NBS

PUBLICATIONS

NBSIR 86-3375

S. G. Fattal

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Structures Division

Gaithersburg, MD 20899

April 1986

Issued September 1986

86-3375

1986

i Proving Ground

NBS

RESEARCH

INFORMATION CENTER NBSIR 86-3375

fj f*

f r\s*\ TENSILE PROPERTIES OF PLEATED

SYNTHETIC ROPE

S. G. Fattal

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Structures Division

Gaithersburg, MD 20899

April 1986

Issued September 1986

Prepared for: U.S. Army

Aberdeen Proving Ground

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

ABSTRACT

Pleated nylon ropes of two sizes and approximately the same

length were tensioned to rupture in a universal testing machine.

Several of the ropes were tested at room temperature. The others

were subjected to specified high and low temperatures before

testing. Deformation measurements of all the specimens were

recorded while testing was in progress. The results were used to

evaluate the breaking strength, ultimate elongation, and load-deformation properties, and to develop criteria for possible

application in the recovery of mired vehicles.

Key words: breaking strength? load-deformation? nylon rope?

pleated rope? pulse loads? specified strength?

stiffness? synthesis? ultimate elongation.

iii
PAGE

CONTENTS

ABSTRACT Ill

1. INTRODUCTION 1

2. DESCRIPTION OF TESTS 1

3. TEST RESULTS 4

3.1 Strength and Elongation 4

3.2 Load-Deformation Behavior 6

3.3 Failure Modes 7

4. INTERPRETAION 8

4.1 Breaking Strength 8

4.2 Stiffness Properties 9

4.3 Synthesis 11

5. SUMMARY AND CONCLUSIONS 14

6. RECOMMENDATIONS 16

7. ACKNOWLEDGEMENTS 17

8. REFERENCES 17

iv
LIST OF TABLES

Page

Table 3.1 Strength and Maximum Elongation 5

Table 4.1 Stiffness Properties of Pleated Rope

in Tension 12

v
LIST OF FIGURES

Page

Figure 2.1 Drawing of the 12,000,000 lbf

capacity universal testing machine 18

Figure 2.2 Special fixtures for attachment of rope

to built-in clevises 19

Figure 3.1 Load-deformation curves of specimens

1, 3 and 5 20

Figure 3.2 Load-deformation curves of specimens

2, 6, 7 and 8 21

Figure 3.3 Load-deformation curves of specimens

9, 10 and 11 22

Figure 3.4 Loan-deformation curves of specimens

12, 13 and 14 23

Figure 3.5 Load-deformation curves of specimens

15, 16, and 19 24

Figure 3.6 Load-deformation curves of specimens

17, 18 and 20 25

Figure 3.7 Failure of 6

4 -mm untreated ropes 26

Figure 3.8 Failure of 80-mm untreated ropes 27

Figure 3.9 Failure of 64-mm heat-treated ropes 28

Figure 3.10 Failure of 80-mm heat-treated ropes 29

Figure 3.11 Failure of 64-mm cooled ropes 30

Figure 3.12 Failure of 80-mm cooled ropes 31

Figure 3.13 General view of 12M lbf universal

testing facility 32

Figure 3.14 Specimen #17 being removed from

cooling box 33

Figure 3.15 Specimen #17 being installed in the

lower clevis 34

Figure 3.16 Specimen #17 under load 35

Figure 3.17 Specimen #17 after rupture 36
1. INTRODUCTION

Twenty pleated nylon ropes were tested in tension to determine

breaking strength and load-deformation properties. The ropes

were about 16-ft (5-m) long, center to center of eyes, and were

of two nominal sizes. Some of the ropes were tested under a constant ambient temperature. Others were heated for a specified

period then tested at ambient, or, were cooled and tested cold.

Synthetic fiber ropes are used in a variety of applications such

as mooring, towing and lifting. The type of rope tested is

intended for use by the U.S. Army to recover military tracked or

untracked vehicles stranded in mire, a ditch, etc. The recovery

is accomplished by transferring the kinetic energy of a moving

vehicle (the recovery vehicle) through the rope to the mired

vehicle as the moving vehicle is brought to a stop by the rope.

Full recovery may require several trials. Starting from rest,

with a specified slack in the rope, the recovery vehicle attains

a certain velocity before the rope becomes taut and exerts a "shock" load upon the stranded vehicle. The process is repeated

until the stranded vehicle is recovered.

The tensile tests conducted at the National Bureau of Standards

(NBS) is part of a testing program undertaken by the U.S. Army to

determine the suitability of eight-strand pleated nylon rope for

kinetic energy recovery applications. Actual use under variable

field conditions is predicated on the resolution of certain key

questions such as, definition of safe working loads (to prevent

premature rupture pf the rope or its attachments) , energy absorption capacity, progressive degradation due to repetitive use, and

degradation as a result of extreme temperature and moisture

conditions, or as a result of accumulation of field debris within

the fibers.

