROVED END FITTINGS 5

3.1 Finite-Element Analysis 6

3.2 Analytical Parameter Study 6

3.3 Selection of Potting Compound 8

3.4 Experimental Prototype Fittings 10

3.5 Design of the Mod 4 End Fitting 10

3.6 Experimental Stress Analysis of the Mod 4 End Fitting ... 11

3.7 Tensile Tests with Mod 4 End Fittings 11

3.8 Further Research on Mod 4 End Fittings 12

3.9 Development of the H3M End Fitting 15

4. INDUSTRIAL SURVEY 18

4.1 The Pultrusion Process 18

5. OPTIMIZATION OF THE MANUFACTURING PROCESS 19

5.1 Evaluation of Product Characteristics 19

5.2 Deficiencies in Existing Products 21

5.3 Manufacture of Experimental Materials 23

5.4 Mechanical Testing Equipment 23

5.5 Preliminary Tests and Results 27

6. PLANS 32

7. REFERENCES 34

8. FIGURES 36

APPENDIX .......

50

i

NON-METALLIC ANTENNA- SUPPORT MATERIALS

First Annual Interim Report

by

Donald E. Marlowe, Nixon Halsey, Richard A. Mitchell and

Leonard Mordfin

ABSTRACT

A program was initiated to develop a nonconductive, f iber-reinf orced-plastic rod material

for use as guy lines and catenaries in large

antenna systems. It is intended that the longterm mechanical properties of the new material,

under adverse weathering conditions, be superior

to those exhibited by the glass-reinforced-polyester rod and rope materials which are available

at the present time. The program centers around

three principal phases: development of an improved manufacturing process, selection of a superior non-metallic fiber-reinforcement material,

and selection of a superior pol3niieric matrix

material.

During the first year of activity on this

program, two new end fittings for f iber-reinforcedplastic rod were developed which out-perform

commercially available fittings and which are generally capable of achieving the full tensile

strengths of available glass-reinforced-polyester

rod materials. These two fittings, which were

developed to enable the tensile strengths of the

rod materials to be accurately determined, include

a shear-type fitting (Mod 4) and a compressiontype fitting (H3M)

Deficiencies in the physical characteristics

of existing glass-reinforced-polyester rod

materials were identified, and modifications to the

manufacturing processes were proposed to overcome

these deficiencies. A subcontract was let for the

manufacture of experimental glass-reinforced-polyester rods using the modified processes.

1
A bank of six JUUUU-ibf-capacity creep

testing machines was designed and is being

assembled. These machines, which will accom- modate 9-ft specimens, are equipped with environmental chambers for stress-rupture tests up to

200°F under saturated humidity conditions.

Key Words: Composite materials; end fittings for

r FRP rod; environmental resistance of FRP rod;

f iber-reinforced-plastic rod; glass-reinforcedpolyester rod; guys, antenna; mechanical properties

of FRP rod; pultrusion; reinforced plastic rod;

stress rupture of FRP rod.

1. INTRODUCTION

Glass-reinforced-polyester rod and rope materials constitute one

of the more interesting and valuable — although lesser known —

products of composite materials technology. These products are generally

fabricated from unidirectional glass fiber roving and a thermosetting

plastic matrix by a process called pultrusion. They are strong, lightweight, nonconducting, corrosion resisting and comparatively inexpensive.

This combination of properties has led to the use of these products in a variety of applications where steel wire rope had previously been employed

almost exclusively. In terms of total volume, the largest application

has been in communications towers and arrays, wherein the use of non- metallic rod and rope as guys and catenaries has permitted the elimination

of the large, expensive, ceramic insulators which are required when steel

wire rope is used.

As frequently happens when promising new products are developed, the

rod and rope were Introduced into service before they were adequately

characterized and without the benefit of carefully compiled design data.

The result has been the occurrence of failures in several field installations. Fortunately, however, most applications have performed satisfactorily.

Some of the reported failures of non-metallic guys and catenaries

involve tracking and flashover damage, and burning, in intense electromagnetic fields. Most of the other reported failures are apparently

mechanical, although environmental effects are probably involved. Ropetype guys and catenaries fail, at locations apart from their fittings, by

a progressive strand-by-strand breakage. The cause of these failures is

unknown but opinions that have been advanced include undetected damage

incurred during installation, wind-induced rubbing of adjacent strands, and

2
strength degradation due to ultraviolet exposure. In the case of rod

materials, failures usually entail splitting or delamination that

progresses along the rod at a slight inclination to the longitudinal axis.

These splits often appear to start in or adjacent to an end fitting but

in many instances they evidently initiate at a transverse crack. Some of

these transverse cracks are particularly interesting because, prior to

splitting, they appear only as hairlines on the surface of the rod; the

surfaces of the crack itself are very smooth. The cause of the transverse

cracks is unknown but most opinions attribute them to some kind of dynamic

loading mechanism.

Although the causes of mechanical failure in these materials are apparently many and varied, at least one generalization can be made. Most

mechanical failures of non-metallic antenna support materials have occurred

after a period of service, ranging from months to years, during which the

materials have been under tension in a moist environment. With this as a point of departure, the National Bureau of Standards embarked upon a threeyear program to develop a new non-metallic f iber-reinforced-plastic (FRP)

rod or rope material which would exhibit superior mechanical properties

and greater resistance to moisture in the form of a hot, humid environment.

This program, which was initiated October 16, 1971, is being carried out in

the Engineering Mechanics Section of NBS under the sponsorship and with the

financial assistance of the Air Force Materials Laboratory. This interim

report covers progress during the first year of activity.

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. SCOPE

The development of a superior rod or rope product centers about three

principal phases of activity.

In Phase I the physical characteristics of existing FRP rod and rope

materials are examined in order to identify deficiencies, if any, which can be traced to improper or inadequate manufacturing practices. The intent,

then, is to have these manufacturing practices corrected or improved, and to

test samples of the improved material in order to assess the significance of

the improvements. The principal result of the activity under Phase I is,

thus, the development of an improved manufacturing process for FRP rod or rope

3
Using the improved manufacturing process, Phase II calls for the

fabrication of FRP rod or rope us^ii^ i t.^iiloro<„meii Ls other than E-glass,

which has been used most commonly. The reinforcing materials are to be

continuous, non-metallic fibers. The properties of the resulting products

are to be evaluated and compared with the corresponding E-glass product.

The improved manufacturing process is again used in Phase III to

fabricate FRP rod or rope with other, potentially better, resin matrix

materials. The resulting products are to be evaluated in comparison with

FRP products fabricated with conventional resin systems.

The evaluations of the experimental materials which are fabricated

in this program are based primarily upon two types of mechanical tests:

tensile tests at normal and high rates of loading, and stress-rupture

tests at 160°F and 200°F under saturated humidity conditions.

Upon completion of the three phases it is expected that the results

obtained will be adequate to specify one or two superior FRP products in

terms of the manufacturing process and the fiber and resin constituents.

In order to facilitate the attainment of this objective, and to promote

the use of the new materials, several supplementary tasks are required

both before and after the three principal phases:

If tensile strengths of the materials are to provide

a meaningful measure of product improvement, it is

necessary to determine the tensile strengths accurately.

However, previous work in this laboratory [1,2] has

shown that the end fittings for these materials, which

are available commercially, are not generally capable of

withstanding the tensile forces which are required to

fracture these materials in a true tensile mode.

