NBSIR 76-1159

Fatigue and Weatherability

Studies of Aramid-Fiber Rope

Slings

Nixon Halsey

Leonard Mordfin

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

September 1976

Prepared for U.S. Army Air Mobility Research and Development Laboratory

Eustis Directorate

Fort Eustis, Virginia 23604

NBSIR 76-1159

FATIGUE AND WEATHERABILITY

STUDIES OF ARAMID-FIBER ROPE

SLINGS

Nixon Halsey

Leonard Mordfin

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

September 1976

Prepared for U.S. Army Air Mobility Research and Development Laboratory

Eustis Directorate

Fort Eustis, Virginia 23604

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

Edward O. Vetter, Under Secretary

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2 . SPECIMENS 2

2.1 Fiber and Yarn Characteristics 2 2.2 Parallel-Strand Rope Construction 2 2.3 Cabled-Strand Rope Construction 2 2.4 Jackets 5 2.5 End Fittings 5

3. PRELIMINARY TENSILE TESTS 6

4. FATIGUE STUDIES 6

4.1 Increasing-Load-Spectrum Tests 9

4.2 Alternating-Block-Spectrum Tests 9

4.3 Residual Strength Tests 14

5. SIMULATED WEATHERING STUDIES 14

5.1 Test Equipment 14

5.2 Tests and Results 16

6. PERMANENT ELONGATION 19

7. DISCUSSION 20

7.1 Breaking Loads 20

7.2 Fatigue Endurance 20

7.3 Weatherability 21

7.4 End Fittings 21

7.5 Synergistic Effects 22

8. CONCLUDING REMARKS 23

9. REFERENCES 24

x
LIST OF TABLES

1. Property Requirements of Sling Legs

2. Typical Properties of Aramid-Fiber Yarn

3. Specimen Descriptions

4. Preliminary Tensile Tests

5. Increasing-Load Fatigue Spectrums

6. Increasing-Load-Spectrum Fatigue Tests

7. Alternating-Block Fatigue Spectrums

8. Alternating-Block-Spectrum Fatigue Tests

9. Tensile Tests of Fatigued Specimens

10. Simulated Weathering Tests

11. Tensile Tests of Artificially Weathered Specimens

ii
LIST OF FIGURES

1. Sling-leg specimens in three tensile capacities, fabricated from

parallel-strand ropes with neoprene- impregnated, braided nylon

jackets and potted, open spelter sockets.

2. Sling-leg specimens in two tensile capacities, fabricated from

cabled-strand ropes with extruded polyethylene jackets. ,

3. Sling-leg specimen (No. 17) with a rope strand that failed in

fatigue and punctured the polypropylene jacket. This is a mediumcapacity specimen, fabricated from cabled-strand rope.

4. High-capacity sling-leg specimens after test. These specimens

were fabricated from cabled-strand ropes with braided polyester

jackets

.

5. Proprietary, experimental, phosphor-bronze end fitting. Fittings

of this kind were used on one high-capacity, cabled-strand, slingleg specimen (No. 25).

6. Parallel-strand sling-leg specimen with failure adjacent to an

end fitting where the potting compound had seeped into the rope

fibers and diminished their flexibility.

7. Fatigue failure of potting head on Specimen No. 8. This is a low-capacity, cabled-strand, sling-leg specimen which was subsequently tested in tension by gripping the remaining portion of

the potting head with wedge grips.

8. NBS high-capacity creep- testing machines, shown here with environmental chambers installed for simulated weathering tests of

pultruded fiberglass rod specimens under tensile load.

iii
'

-

.
FATIGUE AND WEATHERABILITY STUDIES OF

ARAMID-FIBER ROPE SLINGS

Nixon Halsey and Leonard Mordfin

ABSTRACT

Tests were carried out on twenty-six sling-leg

specimens, fabricated from aramid-fiber ropes, as part of a program to develop improved external cargo

slings for helicopters. The ropes included both

parallel-strand and cab led- strand varieties, in three

different tensile capacities. The tests were intended

to evaluate the reductions in rope strength caused by

fatigue loading and by exposure to simulated weathering.

It was found that the fatigue capabilities of some of

the ropes may not be entirely satisfactory; but, because

of inadequacies in the end fittings installed by the

rope manufacturers, thorough evaluations were not possible. The results indicate that the performances of

the ropes could be enhanced through the use of more

efficient end fittings, and suggestions for the development of such end fittings are offered.

Key Words: Aramid-fiber rope; breaking load, rope;

cable, aramid-fiber; end fittings, rope; fatigue damage;

rope, aramid-fiber; slings, cargo; terminations, rope;

weathering, simulated.

1. INTRODUCTION

This report presents the results of one phase of a program to

advance the technology of external cargo slings for helicopters. The

objective of the program is to develop a family of cargo sling assem- blies which offer improved handling characteristics and significant

weight reductions. The work described here was intended to explore the

potential improvements which could be achieved through the use of a newly developed, synthetic fiber material for the slings. To this end,

tensile, fatigue and simulated weathering tests were carried out on prototype sling legs, in three tensile capacities, fabricated from two

varieties of aramid-fiber rope.

This investigation was carried out in the Engineering Mechanics

Section of the National Bureau of Standards (NBS) under the sponsorship

and with the financial assistance of the U. S. Army Air Mobility Research

and Development Laboratory (AAMRDL)

The test conditions for the tensile and fatigue tests were specified by AAMRDL. The simulated weathering tests were designed by NBS.

1
2. SPECIMENS

The prototype sling-leg specimens, consisting of aramid-fiber ropes

with end fittings attached, were provided to NBS by the sponsor in two

different rope configurations and in three tensile capacities, making

six rope styles in all. Variations in rope jacketing materials and in

end fittings raised the total number of specimen styles to nine. One of

the two rope configurations consisted of rope strands gathered together

in parallel, while in the other the rope strands were cabled together.

The tensile capacities and related information are given in Table 1.

2.1

Fiber and Yarn Characteristics

The ropes were manufactured from aramid-fiber yarns consisting of

1.5-denier filaments. Typical yarn properties are given in Table 2.

The singles yarns used for all but one of the rope styles were comprised

of approximately 1000 untwisted filaments and had a linear density of

1500 denier. The high-capacity parallel-strand ropes utilized 15 000-

denier singles yarns containing approximately 10 000 filaments. These

denier values have the following equivalents in other engineering units:

denier lb per 1000 f t mg/m

1.5

1500.

15 000.

0.00011

0.11

1.1

0.17

170.

1700.

2.2

Parallel-Strand Rope Construction

Constructional yarns for the parallel-strand ropes were formed by

plying several singles yarns together. Each rope strand was assembled

from ten constructional yarns twisted together in a direction opposite

to that of the individual constructional yarns. The several rope strands were gathered together without twist and enclosed in a jacket. In order

to achieve a relatively torque-free product, each rope incorporated some strands with right lay and some with left lay. The low-capacity rope,

for example, had two strands twisted right and two twisted left. The

medium size had nine strands in all and the high-capacity rope, fourteen

strands

2.3

Cabled-Strand Rope Construction

Constructional yarns for the cabled-strand ropes were formed from

groups of singles yarns gathered together and impregnated with polyurethane.

The low-capacity rope was comprised of nineteen rope strands, each

containing seven constructional yarns twisted together with left lay.

The twelve outer rope strands were wound with right lay around six

strands which were wound with left lay around one center strand.

2
:able 1 - Property Requirements of Sling Legs

. . - — ’ 1 ' ' ' ’ . —

.

