NBSIR 76-1069

Environmental Effects on the

Strength of A Glass FiberReinforced- Plastic Rod

Material

Nixon Halsey and Leonard Mordfin

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

Final Report

March 1976

Issued July 1976

Prepared for United States Information Agency

Washington, D. C. 20547

NBSIR 76-1069

ENVIRONMENTAL EFFECTS ON THE

STRENGTH OF A GLASS FIBERREINFORCED-PLASTIC ROD

MATERIAL

Nixon Halsey and Leonard Mordfin

Engineering Mechanics Section

Meclianics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

Final Report

March 1976

Issued July 1976

Prepared for United States Information Agency

Washington, D. C. 20547

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

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistarft Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

CONTENTS

Page

1. INTRODUCTION 1

2. MATERIALS AND FITTINGS 3

2.1 Specimen Material 3

2.2 End Fittings 5 2.3 Test Specimens 10

3. PRELIMINARY TESTS AND MEASUREMENTS 10

3.1 Dimensional Measurements 10

3.2 Tensile Tests 12

3.3 Diametral Compression Tests 14

3.4 Hardness Measurements 15

4. environment:AL STRESS -RUPTURE TESTS 15

4.1 Testing Equipment 15

4.2 Tests and Results 19

5. RESIDUAL STRENGTH TESTS 23

6. DISCUSSION 25

7. CONCLUSIONS 27

8. REFERENCES 28

APPENDIX A. Ultraviolet Radiation Calculations 30

APPENDIX B. Failure Analysis of a Tower Guy 31

i
LIST OF TABLES

Page

Table 1 - Rated Properties of Rod Material 4

Table 2 - Rated Properties of 1/2-in (13-inm) Diameter Rods 5 Table 3 - Tensile Tests of Rod Specimens 13

Table 4 - Diametral Compression Tests of Rod Specimens 16

Table 5 - Environmental Stress-Rupture Tests of Rod Specimens 21-22

Table 6 - Residual Strength Tests of Rod Specimens 24

LIST OF FIGURES

Figure 1 - Commercially Available, Mechanical, Compression 7 Type End Fitting.

Figure 2 - Commercially Available, Potted, Compression-Type 7 End Fitting.

Figure 3 - NBS Aluminum-Block End Fitting. 8

Figure 4 - NBS Type H3M End Fitting. 8

Figure 5 - Proprietary, Potted, Compression-Type End Fitting. 9

Figure 6 - Commercially Available Cruciform Wedge for 11

Potted, Compress ion- Type End Fittings.

Figure 7 - NBS High-Capacity Creep-Testing Machines Equipped 18

for Environmental Stress-Rupture Testing.

Figure 8 - Failed Section of Glass Fiber-Reinf orced-Plastic 32

Tower Guy.

li
ENVIRONMENTAL EFFECTS ON THE STRENGTH OF

A GLASS FIBER-REINFORCED-FLASTIC ROD MATERIAL

by

Nixon Halsey and Leonard Mordfin

ABSTRACT

Environmental stress-rupture test methods were

developed for studying the weatherability of materials

under stressed exposure to marine atmospheres and intense sunshine. These laboratory test methods were

applied to a coated, pultruded, glass f iber-reinforcedplastic rod product, with the marine atmosphere simulated

by a saltwater fog and intense sunshine simulated by

ultraviolet radiation. Static tests of the material,

conducted both before and after the environmental ex- posures, were used to characterize both the as-received

mechanical properties of the product and the reductions

in tensile strength attributable to the weathering actions. The experimental results essentially confirmed

the rod manufacturer's contention that the effects of

sunlight and weathering on the product range from none

to slight. For applications as antenna guys, the re- sults suggest that the strength limitations imposed on rods by conventional end fittings, and by the fact that

rods are frequently bowed rather than straight, tend to

overshadow any strength reductions attributable to en- vironments of the types considered here.

A failure analysis of a tower guy, fabricated from

material of the type tested in this investigation, is

given in an appendix.

Key Words: Antenna-support materials; environmental

degradation; guys, nonmetallic; marine atmospheres;

plastics, fiber-reinforced; pultrusions; stress-rupture

testing; sunlight; ultraviolet radiation; weatherability.

1. INTRODUCTION

A factor which has limited the use of plastics in outdoor applications is their susceptibility to degradation under weathering conditions.

One of the principal objectives of polymer research, therefore, has been

the development of plastics with superior resistance to thermal, hydrolytic

and photolytic degradation. Evaluations of the anticipated weatherability

of plastics are most commonly accomplished through the use of accelerated

laboratory tests which expose the materials to elevated temperature, high

humidity, simulated rain, ultraviolet radiation, salt spray, etc. While
it is recognized that a direct correlation does not exist between these

intensified environments and true weathering conditions, experienced

researchers have nonetheless been able to make reasonable predictions

from the test results, particularly for comparative purposes. This approach, together with data derived from real-time outdoor weathering

tests, has been adequate, for the most part, for the design of acceptable

plastic components for those outdoor applications for which plastics have

traditionally been used. Examples of such applications include patio

furniture, rainwear and footwear, glazing, certain items of playground

equipment, tarpaulins and pond liners, etc. In none of these applications is the plastic a structural member and the mechanical stresses to

which it is subjected are either low or infrequent.

The rapid development of continuous-fiber-reinforced plastics has

changed this picture. Plastics, heavily reinforced with oriented glass

fibers, exhibit mechanical properties which make them attractive for

structural, load-bearing members. Pultrusions, in particular, offer

strength/weight characteristics that make them directly competitive with

aluminum and steel extrusions and rolled sections for applications requiring unidirectional strength. Guys and guy insulators for antenna masts

and towers are noteworthy examples, where the dielectric properties of

glass-reinforced plastic provide an additional advantage over steel.

Quite understandably, most laboratory test methods which have been

developed over the years for studying the weatherability of plastics (see,

for example, ASTM Designations D756, D1499, D1501 and D2565 [1]*) are inadequate for structural plastics because they make no provision for applying mechanical stresses to the specimens while they are exposed to the

simulated environments. It is logical to expect that the weatherability

of reinforced plastics would be strongly influenced by the presence of

substantial stresses.