Hence, there has been a need, first, to develop end

fittings, for use in the laboratory, which can gen-'

erate the full tensile strengths of the materials.

A second task which has been pursued prior to Phase I is a study of the existing manufacturing practices

for FRP rod and rope. The results of this study are required to enable potential improvements, in the

manufacturing practices, to be devised. This study

will also permit the identification of one or more

manufacturers having the capability and the willingness to Institute the improved manufacturing

practices.

4
At the completion of the three primary phases of

activity, a task will be undertaken to generate

design data for the new material or materials

developed in this program. These data are re- quired in order to facilitate the use of these

materials either in new systems or as replacements in existing systems.

3. DEVELOPMENT OF IMPROVED END FITTINGS

It has been established [2] that commercially available end fittings

for FRP rod and rope are not generally capable of developing the full

tensile strengths of these materials. In antenna-support applications it

is not necessary that the end fittings have this capability since, in

service, the guys and catenaries are loaded only to a fraction of their

short-time tensile strengths. In a program such as this, however, in

which there is need to measure the tensile strengths of FRP rod and rope

materials, it is necessary to have end fittings which will permit the full

tensile strengths of the materials to be attained.

In this task, laboratory prototypes of two new end fittings were

developed which are markedly superior to any of a variety of commercial

end fittings with which they have been compared. While the development of

these fittings was completed in the present program, much of the basic

work which went into the development was initiated or pursued in prior

studies performed in this laboratory. These prior studies were carried

out under the sponsorship and with the financial assistance of the Army

Electronics Command, the Coast Guard, the Naval Facilities Engineering

Command, Rome Air Development Center (USAF) , the United States Information

Agency and the National Bureau of Standards.

The basic approach to the development of improved end fittings

consisted of an analytical parameter study of the problem and an experimental evaluation of trial prototype designs, which were pursued con- currently. The analytical study involved the axisymmetric , finiteelement stress analysis of a broad class of end connections consisting of

a metal sleeve joined to the FRP 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

5
the other study. Some aspects of the analytical study are reported in

.greater detail elsewhere [3].

3.1 Finite-Element Analysis

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

formulation used [4] was developed for the case of isotropic elastic

materials. That earlier formulation has been modified in the end-fitting

analysis in accordance with the assumption that the FRP rod is homogeneous and transversely isotropic as defined by five independent elastic

constants. 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 coramon boundaries of adjacent elements. In the end-fitting analysis

elasLJC strains within the elements are assumed to be uniform, thus

assuring displacement compatibility along common element boundaries.

Figure 1 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 FRP 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 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 2, 3, and 4 give, respectively, plots of the analysis

mesh, the longitudinal normal stress and the longitudinal shear stress

for the Mod 4 end fitting which will be described later.

3.2 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 5. 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.

6
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 FRP 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

bond-shear 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 Ibf/in^ , greater stiffness 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 & 6. Elastic modulus and length of outer potting compound.

A combination of two potting compounds having different

elastic moduli produces two bond-shear 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 & 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.

7
The two most significant findings of this parameter study are:

1. The potting compound snoulu have a relatively low modulus

of elasticity, i.e., it should be flexible.

2. The potting compound layer should be relatively thick.

Both of these findings appear to be entirely logical but they were not

intuitively obvious prior to the parameter study. In particular, it is

known that adhesives with high bond strengths generally have relatively

high moduli of elasticity, and that adhesives generally exhibit higher

bond strengths when applied in thin layers. However, the parameter

study showed that a flexible, thick potting compound allows the tensile

load to be transferred from the fitting to the rod gradually, over a long

length, without developing a high peak bond-shear stress at the inner end

of the fitting. Thus, a flexible, thick potting compound performs better

in this application than a stiff, thin potting compound which may have a greater bond strength.

3.3 Selection of Potting Compound

On the basis of previous work in this laboratory, the selection of an appropriate potting compound was focused on a single series of formulations. These consist of a 100-percent-reactive epoxy resin adhesive and

an activator. The system requires a 2-h cure at 165°F. The two components

may be combined in different resin/activator ratios to provide different

mechanical properties. The designations and some of the properties for

several mixture ratios are given in Table 1.

A simple, shear-type end fitting was designed and fabricated to

evaluate the relative pullout shear strengths of the several CW potting

compounds. This fitting, called the Mod 3, is shown in Figure 6. It

consists of a cylindrical metal sleeve which is attached to an FRP rod

with a thick layer of potting compound.

A series of twelve tensile tests was performed using 1/2-in diameter,

high-strength FRP rod. All twelve specimens failed by rod pullout and the

results are given in Table 2. Although there is great scatter in these

data, the relatively high strengths of the specimens with the most flexible

potting compound are significant. These results support the analytical

parameter study which showed that a more flexible potting compound produces

significantly lower peak bond-shear stresses. Finite-element analyses of

the Mod 3 fitting indicate that 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.

8
Table 1 - Mechanical Properties of CW-Series Potting Compounds

Resin/activator ratio

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

Designation C2W3 ClWl C3W2 C2W1

(a) Bond strength , Ibf/in^ 2450 3280 -

Compressive strength^^\ Ibf/ln^ 32200 20700 —

Elongation , percent 8.0 7.0

(b ) -

^ Elastic modulus , Ibf/in^ x 10 0.51 2.75 3.16 -

(a) According to the manufacturer.

(b) From compressive tests on 0.5- in

laboratory.

diameter specimens performed in this

Table 2 - Tensile Tests with Mod 3 Fittings on 1/2-in FRP Rod

Test Potting ,(a) compound Maximum

No. Designation Elastic modulus load

Ibf/in^ Ibf

71169

71170

71171

Average

C2W3

C2W3

C2W3

0.51 X 10

0.51

0.51

5 19850

15500

18650

18000

71166

71167

71168

Average

ClWl

ClWl

ClWl

75

75

75

12200

8500

19250

13300

71172

71173

71174

Average

C3W2

C3W2

C3W2

16

16

16

8000

14350

9400

10600

71175

71176

71177

Average

C2W1

C2W1

C2W1

12350

17500

18100

16000

(a) See Table 1.

9
On the basis of these results the C2W3 potting compound was selected

for use in the development of improved end fittings.

3.4 Experimental Prototype Fittings

A variety of gripping concepts investigated prior to the analytical

parameter study gave disappointing results in tensile strength tests. When

the results of the parameter study became available, these early attempts

were abandoned and the development effort was redirected along two parallel

but differing approaches to the problem of achieving high strength in a potted end fitting. In one approach, the length of the interface is increased as a means of raising the total shear-carrying ability of the

fitting. This resulted in the development of the Mod 4 fitting. In the

second approach, compressive stresses are applied normal to the interface

in order to enhance its shear strength. This resulted in the development

of the H3M fitting. The development of these two laboratory prototype

fittings are described below.

3.5 Design of the Mod 4 End Fitting

The Mod 4 end fitting is a shear-type potted end fitting which is,

basically, a longer version of the Mod 3 end fitting which was discussed

above. The Mod 4 fitting. Figure 7, consists of a 1-1/2-in diameter rod

of aluminum alloy, 18-in long, which is drilled out axially to a depth of

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

12-in, leaving a uniform 1/8-in wall thickness. To mount the fitting on an

FRP rod, the end of the rod, after abrasive cleaning, is set into the

3/4-in hole in the base of the cavity. The rod is carefully aligned parallel and concentric with the fitting and the remaining cavity is filled

with the potting compound. The fitting was designed primarily for 1/2-in

diameter rod, which leaves a 3/8- in thickness of potting compound around

the rod.