» - — — —

(a.) Capacity Rated Rated Required fatigue life

lesignation breaking load working load No. of cycles Frequency

lbf (kN) lbf (kN) Hz

low 17 500 (78) 10 000 (44) 5 082 000 2.57

medium 36 000 (160) 25 000 dll) 4 326 300 2.19

high 56 000 (249) 40 000 (178) 3 330 000 1.68

(a) Spectrum loading conditions.
Table 2 - Typical Properties of Aramid-Fiber Yarn

(a)

Specific gravity

Filament diameter

Filament denier

Break elongation (b)

(b) Tensile strength

Young's modulus

1.44

0.00047 in

1.5

3 to 4 percent

400 000 lbf/in2

9 x 106 lbf/in2

(12 pm)

(2.8 GPa)

(62 GPa)

(a)

(b)

According to

Based on dry

yarn manufacturer,

yarn test.

4
The medium-capacity rope consisted of six rope strands wound with

left lay around an independent rope core. Each of these strands was composed of nineteen constructional yarns twisted together with right

lay. The rope core consisted of nineteen strands, each containing seven constructional yarns. The twelve outer strands of the rope core were

wound with left lay on six strands which were wound with right lay on

the center strand.

The high-capacity cabled-strand rope also consisted of six rope

strands wound with left lay around an independent rope core. Each of

these strands was comprised of 37 constructional yarns (18 right, on 12

left, on 6 left, on 1). The rope core consisted of seven strands, six

wound right on one. Each of these strands was comprised of seven con- structional yarns twisted together with left lay.

2.4 Jackets

The parallel-strand ropes were all jacketed with black braided

nylon that had been treated with neoprene to reduce moisture penetration

(Figure 1)

The low-capacity cabled-strand ropes had extruded polyurethane

jackets (Figure 2a). One out of three medium-capacity cabled-strand

ropes also had an extruded polyurethane jacket (Figure 2b); the other

two had thicker, polypropylene jackets (Figure 3) . The high-capacity

cabled-strand ropes all had white braided polyester jackets (Figure 4)

2.5 End Fittings

All of the specimens were furnished with potted, compression-type

(conical-basket) end fittings that had been installed by the rope

manufacturers.

The parallel-strand ropes used commercial, forged-steel, open

spelter sockets of the type that is conventionally used for steel wire

rope (Figure 1) . The tops of the baskets had been built up with tape to

provide a longer potting length. Where the ropes entered the sockets,

the jackets were reinforced with tape.

The low-capacity cabled-strand ropes were fitted with commercial

potting heads of a type that is widely used for synthetic-fiber rope

(Figure 2a). In the medium size, two cabled-strand ropes were furnished

with similar, but larger, potting heads that had been reinforced with

conical metal sleeves (Figure 2b) ; and one cabled-strand rope was fitted

with open spelter sockets similar to those used on the parallel-strand

ropes. Experimental end fittings were used on the high-capacity cabledstrand ropes. Two of these ropes were furnished with steel fittings of

the type shown in Figure 4, bottom, and one had proprietary phosphorbronze fittings such as that illustrated in Figure 5.

5
The descriptions of the specimens are summarized in Table 3. The

table also gives the diameters of the ropes, and the numerical designations of the specimens of each style which were subsequently tested.

3. PRELIMINARY TENSILE TESTSl

Specimens of four different rope styles were subjected to tensile

tests in order to obtain some preliminary indications regarding the

response of the specimens to tensile load. These tests were performed

in universal testing machines at a crosshead speed of 0.75 in/min (0.32

mm/s) . Each specimen was subjected to five or ten cycles of load, from

zero to a predetermined load level and back to zero, before the load was

increased to failure. Each specimen failed near one of its end fittings

(see, for example. Figure 6) where the resin, which had been used to pot

the fittings, had seeped into the rope fibers outside of the fittings

before it was cured. It is not obvious whether this seepage was due to

normal flow under the influence of gravity, or to a wicking action of

the aramid fibers, or both. In any event, the subsequent curing of the

resin destroyed the normal flexibility of the ropes near their end

fittings.

The breaking loads attained by the specimens are given in Table 4,

together with the rated breaking loads. It may be seen that the specimens failed, more often than not, to attain their rated loads. In the

worst case, Specimen No. 1, the actual breaking load was only 88 percent

of the rated value. Interestingly, however, the only specimen which

actually exceeded its rated value (No. 2), was nominally identical to

this one.

The table also lists the free lengths of the specimens, measured

both before and after the pre-cycling of load. (Free length is defined

as the length of a specimen between end fittings.) Differences between

the original free lengths of specimens of different styles reflect

differences in the lengths of their end fittings, since all of the

specimens were manufactured to have the same approximate length between

the clevis pins on their end fittings.

4. FATIGUE STUDIES

Specimens of several styles were subjected to axial tension fatigue

tests using spectrum loading. Two different kinds of block spectrums

were specified. The first consisted of three blocks of cycles, applied

in order of increasing load levels. The second consisted of two blocks

which were applied alternately. All of the tests were carried out on a 24 hours-per-day basis, in a 50 000- lbf (222-kN) capacity, electrohydraulic

fatigue testing machine [1],

6
Table 3-

Specimen

Descriptions cg . E co •H O o z Ga

t/j

CObOc

•H

4-1

4-1 •H

MH-aGW

4-1 G

-Xo

ad

•”3

X

GG

4JGe

c0 •Hoco

•H

4J

g o a 3 o G

pS 4-4 COGo

CJ

COG

<-H (0

G

bOGN

C

G i-l

H >-, CO

C/0G

rH tJO •H

in id >> G a 4-» d) Cd rl H CJI G

G—'

G bO

C

G- *HO

O 4h -H

Pi

G 4-1 O G a g

rH rH

#4 CN 00 •V rH LT) o #s rH CH i * n rH r^H CO vO , r rH CN rH CO r^. CN

m CN * CN r. CN 1—

rH CN

rH I"". - 00 * « O CN

Cs| MO rH <r

•* #\ #S rH m ctn CO

CO CO CO CO

4-J 4-1 4-1 4-1 d) 04 04 CO CO CO G

G

-X rX -X X)

X X) -X l"H

N

Oa cj

GG

G o GG o o o G G G o G O

CO CO cj -G rC jo CO 4-1

G

co -G

G

GG bO b0 bO

G

04 04 04

C

C

G

G • •

4-1 4-) 4-4 •H •H •H 4-1 rH i—1 t—1 r-i i—1 4J 4-> 4-4 i—1 4-1 4-J d) 04 04 4-1 4J 4-1 G G- Gg- CO- G- O o o G- X X

C0 CO CO D- a- G- CO

GG

G

•HG

d> •HG

d) X

X

XX

G

/-—N .—V •H •H

G

0) <DG

GG

G

G

G

'w'

GG

CG

G

G 4-1 T3 X G G G G -G JO CO •H •H •H G G rH i—1 03 G rG JO >> In G G G U G 4-1 4-> o- O- G G G. rO -Q G G o O 4-J 4-1 O G G G G CO CO G C G C 3 3 G- G- G G o O O kO kO Sn m tn G

rH 1 1 r-i rH rH rH rH rH rH

G

rH

O o O o O

O CO- c G a G- Oh G- G- G- • CO

CO

C

C

G

“S /•“S ^•s /-N r-N -—

o G

rH r^. CN CO r" <f •H tn rH rH CN I—

iH rH i—CN CN 4-1

W >w> \«/ •—

V - s_r

G i—

G G

vO cO v£> vG vO M0 3 G

v£> rH rH rH i—

rH i—

bO o

rH 00 CN \ -—, "