In response to this deficiency, the National Bureau of Standards

adopted an environmental stress-rupture test method as a means for studying

the accelerated weathering characteristics of reinforced plastic guys and

guy insulators. In this test method the specimens are subjected to constant, predetermined tensile loads while they are simultaneously exposed

to simulated, severe environments. Prior to the investigation described

herein, the environments examined in this way included elevated temperature and elevated temperature plus high humidity. The objective of the

present investigation was to examine the effects of a simulated marine

atmosphere and simulated sunshine on the long-term durability of a reinforced plastic guy material.

*Numerals in brackets refer to similarly numbered references listed

in Section 8 of this report.

2
This investigation was carried out in the Engineering Mechanics

Section of NBS under the sponsorship and with the financiaJ assistance

of the United States Information Agency. The authors gratefully acknowledge the assistance of Messrs. Raymond G. Russell and Oscar 0. Owens in

the conduct of the program. The authors also extend their thanks to the

Nupla Corporation, and to Mr. Elbert Davis in particular, for furnishing

two pairs of their proprietary high-strength end fittings for use in this

work.

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. It is recognized, however, that the American manufacturers and users of reinforced plastic guys use U.S. customary units almost ex- clusively at this time. For the convenience of this group, therefore,

U.S. customary units have been used in this report followed, in each

instance, by the SI equivalent in parentheses. Factors for converting

between the two systems may be found in the ASTM Metric Practice Guide

[2].

2. MATERIALS AND FITTINGS

2.1 Specimen Material

The reinforced plastic rod material selected for use in this investigation is a 1/2- in (13-mm) diameter pultruded rod product which is widely

used in antenna guying applications. According to the manufacturer, the

product has been in production for more than ten years, during which time

its composition remained essentially unchanged.

The rod consists of unidirectional, continuous E-glass (electrical

grade) roving impregnated with a resin binder. The rod is coated for re- sistance to ultraviolet radiation. The roving consists of fibers that are nominally 0.00052 in (0.013 mm) in diameter (Type K) . According to

tests performed for the manufacturer by an independent laboratory, the

roving has an average tensile strength of 206 000 Ibf/in^ (1420 MPa)

The resin binder is believed to be an orthophthalic polyester, based

on maleic anhydride and propylene glycol, and cross linked with styrene.

The coating, nominally 0.005 to 0.015 in (0.1 to 0.4 mm) thick, is an epoxy resin heavily loaded with titanium dioxide.

Some of the rated properties of the rod materials, and the 1/2-in

(13-mm) size in particular, are given in Tables 1 and 2 respectively.

3
(a) Table i - Rated Properties of Rod Material

2 Tensile strength >100,000 Ibf/in f>690 MPa)

Modulus of elasticity 6.34 x 10 ibf/in (43.7 GPa)

in tension

Specific gravity 1.85 to 2.05

Coefficient of 2.67 x iD^ in/in/°F (1.48 ym/m/°C)

thermal expansion

Maximum operating 250°F (120°C)

temperature (continuous)

Water absorption ' 0.02 percent by weight (mass)

in 24 h

Effect of sunlight none to slight

and weathering

According to the manufacturer

4
Table 2 - Rated Properties of 1/2-in (13-inm) Diameter Rods

Diametral tolerance +0.062, -0.000 in (+1.6, -0.0 mm)

Unit weight (mass) 0.17 lb/ft (0.25 kg/m)

Glass content 77±3 percent by weight (mass)

Minimum breaking strength:

with standard fittings 20,000 Ibf (89 kN)

with special fittings 28,000 Ibf (125 WO

Maximum elongation at 0.7 percent

6700 Ibf (30 kN)

According to the manufacturer

5
2.2 End Fittings

One of the most critical features affecting the tensile loadcarrying capability of a reinforced-plastic rod is the type of end

fitting which is affixed to it and through which the load is applied

to it. Five different types of end fittings were used in various parts

of this investigation.

The Type R/V end fitting (Fig. 1) is a basket-type mechanical end

fitting for reinforced plastic rod materials. 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. The Type R/V end fitting is available commercially. Its principal

advantage is the ease and rapidity with which it may be installed.

The Type R/P end fitting (Fig. 2) is a basket-type, potted, conical

compression fitting which is intended primarily for synthetic ropes. The

outward appearance of this fitting is quite similar to that of the Type

R/V end 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. The

Type R/P end fitting is available commercially.

The aluminum-block end fitting (Fig. 3) is a potted compression fitting that was developed at NBS [3] . This fitting is 2-5/8 x 3-1/4 x 6 in

(67 X 83 X 150 mm) long and is machined from 2024-T4 aluminum-alloy stock.

The block is split in half longitudinally and bolted together, and then a conical cavity is machined out. The cavity is 2-1/4 in (57 mm) in diameter at the large end, tapering to a short cylindrical section 5/8 in

(16 mm) in diameter.

The H3M end fitting (Fig. 4) is a more or less streamlined version

of the aluminum-block end fitting that was also developed at NBS [4,5] in

order to reduce the costs of machining and to facilitate its use in a testing machine. It consists of a single piece of 2024-T4 aluminum-alloy

round rod, 2-3/4 in (70 mm) in diameter and 6-1/2 in (165 mm) long. The

conical cavity is 2 in (51 mm) in diameter at the large end, tapering to a 1/2-in (13-mm) long cylindrical section that is 3/4 in (19 mm) in diameter .

The Type S end fitting (Fig. 5) is a proprietary industrial development. Like the Type R/P, the aluminum-block, and the H3M, it is a potted

compression end fitting. It is machined with an integral clevis for attachment with hardened steel pins.

6
Figure 1. Commercially available, mechanical, compression type end fitting.

(NBS designation: Type R/V.)
8
/V

9
2.3 Test Specimens

The test specimens were cut from long lengths of the rod material

that had been procured in three different batches. A few exploratory

tests were performed on specimens from a quantity of the rod material

(Batch 0) that had been available in the laboratory as surplus from a previous investigation. Subsequently, a 300-ft (91-m) length of the

rod material (Batch 1) was procured from the manufacturer in a continuous

7.7-ft (2.3-m) diameter coil. Finally, 324 ft (99 m) of the rod material

(Batch 2) were procured from the manufacturer in 18-ft (5.5-m) lengths.