The first Mod 4 fittings tested were machined from a low-strength

aluminum alloy and they failed, in tension, near the base of the cavity

at loads between 27900 and 30250 Ibf. Subsequent fittings were machined

from high-strength aluminum alloys, e.g., 7075-T6, and further failures

of the fitting have not been experienced.

10
3.6 Experimental Stress Analysis of the Mod 4 End Fitting

A tensile specimen was fabricated by mounting two Mod 4 fittings on a length of 1/2-in diameter, high-strength FRP rod using the C2W3 potting

compound. The specimen 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 (failure) Ibf. Seventeen

strain gages were located on the surface of one end fitting and the remaining

nine gages were mounted on the FRP 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 potting

compound.

Figures 8 and 9 are plots of longitudinal strain on the surface of the

instrumented fitting as predicted by the linear elastic finite-element 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 9 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. These two cycles were applied less than one hour after completing

Load Cycles 2 and 4, (which had maximum values of 10000 Ibf and 22000 Ibf,

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 Ibf 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 8 or 9 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 bondshear 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.

3.7 Tensile Tests with Mod 4 End Fittings

The performance of the Mod 4 end fitting was evaluated by means of

tensile tests on two commercially available FRP rod materials of 1/2-in

11
diameter. Both are fabricated from unidirectional, continuous, E-glass

roving and a polyester resin matrix. The individual glass fibers are 0.00051 in in diameter (Type K) . The manufacturing process for one of these

materials. Material E, results in the formation of an integral gel coating

of the matrix material on the outer surface of the rod. The resin system of

the second material. Material N, is reported by the manufacturer to be an "epoxy-modif led" polyester. Material N rod is coated with epoxy that is

heavily loaded with titanium dioxide for resistance to weathering and ultraviolet radiation. Thickness of the coating is 0.005 to 0.015 in.

The results of twenty tensile tests using Mod 4 fittings on 1/2-in

diameter Materials E and N rods are given in Table 3. The low-modulus C2W3

potting compound was used on all specimens. Two of the seventeen Material E

specimens failed in the free length at loads of 32850 and 31500 Ibf. The

other fifteen 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 all of the Material E specimens was 30100 Ibf, which

is more than 90 percent of the loads attained by the specimens which failed

in their free lengths. This indicates that the full tensile strength of

Material E is approached with the Mod 4 fittings. •

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

Mod 4 end fittings was only 20100 Ibf. 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.

A comparison of these results with those obtained with commercial end

fittings [2] 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 although it does not,

apparently, allow the full tensile strength of Material N to be approached.

3.8 Further Research on Mod 4 End Fittings

Several approaches were explored in an attempt to improve the performance of the Mod 4 fitting on Material N. In the first of these approaches

the effects of increasing the relative size of the Mod 4 fitting were

examined. In order to avoid the expense of machining larger fittings an initial test series was carried out using the standard Mod 4 fitting on Material N rods of smaller diameters, i.e., 3/8 and 1/4 in. The results of

these tests are given in Table 4. These results show that one of the 1/4- in

diameter specimens did, indeed, fail in the free length. This suggests the

possibility that a larger end fitting of the Mod 4 type would more nearly

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

Accordingly, a scaled-up version of the Mod 4 was designed and several such fxttmgs were fabricated. These have an overall length of 24 in a potted length of 18 in, an outer diameter of 2 in and a wall thickness of

12
Table 3 - Tension Tests with Mod 4 End Fittings on 1/2-in FRP Rod.

Potting compound; C2W3.

Test Maximum

No, Material load Failu

Ibf

71182 E 31350 (a)

71183 e' 32850 (b)

71184 E 25750 (a)

81242 E 27000 (a)

81243 E 28600 (a)

81244 E 32600 (a)

81245 E 29600 (a)

81246 E 29000 (a)

81247 E 31800 (a)

81248 E 32750 (a)

81249 E 30950 (a)

81250 E 31100 (a)

81251 E 31500 (a)

91261 E 23250 (c)

101315 E 30100 (a)

101333 E 31850 (a)

101404 E 31500 (b)

Average

30100

81240 N 23700 (a)

81241 N 17850 (a)

91263 N 18900 (a)

Average

20100

(a)

^ ^Pullout.

Failure in the free length.

(c)

Pullout; potting compound did not appear to have been adequately cured.

13
Table 4 - Tensile Tests of Small-Diameter Material N Rods with Mod 4 Fittings.

Test

No.

Nominal

rod

diameter

Maximum

load

Nominal

maximum

stress

Ibf/in^

Fai lure xn Ibf

91262

92266

3/8

3/8

13100

9800

119000

89000

(a)

(a)

92264

92265

1/4

1/4

3000

7550

61000

154000

(a)

(b)

(a)

(b)

Pullout.

In the free length.

Table 5 - Evaluation of Other Potting Compounds with 1/2-in FRP Rod.

Test

No.

Rod

material

Potting

compound

(a)

Maximum

load Failure

Ibf

101400

101402

101401

101405

E

E

E

N

PC23

PC8

PB5

PBS

22900

16000

12850

7650

(b)

(c),(b)

(b)

(b)

(a) PC23 is a flexible epoxy laminating resin which is prewarmed and mixed in

the ratio 2 parts resin to 3 parts hardener. PCS is a two-component , 100-

percent solids, liquid epoxy pouring compound with a prescribed mixture.

PB5 is a two- component , flexible, polysulfide adhesive with a 1;1 prescribed

mixture requiring extensive mixing.

^^^Pullout.

(c) Bond between potting compound and fitting failed at 1500 Ibf, then potting

compound was gripped directly.

14
0.25 in. Two tensile tests were performed using these fittings on 1/2-in

diameter rod of Material N. The results showed no improvement over those

obtained with the standard Mod 4 fitting and this approach has not been

pursued further.

A second approach which was explored involved an attempt to find a potting compound which is superior to C2W3. Four tensile tests involving

three promising potting compounds were carried out using Mod 4 end fittings

and 1/2-in diameter FRP rod. The results, Table 5, indicate that none of

these compounds performs as well as C2W3.

In the third approach, wedges were driven into the ends of Material N

rods prior to potting in Mod 4 fittings. This was done in an attempt to

achieve a degree of mechanical locking which would enhance the pullout

strength. Two tests culminated in pullout failures at loads not markedly

different from those attained without the wedges.

Several numerical experiments were conducted on the computer in an effort to obtain a greater understanding of the process of load transfer

from the end fitting, through the potting compound, to the rod. This effort

indicated the feasibility of incorporating the nonlinear properties of polymeric potting compounds into the finite-element analysis. It was found that

a relatively coarse finite-element mesh can give sufficiently accurate

linear solutions for bond-shear stress and surface strain, and that a series

of these linear solutions could be used in a direct-iterative procedure to

approximate nonlinear response. This type of nonlinear analysis has been

used in this laboratory to study composite-reinforced cutouts in metal

sheet [5]. However, it was determined that this approach could not be

effectively pursued within the resources available on the present program.

3.9 Development of the H3M End Fitting

Since the Mod 4 end fitting was found to be incapable of developing

the full tensile strength of Material N, it was decided to investigate the

performance of compression- type, rather than shear- type, end fittings for

this material. Accordingly, a series of tension tests was performed using

two different compression- type potted end fittings. One of these is a commercially available fitting and the other was developed in this laboratory

in an earlier program.