The NBS tests provide data on the strength, stiffness and energy

absorption capacity of pr eviously-untensioned ropes under

monotonic, quasi-static tensile loading to rupture. The

information is needed to assist the U.S. Army in the design of

selective field tests and subsequent upgrading of their product

compliance requirements. The ropes were supplied by Marlow Ropes

Ltd. 1

2. DESCRIPTION OF TESTS

The tests were conducted using the NBS large scale testing

facility [1] . The universal testing machine has a rated capacity

1 The manufacturer's or supplier's name is mentioned for

product identification purposes only, and does not

constitute endorsement of the product.

1
of 6000 kips (26,700 kN) in tension and twice as much in

compression. It is equipped with a stationary (locked) tension

crosshead at the top and a movable (sensitive) crosshead below

that, as shown in figure 2.1. In tension tests the specimen is

mounted between the two crossheads by means of. clevises.

Displacement-controlled loading is accomplished by lowering the

sensitive head which can be actuated either hydraulically or

mechanically.

The existing clevises were equipped with 16-in (406-mm) pins

which were too large to accommodate the eyes of the ropes. To

test the ropes, special fixtures were fabricated consisting of a 6-in (152-mm) diameter by 12-in (305-mm) long pin, a pair of

10-in (254-mm) O.D. by 2-in(51-mm) thick by 7-in (178-mm) long

steel tubing, and a pair of 16-in (406-mm) O.D. by 3-in (76-mm)

thick by 8-in (203-mm) long steel tubing for each clevis. The

special fixtures were designed to keep bearing, shear and

flexural stresses of the pin within the allowable limits under

the maximum anticipated breaking load of the rope. A sketch of

the attachments is shown in figure 2.2.

Except where noted otherwise, the tests were performed in

accordance with Test Method 6015 of Federal Standard 191A [2] . The standard states that the pins shall be of sufficient size and

held in a manner to assure breaking of the specimen in the free

length. All specimens failed in the free length.

The Standard calls for a measuring device graduated in 1/8-in

(3.21-mm) increments. During the tests the displacement of the

gage marks were monitored at discrete load intervals by two

theodolites. The resolution of the theodolites was within the

specified tolerance by several orders of magnitude. The theodolites were aimed at two bull's eyes flanking the center of the

rope and about 30-in (760-mm) apart as called for in the

Standard. The initial gage length was recorded after loading the

specimen to one percent of its specified breaking load in

accordance with the Standard. The distance center-to-center of

pins was measured at one percent of specified breaking load and

at actual breaking load.

To examine extreme temperature effects the ropes were conditioned

prior to testing as follows.

Four specimens of each size (64 and 80 mm; 2.52 and 3.15 in) were

tested at 72°F (22°C) , which was the constant ambient temperature

of the laboratory. The Standard calls for triplicate tests of

each size. The additional specimens of each size were initially

tested as trial runs to validate the test setup. As both these

tests turned out to be successful, their results were

consolidated with the corresponding triplicate tests at ambient

temperature.

2
Three specimens of each size were kept for five days in a thermostatically-controlled heating chamber at 165°F (74°C) . They were then cooled to ambient temperature for one or two days

before testing. The temperature of the specimens was monitored

by thermocouples during the cooling process to assure that

ambient temperature is attained before testing.

Three specimens of each size were cooled in a foam box packed

with dry ice to temperatures ranging from -71°F to -97°F (-57°C

to -72°C) . While in the box, their temperature as well as the

air temperature within the box were monitored by thermocouples.

They were then removed from the box and installed in the test

machine in about 10 to 20 minutes before the start of the actual

testing. The temperature of two ropes were also monitored during

the tests.

For low-temperature experiments, the Standard specifies that the

specimens be cooled to a temperature of -65°F + 5°F (-54°C + 3°C)

and tested cold. This provision is difficult to implement in a

laboratory where the controlled environmental temperature is at

72°F (22°C) . In consultation with the Sponsor, several schemes

were considered, such as testing the specimen within an enclosure

in which the air temperature would remain at -65°F (-45°C) during

testing. None of the schemes turned out to be feasible because

first, the explosive failure of nylon rope would shatter the

containment device, and second, any confinement would prevent

monitoring of gage deformations during testing. Two of the

specimens were tested while wrapped in double aluminum foil. This

scheme was abandoned after discovering that the foil was not very

effective in keeping the temperature from rising during the time

interval needed to mount the rope, and the breaking of the rope

caused an excessive amount of aluminum debris.