Specimens for use in tensile or stress-rupture tests consisted of

lengths of the rod material with end fittings on the ends. The procedure

for mounting any of the four different kinds of potted compression end

fittings was essentially the same. The surface of the potting cavity in

the end fitting was first treated with an epoxy-release agent. The fitting was slipped over the end of the rod and a fluted, cruciform wedge

(Fig. 6) was driven into the end of the rod, axially and concentrically.

This divided the rod end into four quarters. The circumference of the

rod, where it emerged from the end fitting, was built up with tape to

provide a seal with the fitting. The rod was then alined concentrically

and parallel with the fitting, in a vertical orientation, and the potting

cavity was filled with a high-strength, flexible potting compound developed

previously [4] to meet the needs of this specific application. The com- pound consists of two parts of a fully reactive epoxy resin adhesive and

three parts of an activator. The mixture was cured in place, with infrared

lamps, for approximately 2 h at 165 °F (74 °C).

The lengths of the tensile specimens were approximately 4 ft (1.2 m)

between end fittings. The lengths of the stress-rupture specimens were

approximately 9 ft (2.3 m) overall.

3. PRELIMINARY TESTS AND MEASUREMENTS

Several series of tests and measurements were performed on specimens

of the rod material from Batches 1 and 2 in order to characterize the

material.

3.1 Dimensional Measurements

The diameters, densities and unit weights (masses) were determined

on samples of Batches 1 and 2. The results of these measurements are summarized here.

10
Figure 6. ConimerciaLly available cruciform wedge for potted, compression-type

end fittings.

11
Batch 1 Batch 2

Diameter, in

(mm)

0.514 + 0.002

(13.05 + 0.05)

0.540 + 0.004

(13.71 + 0.10)

Density, Ib/in^

(Mg/m3)

0.074 + 0.0005

(2.05 + 0.02)

0.0733 + 0.0005

(2.03 + 0.02)

Unit weight, lb/ft

(Unit mass, kg/m)

0.184 + 0.001

(0.275 + 0.001)

0.200 + 0.003

(0.298 + 0.004)

Several conclusions may be drawn from these measurements. The material

was quite uniform within each batch but there were significant differences

between the batches. The diameters and the densities were within the ranges

specified by the manufacturer (Tables 1 and 2)*, but the unit weights (masses

exceeded the rated value. (This finding is consistent with earlier measurements made on another batch of this material [ 6] . ) The diameter of Batch

2 was greater than that of Batch 1, but the density was less. This suggested

that the quantity of glass reinforcement may have been the same in both and

that the additional weight (mass) of Batch 2 consisted of resin binder. On

this basis the tensile breaking loads of the two batches (which depend primarily on the quantities of glass reinforcement present) were expected to

be essentially the same.

Tensile tests were performed to evaluate the static breaking loads of

the rod from Batches 1 and 2. These tests were carried out in a horizontal,

universal testing machine having a capacity of 100 000 Ibf (450 kN) [ 7] . A crosshead speed of 0.75 in/min (0.32 mm/s) was used.

The results of these tests are given in Table 3. The results of the

first two tests listed in the table are of questionable value since it was noted that the potting compound in the end fittings, for some unknown reason, had not cured properly. These tests, as well as all of the others on

rod from Batch 1, all culminated in failures which initiated in an end fitting. Thus, it is probable that the full strength of the rod was not attained. The occurrence of this type of failure is attributed to the fact

that the specimens were bowed. Batch 1, as noted earlier, was procured in

a single 7.7-ft (2.3-m) diameter coil. Upon unraveling the coil it was

found that the rod tended to remain in a coiled condition, exhibiting a residual radius of curvature of approximately 14 ft (4.3 m). It is virtually impossible to introduce a tensile force into a bowed rod uniformly

and, under the action of the applied tensile loads, the rod specimens

*Note that the density, in Mg/m^, is numerically equal to the specific

gravity.

3.2 Tensile Tests

12
Table 3 - Tensile Tests of Rod Specimens

Batch Specimen End

No. No. fitting Breaking load

Ibf (kN)

X iJNX njPl oil nn \ y J . y

J. ilN Z njFl \. yo . d;

X iiN J njn. 9 /. n 1

•PM /. iiN4 ( 1 r\i, 9 \

11 iJND Ai-biOCK / 1 1 o r\\ J . U ) 1 TN6 Al-block 25100 (111.6)

2 NRlPclTl H3M 20300 ( 90.3)

2 NR2PclT2 H3M 22600 (100.5)

2 NR2PcXT3 H3M 24850 (110.5)

2 NR2PcXTA H3M 24450 (108.8)

2 NR2Pc7T5 H3M 25250 (112.3)

2 NR2Pc8T8(a) S 23300 (103.6)

(a) This specimen had a free length of 6 ft (1.8m) rather

than 4 ft (1. 2iii)

13
tended to straighten. This created longitudinal shearing stresses in

the specimens, which caused them to split longitudinally, starting from

a point inside one of the end fittings.

Because of this deficiency in the rod from Batch 1, which also

caused difficulties in the stress-rupture tests (to be discussed later),

the second batch of rod (Batch 2) was procured, this time in uncoiled

lengths. Unfortunately, this material was also bowed, although not as severely as the material in Batch 1. Nevertheless, the degree of bowing

was sufficient to cause all of the tensile tests of this material (Table

3) to also culminate in failures which initiated in the end fittings.

Table 3 shows that all of the rod specimens exhibited tensile breaking

loads exceeding 20 000 Ibf (89 kN) , which is the manufacturer's rated value

with standard end fittings. None of the specimens attained 28 000 Ibf

(125 kN) , which is the manufacturer's rated value with special fittings.

While this value possibly could have been reached with straight specimens

that failed in their free lengths, it is also true that the highest breaking

load ever achieved in this laboratory on 1/2-in (13-mm) diameter rod from

this manufacturer is 26050 Ibf (116 kN) [ 4] . This load was attained with

a straight specimen of the rod material which was tested with aluminum-block

end fittings and which failed in its free length. (It should be pointed out

that the potting compound used in the end fittings for these tests was different from that which the rod manufacturer uses.)

3.3 Diametral Compression Tests

The dielectric strength of dry samples of rod, of the type studied in

the present investigation, has already been shown [3] to be quite high in

comparison with that of other glass-reinforced-plastic rod materials [8].