•H •H "—

tH \ —s i—

rH rH

mm 14H 4-> rH 1^- rH rH rH i—

i—rH

C

O o 3

4-J 4-1 r-v /—s ^-N X CO G

OH 4H 14H bO bO

GG -X ' - '—

-S ' G O o u o CJ CJ CJ G • a G

O

O o pci PS PS PS PS 4-4 rH •r~) i—

rH rH nM

M t-HM

M CO mh 1 G o bO

XXXX O' cy> O' nX i—

G

rH rH i—

cnm

G JOG •H

-cf <y> -d" ON <—

G

GX 1—

1—

X X X X X JO H -2 3 G E i—

lO M0 iD vO x> CJ

G 3 CJ • •H G C • G '—

•H

O

G

X

G

X

GX o o o o o O O o o X > G O' o o o o o o O o o G •H /—n 4J i—

nm in o m m in m m in G bO CO G

rH i—rH t—

i—i—

rH i—

G

X E

rt

—

m 4-1G

G

G • GG

rH CO

GG •HXG

G 1 G G X G O O

rH 4J 4J CJ CJ G CO CO G G i—

bO

G

G

GG E E E E rH G G 4-1 bO G. G- 3 3 3 3 G •H (0- 3 G 0 O

•pH -C •rH •H •H -G JOG 4-J o o GG

G 2 X bO Ss X X X bO b0 G G G G- o G •H o G G G •rH •H CO- G G E 4-1 4J rH E -G rH B E E JG J3 <4H 4-1 •H G G

CO rHOG i G

G

4-1

G E GXX

CJ

G •H

GGG

CO •H

G

X

GGG i—

i—t—

G G -2 o G G- G- G G G G G E G G- G G t—

i—

rH

X

XXX

X

X 4h E 3 GOXX i—

rH rH

G

GGGG

GG 3 SS COG

GC

G

G

G i—

i—

pH i—

rH i—

PS S <d S3

MM

GGG Xi ^3 JO JO JO JO /r- ^—\ G G G G G G G G G G JO o X

G MH bO

Q- O- aa CJ CJ CJ CJ CJ

—- —^ -—- v~^

N-^ 7
Table 4-

Preliminary

Tensile

Tests

X) Z—N Z“N o

o3 Z“N CO o • O z • • o

od i—

00 co 40

03 40 00 rH d bO 'w' d d

cn -h o O o

o3

M in in o

03 o3 4-1 CO 4D o S 03 43 u r-H LO 00 40

43 rH rH on

/-n 43 o CN ZN d bO g 00 00 o

bO

d

g o o C d -H N_z rH iH 03

ai a;

h N_Z N_Z n«z rH 4) G

44

N

m 4C CN

03 0!

O

d • • • 0) •H CN CN 00 d on

03 • 0) 0 4-1.—

ZOO in

mm

No

03 Z—\ Z“N

03 Z~N rH O Z • • • rH 4*! (—

CO CO

rH CO CO

bO V-/ vz d

•H o o o

rH 4-i o o o o m m m n CN r-* o

43 Z"N z-s

-U» o Z“N bO bO g 00 00 d 03 d § o o 40

0)d -H N«Z 1—

1—1 03 i—1 O '

1 '—

N>z N«Z

4-4

O

0) <u >-. <rm rH

03 43u

d

d •rH CN Osl 00

Pn <r CO

0) , d 1 *H d bC n

03

d o 03 03 03 0) rl n) h g

03

03 Z-N O Z“N /—V O

rH z 00 00 40

03 rH

0) bO v«z s-/ 'w' u d

0) -H o o o

0d 4«S 4-1 o o o

03 43m

m o

03 rH d r^» 40

43 rH rH CO i d O 03 • 03 a O rH CN m

CX -rl Z

C/3 1 iH 1—

1—

p d 03 03 03 Q3 bO o rH 1 1 1 1 CX -H . H < 1 1 1 rH o 4-1 u 03 03 03 d d m d S-i d O U 03 03 03 o cx cx du

—Z“N

m l—

Z-N z—

• • o o m • • CN CN

CN CN

'•w' N-Z vz o O o o

in o o

rH 40 cn CO i—

o r-* m in rH rH

Z“N Z-N Z“N cn nT o 40 m 40 r- • 00 CO 40 40 1 1 n_z N-Z N-Z N-Z rH 40 o 40 • • • * 0) CO <r 40 40 d

cn CO CN CN •H • o X 03 -o e

rH o o d

rH rH UO

m 03 -H d d

•H 03 x: 03 O d

03 Z“N Z-N a "d 40 40 Z-N Z—

03 • • rH NT 00 d • • d 03 CN CN 1—

CO •H >

rH rH rH CO

d

O

W vz N_Z N-Z cn cx

03 o o o o d tn o o o o rH o o in m 03 i—

CN 03 03 00 CO d 03 CN CN 03 -d 5 -H O d

CX 0! Z-N Z-N Z-N ZN 1 d

rH rH o 40 S 03 40 40 r-. 03 N

00 00 40 40

d jd

N-Z N_Z o

cn •> rH C33 o 40 rH - 0) <n CO 40 40 rH d

cn CO CN CN 03

d

03

O

•rl

N

d -H d d

03

O

43 43 03 03 > n N >4 d d

•rH *H

Z"N z-n o a

04 03 Z-N Z-N 03 0! <r 00 00 a cx CN CN 0! 03 '—

v-/ NZ w O O o o o o Z-N Z-N o o o o o o m m

40 40 r^. co <r m m i—1 rH <r • • rH O N-Z VZ o 4-1 44

<33 rH CO <1* rQ Xl 1 1 rH o o o o o o rH rH •4 C4 03 0) o o rH 3 1 03 03 o o rH rH 03 0) <T rH

03 03 1 1 1 1 d d 43 43

03 0) 03 03 • • CX C4 o 03 03 43

04

•x i 8

Numerals

in

square

brackets

refer

to

similarly

numbered

references

cited

in

Section
4.1 Increasing-Load-Spec trum Tests

Three increasing-load spectrums were used, as given in Table 5.

Within each block of each spectrum, the minimum load level is 60 percent

of the maximum load level. Spectrums B and C, intended for the mediumcapacity specimens, have identical load levels and differ only in the

numbers of cycles; Spectrum C has approximately one-fourth the number of

cycles as Spectrum B. Spectrum A was intended for the low-capacity

specimens. Expressed as percentages of the respective rated working

loads, the load levels in Spectrum A are identical to those in Spectrums

B and C.

Five specimens were tested using these spectrums. The results are given in Table 6. Malfunctions of the automatic spectrum-programming

equipment for the fatigue machine led to several deviations, in numbers

of cycles, from the specified spectrums; these are cited in the footnotes to the table. The results show that only one specimen, a lowcapacity parallel-strand rope, survived the fatigue tests without

failure. A low-capacity cabled-strand specimen and three mediumcapacity parallel-strand specimens all failed before completing the

specified test spectrums. In each of these cases, however, the failure was due to an end fitting and not to a deficiency in the rope itself.

4.2 Alternating-Block-Spectrum Tests

These tests consisted of basic constant- load-amplitude fatigue

tests which were interrupted at periodic intervals with the application

of five cycles to a higher load level ("spikes"). The interruptions

were applied after each 50 000-cycle interval whenever such interval

occurred during regular working hours (8:30 am - 5:00 pm, Mon-Fri)

.

Table 7 lists the basic and the spike load levels for each rated capacity of the specimens. The maximum load levels of the basic blocks

represent the same approximate percentage of the respective rated

working loads. In the spectrums for the low- and medium- capacity specimens, the minimum loads are the same for the basic blocks and the spike

blocks; for the high-capacity specimens, the minimum load of the spike

block equals the maximum load of the basic block.