The detachment of bull's eyes at low temperature was another

difficulty encountered in the tests of cooled specimens, for the

first cooled specimen (a 64-mm or 2.52-in rope), gage length

deformation as well as pin-to-pin elongation were monitored

during the test. The center-to-center of pin displacements were

measured by sighting the scale of a retractable tape attached to

the sensitive crosshead. The free end of the retracting tape was

attached to a main column of the test machine such that during

the course of testing, the downward moment of the crosshead could

be monitored at discrete load intervals by sighting the uncoiling

tape at the exit point of its casing. The method proved reliable

and was adopted in the subsequent tests of the cooled specimens

as an alternative to the measurement of gage elongation.

• As noted earlier, the sensitive head can be actuated either

hydraulically or mechanically. The hydraulic system has a maximum usable stroke of 5 ft (1.5 m) and a displacement rate of

approximately 3 in/min (76 mm/min) . The mechanical system

operates at a displacement rate of approximately 12 in/min

3
(305min/min) • Its maximum available stroke depends on the initial

length of the specimen. The maximum available distance center to

center of clevis pins is 45 ft (13,7m). This is more than the

30-ft (9-m) estimated length of rope at breaking load.

The nylon ropes consist of eight strands, twisted clockwise and

plaited in pairs. In the 80-mm rope each strand consists of 56

sets of yarns twisted counter-clockwise. Each set consists of

three yarns twisted clockwise. Each yarn consists of nylon

filaments. In the 64-mra rope, each strand consists of 44 sets of

yarns twisted counter-clockwise. Each set consists of four yarns

twisted clockwise. The number of filaments in a yarn is less

than used to prepare the yarns for the 80-mm rope. Thus, to

obtain a certain specified size of rope the number of yarns in a strand and the number of filaments in a yarn are altered.

Plaiting introduces a repetitive pattern or a "pitch" in the

rope. Since plaiting does not introduce a constant twist, the

orientation of the filaments with respect to the axis of the rope

changes continuously within a pitch. The manufacturer's

specified breaking strength is 72000 kilos (158.7 kips) for the

64-mm rope, and 110,000 kilos (242.5 kips) for the 80-mm rope.

In the initial tests (specimens 1 through 10), the ropes were

loaded mechanically up to about 50 percent of breaking load. The

machine was then switched to the slower hydraulic system for the

remainder of the test. Specimens 11 through 20 were loaded

mechanically throughout the entire loading range to examine any

effects that might occur as a result of the faster loading rate.

The hydraulic system could not be used exclusively because the

total elongation at breaking load would have exceeded the

available stroke.

3. TEST RESULTS

3.1 Strength and Elongation

Table 3.1 shows the breaking strength and the corresponding

maximum elongation of the individual ropes. For specimens 1 through 15, the elongation of the gage length as well as the

length center-to-center of the 6-in (152-mm) diameter clevis pins

(pin-to-pin or PTP length, in brief) were measured. Actual gage

lengths and PTP lengths under one percent specified breaking load

are indicated in the table. Except as noted in the table, the

percent elongations of gage and PTP lengths were calculated from

the total elongation measured at breaking load. Where the last

measurement was taken at a load less than the breaking strength,

that load has been specified in a footnote to the table. For

specimens 16 through 20 only PTP length elongations were

measured. Where a measurement was lost due to any cause,

attention is drawn by an appropriate footnote to the table.

4
Table 3.1

Breaking Strength and Maximum Elongation

T ROPE size Breaking Gage Length in.

)

ci+-> •H ,o-Pin Length (ft .

)

P, NO. mm st renath 1% Id. brkg. Id. % elong. 1% Id. bkg. Id. % elono.

1 64 210.

0

k 31.30 a 53.98 72.4 - -

3 64 205.

0

k

? c 34.51 ? c 17.91 26.22 46.4

4 64 184.

5

k 31,10 d

? d

? d 18.38 26.63 44.9

5 64 192.