In an earlier study [9] it was shown that an approximate relationship

exists between the dielectric strengths of wet samples of glass-reinforcedplastic rod materials and the transverse tensile strengths of the (dry)

materials. The basis for this relationship appears to stem from the fact

that the wet dielectric strength and the transverse tensile strength are both influenced by the quality of the fiber /matrix bond in the material.

Since electrical properties as well as mechanical properties are important

in antenna guying applications, measurements of transverse tensile strength

were performed in the present investigation in order to obtain some indication of the wet dielectric strength of the rod product being studied.

An indirect measurement of the transverse tensile strength of glassreinf orced-plastic rod products can be obtained by means of diametral com- pression tests. In this well-known test method for brittle materials [ 10] a cylindrical disk-shaped specimen is compressed diametrically between two

flat platens. Under the proper conditions [11] the test culminates in a tensile fracture along the loaded diameter, and the transverse tensile

strength a of the rod is calculated from the relation

14
t irDt

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

disk and t is its thickness.

Disk specimens, approximately 1/4 in (6 mm) thick, were sliced from

the rods using a water-cooled, diamond cutoff saw. Care was taken during

this cutting process to prevent fraying of the edges. The coolant was carefully blotted from the disks which were then allowed to dry, under

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

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

was mounted, on edge, in a compression subpress [12] and loaded to fracture in a universal testing machine.

The results of these tests are given in Table 4. Although the

variability of the test results was greater for Batch 2 than for Batch

1, the average transverse tensile strengths of the two batches were not

excessively inconsistent, i.e., 1740 Ibf/in^ (12.0 MPa) for Batch 1

versus 1600 Ibf/in^ (11.0 MPa) for Batch 2*. The difference between

these two values is not particularly significant in view of the testto-test variabilities observed. On the basis of the relationship discussed above, these values suggest that the rods had wet dielectric

strengths within the range observed for other glass-reinf orced-plastic

rod materials.

3.4 Hardness Measurements

Hardness measurements were made on samples of the rod product using

Procedure A of ASTM Designation D785 [1] . The measurements showed a hardness of 75 + ^ for Batch 1 and 71+5 for Batch 2, on the Rockwell E scale.

Ordinarily, hardness measurements of this kind can be used to give some indication of the degree of cure in plastics products. However, since the

rod product was coated, the measurements in this case reflected the hardness of the coating and gave no meaningful indication of the degree of cure of the resin binder in the rod.

4. ENVIRONMENTAL STRESS-RUPTURE TESTS

4.1 Testing Equipment

As pointed out in the Introduction, environmental stress-rupture tests

were selected as the means to be used to assess the weatherability of the

*The anomalously low value measured in Test No. 4 of Batch 2 (Table 4) was not included in the calculation of the average.

15
Table 4 - Diametral Compression Tests of Rod Specimens

Batch Test

No. No. Transverse tensile strength

Ibf/in^ (MPa)

1 1 1590 (11. 0)

1 - 2 1740 (12.0)

1 3 1720 (11.9)

1 4 1770 (12.2)

1 5 1570 (10.8)

1 6 1940 (13.4)

1 7 1830 (12.6)

2 1 1810 (12.5)

2 2 1540 (10.6)

2 3 1610 (11.1)

2 4 1030 ( 7.1)

2 5 1340 (9.2)

2 6 1660 (11.4)

2 7 1620 (11.2)

2 8 1900 (13.1)

2 9 1340 ( 9.2)

16
rod product. These tests were performed in a series of creep-testing

machines (Fig. 7) that had been designed and assembled earlier for en- vironmental stress-rupture testing [4]. Each of these machines has a tensile load capacity of 30 000 Ibf (130 kN) , which is applied by dead

weights (masses) acting through a 100:1 compound lever system. Speci- mens up to 9 ft (2.7 m) long, including the end fittings, can be ac- commodated. The overall height and length of each machine are approximately

7 ft (2 m) and 12 ft (4 m) , respectively.

Each machine may be equipped with any of a variety of test chambers

for providing the required environment to the test specimen while it is

under tensile load. The housing for each of the test chambers used in

this investigation consisted of a 10-in (0.25-m) diameter asbestos-cement

pipe, 4 ft (1.2 m) long. The specimen passes concentrically through the

chamber with its ends, including the end fittings and the pull rods, re- maining outside the chamber. Three types of chambers were employed; one

for providing elevated temperature, one for providing a simulated marine

atmosphere, and one for simulating the damaging effects of sunshine.

The elevated temperature chamber contains a brass tube, mounted con- centrically, which extends nearly the full length of the chamber. The

ends of the chamber are fitted with transparent plastic windows. The purpose of the brass tube and the end closures is to provide more uniform

temperature distributions on specimens. Heat was supplied by twelve tubular

infrared lamps affixed to the inside of the asbestos-cement pipe, outside

of the brass tube. Each lamp had a capacity of 500 W at 120 V. The temperature distribution in the chamber was kept uniform within 2 °F (1 °C)

by regulating the power to the lamps. The test temperature was maintained

constant within 5 °F (3 °C) with a temperature controller.

A marine atmosphere was simulated by means of a saturated-humidity

salt-fog environment. The test chamber for this purpose was identical

to the elevated-temperature chamber except that an atomizing nozzle was passed through a small hole in one of the plastic windows. The nozzle

was powered by a pneumatic mist generator and delivered a soft, finely

divided spray of saltwater to the interior of the chamber. The nozzle

was directed so that the spray did not impinge directly on the specimen.

The saltwater supply for the mist generator was a standard solution [13,

14], prepared by dissolving 5 parts by weight (mass) of sodium chloride

in 95 parts of distilled water. The rate of consumption of the solution

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

condensate on the windows of the chamber. The chamber was operated at

an elevated temperature of 125 °F (52 °C) for reasons which will be ex- plained later.

It has been established, from both theoretical considerations [15]

and experimental measurements [16] , that the degrading effect of sunshine

on plastics is due, almost entirely, to that portion of the solar spectrum

that lies in the near ultraviolet range. For this reason sunshine was

17
Figure 7. NBS high-capacity creep-testing machines equipped for environmental

stress-rupture testing.