The results of alternating-block-spectrum fatigue tests on seven specimens are given Table 8. Two of the specimens were of the parallelstrand variety. One of these, a low-capacity rope, pulled out of one of

its end fittings prematurely. The other, an intermediate-capacity

specimen, survived the entire fatigue test without failure.

Of the five cabled-strand specimens which were tested, three,

representing one of each tensile capacity, survived without failure. A

fourth, of medium-capacity, suffered a partial failure (Figure 3), but

nevertheless survived the entire test. The fifth, a high-capacity

specimen, failed in its free length relatively early in the test (Figure

4a) .

9
Table 5 - Increasing-Load Fatigue Spectrums

Spectrum

A

Spectrum

B

Spectrum

C

First Block

Maximum load, lbf 2500 6250 6250

(kN) (11.1) (27.8) (27.8)

Minimum load, lbf 1500

' 3750 3750

(kN) (6.7) (16.7) (16.7)

No. of cycles 508 200 432 630 108 158

Second block

Maximum load, lbf 5000 12 500 12 500

(kN) (22.2) (55.6) (55.6)

Minimum load, lbf 3000 7500 7500

(kN) (13.3) (33.4) (33.4)

No. of cycles 508 200 432 630 108 158

Third block

Maximum load, lbf 10 000 25 000 25 000

(kN) (44.5) (111.2) (111.2)

Minimum load, lbf 6000 15 000 15 000

(kN) (26.7) (66.7) (66.7)

No. of cycles 3 811 500 3 244 725 811 181

Total No. of cycles 4 827 900 4 109 985 1 027 497

10
Table 6-

Increasing-Load-Spectrum

Fatigue

Tests

cn

54303

Pi

m3

iH X) 3 3

>s -H O i—I

cx i—I cx cd 3

44oH

u I C

CO (1) 0)

<uM 3 H 4-i cr3

d)CX

CO

43

4400c33

<054 pH

<u iH

U

•H *H

CO 3 C 3

a)a

H COo I c a 3 • 3 0 o

cx -h js CO I3 (3 3 00 o

CX -h

PiC « 8 * NK

rO o CJO 4-1 . vO ( / , 00 H

IX44O

cd 0) vO o cO

O rcr\ rH to CN 43 o 00 o H

oo 00 rH CN cO 3

a\ -<r r^- 00 00 0) m CN co CN o *H <r i—i

rH CN 05 • 053

CJ 44 oI—1

05 rO 43

4-1

X

CTc r^. 54

O o o rH o m •H 44 • . • • • 43 3 o o CN o CN 44 •H rH 1—1 1—1 C X

•H 05CX

CO 05 ' 05 05 i—1

CO CJrO X < PQ PQ CJ < CJ 3

43

mo X33

CO

O o CX

/•"s

m 0

r-s—\/—s o

ON o o CO CJ o 00 ~cr vD t—1 X CTc cO X 00 s—

CwT v_/ c 3

id •H

44 CN cO CO O *« 44 • . . • 4* O

CO 00

X r-^ vO CJ CX co CO CN o1—1 3

43 33 X 43 3 • 5 O 05 o 3 00

0) 3 3 0 0 0 CO rH •H

35 3 3 •H 3

•H •H •H 3 3 44 X) X X S H 44 •H o 05 05 0) o 44

rH 0 0 0 rH CO 44 3 3 X

iH

O (3 3 43 3

>r 44 3 -H

O

Sc 44 1—1

<r x) 4-» CN cO rH 00 m 3 3

rH i—1

44 3 • CO 3 3 X

00 C3 3 3 m -h •r-) 54 s X 0

co 3 3 CO 3 3 3 X 44 X 3 0 3 0

rH rH rH ! 1 rH 44 1 1 3 3 3 3 3 3 *H 44 rH rH rH rH X (3 3 3 O

r—1 \—1rH rH 3 H

H fH S5 3 3 3 3 rH 3 3 3 3 43 3 3 3 3 3

CX CX CX CX CJ 3 43 3 X

11

Includes

624

001

cycles

in

first

block.

Fracture

of

lip

on

potting

head

of

one

end

fitting

(Fig.

7).
Table 7 - Alternating-Block Fatigue Spectrums

Tensile capacity of specimen

low medium high

Basic block

Maximum load, lbf 6800 17 000 27 270

(kN) (30.2) (75.6) (121.3)

Minimum load, lbf 6200 15 900 24 930

(kN) (27.6) (70.7) (110.9)

Frequency, Hz 5 5 4

No. of cycles per block 50 000 50 000 50 000

Spike block

Maximum load, lbf 10 270 21 000 33 600

(kN) (45.7) (93.4) (149.5)

Minimum load, lbf 6200 15 900 27 270

(kN) (27.6) (70.7) (121.3)

Frequency, Hz 2 2 2

No. of cycles per block 5 5 5

Total No. of cycles 3 456 000 2 950 000 2 264 000

12
Table 8-

Alternating-Block-Spectrum

Fatigue

Tests

coh3B

<u & o o o 00 oo o <r o o o CM 00 MO i—4 O' o o o 00 rH 3

X) 4J o vO o 1—1 CO co

<u o CO 40 inmm 00 r^

•H 4J O' O' O' O' CM <r

T—

Dr CM co CM CM CM acO 3

CD

CD •H LO o m o Om o rH a CO 00 n> 'O m tH lO O CD

Sn| a

4-4 CM o m o 00 OO o o rH CM CM <r n- O'. o

CJ O' O' O'. O' t'- rH O' • •H o CD I"- CT\ in O' i—(

CO co

S3 CO co in

m 00 oJO O' O' <r o\ O' CM <r

CM CO CM CM CM 3o

jc

4J •Hb3

•U33

•He3

4J i—

4-1 333

a*

a)co JO3

co

4-1 CD

a) H

co

oo

•H

Dr

CD

a) /s 3

,3 rH /-v /—N 00 s So

4-1

g 00 o 00

m rH CM

m CJ 00 g o o m in rH m 43 3 rH tH 43 vD rH O' O

a) v-/ 'w' 'w' • O rH 003

a) 00 <r O' co o vD o •H

O 3 3 4-1 H •H CM O' m m <fr o> 00 4-> • pH CO CM CM <r CM CO •H

4H i—i 3

MT

X3 r—

• 3 3 00

4-J •H 4H 3 O • B 'w' 3 >, 3 •H rH 4J B B B 4-1 3 X -3 •H *H 3 3 3 3 3 o 4-J CD CJ •H •H •H -3 -3 O rH H 00 C cu 5 03 5 X X) 00 00 *H Dr 3 3 Dr o 3 o 3 3 •H •H 00 3 a 3 H 3 rH B rH B B -3 rC 3 4H 3 t 1 u H

rH

H

u 3 i—

3 3 3 3 O 4J H

Dr -3 4H 4-4 4J 3 c •H 3

a> -D & X •H B • 3 • •H O 'd- m CO 'O n- in CD X X 1 1 3 X

CJ 25 rH rH rH rH rH CM CM 3 3 •HH 3

cu 1 1 4J 3 3 1 1 a •H 3 4-4 rH *H

CD 3 3 •H 3

4H •H

X 3 4H s 3 4-4 3 u 3 3 3 3 H >-l 3 s U 4-1 3 B

•H CD -3 •H • rH rH CJ 4J 4J a

00 (U 3 3 CD 3 H 3

CU -H rH 1 1 03 X) X)XX Dr 3 3 3 Dr

D- 4-4 rH 1 1 3 3 3 3 3 CD

H

O 4-1 CD

O 3 3 3 rH rH rH vH 1 1 O H V4 -a jo 3 J0 -a

CJ 3 3 3 3 3 3 3 • • • • Dr O- u O 3 CJ CJ 3 -D o X

13
4.3 Residual Strength Tests

The residual strengths, of the seven specimens which had survived

the fatigue tests, were measured with tensile tests. These tests were

performed in a 400 000- lbf (1.78-MN) capacity, screw-powered testing

machine [1] at a crosshead speed of 0.75 in/min (0.32 mm/s). Each

specimen was subjected to ten cycles of load, from zero to a predetermined load and back to zero, before the load was increased to failure.