5

k 30.62 50.27 64.2 17.88 26.58 48.7

oLU 2 80 321.0 33.00 52 . 83 b 60.1 18.73 28.08 49.9

l—<UJ 6 80 297.5 31.06 51.60 60.1 17.38 26.41 52.0

q:t—2 7 80 295.5 29.88 49.62 66.1 19.28 29.13 51.1

=5 8 §0 311.0 30.72 51.65 68.1 18.73 28.33 51.3

9 64 210.0 30.22 50 ,70 b 67.8 17.53 27.03 54.2

QLU 10 64 217.0 30.00 51.78 72.6 17.50 26.63 52.2

h-< 11 64 209.5 30.50 50.11b 64.3 17.90 26.91 50.3

oc 12 80 323.2 29.49 51.59 76.2 17.78 27.43 54.3

<LU 13 80 319.0 30.33 51. 41b 69.5 18.13 27.35 47.0

14 §Q 320.0 30.03 52.62 75.2 17.81 27.48 54.3

15 64 215.0 33.83 53 . 89 b 62.7 18.57 27.22 46.6

16 64 232.3 _ e - - 18.60 27.35 47.0

19 $4 224.0 _ e — — 18.35 26.93 46.8

QLU

_J 17 80 335.5 _ e - - 18.70 28.05 49.7

O

O

o 18 80 329.0 _ e - - 18.27 27.43 50.1

-.20 —8.Q. 336.5 _ e - - 19.16 -2$iP5L- 46.2

ige Length and Pin-to-Pin Length at; 1600 lb. Rope 1; others, at 1600 lb

>4 mm) (2600 lb (80 mm)-

ist measurement at: 330 k (#2); 200 k (#9); 200 k (#11); 200 k (#15).

ill's eye of top of gage became detached during test.

ie theodolite adjustment was destroyed,

ily Pin-to-pin elongations measured throughtout test.

Conversion Factors

1 kip = 4.45 kN

1 in. = 25.4 mm

1 ft. = 0.305 m

5
3.2 Load-Deformation Behavior

Load-deformation measurements were taken at discrete load

intervals throughout the entire loading range. The deformations

were gage length elongations for specimens 1 through 15, and PTP

length elongations for specimens 16 through 20. This information

is presented as load versus percent elongation plots of the

individual ropes in figures 3.1 through 3.6. Replicate plots of

a given size rope and specified temperature are shown on the same

figure. The average curve of the replicate tests is shown as a solid line. The average percent elongation at 75 percent of

specified breaking load is also shown. Other comments specific

to these curves follow.

Among specimens 1,3, 4, 5 (64-mm ropes tested at ambient

temperature) the load-deformation plots for specimens 1 and 5 are

the only ones available (see footnotes table 3.1). These are in

fairly close agreement as seen in figure 3.1. The information

for specimen 4 was not meaningful because one of the theodolites

went out of adjustment after sighting the initial gage length

reading. In the case of specimen 3, the actual gage length is

not known because one of the bull's eyes became detached after

the initial gage length reading had been taken. After the bull's

eye was lost, however, the theodolite kept tracking an identifiable target until rupture. The load-deformation curve for

specimen 3 shown in figure 3.1 is developed by assuming an

arbitrary initial gage length. It is seen that if the elongations of specimen 3 were reduced by the ratio of percent

elongation of specimens 1 and 5 to percent elongation of specimen

3 at breaking load, the load-deformation curve of specimen 3 will

not be appreciably different from those of specimens 1 and 5.

The load deformation curves for specimens 2, 6, 7, 8 (80-mm ropes

tested at ambient) are in good agreement, with specimen 2 showing

slightly greater stiffness than the rest (figure 3.2). The heattreated specimens of either size (specimens 9-11 and 12-14), also

show close agreement in their load-deformation properties

(figures 3.3 and 3.4).

Figure 3.6 shows load versus percent PTP length elongation of

specimens 17,18,20 (80-mm ropes cooled and tested cold). The

temperature of specimen 17 was -83°F (-64°C) at the time it was

removed from the cooler. It was tested 40 minutes later.

Specimen 18 registered a temperature of -88°F (-67°C) when it was

removed from the cooler. At the start of testing 23 minutes

later, a thermocouple embedded at the center. of the rope

registered a temperature of -43°F (-42°C) . The temperature was

monitored during testing which lasted 10 minutes. The last

reading, taken at rupture was 16°F (8°C) . The temperature of

specimen 20 was -103°F (-75°C) when removed from the cooler. No

other temperature readings are available for specimens 17 and 20

because the thermocouples detached during handling of these

6
specimens. According to the diagrams shown in figure 3.6, these

temperature differences did not have a significant effect on

load-deformation properties.

Figure 3.5 shows the plots for specimens 15, 16, 19 (64-mm ropes

tested cold). When first removed from the cooler, specimens 15,

16, and 19 registered temperatures of -70, -74 and -97° (-56, -59

and -72°C) , respectively. The temperature of specimen 16 ranged

from -5°F to 45°F (-21°C to 7°C) during testing. The temperatures of the other specimens were not monitored. The nearly

identical curves for specimens 16 and 19 once again demonstrate

the insensitivity of load-deformation properties to initial

temperature differences between the replicate specimens. On the

other hand, figure 3.5 indicates a major difference between PTP

elongations (specimens 16 and 19) and gage elongations (Specimen

15). Most of the differences occurs at 10 percent breaking load

at which point the elongation of the gage length is more than

twice that of the PTP length. Beyond that load level, however,

all the specimens exhibited similar stiffness properties until

rupture.