18
simulated, in this study, by ultraviolet radiation. The test chamber

consisted of twelve 48-in (1.2-in) tubular fluorescent lamps uniformly

spaced around, and affixed to, the inner surface of the asbestos cement

pipe. Six of the fluorescent lamps were sunlamps, and were mounted alternately with six blacklight lamps. The peak spectral emission of the

sunlamps is at a wavelength of approximately 310 nm* and that of the

blacklights is at 350 nm [17]. By combining the two kinds of lamps a total spectral distribution is obtained that closely approximates the

solar spectrum in the ultraviolet region below 360 nm [18], where the

peak activation spectra of most polymers, including polyesters (325 nm)

,

lie [19].

The ultraviolet energy impinging upon the test specimen was mon- itored with a commercial ultraviolet intensity meter that is sensitive

to wavelengths from 300 to 400 nm. It was found that the energy distribution was quite uniform over the surface of the specimen within the

chamber. At the start of a test the intensity averaged about 10.3 mW/in^

(16 W/m^) . This value diminished to about 9.7 mW/in^ (15 W/m^) after

2000 h due to normal deterioration of the lamps. New lamps were installed

at the start of each test.

Since fluorescent lamps emit a small amount of infrared energy, no windows were used on the ends of the test chamber in order to minimize

heating effects. In this way the temperature of the specimen was observed to remain at about 150 + 5 °F (66 + 3 °C) throughout each test.

4.2 Tests and Results

In an earlier study of the stress-rupture properties of the rod product, which was limited to tests of relatively short duration, it was con- cluded that humidity did not have a seriously damaging effect on the durability of the material [6]. Later work on other rod products, using tests

of longer duration, tended to refute this conclusion, however, [9]. It is

now believed that humidity does, indeed, have a degrading effect but that

the effect becomes significant only after a period of time sufficient to

allow a substantial amount of moisture to diffuse into the interior of

the product where it can attack the fiber/matrix interfaces. For this

reason the environmental stress-rupture tests in the present program were

allowed to run for 2000 h before termination unless, of course, failure

occurred sooner.

The original test plan called for tests to be performed in the ambient

laboratory atmosphere, as well as in a simulated marine atmosphere and simulated sunshine, to provide baseline data for evaluating the degradation

*1 nanometer (nm) = 10 angstroms

19
attributable to the latter tv;o environments. The test conditions and

results are given in Table 5 and are discussed below.

Tests Series No. 1 . Three exploratory tests, in air, were performed

on specimens from Batch 0 while awaiting delivery of the material procured

specifically for this investigation. The first two of these tests were

carried out in the ambient laboratory atmosphere. Following this it was learned that the laboratory heating and air conditioning system would be

shut down during evenings and weekends as part of the Bureau-wide energy

conservation program. Recognizing that this could introduce some serious

temperature fluctuations in the laboratory environment, it was decided to

conduct all subsequent "ambient" tests at a temperature of 80 °F (27 °C)

,

which is about the lowest elevated temperature which can be maintained

uniformly with the elevated- temperature environmental chamber.

The first two tests in this series, conducted at 16 000 Ibf (71 kN)

using Type R/P end fittings, culminated in failures which initiated in

the fittings. Since failures of this kind do not necessarily provide an indication of the durability of the specimen material itself, the third

test was performed with H3M end fittings. This test was terminated after

2000 h without any evidence of damage in the specimen and, although it was conducted at a lower load level (15000 Ibf, 67 kN) , it suggested that the

H3M fittings would be preferable, in this application, to the Type R/P

fittings

At this point the Batch 1 material arrived and further testing of

Batch 0 material was discontinued.

Test Series No. 2 . Tests were performed on Batch 1 specimens in

air, at 15 000 and 14 000 Ibf (67 and 62 kN) , using H3M end fittings. The

test at 15 000 Ibf (67 kN) culminated in a failure in one of the end fittings which appeared to have been induced by the bow in the rod. This

confirmed the observations in the tensile tests (Section 3.2) and led

to the procurement of Batch 2 and the abandonment of further testing on Batch 1.

Test Series No. 3 . As pointed out earlier, the rod material in

Batch 2 was also bowed and, as it turned out, this precluded the attainment of failures within the free lengths of these specimens in the stressrupture tests. Five tests were performed in this series using three different types of end fittings but failure in each case initiated in an

end fitting.

Test Series No. 4 . In a further attempt to obtain meaningful stressrupture data for the material in an air environment four more tests were

performed, this time using an environmental temperature of 125 °F (52 °C)

Since the end fittings in these tests remain outside of the environmental

chamber it was felt that this change might enable failures to be obtained

in the free lengths of the specimens. Such was not the case but some

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22
degree of success was nevertheless achieved. It was found, by using the

Type S end fittings, that the rod material was capable of sustaining a tensile load of 15 000 Ibf (67 kN) at 125 °F (52 °C) for 2000 h witiiout

any visible evidence of damage or deterioration. This is consistent

with earlier findings [6] which indicated Long-term stress-rupture

strengths of 16000 Ibf (71 kN) at room temperature and 14000 Ibf

(62 kN) between 150 and 200 °F (66 and 93 °C) for this product.

Test Series No. 5 . In order to facilitate comparisons with the

stress-rupture tests in air, the stress-rupture tests in a salt- fog en- vironment were also performed at 125 °F (52 °C) . Two tests were performed,

one at 13000 Ibf (58 kN) with Type R/P end fittings and one at 15000 Ibf

(67 kN) with H3M end fittings. Both tests were terminated after 2000 h

without failure. Upon removal of the test specimens from their environmental chambers it was observed that the specimen which had been tested

at the lower load level exhibited evidence of damage. In two locations

near the midlength of this specimen the coating had begun to split away,

longitudinally, taking some glass fibers with it. Strangely, the specimen which had been tested at the higher load exhibited no signs of damage.

On the basis of the evidence available it is not clear whither the damage

in the specimen tested at 13000 Ibf (58 kN) was due to the simulated marine

atmosphere or, simply, to the bow in the specimen. However, since none of

the specimens which had been tested in air had exhibited damage of this

kind although they too were bowed it is surmised that the simulated marine

atmosphere had contributed, at least, to the observed damage.