For Specimen No. 8, which had previously suffered a fatigue failure of one of its end fittings (Figure 7), the potting head was clamped directly

into the tapered wedge grips of the testing machine. The teeth marks

seen on the potting head in the figure are the result of this gripping.

The results of the tests are given in Table 9. Of the seven specimens tested, two failed in their free lengths at loads substantially

below their rated breaking loads. One of these. Specimen No. 17 (Table

8) had experienced a failure of one of its strands in its fatigue test,

and the other. Specimen No. 8 (Table 6), probably sustained shockloading damage in the recoil from the fatigue failure of its end fitting.

The other five specimens all failed in or near an end fitting. The

breaking loads for four of these specimens closely approached, or ex- ceeded, the respective rated breaking loads. The fifth, Specimen No.

26, a high-capacity cabled-strand specimen, failed at only 78 percent of

its rated breaking load. The failure occurred adjacent to both of its

end fittings despite the fact that there had been no seepage of potting

compound from the fittings. See Figure 4b.

5. SIMULATED WEATHERING STUDIES

The degrading effects of weather were simulated by means of environmental stress-rupture tests in which sling-leg specimens, under constant

tensile loads, were exposed to accelerated weathering conditions. These

conditions, adapted from Procedure C of ASTM Designation D1501 [2],

consisted of the following sequence of environments in each 24-hour

period:

2 h in an ambient temperature, saturated humidity environment;

2 h under heat and ultraviolet light;

2 h under saturated humidity as above;

18 h under heat and ultraviolet light.

This 24-hour cycle was repeated without interruption for a total duration of 240 hours unless the specimen failed sooner.

5.1 Test Equipment

The required tensile loads were applied to the specimens by 30 000-

lbf (130-kN) capacity, horizontal, dead-weight, lever-arm, creep- testing

machines (Figure 8) which can accomodate tensile specimens up to nine

feet (2.7 m) long [3]. The test environments were provided by environmental chambers installed in the machines.

14
Table 9-

Tensile

Tests

of

Fatigued

Specimens

CO

4*:3

CO cd 3 3 no 3 a 4-4 6

a) pi / r

\ /N “V

33m /*N /""N

O CO CO

cO vO • o . • • O z • p- • • p-* m

33 rH 2 av

m CM ONm <* <Ti 3 ''w' r>* rH vO i—i

rH rH 3 60 v-/ v«/ v ' •—

3 3

CO •rH

OO o o o o o

cO 4*

O o o in o m o

CD 3 av -<r rH co <r 02 S (D rD 3 rH m r^ in CM CO • 42 rH CO rH rH co CO 43 .

4-1 42

60 4-1 3 60 3 3

rH 3

42 “N /^S r\ rH

4-1 /T*N r^. /*-n rH id 3

so 60

g o rH cn vD CM 3 3 3 3 3 g rH /TV /'-'S r^ p- rH O 3 3 3 <D •rl 'w' rH 42 rO vO 45 rH i—I

4-1 3

rH 4-1 rH v-/ s—/ ' N'—'' '—

'w'

—

4-4

4-4

O

O

CU 3 5o VO

m r- -d- 4-1 3 3 a c • • • • . 3 •rH U •H CO 42 rO vO 2d m o •H

pH 'd' CM CN <r 33

33 3 3 i—1 a •rl 3 3 3 4-4 to 33 -a /—s

/-N /-s /T-S 2d 33 3 3 00 1—1

oo r-l rH . 3 3 O * • • • . • <r 4= rH r^* O r- o o CN rs

CO 00 CO CO 00 00 i—i

33 CO

60 V— s-/ s-/ v—/ 'w' i-/ 3 . 3 3

•H o o o O o o o O

rH o o o O o o o a 3 o M-l m o <r m o o o B O

JO 00 00 00 o •3 o tH 00 00 oo 00 CJ 4J 1—I

rH rH CN CJ 60 3 3 C/3 •rH

4-J 3

42 /—N

x—s

4H 3

4-»

m 20 /T-S vO 2dO

W

SO 60

g o rH rH 00 rH

a 3 <D 3 g rH O /T-S 45 rH o . 3 3 •H 'w' rH rH x> vO rH rH 3 3 so

t-H

O rH 2-^ 'w' 'w' •—* 'w*' 2—^ 3 60 3

<4-4 CJ 3 3 •rl 3 a) >2 m o < co rH o -3 B 4J 3 42 CJ c • • • • • • S 3 4-J 3 •H CO o X* vO vd m o 33 •r-l <r •O' CM CN 60 4H 3 W

•H 3 33

4-1

O 3

4-1 •rl 3

33 'rH > 3 -rs /TS 4H 3 3 O IS o X—N o O 02 3 3

rH 00 2d 00 00 Id VO <r 33 <2-

O

33 s-/ r^ rH n- 1—1

rH CN 3

CD 60 S-*' s— •^^

w •—

3 33 3

4-» 3 3 33

cO •H o o OO o O

O 3 3 •rl Pi 44! o O o O o o o 3 •rl 3 3 4-) m O m m o o o 3 3 3 3 JO 3 4H •H 3 rH 45 2d vO 2d m

42 T—1

co rH rH CO CO in 33 3 33 3 w 3

rH • rH l 3 •H 33 33 •rl o CD • 3 3 3 3 3 6 O CM m 00 CO 20 r^. 2d 4-4 3 42 4H a •H z rH rH rH rH rH CN 3

co 3 w 3 3 3 3 3 3 B 3 B B

•rl 6 •r-l •rl 1 rH rH 3 CJ O 3 3 0) 3 3 4-1 3 3 3 60 o rH rH T3 XI 33 n0 33 a o a (X

a. •rH «H rH rH Q) <D 3 <D 3 CO 2 CO C/2 o 4-4 Jj cd 3 rH rH rH rH i—i

Pi 3 cd

U 3 X* 42 X* 42 o u cd 3 cd cd 3 cd 3 • . • • CJ a a a o CJ CJ U 3 42 CJ 33 3 3 3 3 B

•rl3 H

4-1

4-1

B -h H 4-1 CJ

15

failed

by

pulling

out

of

one

end

fitting.

failed

adjacent

to

both

end

fittings

(see

Figure

4b).

Potting

compound

had

not

seeped

out

of
The housing for each chamber was a 10- in (0.25-m) diameter asbestoscement pipe, 4 ft (1.2 m) long. The chamber was mounted in the machine

such that the specimen, with pull rods attached, passed concentrically

through it with approximately one-half of the specimen, including one

end fitting, located inside it. The other half of the specimen and its

end fitting remained outside of the chamber. The ends of the chamber

were fitted with transparent plastic windows having central holes to

enable the pull rod on one end, and the specimen on the other, to pass

through. Ultraviolet light was supplied by twelve 48- in (1.2-m) tubular

fluorescent sunlamps which were uniformly spaced around, and affixed to,

the inner surface of the asbestos-cement pipe.

The saturated humidity environment was provided by an atomizing

nozzle mounted 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 distilled water to the interior of the chamber.

The nozzle was oriented so that the spray did not impinge directly on

the specimen. The rate of water consumption was adjusted by regulating

the air flow rate to maintain a light film of condensate on the windows

of the chamber.