The Type R/P end fitting, Figure 10, is a commercially available,

basket type, potted, compression fitting which is intended primarily for

synthetic ropes. The conical basket, or potting head, is fitted with a yoke and bail for attachment to thimble-eye type hardware. To mount the

Type R/P fitting on an FRP rod, a fluted, cruciform wedge (Fig. 11) is first

driven axially into the end of the rod. (These metal wedges are available

from the manufacturer of Material N.) The inner surface of the potting

head is treated with an epoxy-release agent, then the rod is alined parallel

and concentric with the fitting, which is then potted and cured. The

15
release agent permits the potted material to slide relative to the potting

head, and under tensile loads the potted end seats tightly into the conical

potting head. This seating process develops significant radial compressive

stresses in the potted end which enhance the strength of the bond between

the rod and the potting compound. The release agent also facilitates removal

of the potted material for reuse of the fitting.

The NBS aluminum block end fittings. Figure 12, were developed in an earlier investigation [1]. These are 2-5/8 x 3-1/4 x 6 in long and are machined from 2024-T4 aluminum-alloy stock. Each block is split in half

longitudinally and bolted together, and then a conical section is machined

out. For use with 1/2-in FRP rod the conical section is 2-1/4 in in diameter

at the large end, tapering to a short cylindrical section 5/8 in in diameter.

The wedging and potting procedures for these fittings are the same as those

described above for the Type R/P end fittings.

These two types of fittings were used in a series of eight tensile

tests on 1/2-in Material N in an earlier study [1]. For those tests conical

wedges (Fig. 11) and ClWl potting compound (Table 1) were used. Except for

one test in which the rod pulled out of the fitting prematurely, all of the

specimens failed at loads between 20500 and 22300 Ibf by pinching off at the

fitting due to the radial compressive stresses.

Four additional tensile tests of 1/2-in Material N were performed in

the present investigation using these two types of end fittings. In these

tests the cruciform wedges were used. The C2W3 potting compound was used

for two of the specimens on the basis of the analytical parameter study.

PCS potting compound (Table 5) was selected for the other two specimens

because of its high compressive strength. The results of the tests are given in Table 6. These results show that the aluminum block end fitting

is capable of developing the full tensile strength of Material N rod and

verified that the C2W3 potting compound is apparently the best of those

which have been tried. The maximum load in Test No. 101411 is the highest

ever achieved in this laboratory on 1/2-in Material N at room temperature.

The aluminum block end fitting was subsequently redesigned in a cylindrical, one-piece configuration, and assigned the laboratory designation

H3M. See Figure 13. These changes were made in order to reduce the cost

of the fitting and to facilitate its use in the testing machine which is

being employed for the tensile tests in this investigation. The changes

have no apparent effect on the performance of the fitting.

Two more tensile tests were performed to examine the relative performances of the conical wedge and the fluted, cruciform wedge when used

with the H3M end fitting. For these tests 1/2-in Material E rod and C2W3

potting compound were employed. The specimen with the cruciform wedges

failed in its free length at 33450 Ibf. This is the highest breaking load

ever achieved in this laboratory on 1/2-in Material E rod. The specimen

16
Table 6 - Tensile Tests of 1/2-in Material N with Compression-Type

Potted End Fittings.

Test End Potting Maximum

No. fitting compound load Failure

Ibf

101408 R/P PCS 21950 (a)

101410 R/P C2W3 21000 (a)

101409 Al block PCS 24350 (b)

101411 Al block C2W3 26050 (b)

(a) Initiated in the end fitting.

^^^In the free length.

17
with the conical wedges pulled out of the end fitting at 32900 Ibf. On the

basis of these results it was decided to continue using the cruciform

wedges, which are available commercially at less cost than the conical

wedges, which had been manufactui -iu tiOuse

4. INDUSTRIAL SURVEY

The objectives of this task were twofold:

(1) To learn the fundamentals of the pultrusion process as

it is used in American industry for the manufacture of

FRP rod and rope materials, in order to identify

individual operations which may need improvement.

(2) To identify some pultrusion manufacturers with 'both the

capability and the interest in working with this laboratory toward the development of an improved pultrusion

manufacturing process for FRP rod or rope.

A notice was placed in the Commerce Business Daily [6] inviting

interested manufacturers to respond. Fourteen responses were received.

On the basis of these responses plus prior contacts, thirteen firms were

surveyed, nine by means of on-site visits and the remainder by correspondence and telephone discussions. The on-site visits involved inspections of

the pultrusion facilities plus discussions of possible means for improving

them.

At the time this survey was carried out, it was found that there were

no firms with the capability for manufacturing 1/2-in FRP rope in production

quantities*. It was decided, therefore, to pursue this program in terms of

FRP rod. Seven of the thirteen firms were identified as having the capability

to manufacture 1/2-in FRP rod in production quantities.

4.1 The Pultrusion Process

The pultrusion process for FRP rod has been described in the literature

[7, 8, 9]. For completeness, the basic features of this process are reviewed

here. Figure 14 is a schematic of the process which illustrates several

features that are common to most manufacturers. It should be noted, however,

that no two manufacturers were surveyed whose processes were identical.

On the left of the figure, the glass rovings are drawn from a creel in

which are mounted dozens of bales or spools of roving. The rovings are guided down into the resin tank where the individual fibers are wetted with

the matrix resin. Following this, the impregnated mass is drawn through one

or more guides and bushings which give the material its desired cylindrical

shape. Finally, the rod is cured in a heated die system. On the right of

the figure is the pulling mechanism which gives the process its name and

which supplies the considerable force that is required. The final product

is either cut to desired lengths or coiled, and allowed to cool.

*This is no longer the case.

18
5. OPTIMIZATION OF THE MANUFACTURING PROCESS

5.1 Evaluation of Product Characteristics

A series of laboratory tests and inspection procedures was devised in

order to evaluate certain important physical characteristics of FRP rod

products. Some of these tests and procedures are applicable to available

products, and were used to identify those characteristics which appear to

be deficient and which could, potentially, be upgraded by appropriate

modifications of the manufacturing process. Some of these tests and

procedures are primarily applicable to the experimental FRP materials which

are to be fabricated in this program, and are intended to indicate the degree

of improvement, in product characteristics, which is achieved through

various process modifications. The tests and procedures are outlined below.

5.1.1 Dimensional Uniformity . Measurements of the average diameter, the

length, and the weight of several sections of each FRP rod material are

made. The densities [10] and the weights per unit length are calculated.

These determinations provide an indication of the uniformity of the product.

5.1.2 Resin Content . The determination of the resin content of FRP rods

is maae by means of a burn-out test similar in principle to the ASTM

ignition loss test [11]. This test, which is intended for use on the experimental FRP materials, involves the use of a continuous-reading scale which is

enclosed in a muffle furnace. The furnace temperature is set at a value at

which the organic resin matrix material is completely decomposed to volatiles

The difference between the original weight of a sample and the minimum weight

achieved during heating is considered to be the resin content.

5.1.3 Fiber Distribution . Microscopic examinations of cross sections of an FRP rod provide a simple means for assessing the uniformity of the distribution of fibers in the rod. By using an image analyzing computer, a measure

ment of the fiber volume-fraction as a function of radial distance from the

center may be obtained.