3.3 Failure Modes

Figures 3.7-3.12 show the specimens after failure. Replicate

specimens are shown in the same figure and are identified by

their numbers. The top ends of the specimens (as installed in

the test machine) are shown on the left side. Figure 3.13 is a

general view of the testing facility with a specimen ready for

testing. The movable work platform is at the center of the

figure. Figures 3.14-3.17 show, respectively, specimen 17 being

removed from the cooling box, being installed in the clevis,

under load, and after rupture.

All the specimens broke suddenly and explosively, releasing fiber

debris at the point of rupture. Popping of (intentionallyplaced) black yarns within the ropes with increasing frequency

provided an indication of approaching failure. In all cases

rupture occurred in the rope. In some cases the point of rupture

was away from the eyes (nos. 1,4,9,11,19). More frequently,

rupture occurred close to or right beyond the eye splice. No

ruptures occurred within the eye. Some of the specimens broke

completely (nos. 1-4,6,7,9,11,19). In the others, a single or

double-twisted strand remained unbroken.

All the specimens exhibited frictional heat damage to varying

degrees, as evidenced by the coloring of the yarns from tan to

dark brown and by their brittle texture. In specimen 1, smoke

was detected near the point of rupture before it occurred. In

most cases scorching was confined to the superficial yarns of the

strands, which had maximum exposure to frictional heat. Opon

rupture, the two segments of rope tended to compress axially

7
towards the respective eyes and then solidify in that state, in

some cases (specimens 10 and 15) the eye became tightly compressed around the pin and had to be sawed off to remove it.

Several of the tests were videotaped or photographed on a time-lapse film.

4. INTERPRETATION

4.1 Breaking Strength

The specified breaking strengths of the 64- and 80-mm ropes are

158.7 and 242.5 kips, respectively. The actual average breaking

strengths (table 3.1) were 198 and 306.2 kips (881 and 1363 kN)

,

respectively, for the specimens tested at ambient temperature

(the coefficient of variation was 0.10 for either size based on

four tests each). The corresponding figures were 212.2 and 320.7

kips (944 and 1427 kN) for the heat-treated ropes, and 223.8 and

333.7 kips (996 and 1485 kN) for the cooled ropes.

The above information indicates that the specified temperature

conditioning of the ropes had no detrimental effect on breaking

strength. In fact, the cooled specimens developed around 10

percent greater strength than the untreated ropes. This may be

attributed to the decrease of frictional heat degradation due to

the lower temperature at the start of the test.

In the case of the heat-treated ropes, strength was exceeded by

about 6 percent. The low strength of specimens 4 and 5 could

have been caused by the abrasion of some of their yarns observed

before testing. If this possibility is taken into consideration,

strength differences between heat-treated and untreated ropes

becomes statistically insignificant. According to the results in

table 3.1, the proximity of the point of rupture to the eye

splice (such failures are identified in section 3.3) had no

effect on breaking strength.

As noted in section 2, two different loading rates were used in

testing specimens 1 through 10. Attention is drawn to specimens

9 through 11 which are replicates. Specimens 9 and 10 were

loaded hydraulically to 100 kips (445 kN) , then mechanically to

rupture. Specimen 11 was tested mechanically all the way. In

this instance (table 3.1), the variation in the rate of loading

does not show any discernible effect on strength (no unusual

scatter compared to the other replicate tests)

In summary, ~ heat treatment had no significant effect on breaking

strength while low initial temperature caused the specimens to

break at slightly higher load than those tested at ambient.

There is some visual indication that superficial damage (such as

caused by prior fictional contact with a rough surface) , could

have an adverse effect on breaking strength. Variation in the

8
rate of loading in the range of 3 to 12 in/min (76 to 305 mm/min)

had no significant effect on breaking strength.

4.2 Stiffness Properties

The load-gage length elongation curves shown in figures 3.1

through 3.5 exhibit a common trend. Initially, large

deformations occurred under small loads. About two-thirds of the

ultimate elongation occurred under 10 percent of the actual

breaking load. Most of the stiffness gain occurred between 10

and 30 percent of the breaking load. Beyond that level, the

stiffness remained fairly constant to rupture.