• The 24-hour exposure cycle was controlled with an automatic cycle

timer

In the first test conducted with this apparatus, it was observed

that temperatures in the environmental chamber reached approximately 200

°F (93 °C) during the sunlamp exposures. In the subsequent tests a small blower was used to circulate air through the chamber during the

sunlamp exposures in order to limit the maximum temperatures to more

reasonable values.

5.2 Tests and Results

Seven tests were performed. The tensile load applied to each

specimen during the environmental exposure was one-half of its rated

working load (Table 1). The results of the tests are given in Table 10.

Four of the specimens survived the simulated weathering tests

without failure. Three specimens, all of the parallel-strand variety,

failed by pulling out of the end fitting which had been exposed to the

weathering. Examination of the specimens following these tests showed

that the potting compound which had been used in the end fittings on the

parallel-strand rope specimens had softened excessively as a result of

the environmental exposures. This effect was not noted for the potting

compound used for the cabled-strand rope specimens.

The four specimens, which survived the simulated weathering tests,

were then tested in tension following the same procedure used for the

specimens which had survived the fatigue tests (Section 4.3). The

results of these tension tests are given in Table 11.

16
Table

10-

Simulated

Weathering

Tests

w3

COB

a> ed3o 3 -H

cn 3

(1) c0 H l-i 3X3

l-i 33

c0

l-i 3aB

a) H

3-i

4-4 4-1 4-1 <4-1

<1- CM r^ o o o o CO CM O O <r V CM CN CM CN CM CM >43

X)

CO CM ON CO

00

CO ooCM X O O On O

On

CO

CMO

CO CO co co co

CMOn

CM 00 ON On

C48

rH33

CO>3

a) 33

•HXI 00

003

•H33

/—N S rs /T-S /S —s

X

XI /N CM CM NO

O CM NO o 3 2; • • • • • • X O CM CM m ON CM m On 3

rH 'w' CM CM

m 00 CM LO 00 3 • 'w' 'w' 'w' Vw-' °H 3 3 3 3

rH 4-1 J0

•H o O o o O o o 4-1

g

cn o O o o o o o 3 Co 3 4-1 o O in o o in o X 3 rQ m m in >N 3 H rH CM o CM o rH rH CN i—

CM i—

1 1 3 3 3 3 3 a 3 3B • 3

,3O /TV /-~S <TN /'"S /-N

O 3 /-v NT co m o o CJ 3

00

g CM o NO r-.

O •H 3 g jo rH ON 00 NO NO ON >

0) 'w' 'w' 'W Vw' V-/ CO 3

rH 3

+i

0) 00 NO CM o CM '—s 3

<u 3 • • • • • • •H 3 •H .o oo m CO m NO 00 Pm

Pm CO CO CM CM CO o 003 m •H3

+i 3

•H

r\ 4-1 cu Sn 3

I—1 3 B

g 1 1 X

•H *H 3 O 3 cn o •H r* •H X kO 3 3 3 S s X) oo & XI 00 a 3 a. o o 3 •H O 3 •H 3 H 3 rH rH B ,3 rH B ,3 3 O 3 3

(X 33 3 O 33 00 3 4-J 3 3 3 •H B • 3 1 1 •H O 00 ON rH CM o CO 3 • rH o z rH rH CM CM CM CM CM C4 X 3 3 S 3 a a 3 3

CO 3 • 3 >N 3 3 X B 3 3 3 3 3 X B cn B 3

rH rH rH rH •H

O 1 1 • 3 3 3 3 X 3- 3 •H 3 00 rH rH rH rH X) X X 3 X o 3 a -h i-H rH t—1 rH 3 3 3 £ 3 2 34 O 4-4 3 3 3 3 rH rH 1 1 P4 3 3 3 3 3 r& x -O o 3 3 3 3 3 3 3 • o 3. Ou (X CX O a 3 x> a

17

Except

125

°F

(52

°C)

during

one

18-h

interval

due

to

blockage

in

air

stream. ±

7.5h;

timer

cut-off

switch

malfunctioned.

Test

terminated

without

failure.
Table

11-

Tensile

Tests

of

Artificially

Weathered

Specimens

G

pH o

XJ • <y\ G —\

vO o • • O 53 o • rH T3 rH Pi CM CN G w A r— rH CN M bO G C v '

cn -H o O o o G Hi o O o o G G <4H co CO ON CSj g G rD

>-l 1—1 vO rH rH HP »—

CO L/0

A

ne

4-1 bO bO /^N

/*v in /N

vO

/—N COo G M G g vO vO o G G -H g CO vO vO rH

rH 4-1 i—

'w' 'w'

G

<4HO

G >v CNJ CM vO l/o

G

aC • • • • p4 •H vO vO av

pH CO CM CN CO

HPub

gG

bOc

•H

vO /X-V /H vO

XJ • CO 1—1 • G <r • » O 52 CM o CM i—1 Pi i—i CO 00 rH '

s '— '— v/ bOC o o o o

•H o o o o rH <4H o U0 o o U HP 00

pH rH 00 00 00 u CM rH CM

vO

VO

00 o

vO

vO

co

vO

vO

CTv On

4HO

G 4J

G

G

r

—

o m CM

g 4-1

G -Q o • • • • •H •H H •H <r vO vO ON a 4H

Eh CO CM CM CO

Ga • T3 m HP C

4J

G

<4H 00 X) o G

O

G ^—

r~\

G 4-1

O /x-N CTv

—

o CTV HP r-H i—

Hj- 00 vO <d- 4H 4-1

M CM r- rH CM 00

G

G

X) bO

v

* v—r v-r '—

G

GG

G

GG

H o

4-1 •H

O o o O

U mhG

G Pi o o o o 4-1 •ro Pi G 4-1 o m o o m C X) G £> •H G

p4 rH vO r^- vO vO • #v hp m i—

CO

m X3 X) X) G G G i—

i—i—1 1 G •H •H •H O G • G G G G g o CM o CO <r MH Mh Mh

CX *H

CO

53 CM CM CM CM

X)

GC o G G u g g

•H

H I i—

1—

O O 3 G G 1—1 G G

G bO

O i—X. x» X) 3 ex ex

CX -H

H rH

GGG PH CO CO

O 4H 4-1

G rH t—

rH

05

G

GH HP rO HP

O

HG

G

G

G • • • U ex a o U G HP a G

4-1 GG

•HBG

4-1 a) x)G

•H

>H

ai 1

hpo(

X)•HW

4-1 3OG

a)

a) hpG

HP

HPO

•H&

bOG

18
In the tension test of parallel-strand Specimen No. 22, one of the

tensile pull rods failed at 46 300 lbf (206.0 kN) , due to a machining

defect which had not been detected "previously . The recoil from this

failure appeared to damage the specimen near the end fitting which had

been exposed to the simulated weathering. In a subsequent re-test, the

specimen failed at this damaged site, at a load of 43 500 lbf (193.5

kN).

One of the three cabled-strand specimens failed, surprisingly,

adjacent to its end fitting which had been outside of the environmental

chamber during the simulated weathering exposure. The other two cabledstrand specimens failed in their free lengths; one at a load close to

its rated breaking load, and the other at a load 9 percent below its

rated breaking load. Removal of the braided polyester jacket from the

latter specimen revealed that the rope, in that portion of the specimen

which had been exposed to simulated weathering, had become discolored.

The fibers, usually yellow, had assumed a deeper, orange tint. This

discoloration was not observed on any of the other specimens tested.