5.1.4 Fiber Straightness and Collimation . Careful examination of the surfac

of an uncoated FRP rod occasionally permits the paths of individual fibers to

be traced for several inches or feet, before the fibers descend into the

interior of the rod. In the fabrication of experimental rod materials it is

intended to add a colored glass tracer strand to the reinforcing rovings on certain runs. By sectioning the rod product at discrete intervals it should

be possible to follow the paths of these tracers reasonably well.

5.1.5 Fiber Wetting . When an FRP rod is properly tested in tension, such

that failure initiates in the free length of the rod, the fracture generally

causes all of the fibers to broom out in a rather explosive fashion. Examination of these fibers can sometimes reveal where and whether they had been

adequately wetted by the resin.

19
A more quantitative indication of the. degree of fiber wetting can,

perhaps, be made by measuring the corona inception voltage on rod samples

which have been saturated with water. This approach is based on the premise

that absorbed water will migrate tc fiVc : 'lesiu interfaces which are not

thoroughly adhered. The following test procedure has been suggested [12]

for some of the experimental FRP rod materials:

(1) Cut a 1/2-in long specimen from 1/2-in diameter

rod, with the ends smooth and parallel.

(2) Boil the specimen in distilled water for 2 h.

(3) Measure the electrical breakdown voltage across

the parallel faces within 15 min after removal

from the water, as follows:

(a) Mount the specimen between 1-in diameter

, . electrodes in an oil bath.

(b) Raise the voltage across the specimen

from 0 to 15 kV at a rate of 0.5 kV/sec.

(c) Hold the voltage at 15 kV for 1 min. If

failure occurs within this interval, record

the failure voltage as 15 kV.

, -^.i. . (d) Increase the voltage further at a rate of

0.5 kV/sec to breakdown and record the

breakdown voltage. (The maximum voltage

from the available equipment is 50 kV.-)

5.1.6 Degree of Cure . Rockwell hardness measurements are made on FRP

materials as an indication of the degree of cure of the resin. For this

purpose Procedure A of ASTM Designation D785-65 [13] is used. Samples are removed from several locations along the length of a rod. A minimum

sample length of 2 in has been established to enable multiple measurements

to be made on the same sample. Not less than five measurements are made

on the cylindrical surface of each sample. Three of these are made on a single generator line. The other two measurements are made on a second

generator line, 90° from the first line, at points intermediate to the

first three test sites.

5.1.7 Transverse Tensile Strength . A measure of the tensile strength of

the resin matrix material in an FRP rod is obtained by means of a diametral

compression test. In this well-known test method for brittle materials [14], a cylindrical disk-shaped specimen is compressed diametrically between two

flat platens. Under the proper conditions [15] the test culminates in a

20
tensile fracture along the loaded diameter, and the transverse tensile

strength a of the rod is calculated from the relation

2P

a =

TTDt

where P is the compressive load at fracture, D is the diameter of the disk

and t is its thickness. The transverse tensile strength of the rod is

considered to be indicative of the tensile strength of the resin matrix

material assuming, of course, that the influence of the glass fibers on the

transverse strength is not significant.

Disk specimens, approximately 1/4-in thick, are sliced from 1/2-in

diameter FRP rods using a water-cooled, diamond cutoff saw. Care is taken

during this cutting process to prevent fraying of the edges. The coolant

is carefully blotted from the disks which are then allowed to dry, under

ambient laboratory conditions, for at least 24 h prior to testing. The

diameter and thickness of each specimen is measured, and the specimen is

mounted, on edge, in a compression subpress [16] which is then loaded to

fracture of the disk in a universal testing machine. A fractured specimen

in the subpress is shown in Figure 15.

5.2 Deficiencies in Existing Products

Some of the tests and procedures described above were applied to

commercially available FRP rod products obtained from five different manufacturers. The following list of product deficiencies were found to be

present, in varying degrees, in all of the products.

1. Fibers are not uniformly distributed throughout the cross- section of the rod.

2. Fibers are not thoroughly wetted by the resin.

3. Microscopic voids exist in the matrix.

4. Fibers are not straight.

5. Fibers are not axially oriented.

6. Resin is incompletely cured.

These deficiencies are discussed below together with proposed modifications

to the manufacturing process which may help to overcome these deficiencies.

5.2.1 Deficiency No. 1 . Photomicrographs, such as that shown in Figure 16,

clearly reveal that the fibers are not uniformly distributed throughout the cross section of a rod. Instead, they seem to be bunched together in

clusters with resin-rich areas between them. It nay be speculated that each

cluster corresponds to the ends of a single roving. If this is, indeed, the case then it might be possible to achieve a more uniform distribution by

21
puitruding a larger number of rovings having a lower end count (i.e., fewer

strands ppr roving) or, perhaps, by using fibers of larger diameter.

5.2.2 Deficiency No. 2 . Examination of the fibers in a rod which had been

fractured in tension reveals that many fibers do not appear to have been

wetted by the resin. Numerous modifications to the manufacturing process

may be proposed which offer promise of facilitating wetting. For example:

a. Use a better coupling agent on the fibers.

b. Use untwisted rovings. (The rovings, as they are drawn out

of bales or off of spools, usually have a certain amount of

twist which inhibits the flow of resin to the individual

fibers.)

c. Lengthen the resin tank or reduce the pulling speed to

provide more wetting time.

d. Preheat the rovings before they enter the resin tank, to

drive off surface moisture.

e. Agitate the resin tank and/or the rovings passing through

the tank.

f . Add a wetting agent to the resin system.

5.2.3 Deficiency No. 3 . Photomicrographs made under high magnification

frequently show apparent voids adjacent to individual fibers. This tends

to confirm the above observations regarding incomplete wetting. However,

apparent microscopic voids are also occasionally observed in the matrix

at locations apart from fibers. It is speculated that these voids may

be created by volatilization of one or more components of the resin system

under the heat of the curing operation. A re^iedy for this deficiency would

probably require an additive or other modification to the resin system.

5.2.4 Deficiency No. 4 . Examinations of fibers on the surfaces of FRP

rods reveals that the fibers are not straight but follow a rather wavy

path. This condition is obviously not conducive to achieving a uniform

distribution of load among the fibers in a tensile application. This may

be another manifestation of the fact that the rovings are twisted initially.

It is also believed that this deficiency could be remedied, to some extent,

by pretensioning the rovings before they go into the shaping and curing dies.

5.2.5 Deficiency No. 5 . This deficiency is related to the previous one. Examination of surface characteristics, particularly on broken rods, reveals

that the individual fibers are not alined with the direction of the rod.

Deviations of 10 to 20° are not uncommon. In this situation rods are clearly

unable to demonstrate maximum tensile strength. It is speculated that this

condition could be improved by providing better collimation for the rovings.

That is, it is believed that better alinement could be achieved if the

rovings were guided in a straight or conical path from the creel to the dies,

rather than forcing them to travel down into the resin tank and then up

again to the dies.

22
5.2.6 Deficiency No. 6 . Hardness measurements on FRP rods have been made,

both before and after post-curing operations. In all cases the post cure produced an increase in hardness. This is taken to mean that the rods, in

the as-received condition, were not fully cured. A fully cured rod should

exhibit no significant increase in hardness during post curing. Perhaps the

manufacturing process for FRP rods should include a post-curing operation

prior to shipment.