The low stiffness under small loads may be attributable to the

slack in the "weave" of previously-untensioned plaited rope and

possibly, to initial creep effects due to the slow displacement

rate used. The consistency in replicate test results suggests

that at a certain load level (about 30 percent, in this case) the

rope becomes fully taut and develops a "characteristic"

stiffness. Since the maximum rate of loading also occurs at this

point and beyond, creep effects beyond the 30 percent load range

may not be significant. This is reinforced by the absence of

kinks in the load-deformation curves in cases where the loading

was temporarily stopped to allow the theodolite readings to keep

pace with the test.

For certain applications, manufacturers suggest that pleated rope

be "stabilized" first by subjecting it to a specified number of

load cycles at a given amplitude. This process minimizes

hysteretic effects, stiffens the rope and renders its loaddeformation response fairly linear. However, partial recovery of

the resulting permanent elongation occurs with time.

In a kinetic energy recovery operation, the rope is subjected to

three or four tugs. It is then stored, sometimes for months,

before the next use. Between the tugs, the rope folds upon

itself when the recovery vehicle backs up towards the mired

vehicle before the next attempt. Perhaps because both these

factors will tend to make a previously tensioned or stabilized

rope slack, the present Standard [2] does not call for prior

stabilization as a prerequisite for testing.

There is, however, another factor that needs to be considered.

According to field tests conducted at the U.S. Army Aberdeen

Proving Ground (APG) , the 'duration of the initial pulse load of a

tug is typically in the neighborhood of two seconds (one second

rise time) [3] . This fast rate of loading is greater by several

orders of magnitude than that achieved in the NBS universal

testing machine.

In a recent field test of an 80-mm rope from the same source as

the NBS specimens, but 70-ft (21-m) long, elongations produced by

9
pulse loads of 120 and 165 kips (534 and 760 kN) were in the

order of 10 and 18 percent, respectively. This indicates that

the soft portion of the response curves obtained from the NBS

tests may not materialize under a pulse-type load.

Because of its short period, it is possible that a pulse load

will engage part of the fibers but will not allow full tautness

to occur. The resulting stiffness would then be somewhat less

than the characteristic stiffness of a fully taut rope obtained

from quasi-static tests. If, for instance, a constant stiffness

is assumed for the average response curve (figure 3.2) between

the data points at 120* and 240*^ (530 and 1070 kN) (this portion

of the curve is virtually linear), the corresponding elongations

would be 8 and 17 percent, respectively. Therefore, on the basis

of this limited information, it appears that the characteristic

stiffness obtained from quasi-static load tests provides an upper

bound estimate on the stiffness obtained from the pulse-type

loads encountered in recovery operations.

The response curves of 64-mm untreated ropes (specimens 1 and 5,

fig. 3.1) are not distinguishable from those which were

heat-treated first (specimens 9,10 and 11, fig. 3.3). At

breaking load, the gage length elongations were likewise in close

agreement, while the PTP elongations of the heat-treated ropes

were, on the average, 12 percent more than those of the untreated

ropes. No specific reason can be given for this last difference,

except to note that the gage length elongations for specimens 9

and 11 were obtained at a load slightly less than their breaking

strength (footnote b to table 3.1), and also, elongations

monitored on a longer portion of the rope (PTP length) tend to

provide a better indication of the overall heat effect than those

obtained from a much shorter segment of rope (the gage length)

Similar observations can be made on the responses of 80-mm

untreated and heat-treated specimens. Differences in the

response curves (figs. 3.2 vs fig. 3.4) as well as gage and PTP

elongations at breaking load (table 3.1) are slight or insignificant. Note that heat treatment had almost no effect on the

characteristic stiffness of either size rope (the slope of the

response curve between approx. 30 and 90 percent of actual

breaking load)

Because of a change in deformation measurements from gage to PTP

length, no direct comparison can be made between the loaddeformation responses of untreated and cooled specimens. The

results for specimen 15 provide a possible link between the two.

Attention is drawn to figure 3.5 where the response curves of the

64-mm cooled ropes are plotted. Beyond a load of about 40 kips

(180 kN) , the curves are virtually identical in slope. In other

words, if the curve of specimen 15 were shifted to the left it

would closely match these for specimens 16 and 19, which were

developed from PTP elongations. The difference in the response

10
curves below 40 kips (180 kN) suggests the presence of a greater

degree of initial slackness in the gage length portion than in

the rest of the rope. An indication of this trend is provided by

the data in table 3.1 where the PTP elongations of specimens 1

through 14 at breaking load are consistently and significantly

lower than the corresponding gage length elongations. Note that

the gage length and PTP elongations of specimen 15 at breaking

load is consistent with those for the 64-mm untreated ropes while

its PTP elongation closely agrees with those of replicate

specimens 16 and 19. These observations lend creditability to

the test results of specimen 15.