Test Series No. 6 . Three tests were performed with simulated sunshine . at a specimen temperature of 150 °F (66 °C). This temperature is not at all

unrealistic for a material exposed to direct sunlight in many parts of the

world [20]. At a tensile load of 13000 Ibf (58 kN) the specimen survived

the entire 2000-h exposure. Upon removal of this specimen from the environmental chamber it was observed to have experienced no visible mechanical

damage but the coating on the rod material, which had originally been

white and glossy, had become dull and pale yellow. Two tests performed

at 15000 Ibf (67 kN) culminated in fractures which appeared to have initiated in the end fittings due to the bow in the specimens. There was no evidence that these failures had been accelerated by the simulated sun- shine environment.

5. RESIDUAL STRENGTH TESTS

Each specimen which survived the 2000-h environmental stress-rupture

tests was subjected to a tensile test to measure its residual strength.

These tests were performed at the same crosshead speed and in the same testing machine as had been used for the tensile tests on virgin specimens. In this case, however, the specimens were 9 ft (2.3 m) long and

were tested with the end fittings that had been affixed for the environmental stress-rupture tests. The results of these tests are given in

Table 6.

23
Table 6. Residual Strength Tests of Rod Specimens .

Test

Batch Series Specimen

No. No. No. Breakine load

Ibf /I XT \ (kN)

0 1 0-3 24500 (109.0)

1 2 SRNl 24350 (108.3)

2 4 NR2Pc4SR7 24250 (107.9)

2 4 NR2Pc9SR10 23700 (105.4)

2 4 NR2Pc9SRll 22400 (99.6)

2 5 NR2Pc7SR8 23100 (102.8)

2 5 NR2Pcl2SR13 25750 (114.5)

2 6 NR2Pc6SR9 21750 (96.7)

Refers to environmental stress-rupture test series in Table 4.

24
Failure, in every test but one, initiated in an end fitting and

appeared to have been induced by the bow in the specimens. In the test

of Specimen No. NR2Pc7SR8, which had sustained some surface damage during

its salt-fog exposure, failure initiated at this damage site. In every

case but one the residual tensile strength was found to Lie within the

range of strength values measured on virgin specimens (Table 3) . The

exception, Specimen No. NR2Pcl2SR13, exhibited a residual tensile strength

in excess of the values measured on virgin specimens. It must be con- cluded, therefore, that the degradation of the specimens, due to their

stressed environmental exposures, was not great, at least in terms of

its effects on tensile strength.

6. DISCUSSION

It is widely recognized that a direct correlation does not generally

exist between the results of accelerated laboratory tests in simulated

environments and real-time performance in outdoor weathering situations.

Within this limitation, however, some general observations can be made

regarding the implications of the test results on the durability of

antenna-support systems in field installations.

A very minimal amount of evidence was obtained which suggests that

the laboratory salt-fog environment may have caused some degradation of

the product in 2214 hours while under a tensile load of 13 000 Ibf (53 kN)

This salt-fog environment was probably more severe than any marine atmosphere which might be encountered in service. The humidity was maintained " at saturation level and the temperature was held at 125 °F (52 °C) . These

conditions produced a more extensive deposition of salt on the rod and

greater diffusion of saltwater into the rod than would be the case in the

field. Thus, it is logical to expect that whatever degradation was ex- perienced in the laboratory would be attained in service only after an exposure time substantially greater than 2214 hours (0.25 years) while

carrying the same tensile load.

The simulated sunshine tests revealed no evidence of strength degradation in the rod even after 2266 hours while carrying a tensile load of

13000 Ibf (58 kN) . In fact, there was some evidence that this exposure

actually raised the tensile strength of the product, presumably by post

curing the resin binder in the rod. Calculations in Appendix A show that,

in terms of total incident ultraviolet radiation, this exposure was equivlent to that which would be experienced by an antenna guy in 2.5 years in

Puerto Rico or in 5.6 years in Alaska.

Of even more significance, perhaps, than the severity of the simulated

environments was the magnitude of the tensile loads which the specimens were

able to carry for more than 2000 hours in these environments. These tensile

loads (13000 Ibf, 58 kN) were substantially higher than those which would

ordinarily be used in field installations on a continuous basis. The rod

25
manufacturer recommends a working load of only 6700 Ibf (30 kN) ; there

is evidence that even lower loads are actually used (see Appendix B)

Stress-rupture behavior is extremely sensitive to load; small reductions

in load characteristically produce large increases in Lime Lo rupture.

It is not at all unreasonable to expect, therefore, that whatever damage

was incurred in 2000 hours at 13 000 Ibf (58 kN) would not be incurred

at half this load level until an exposure many times longer had been

sustained

In planning the test program described herein it had been hoped

to acquire definitive data on the effects of certain severe environments on the strength of a pultruded rod product. Unfortunately, this was not to be the case. The inability to accurately measure the strength

of the product, in tensile as well as in stress-rupture tests, precluded

the attainment of the original objective. In every case but one, failure

in the test specimens resulted from the bow in the rod and/or from the

inability of the end fittings to allow the full strength of the product

to be attained. In retrospect, however, this observation may, in fact,

be of greater importance than the considerations discussed above.

In effect, the minor amounts of environmental degradation which the

rod specimens experienced in this test program were rendered inconsequential

by the strength limitations already introduced by the end fittings and by

the bow in the rods. Two out of the three batches of material which were

examined in this investigation were bowed and this suggests a reasonable

likelihood that batches of the material procured for use in structural

systems will also be bowed. Similarly, the types of end fittings which

will be used on the rod material in structural systems, particularly in

antenna-support systems, are not likely to be superior to those used in

this investigation, in terms of their capability to allow the rods to

resist high tensile loads. It appears, therefore, that in the design

of such systems, the environmental degradation of the rod material due

to a marine atmosphere or to intense sunshine need not be addressed

explicitly so long as the effects of bow and the limitations of the end

fittings are properly accounted for. One way of doing this is, simply,

to base the design on the manufacturer's rated breaking strength value

for rod with standard end fittings (Table 2) rather than on the true

strength of the rod, whatever that might be. The breaking loads which

were measured in twenty tensile tests in this investigation (Tables 3 and 6) on both virgin and environmentally exposed specimens using four

different kinds of end fittings were, in every case, greater than this

rated value (20 000 Ibf, 89 kN) . This is not to say that the working

load should be set equal to the rated load—certainly, the long-terra

strength of the rod is less than its static tensile strength—but,

rather, that the factor which has been used to establish the working

load from the rated load need not be modified to compensate for degradation due to environments such as those examined here.