5.3 Manufacture of Experimental Materials

A Request for Proposals (RFP) was prepared for the procurement of

experimental FRP rod materials, pultruded with improved manufacturing

processes. The number of potentially beneficial modifications to existing

manufacturing processes, which are mentioned above, is greater than that

whicn could properly be investigated within the resources available to this

program. Nevertheless, a discussion of all of these modifications was

included in the RFP, which invited potential subcontractors to exercise

ingenuity in proposing improved manufacturing processes for the pultrusion

of the experimental materials.

On the basis of the Industrial Survey, the RFP was issued to four

firms which were considered to have both the capability and the interest

to work with this laboratory toward the improvement of the manufacturing

processes for 1/2-in FRP rod. Additional proposals were solicited via an announcement in the Commerce Business Daily [17]. Proposals were received

from three firms and the subcontract was awarded to one of these.

The subcontract calls for the manufacture of fourteen runs of 1/2-in

FRP rod using E-glass roving (Table 7) and a polyester resin matrix. The

polyester (Table 8) is the isophthalic equivalent of an orthophthalic resin

frequently used by the subcontractor. The processing conditions for the

first eleven runs were designed, by a statistical techique, to facilitate

evaluation of the effects of nine different processing parameters. The

processing conditions for Runs 1 through 11 are given in Table 9. The

processing conditions for Runs 12, 13 and 14 will be selected at a later

date. A colored tracer strand will be incorporated into most of the runs

to ease the problem of determining fiber straightness and alinement.

5.4 Mechanical Testing Equipment

The mechanical properties of the experimental materials will be

evaluated by means of room temperature tensile tests, at both normal and

high loading rates, and by stress-rupture tests, at 160 and 200°F, under

saturated humidity conditions.

The tensile tests at normal rates will be performed in a 100000-lbf

capacity, horizontal, hydraulic, universal testing machine [20]. A

23
(a) Table 7. — Characteristics of Glass Fiber Rovings

Type: E~glass , continuous.

Fiber diameter: K (0.00052 in, nominal).

Fibers per strand: 400, nominal.

Roving yield:

(h

End count Nominal yield

yd/lb

. . 30 123±6

60 61.5±3.1

Type of binder: Silane (proprietary formulation).

Percent of binder: 0.55±0.15, by weight of glass.

Tensile strength^'^^ : > 200,000 Ibf/in^

(a) According to the manufacturer.

Number of strands per roving.

^^-^Per ASTM Designation D-2343 [18].

24
Table 8. — Characteristics of Isophthalic Polyester Resin

Monomer

Vis cosity

Specific gravity:

Reactivity

Cure data^^^

Gel time

Time to peak:

Peak temperature;

Styrene, approximately 30 percent.

2'3-27 poises at 77°F.

1.12 - 1.13 at 77°F.

Medium

3-4 minutes.

4.5 - 5.5 minutes.

390-410°F.

(a) According to the manufacturer, for matched-metal-die molding applications

^^^Per SPI procedure [19].

25
CO !-l O 3

PM CJ

O

2i O2; o o o o3

O3

O

123 O 0) 3

O

•H

4-) COB

o

J5 O o o o o o o

CO O (1) o

'iZ >i

o

•H d

C O

O -H

CD

CO CO

V-l -U

4J iH bC

CO

o O

J3

CD cn

01 o 0) S PH o

CO

0) o

cn

QJ o o

O CO 5 a

C CO CU 0)

bO 00 00 00 C c

•u u o o o u o u

CO CO ,-1 cy3 1-J CO CO CO CO

oao

PS-- •H *->

4-> fl U OJ

CU 00 :s CO

o o

la

CO cn 0) CU o

>^ >-i 2: o3

cn

OJ

cn0) o o o

o

OJ cn

OJ o o o

cn cn cn cn

CU (U (U CU o o o o

>H >-< 2;

4J

o o o o o o o o o o o O

00

0) Ci U -H

CO >

•H O

cn cn CO CO cn cn CO cn

0) (U CU 0) o O O (U (U (U

>-• 3 J3 2; >-' >^

CO

(U

(U

00

CJ •H>

o

CO CO CD cn cn cn CD CD CD o a; CU CU CU (U <u (U 0) CU 2; >^ tH >-< >^ >> >^ tH

3 O 00 CO

26
crosshead speed of 0.75 in/min will be used. The tensile tests at high

rates will be performed in a programmable, servo-controlled, electrohydraulic, dynamic testing machine [20]. The peripheral apparatus needed

to carry out the latter tests has not yet been assembled.

The stress-rupture tests will be performed in six creep-testing

machines that were designed and are being assembled for this task (Fig. 17).

Each of these machines has a load capacity of 30000 Ibf, which is applied

by dead weights acting through a 100:1 compound lever system. Specimens

up to 9 ft long, including the end fittings, can be accommodated. The

overall height and length of each machine are approximately 7 ft and 12 ft,

respectively.

Each creep- testing machine is equipped with an environmental test

chamber for providing the required temperature and humidity conditions. The

chambers are cylindrical in shape, consisting primarily of a 10-in diameter

asbestos-cement pipe, 4 ft long, with transparent plastic windows at the

ends. Each chamber contains a brass tube which extends nearly the full

length of the chamber. The purpose of the tube is to provide more uniform

temperature distribution on the specimen, which passes concentrically

through the tube. The ends of the specimen, including the end fittings and

the pull rods, remain outside of the chamber.

Heat is supplied to each chamber by twelve quartz-tube, infra-red lamps

which surround the brass tube. Each lamp has a capacity of 500W at 120V.

The temperature distribution in the chamber is adjusted by regulating the

power to the lamps. The test temperature is maintained with an electronic

temperature controller.

Each chamber is furnished with an air-powered mist generator to provide

the required saturated humidity conditions. The rate of water consumption

is adjusted by regulating the air flow rate to maintain a light film of

condensation on the windows of the chamber.

5.5 Preliminary Tests and Results

To establish suitable values for the initial processing conditions for

the experimental FRP rods, the subcontractor manufactured and furnished a preliminary run of 6-ft long, 1/2-in diameter FRP rods, designated Run 0,

The processing conditions for these rods are given in Table 10. The Shore D

Durometer hardness for all of these rods was reported, by the subcontractor,

to be in the range 91.0 to 94.5 and the glass content was estimated at 71.9

percent by weight.

Most of the rods were carefully weighed and measured in this laboratory

and the densities were calculated. These calculations showed that the density

was not significantly affected by the three processing parameters that were

27
Table 10. — Processing Conditions for Rxm 0.

No. of rovings : 22, 60-end twisted.

Preheat: 255-260°F.

Ambient rh : 60-63 percent.

S;.ecimen

No. D • (a) Resm ' Pulling)

speed

in/min

Die

temperature

°F

Post. V Cure

Colored

tracer

OlA I 9 325 Yes No

OIB I 9 325 Yes No

OIC I 9 325 No No

OlD I 9 325 No No

02A I 12 350 Yes No

02B I 12 350 Yes No

02C I 12 350 No No

. 02D I 12 350 No No

03A 0 9 340 Yes Yes

03B 0 9 340 Yes Yes

03C 0 9 340 No Yes

04A 0 12 365 Yes Yes

04B 0 12 365 Yes Yes

04C 0 12 365 No Yes

04D 0 12 365 No Yes

Medium reactivity polyester. I, isophthalic; 0, orthophthalic.

Indicative of wetting time.

16 h at 275°F.