6. PERMANENT ELONGATION

The specimens tested in this investigation usually sustained some permanent (or quasi-permanent) elongation as a result of the tensile

loads which were imposed upon them. The measurements of free length,

given in Tables 4, 6, 8, 9, 10 and 11, permit the permanent elongations

to be calculated. These calculations suggest the following conclusions,

which must be considered only approximate and unsubstantiated, since the

volume of data upon which they are based is very sparse.

The application of five or ten load cycles up to approximately onehalf of the rated breaking load of a specimen produced permanent elongations

of 0 to 1.5 percent. The magnitude of the elongation tended to increase

with rope diameter and, surprisingly, was not heavily dependent on whether the cycles were applied to a previously untested specimen, to one which had been fatigued, or to one which had been exposed to simulated

weathering under load for 240 hours.

The fatigue tests produced permanent elongations which also tended

to increase with rope diameter. These elongations ranged up to 1.5

percent for parallel-strand ropes, and up to 5.3 percent for cabledstrand ropes.

The simulated weathering exposures produced permanent elongations

(i.e., creep) of less than 3 percent. These elongations did not appear

to be very dependent on rope configuration or rope diameter.

The largest total permanent elongation was observed on high-capacity

cabled-strand Specimen No. 26, which sustained 5.3 percent elongation in

its fatigue test and another 1.0 percent in the ten load cycles applied

prior to its subsequent tensile test (Tables 8 and 9) . The largest

19
permanent elongation observed on the parallel-strand specimens was 3.0

percent on Specimen No. 22. This high-capacity specimen experienced 2.7

percent elongation as a result of its simulated weathering test and

another 0.3 percent in the ten load cycles applied prior to its subsequent

tensile test (Tables 10 and 11).

7. DISCUSSION

7.1 Breaking Loads

The tests which were performed to evaluate the tensile breaking

loads of the specimens in their as-received conditions (Table 4) all

culminated in failures which were attributable to inadequacies of the

end fittings. Because of this, the true strengths of the aramid-fiber

ropes were not determinable and a comparison of the relative strengths

of the parallel-strand and the cabled-strand ropes was not possible. In

spite of the undesirable failure mode, some specimens of both configurations attained over 98 percent of their rated breaking loads before

failure.

7.2 Fatigue Endurance

Out of twelve specimens subjected to fatigue loading, only one failed to endure the imposed fatigue spectrum because of damage incurred

in the rope itself. All of the other specimens either survived the

entire spectrum or failed due to an end-fitting inadequacy. The single

rope failure occurred in a high-capacity cabled-strand specimen (No. 25,

Table 8). However, another high-capacity cabled-strand specimen survived

the same fatigue spectrum. So it is not known, on the basis of these

limited data, whether or not the lone rope failure reflects a weakness

in fatigue that is characteristic of this rope style. No high-capacity

parallel-strand rope specimens were tested in fatigue, so a direct

comparison of the two high-capacity configurations in fatigue is not

possible.

In tensile tests of the seven specimens which survived their

imposed fatigue spectrums, six failed due to end-fitting inadequacies

and only one (Specimen No. 17, Tables 8 and 9) due to fatigue damage

sustained by the rope itself. This medium-capacity cabled-strand

specimen attained only 90 percent of its rated breaking load. Another

medium-capacity cabled-strand specimen attained 98 percent of its rated

breaking load before pulling out of one of its end fittings so here,

too, it is not known whether or not the lower value reflects some inherent

weakness that is characteristic of this rope style. A medium-capacity

parallel-strand specimen also attained more than 98 percent of its rated

breaking load following fatigue cycling. In fact, most of the specimens

which had survived their imposed fatigue spectrums attained more than 98

percent of their rated breaking loads before failing due to end-fitting

inadequacies. A notable exception was the high-capacity cabled-strand

20
specimen, referred to above, which had survived the fatigue spectrum.

In its subsequent tensile test, thijs specimen (No. 26, Table 9) failed

at an end fitting at only 78 percent of its rated breaking load.

Since no high-capacity parallel-strand specimens were studied in

fatigue, there are no data to indicate whether or not the fatigue

strength of the parallel-strand configuration might be better than that

of the cabled-strand configuration in the high-capacity size.

7.3 Weatherability

Out of seven specimens subjected to simulated weathering conditions

under load, three failed due to end-fitting inadequacies and four survived

the entire 240-hour exposures. In subsequent tensile tests of these

four specimens, only two, both having cabled-strand configurations,

failed in a manner which reflected the true residual strengths of the

ropes (Table 11). One of these, a low-capacity specimen (No. 20),

attained 99 percent of its rated breaking load but the other, a highcapacity specimen (No. 24), only reached 91 percent of its rated breaking

load before failing in its free length. The latter specimen was the

only one subjected to simulated weathering which was not fabricated with

a relatively impervious jacket, and the rope in this specimen was the

only one which had visibly degraded due to the applied environment. Two

other tests culminated in failures at even lower percentages of the

rated breaking loads, but the modes of failure do not permit any con- clusions to be drawn regarding the relative performances of the

parallel-strand and the cabled-strand configurations.

7.4 End Fittings

All of the parallel-strand specimens were fitted with potted, open

spelter sockets. The failures of all but one of these specimens was clearly attributable to inadequacies related to the fittings. Two such

inadequacies were predominant. One was the potting technique, which

permitted potting compound to seep into the rope fibers outside of the

fitting before it was cured, thereby diminishing the flexibility of the

rope near the fittings. The second inadequacy was the potting compound

itself. While the potting compound exhibited the flexibility, under

normal environmental conditions, that NBS work has shown to be desirable

[4], its degradation under the simulated weathering exposures was

excessive.

These two inadequacies were not as predominant in the end fittings

used on the cabled-strand specimens. Seepage of the potting compound was a serious problem only on the low-capacity specimens. It appeared

as though the manufacturer of the cabled-strand specimens had somehow

solved the seepage problem before assembling the medium- and highcapacity specimens. The potting compound used on the cabled-strand

specimens did not appear to suffer serious degradation under the simulated

weathering conditions; but, because it was rather hard, it may have

21
contributed to a number of other undesirable failures. Two specimens

failed adjacent to end fittings where seepage had not occurred, and one failed inside an end fitting. These failures occurred at loads corre- sponding to 78, 89 and 101 percent of the respective breaking loads of

the specimens*, and appeared to have been induced by the stress concentrations imposed by the end fittings. NBS work has shown that these

stress concentrations can be reduced by using a more flexible potting

compound [4]. Of course, it is not immediately obvious that a suitable

potting compound can be formulated which is flexible under ordinary

environmental conditions yet does not degrade rapidly under severe environments

End-fitting parameters other than those related to the potting

compound also merit discussion. It seems unreasonable to expect that

commercial end fittings, designed for use on steel wire rope or on lowmodulus, synthetic fiber ropes, would necessarily perform well on aramidfiber rope. Most of the end fittings used in this investigation fall

into these categories. However, two of the end-fitting types used in

this investigation (Figures 4b and 5) were apparently designed specifically for the sling-leg specimens. Some measure of success was thereby

achieved, since in three tests involving these end fittings, two culminated in failures which occurred in the free lengths of the ropes.

Going a step further, it is not even true that end fittings of the

potted compression variety are necessarily the best for high-strength

ropes. Studies involving fiberglass rope, for example, have shown that

dead-end guy grips and potted, shear-type end fittings** both performed

better than potted compression- type end fittings [4]. Dead-end guy

grips, unfortunately, exhibited low fatigue strengths when the cable

specimens, to which they were attached, were cycled between a slack and

a taut condition [5]. However, the fatigue failures occurred in the

outside loops of the dead-end guy grips and suitable reinforcement

could, perhaps, rectify this deficiency. The potted shear-type end

fittings have not been evaluated under fatigue loading.