Table 4.1 provides estimates of the stiffness properties in the

near-linear range of the response curves. These were calculated

by assuming a linear response between data points at 80 and 160

kips (356 and 712 kN) for the 64-mm ropes, and 120 and 240 kips

(534 and 1070 kN) for the 80-mm ropes. The load-strain ratio is

the slope of this linear portion divided by 100. The (characteristic) stiffness is the load-strain ratio divided by the

appropriate gage or PTP length. The listed properties were

calculated using average response curves of replicate tests.

The data in table 4.1 yields weighted average load-strain ratios

of 956 kips (4254 kN) for the 64-mm ropes, and 1346 kips (5990

kN) for the 80-mm ropes. The changes in load-strain ratios due

to heat-treating or cooling the specimens before testing are not

significant, in light of the inherent scatter in the test results

and the approximation introduced by the assumption of linear

response. The trend in the characteristic stiffnesses is the

same. Note that since stiffness is inversely proportional to

length, a stiffness based on the PTP length will be less than

that based on the gage length by the ratio of their lengths.

This explains the low stiffness values of the cooled ropes (table

4.1) determined on the basis of the PTP length.

In summary, pleated nylon rope develops a characteristic

stiffness within the range of approx. 30 to 90 percent of actual

breaking load. This stiffness appears to be an upper bound

estimate of the stiffness under a pulse-type load occurring in

energy recovery applications. This observation is preliminary

and needs further verification by selective field tests.

Subjecting the rope to a specified high temperature for a specified length of time before testing at ambient temperature

did not alter its mechanical properties under quasi-static

tensile loading. Similarly, cooling the specimen to a specified

temperature and testing it cold did not have a discernible effect

on these properties.

4.3 Synthesis

Several factors will influence the choice of a rope suitable for

kinetic energy recovery application. The right combination of

11
Table 4.1

Stiffness Properties of Pleated Rope in Tension

Size and

Temperature

Conditionina

Load-strain

Ratio

kiDS (kN)

Stiffness

k/ft (kN/m)

64 mm

Untreated

875 341

(Specimens

1 and 5)

(3894) (463)

64 mm

Heat Treated

975 387

(Specimens

9.10.11)

(4338) (525)

64 mm

Cooled

1126 400

(Specimen

15)

(5011) (543)

64 mm

Cooled

1026 56

(Specimens

16 & 19)

(4343) (72)

80 mm

Untreated

1481 571

(specimens

2. 6.7. 8)

(6590) (775)

80 mm

Heat Treated

1319 528

(Specimens

12.13.14)

(5870) (716)

80 mm

Cooled

1237 66

(Specimens

17.18.20)

(5505) (90)

12
size and length of rope will allow the required amount of energy

transfer to recover the mired vehicle, while keeping the

amplitude of the pulse load within a safe limit to avoid

premature rupture of either the rope or its attachments and to

inhibit frictional heat degradation.

According to the NBS tests, pleated nylon rope develops a

characteristic stiffness in tension under a slow rate of

loading. From the limited information available from field tests

at APG, it appears that the characteristic stiffness provides an

upper bound estimate of the stiffness occurring under a

pulse-type loading in a kinetic energy recovery operation. If

this observation could be further corroborated by additional

field tests, the characteristic stiffness can be used as a

criterion in the selection of an appropriate length for a given

size rope.

Suppose, for example, the length L of a given size rope is

desired. Assuming linear response, the stiffness k of the rope

is given by

K (4.1)

where P is the tensile force and a is the corresponding elongation

in length L. The notation L

Q and k Q designate the length and

stiffness of a specimen of tne same size obtained from tensile

tests (for instance, k Q

= 571 kips/ft for the 80-mm untreated

rope based on the average PTP length of 18.53 ft, tables 3.1 and

4.1)

The strain energy Us under load P is then.

U,

Pa

2 2k 2k.o o

This yields an estimate of the desired length.

L = 2k

o

L

ous

(4.2)

(4.3)

Assuming full energy transfer, the strain energy Os will be equal

to the kinetic energy of the recovery vehicle, Uk = mvz/2, where

m is the mass and v is the velocity of the vehicle just before it

exerts a pulse load on the rope. Load P will be governed by

considerations of maximum safe load to avoid failure in the rope

or its connections during recovery, and to minimize frictional

heat degradation in the material.

13
The compliance testing requirements of the present Standard can

be upgraded in certain areas to reflect the findings of the NBS

tests noted above. For instance, the requirement for recording

elongations at 75 percent of the specified minimum. breaking

strength will not yield meaningful results because the large

deformations occurring under low static loads in a previouslyuntensioned rope will distort the results significantly. Since

there is partial evidence that such large deformations do not

occur under pulse loads, it would be more meaningful to require,

instead, deformation measurements to be taken at two distinct

load levels, expressed as percentages of specified breaking

strength. The selection of the two load levels should be made in

such a manner as to yield reliable information on the characteristic stiffness of the rope. For the ropes used in the NBS

tests, data points at 40 and 80 percent actual breaking load (or

50 and 100 percent specified breaking load) provided reasonably

good estimates of the characteristic stiffness.