28
varied, i.e., type of polyester resin, pulling speed, and post cure. The

density of all of the rods was 0.070 Ib/in^, within one percent. This is

comparable with other, commercially available, FRP rod products [2].

The surface hardness and the transverse tensile strength were also

measured on most of the rods. The results of these measurements are given

in Table 11. Although these data are sparse and incomplete they provide

some indication of the effects of the process parameters. Changing from an orthophthalic to an isophthalic polyester resin produces an increase of

roughly 25 percent in the hardness but, surprisingly, an equivalent decrease

in the transverse tensile strength. Raising the pulling speed from 9 to

12 in/min also increases the hardness and decreases the transverse tensile

strength, but the magnitude of these changes is small and probably not

significant. Adding a post cure produces the most significant change, an increase of approximately 35 percent in both the hardness and the transverse

tensile strength.

It is, perhaps, noteworthy that the transverse tensile strengths of

these rods are considerably less than those of other, commercially available,

FRP rod products. The transverse tensile strengths of three other pultruded

rod products were found to be in the range 2200 to 3300 Ibf/in^.

A series of tensile tests was performed to determine which of the end

fittings would perform best on this material. These tests were carried out

on several of the rods from Run 0, plus one additional rod (designated 00)

which had been selected at random from the subcontractor's stockpile. The

results of these tests are given in Table 12. Comparing the results of the

first three tests listed with those of the last five shows that the H3M end

fitting is superior to the Mod 4 end fitting for this material, and confirms,

once again, that the C2W3 potting compound performs better than the PCS

potting compound in this application. Of the five specimens tested with the

H3M end fitting and C2W3 potting compound, none pulled out of the fitting,

three failed in the free length and two failed in tension in the end fitting

at loads which appear to be very close to the full tensile strengths of the

materials.

While not statistically significant, the results of the last five tests,

in comparison with Table 10, also suggest that neither pulling speed nor post

curing affects tensile strength to any meaningful degree, and that a specimen

(03A) manufactured with the orthophthalic resin has slightly less tensile

strength than those manufactured with the isophthalic resin.

Since the end fittings in stress-rupture tests need not be capable of

withstanding the full tensile strength of the material, four short-time

stress-rupture tests were performed to evaluate the suitability of commercially available end fittings for this work. These tests were carried out

under saturated humidity conditions. The specimens were maintained at the

test temperature and humidity for one hour prior to the application of load.

29
Table 11. — Physical Properties of

Specimen Average

No« diameter Density

in Ib/in^

u.uoy

0.0698

02A 0.5019 0.0696

02B , C.5016 0.0696

02C ^ •

. 0.5006 0.0699

02P 0.5020 0.0695

03A 0.4983 0.0705

0'3B

' 0.5010 : 0.0698

03C 0.4979 0.0707

04A 0.5018 0.0695

04B 0.5002 0.0700

04C 0.5008 0.0699

04D 0.49 78 0.0707

0 Specimens

Transverse

Rockwell tensile

Weight hardness strength

lb/ft Rj, Ibf/in^

0.165 62 1375

0.165 46 1320

0.165 " — 1500

0.165 61 1470

0.165 — 1020

0.165 50 1450

0.165 — 1800

0.165 — 1870

0.165 31 1630

0.165 54 1920

0.165 ~

0.165 56 1780

0.165 33 1540

30
Table 12. — Results of Preliminary Tensile Tests

Specimen

No.

End

fitting

Potting

compound

Free

length

Maximum

load

Ibf

Failure

00

OlA

03B

Mod 4

Mod 4

H3M

C2W3

C2W3

PC8

22

59

57

19100

22450

21850

(a)

(a)

(a)

OIB

OlD

02B

02D

03A

H3M

H3M

H3M

H3M

H3M

C2W3

C2W3

C2W3

C2W3

C2W3

57

57

58

51

58

28100

28250

28050

28150

27200

(b)

(c)

(c)

(b)

(b)

(a)

(b)

Pullout.

Failure in free length.

(c) Failure in end fitting,

31
The test results are given in Table 13- These results indicate that the

Type R/P end fitting with the C2W3 potting compound is adequate for this

application. The results also suggest that the rods fabricated with the

isophthalic resin are more resistant to stress rupture than the rod (04C)

fabricated with the orthophthalic resin, and that the stress-rupture

strength is greater at 160°F than at 200°F.

.'^ 6. PLANS

The following summarizes the projected activities during the second

year of effort on each of the three principal phases of this program:

Phase I - Experimental FRP rod materials, pultruded in accordance with

specified modified manufacturing processes, will be procured and laboratory

tested. Recommendations for an improved manufacturing practice for FRP rod

will be formulated.

Phase II - Experimental FRP rod materials, fabricated with one or more potentially superior fiber-reinforcing materials, will be procured and

laboratory tested. The degree of improvement which is achieved, relative

to glass-reinforced rod materials, will be assessed.

Phase III - Plans will be made for the procurement of experimental FRP

rods, fabricated with two or three potentially superior polymeric matrix

materials . ......... ******

The authors gratefully acknowledge the valuable contributions of Mrs. Ruth

M. Woolley, Mr. Oscar 0. Owens and Mr. Raymond G. Russell of the Engineering

Mechanics Section, NBS, and of Dr. Fred A. Yeoman, Mr. John S. Hudock and

Mrs. Donna K. Nowakowski of the Westinghouse Electric Corporation.

32
Table 13. — Results of Preliminary Stress-Rupture Tests

Specimen End ... Test Tensile Time to

No. fitting temperature load rupture Failure

°F Ibf h

OIC R/P 200 13000 0.27 (c)

04C R/P 200 12000 0.03 (c)

02C R/P 200 11000 1.6 (c)

02A R/V 160 11000 4,2 (d)

(a) All tests performed under saturated htunidity conditions.

^^^Type R/P end fittings used with C2W3 potting compound and cruciform wedges,

Type R/V end fitting is a mechanical, wedge- type, compression fitting.

(c) Failure in free length, in the test chamber,

Failure in the end fitting.

33
7 . REFERENCES

1. Halsey, N., Properties of Glass Fiber-Reiniorced 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 GuyInsulator Systems of Loran C Towers, NBS Report 10 261 (June 1970).

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

Rope Materials and Associated End Fittings, NBSIR 73-129 (Dec. 1972).

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

Fittings for FRP Rod and Rope, presented at ASCE Specialty Conference

on Composite Materials, Pittsburgh, Nov. 1972 (to be published).

4. 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).

5. Mitchell, R. A., Woolley, R. M. and Chwirut, D. J., Composite-Overlay

Reinforcement of Cutouts and Cracks in Metal Sheet, NBSIR 73-201

(Feb. 1973).

6. Anon., Research and Development Sources Sought, Optimization of FiberReinforced Plastic Rod and Rope, Commerce Business Daily, Nov. 2, 1971,

p 13 (Superintendent of Documents, Washington, D. C).

7. Goldsworthy, W. B. , Continuous Production Methods, Chap. 19, Handbook

of Fiberglass and Advanced Plastics Composites , ed. G. Lubin, pp 485-

493 (Van Nostrand Reinhold Co., N. Y., 1969).

8. Morrison, R. S., Pultrusion, Modern Plastics Encyclopedia 1972-1973 , ed. S. Gross, pp 642-644 (McGraw-Hill, N. Y., 1972).