26
7. CONCLUSIONS

Environmental stress-rupture test methods were developed for studying

the weatherability of materials under stressed exposure to a simulated

marine atmosphere and simulated intense sunshine. These test methods

were used to examine the environmental degradation of a coated, pultruded,

glass f iber-reinforced-plastic rod product. The experimental results

essentially confirmed the rod manufacturer's contention (Table 1) that

the effects of sunlight and weathering on the rod product are "none to

slight". These results suggest that for antenna guying applications,

the strength limitations imposed on the rod by the use of conventional

end fittings, and by the fact that the rod is frequently bowed rather

than straight, tend to overshadow any strength reductions attributable

to environments of the types considered here.

27
8 . REFERENCES

1. 1975 Annual Book of ASTM Standards , Part 35 (American Society for

Testing and Materials, Philadelphia).

2. Standard Metric Practice Guide , ASTM Designation E 380-72

(American Society for Testing and Materials, Philadelphia, 1972).

3. Halsey, 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 10261 (June 1970).

4. Marlowe, D. E., Halsey, N. , Mitchell, R. A. and Mordfin, L.

,

Non-Metallic Antenna-Support Materials, First Annual Interim Report,

NBSIR 73-233 (April 1973).

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

Fittings for FRP Rod and Rope, J. Engineering Mechanics Div. ASCE 100 , No. EM4, pp 687-706 (Aug. 1974).

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

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

(Dec. 1972) .

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

the Engineering Mechanics Section, National Bureau of Standards,

Washington, D. C, NBS Special Publication 370 (1973).

8. Halsey, N. , Marlowe, D. E. and Mordfin, L., Non-Metallic AntennaSupport Materials, Second Annual Interim Report, NBSIR 74-557 (June

1974)

9. Halsey, N. , Marlowe, D. E., Mitchell, R. A. and Mordfin, L. , Non-Metallic

Antenna-Support Materials, Final Report, AFML Technical Report, in press.

10. Standard Method of Test for Splitting Tensile Strength of Cylindrical

Concrete Specimens, ASTM Designation C496-71, 1975 Annual Book of

ASTM Standards , Part 14, loc. clt .

11. Mordfin, L. and Kerper, M. J., Strength Testing of Ceramics—

Survey, Mechanical and Thermal Properties of Ceramics , ed. J. B. Wachtman, Jr., NBS Special Publication 303, pp 243-262 (1969).

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

Tests, NACA TN 912 (1943).

28
13. Standard Method of Salt Spray (Fog) Testing, ASTM Designation

B117-73, 1975 Annual Book of ASTM Standards , Parts 9, 10 and 27,

loc. clt .

14. Salt Fog, Method 509, Military Standard Environmental Test Methods,

MIL-STD-810B (15 June 1967).

15. Johnson, F. S., Solar Radiation, Chapter 5, Space Materials Handbook,

ed. C. G. Goetzel et al , 2nd edit.. Technical Documentary Report

No. ML-TDR-6A-40, pp 31-37 (Air Force Materials Laboratory, Jan 1965).

16. Dreger, D. R., How Dependable are Accelerated Weathering Tests for

Plastics and Finishes?, Machine Design 45, No. 29, pp 61-67

(Nov 29, 1973).

17. Hirt, R. C, Schmitt, R. G., Searle, N. D. and Sullivan, A. P.,

Ultraviolet Spectral Energy Distributions of Natural Sunlight

and Accelerated Test Light Sources, J. Optical Society of America

50, No. 7, pp 706-713 (July 1960).

18. Cipriani, L. P., Giesecke, P. and Kinmonth, R. , New Way to Peg

Plastics Aging, Plastics Technology 11 , pp 34-36, May 1965.

19. Hirt, R. C. and Searle, N. Z., Energy Characteristics of Outdoor

and Indoor Exposure Sources and Their Relation to the Weatherability

of Plastics, in Weatherability of Plastic Materials (J. Applied

Polymer Science, Applied Polymer Symposia No. 4), ed. M. R. Kamal

,

pp 61-83 (Interscience Publishers, N.Y., 1967).

20. Ohman, H. L., World Areas with Higher Temperatures than the Yuma

Proving Ground during Summer (Note 2 of "Studies to Aid TECOM in

Analyses of Environmental Risks to Materiel"), U. S. Army Engineer

Topographic Laboratories Report No, ETL-RN-74-6 (Fort Belvoir,

Aug 1974).

21. Searle, N. Z. and Hirt, R. C, Ultraviolet Spectral Energy

Distribution of Sunlight, J. Optical Society of America 55 , No. 11,

pp 1413-1421 (Nov 1965).

22. U. S. Army Test and Evaluation Command, Common Engineering Test

Procedure "High Temperature With Solar Radiation", Report No.

MTP 5-2-593 (Aberdeen Proving Ground, 1 July 1971).

23. Masters, L. W. and Wolfe, W. C, The Use of Weather and Climatological Data in Evaluating the Durability of Building Components

and Materials, NBS TN 838 (Aug 1974).

24. Pohlmann, E. F., letter to NBS, November 14, 1973.

29
APPENDIX A

Ultraviolet Radiation Calculations

Let be the ultraviolet radiation intensity which was incident

upon the rod specimen in the simulated sunshine environmental chamber.

Then the total ultraviolet energy which struck a length L of the specimen

during a time interval tj^ was

= TTDLq^t^ (A.l)

where^D is the r^d diameter. Using D = 0.5 in (12.7mm) and q^ = 10.0

raW/in (15.5 W/m ), and converting to consistent units, it is found that

the total ultraviolet energy which impinged upon the specimen in 2266 h was 417 W.h per foot of rod length (4.93 MJ per meter of rod length).

Consider, now, an antenna guy rod in a real field application.

Let q be the average intensity of solar radiation at this location on the earth's surface, and let f represent that fraction of this intensity

which is concentrated in the ultraviolet range. Then the total ultraviolet

energy which strikes a length L of the rod during a time interval t is

U = DLfqtsine (A. 2)

where 9 is the angle between the sun's rays and the axis of the rod.