7.5 Synergistic Effects

In this investigation the effects of fatigue loading and of weathering were evaluated separately, using different specimens, although a helicopter sling in service experiences both effects simultaneously. It

may be well to note, therefore, that tests of fiberglass ropes revealed

significantly reduced fatigue strengths following exposures to simulated

weathering conditions [6].

*Specimen Nos. 26 (Table 9), 24 (Table 11) and 13 (Table 9), respectively.

**NBS Mod 4.

22
8 . CONCLUDING / REMARKS

As part of a program to develop improved external cargo slings for

helicopters, a test program was carried out on prototype sling-leg

specimens fabricated from aramid-fiber ropes with potted, compression

end fittings. The ropes included both parallel-strand and cabled-strand

varieties, in three different tensile capacities. The test program was intended to provide data on the actual tensile breaking loads of the

ropes and on their resistances to fatigue damage and to weathering. Most

of the specimen failures incurred in the tests were directly attributable to inadequacies in the end fittings. This fact, together with

the sparse amount of data acquired (only twenty-six specimens were

tested), prevented complete evaluation of the maximum capabilties of the

ropes themselves. Therefore, the following observations are offered in

the absence of fully substantiated conclusions.

1. No evidence was obtained which indicates that new sling

legs with adequate end fittings would be incapable of

meeting the specified breaking load requirements.

2. Some variability was observed in the performance of

the cabled-strand ropes, which suggests that their

resistances to fatigue damage — particularly in

the medium- and high-capacity sizes — may not be

entirely adequate for the intended application.

This observation should not be interpreted as necessarily favoring the parallel-strand configuration,

since the data obtained on the latter were simply

inadequate for a comparable judgement. The endurance

of sling legs fabricated with either rope configuration,

and particularly the parallel-strand variety,

could be improved with more adequate end fittings.

3. No evidence was obtained which indicates that the

sling legs could not demonstrate acceptable weather

resistance if terminated with adequate end fittings

and protected with relatively impervious jackets.

Regarding the latter, the weathering protection

provided by a braided jacket appeared unsatisfactory in comparison with that which was provided

by extruded polyurethane and neoprene- impregnated,

braided jackets.

This test program was undertaken because aramid-fiber ropes appear

to offer advantages, in terms of strength/weight and flexibility properties, for airborne cargo sling applications. The results which were

obtained do not contradict this premise and it is felt, therefore, that

further development and testing of aramid-fiber slings is warranted.

This effort should concentrate, primarily, on the development of improved

end fittings tailored to the characteristics of aramid-fiber rope as well as the demands of the intended application.

23
9 . REFERENCES

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

Engineering Mechanics Section, National Bureau of Standards,

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

2. American Society for Testing and Materials, Standard Recommended

Practice for Exposure of Plastics to Fluorescent Sunlamp, ASTM

Designation D1501-71, 1975 Annual Book of ASTM Standards , Part

35 (1975).

^

3. 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).

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

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

(Dec. 1972).

5. Halsey, N. and Quindry, T. L., Relative Performance of Combinations

of Guy Grips and Attachments, NBS Report 10849 (May 1972).

6. 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).

24
Figure 1. Sling-leg specimens in three tensile capacities, fabricated

from parallel-strand ropes with neoprene-impregnated, braided

nylon jackets and potted, open spelter sockets. (Disregard

the numerals on the sockets; they are not specimen designations.)

Figure 2. Sling-leg specimens in two tensile capacities, fabricated from

cabled-strand ropes with extruded polyethylene jackets.

(a) , above: low-capacity specimen (No. 3) with potting head.

(b) , below: medium-capacity specimen (No. 23) with reinforced

potting head.
.

,
Figure 3. Sling-leg specimen (No. 17) with a rope strand that failed in

fatigue and punctured the polypropylene jacket. This is a medium-capacity specimen, fabricated from cabled-strand rope.

Figure 4. High-capacity sling-leg specimens after test. These specimens

were fabricated from cabled-strand ropes with braided polyester

j ackets

.

(a), above: Specimen No. 25 with free-length failure incurred

in fatigue.

Specimen No. 26 with failure adjacent to experimental

end fitting, incurred in tension test following

fatigue loading.

(b ) , below
.

,
Figure 5

Figure 6.

Proprietary, experimental, phosphor-bronze end fitting. Fittings

of this kind were used on one high-capacity, cabled-strand

,

sling-leg specimen (No. 25).

Parallel-strand sling-leg specimen with failure adjacent to an end fitting where the potting compound had seeped into the rope

fibers and diminished their flexibility.

Figure 7. Fatigue failure of potting head on Specimen No. 8. This is a low-capacity, cabled-strand , sling-leg specimen which was subsequently tested in tension by gripping the remaining portion

of the potting head with wedge grips.
' *
X

CD

T“H • r-4 X) 3 CO

4-1 O

Cn rH C

•H (D

T“i

C0 *rt

>-i CO

cu e

43 <D

43 VJ 3 CD X)

i—t CO

cO 3

cnC

CDB

•HO

CDa

cn X

o

5 cn

cn

CD co

j-i i—

CD 00 X U

CD C 40 5 -H O cm

43

cn x

CD

•> X

cn 3

CD J-i 3 4-4 •H i—

43 3

CJ 0-

cOe cmo

00C cn •H -U M cn

cn cd

CD 4-1 4-1

i oo

a- c

CD *rl CD U

3 CD

CJ 43

4-1 cO

4-) 3

•H 3

3

cO X

CD 3

3 4-1 3 3

1 -H

43 3

00 6

•i-l 'rH 43 3

cm 3

CP o 2 CM

oo3

3

3

00

•rH CM

NBS 1HA (P_v. 7 - 73 )

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 76-11592. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Fatigue and Weatherability Studies of Aramid-Fiber

Rope Slings

5. Publication Date September 1976

6. Performing Organization Code

7. AUTHOR(S)

Nixon Halsey and Leonard Mordfin

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project /Task/Work Unit No. Cost Center 2130545

11. Contract/Grant No.

AMRDL 75-2

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

U.S. Army Air Mobility Research and Development Laboratory

Eustis Directorate

Fort Eustis, Virginia 23604

13. Type of Report & Period Covered

Final, 4/75 - 12/75

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.)

Tests were carried out on twenty-six sling-leg specimens, fabricated from

aramid-fiber ropes, as part of a program to develop improved external cargo

slings for helicopers. The ropes included both parallel-strand and cabledstrand varieties, in three different tensile capacities. The tests were

intended to evaluate the reductions in rope strength caused by fatigue loading

and by exposure to simulated weathering. It was found that the fatigue

capabilities of some of the ropes may not be entirely satisfactory; but,

because of inadequacies in the end fittings installed by the rope manufacturers,

thorough evaluations were not possible. The results indicate that the performances of the ropes could be enhanced through the use of more efficient end

fittings, and suggestions for the development of such end fittings are offered.

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

)

Aramid-fiber rope; breaking load, rope; cable, aramid-fiber; end fittings, rope;

fatigue damage; rope, aramid-fiber; slings, cargo; terminations, rope;

weathering, simulated.

18. AVAILABILITY [X ] Uni imited

[ ] Eor Official Distribution. Do Not Release to NTIS

'} Order From Sup. of Doc., U.S. Government Printing Office Washington, O.C. 20402, SD Cat. No. C 1 4 PT Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS 21. NO. OF PAGES

(THIS REPORT)

UNCL ASS1FIED 34

20. SECURITY CLASS 22. Price (THIS PAGE)

UNCLASSIFIED $4.00

USCOMM.OC 29042-P74
- -

_

,

'

'