9. Hardesty, E. E., Advanced Composites, the On-Going Transition from

Handmade Prototypes to Machine-Fabricated Components, Proc. SPI

Reinforced Plastics/Composites Institute , Washington, Feb. 1973,

28th Annual Conference, Section 15-D (Society of the Plastics Industry,

N. Y., 1973).

10. Anon., Specific Gravity from Weight and Volume Measurements, Method

5012, Plastics: Methods of Testing , Fed. Test Method Std. No. 406

(General Services Administration, Washington, Oct. 5, 1971).

11. Anon., Standard Method of Test for Ignition Loss of Cured Reinforced

Resins, ASTM Designation D 2584-68, 1972 Annual Book of ASTM Standards , Part 26 (Am Soc Test Mat, Philadelphia, 1972).

12. Hudock, J. A., private communication.

34
13. Anon., Standard Method of Test for Rockwell Hardness of Plastics

and Electrical Insulating Materials, ASTM Designation D 785-65,

1972 Annual Book of ASTM Standards , Part 27, loc. cit .

14. Anon., Standard Method of Test for Splitting Tensile Strength of

Cylindrical Concrete Specimens, ASTM Designation C 496-71, 1972

Annual Book of ASTM Standards , Part 10, loc. cit .

15. Mordfin, L. and Kerper, M. J., Strength Testing of Ceramics - A

Survey, Mechanical and Thermal Properties of Ceramics , ed. J. B.

Wachtman, Jr., NBS Spec Publ 303, pp 243-262 (1969).

16. Aitchison, C, S. and Miller, J. A., A Subpress for Compressive Tests,

NACA TN 912 (1943).

17. Anon., Development of Improved Manufacturing Techniques for Pultruded

Glass-Reinforced Plastic Rod, Commerce Business Daily, May 8, 1972,

p 1, loc. cit .

18. Anon., Standard Method of Test for Tensile Properties of Glass Fiber

Strands, Yarns, and Rovings Used in Reinforced Plastics, ASTM

Designation D 2343-67, 1972 Annual Book of ASTM Standards , Part 26,

loc. cit .

19. Anon., Suggested SPI Procedure - Procedure for Running Exotherm

Curves Using The Block Test Method, Proc. SPI Reinforced Plastics/

Composites Institute , Washington, Feb. 1969, 24th Annual Conference,

loc. cit . (1969).

20. Chwirut, D. J. (coordinator). Research and Testing Facilities of the

Engineering Mechanics Section, National Bureau of Standards,

Washington, D. C. , NBS Spec Publ 370 (1973).

35
36
37
38
39

Figure 6 - Mod 3 experimental end fitting. Overall length, 12 in.

0 4 8 12

DISTANCE FROM INNER END , IN .

Figure 9 - Longitudinal normal strain on the outer surface

of a Mod 4 end fitting, high tension loads.

Points are experimental; curves are analytical.

43
• (U

/"-^ QJ •U

y-i 15

cu

B

rH

u •

y o

(1) •H bO o

<u

13 o 1—1

CO

CJ 0) •H

=J

O rH

4-1

0)

bO

•H

Pt4

00

•Hu

•H

M-l T3C

0)

P-i

0)>. HI O

0)

00

•H

44
Figure 12 - Aluminum block experimental end fittings.

45

LU

LUU

47
Figure 15 - Fractured disk specimen of FRP rod in subpress after

diametral compression test.
XiaW31S[A

lBno±:JBxn93nI srfj

nsD is 3mA s

1 -o

ure 17 - One of six high-capacity creep testing machines for

stress-rupture testing of long, FRP rod specimens

under elevated temperature and saturated humidity

conditions

49
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:

Quantity

Force

Length

Mass (weight)

Plane angle

Temperature

Density

Roving yield

Speed

Stress

Viscosity,

dynamic

Weight per unit

length

U. S. Customary

Unit

pound-force

(Ibf)

inch (in)

foot (ft)

yard (yd)

pound (lb)

pound (lb)

degree (°)

degree Fahrenheit (°F)

Ib/in^

yd/lb

in/min

Ibf/in^

poise (P)

lb /ft

SI Unit

newt on (N)

centimeter (cm)

meter (m)

meter (m)

gram (g)

kilogram (kg)

radian (rad)

degree Celsius

(°C)

g/cm^

m/kg

m/s

N/m^

N's/m^ or , . Pa-s^^^

kg/m

Conversion Factor

1 Ihf = 4.448 N

1 in = 2.54 cm

1 ft = 0.3048 m

1 yd = 0.9144 m

1 lb = 453.6 g

1 lb = 0.4536 kg

1° = 0.01745 rad

°C = (5/9)(°F-32)

1 Ib/in^ = 27.68 g/cm^

1 yd/lb = 2.016 m/kg

1 in/min = 4.233_x

10 V/s

1 Ibf/in^ = 6895 N/m^

1 P = 0.1 N-s/m^

1 lb/ft = 1.488 kg/m

(a)

1 pascal (Pa) = 1 N/m^

50
U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. 2. Gov't Accession

No.

3. Recipient's Accession No.

i. Tn Lb AXD SUH'l l'l LE

IN^JIL 1 »w Ud J. J_ JU<-. iT.il LtillLLcl iJ Lip U U X L I Ld LtdL XcdXo • First Annual Progress Report

5, Publicatioti Date

6. Performing Organization Code

7. AL'TllOrvCS) D.E. Marlowe, N. Halsey, R.A. Mitchell and L. Mordfin

8. Performing Organ i;:ati'jn

9. PERl'ORMLVG ORG AN'IZATION N.A.ME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/ Work Unit No. 2130441

11. Contract/Grant No. D.O. F33615-72-M-5000

12. Sponsoring Organization Name and Address Air Force Materials Laboratory

Wright-Patterson Air Force Base, Ohio 45433

13. Type of Report & Period Covered Tnterim

10/16/71 - 10/15/72

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.) A program was initiated to develop a non-conductive FRP rod material for use as guy

lines and catenaries in large antenna systems. It is intended that the longterm

structural properties of the new material, under adverse weathering conditions, be

superior to those exhibited by the GRP rod and rope materials which are available at the

present time. Tx-7o" new end fittings for FPvP rod were developed which out-perform

commercially available fittings. These two fittings, which enable the tensile strengths

of the rod materials to be accurately determined, include a shear-type fitting and a compression- type fitting. Deficiencies in the physical characteristics of existing GRP

rod materials xjere identified, and modifications to the manufacturing processes were

proposed to overcome these deficiencies. A subcontract was let for the manufacture

of experimental GRP rods using the modified processes. A bank of six high-capacity

creep testing machines was designed and is being assembled. These machines, xjhich will

accomodate long specimen lengths, are equipped with environmental chambers to permit

stress-rupture testing under elevated temperature and saturated humidity conditions.

1 7. KEY WOliDS (-Alphabetical order, separated by semicolons) Composite materials; end fittings for FRP

rod; environmental resistance of GRP rod; f iber-reinforced-plastic rod; glass-reinforced

plastic rod; guys, antenna; mechanical properties of GRP rod; pultrusion; reinforced

. n las tic rod: st -

' ^ n '^"^"^

i. astic rod: stroGo rupture of GRI' rod-: 3-. A\ A1LAI3ILITY STATEMENT ^ ^ ^

i-

[ic] UNLIMITED. Release to NTIS

r'J FOR OFFICIAL DISTRIDUl ION. DO NOT RELEASE

TO NTIS.

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20. SECURITY CLASS

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