Note that this relationship differs from Eq . (A.l) because in this case

the energy incident on the rod is determined by the projected area of the

rod normal to the sun's rays whereas in the environmental chamber the

ultraviolet lamps radiate to the entire surface of the rod.

The fraction f has been variously estimated at 1 to 7 percent [21],

2 percent [16], 3 percent [22] and 5 percent [23]. The angle 9 depends

upon the orientation of the guy and the instantaneous location of the

sun. When the rod is normal to the sun's rays sin 9 is unity and when

it points directly at the sun sin 9 is zero. For long-term averages

it is probably adequate, for present purposes, to take f as 0.04 and

sin 9 as 1/2. With these values and Eq (A. 2) the total ultraviolet

radiation which impinges on a unit length of guy rod over a one-year

exposure in various locations may be calculated using available tabulations of q [23]. The results are as follows:

Average intensity of Annual u-v energy on Location solar radiation, q unit length of guy, U

mW/in2 (W/m2) W-h/ft (MJ/m)

San Juan, P. R. 160 (248) 168 (1.99)

Las Vegas, Nev. 157 (244) 165 (1.95)

Washington, D. C. 109 (170) 115 (1.36)

Seattle, Wash. 94 (146) 99 (1.17)

Fairbanks, Alas. 71 (110) 75 (0.88)

30
APPENDIX B

Failure Analysis of a Tower Guy

During the course of the investigation, the sponsor submitted to

this laboratory a segment of a tower guy that had failed in service

[24]. The diameter (1/2 in, 13mm) and the manufacture of the guy

were nominally identical to those of the rods tested in this investigation.

The tower is located in the hot, moist climate of Thailand but,

being situated about 60 miles (97km) from the coast, it is not exposed

to a marine atmosphere. It is supported by numerous guys in parallel

pairs. The particular guy in question was 4A8 ft (137m) long and was oriented at approximately 45 degrees to the horizontal. It was designed

to carry a tensile load of 2000 Ibf (8.9 kN) under static conditions and

a peak dynamic load of 5575 Ibf (24.8 kN) under full wind conditions.

During a routine inspection of the facility about six years after

installation the guy was observed to have suffered a transverse crack,

approximately 30 ft (9 m) from its upper end, which penetrated almost

halfway through the rod cross section. The opposing surfaces of the

crack were quite jagged and uneven, not at all resembling the hairline

cracks that occasionally afflict materials of this kind. Clean longitudinal splits extended almost 3 ft (1 m) in both directions from the base

of the crack. See Figure 8.

Since this particular guy, alone out of many, sustained damage it is

considered unlikely that weathering was a significant causative factor.

The absence of any evidence of burning in the vicinity of the crack tends

to rule out the possibility that it was caused by a lightning strike.

On the basis of these observations, plus the fact that the longitudinal splits did not extend into the upper end fitting, it is considered

likely that the transverse crack was the result of local damage inadvertently

introduced prior to, or during, installation, or by impact with a stone

or other missile after installation. However, being in a relatively

inaccessible location, the crack was probably difficult to detect by

routine inspections until the fatiguing action of wind-induced vibrations,

coupled with the low longitudinal shear strength of the material, produced

the longitudinal splits which enabled the crack to widen. Worn areas

at the edges of the crack suggest that it had been subjected to rubbing,

due to wind-induced vibrations, for a considerable length of time.

Largely as a matter of curiosity, hardness and diametral compression

tests were conducted on samples cut from the guy rod. These tests showed

an average hardness of 75 on the Rockwell E Scale and an average transverse

tensile strength of 2400 lbf/in2 (16.5 MPa) . The hardness measurement, as pointed out earlier, is relatively meaningless since it was determined

31
^igure 8. Failed section of glass f iber-reinforced-plastic tower guy.

32
on the rod's coating material rather than on the resin matrix. The

transverse tensile strength value, on the other hand, is interesting

because it is about 24 percent higher than the highest value measured

on virgin material (Table A). This is consistent with observations

made on other guys which have been removed from service and suggests

the existence of a natural curing process in the polyester matrix which

apparently goes on for months or years after fabrication.

33
NDi-l I4A (K _ V. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. I'UHI.K A TION OK Ki-.POR I" NO.

NBSIR 76-1069

2. (lov't Accc s.sion No.

3. K( ( inicnt's Acics.sion No.

4. TITI.K AND SUBTITI.I- Environmental Effects on the Strength of a Glass FiberReinforced-Plastic Rod Material

5. Publ icat ion Date March 1976

6. Performing Organi7.ation Code

7. AUTHOR(S)

Nixon Halsey and Leonard Mordfin

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAMF AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Ta.sk/Work Unit No. Cost Center 2130444

11. Contract /Grant No. IA-16742-23

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

United States Information Agency

Washington, D. C. 20547

13. Type of Report & Period

Covered

Final, 8/73 - 2/75

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

Environmental stress-rupture test methods were developed for studying the weatherability of materials under stressed exposure to marine atmospheres and intense sunshine.

These laboratory test methods were applied to a coated, pultruded, glass fiber-reinforce

plastic rod product, with the marine atmosphere simulated by a saltwater fog and intense

sunshine simulated by ultraviolet radiation. Static tests of the material, conducted

both before and after the environmental exposures, were used to characterize both the

as-received mechanical properties of the product and the reductions in tensile strength

attributable to the weathering actions. The experimental results essentially confirmed

the rod manufacturer's contention that the effects of sunlight and weathering on the

product range from none to slight. For applications as antenna guys, the results

suggest that the strength limitations imposed on rods by conventional end fittings, and

by the fact that rods are frequently bowed rather than straight, tend to overshadow any

strength reductions attributable to environments of the types considered here.

A failure analysis of a tower guy, fabricated from material of the type tested in

this investigation, is given in an appendix.

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

)

Antenna-support materials; environme ital degradation; guys, nonmetallic; marine

atmospheres; plastics, fiber-reinforced; pultrusions; stress-rupture testing; sunlight;

ultraviolet radiation; weatherabil i ty

.

18. AVAILABILITY [X] Unlimited

I I For Official Distribution. Do Not Release to NTIS

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