The definition of a safe working load depends on the strength of

the rope, the strength of the connections and the load-capacity

of the rope within which no permanent damage will occur. The NBS

tests provide information on the first of these factors. The

average breaking strength of the specimens exceeded the specified

strength by 25 percent in the case of the untreated ropes of both

sizes (section 4.1). The corresponding coefficient of variation

was around 10 percent. Therefore, the specified strength appears

to be reasonably conservative. In addition, the eyes of the test

specimens had sufficient strength to develop the strength of the

rope. However, it is not known whether this will also be true if

a different pin size were used.

The Standard should stipulate that the connections should be

strong enough to develop the strength of the rope. This will

require verification through independent testing of the connections. The last factor, the maximum load that can be applied to

the rope without causing material degradation, can be addressed

by extending the scope of testing to include cyclic loading

followed by loading to rupture. In such investigations the

principal test variables would be the amplitude of the cycled

load, the number of cycles, and possibly, the time interval at

zero load between consecutive cycles.

5. SUMMARY AND CONCLUSIONS

Pleated nylon ropes of two sizes and approximately the same

length were tested in tension to failure in a universal testing

machine. Some of the ropes were tested at the ambient

temperature of the laboratory. To study high and low temperature

effects, the remaining specimens were subjected to a specified

temperature for a specified number of days and tested at the

14
ambient temperature, or, they were cooled to a specified

temperature and tested cold.

The tests and pre-conditioning for temperature effects were

carried out in accordance with the U.S. Army testing

specifications for synthetic rope. The Army intends to use the

ropes to recover mired military vehicles provided certain

conditions are met. The NBS tests were part of a broader

program, currently being implemented by the Army, to determine

the suitability of pleated synthetic rope for recovery

applications. This report documents and interprets the NBS test

results in the light of this intended use. Included are data on

breaking strength, corresponding elongations, and load-deformation properties. The following conclusions are drawn based on

the observed behavior of the ropes in tension.

The mean breaking strength of the untreated specimens exceeded

the specified strength by 25 percent. The coefficient of

variation was 0.10 based on four replicate tests of each size.

The mean breaking strengths of heat-treated and cooled specimens

were about 6 and 9 percent above the mean breaking strength of

untreated rope, respectively. The coefficient of variation based

on triplicate tests varied from 0.03 to 0.09, depending on size

and type of treatment. The slightly higher strengths compared to

untreated rope cannot be given much significance because of small

sample size. However, it is reasonably safe to assume that the

specified temperature treatment will have no detrimental effect

on strength.

Similarly, the prescribed high and low temperature treatments had

no significant effect on the load-deformation properties of the

specimens. It is therefore concluded that simulated vehicle

recovery tests in the field need not be concerned with ambient

temperature fluctuations, specially since it is very unlikely

that the extreme temperatures used in the tests will be

encountered in the field.

All the specimens exhibited consistent load-deformation

properties. In specimens where gage length elongations were

measured, about two-thirds of the ultimate elongation occurred

within 10 percent of breaking load. Between 10 and 30 percent of

breaking load, the specimens developed full stiffness which

remained nearly constant up to breaking load. In specimens where

pin-to-pin (PTP) length elongations were measured, about onethird of the ultimate elongation occurred under 10 percent of

breaking load. Otherwise, the response was nearly identical to

those based on gage length elongations. The difference is

attributed to differences in the initial slackness of the rope as

a whole, versus that present in the gage portion of the rope.

15
According to the test results, a given size rope will develop a characteristic stiffness when tensioned at a low displacement

rate (compared to the high flexibility of the specimen) typical

of a universal testing machine. Good agreement between the

characteristic stiffnesses obtained from gage and PTP elongation

measurements of replicate specimens provides a degree of

confidence that this is a dependable or "stable" property.

On the basis of limited field tests by the U.S. Army, there is

some evidence that the characteristic stiffness provides an upper

bound estimate on the stiffness that develops under a pulse-type

load occurring in mired vehicle recovery operations. Subject to

further verification through additional field tests, the

characteristic stiffness could be used as a key parameter in the

selection of an appropriate size and length of rope for use in

recovery applications.

The tests show that elongation measurements at 75 percent of and

at breaking load currently required by the Army specifications

are distorted considerably due to the initial slack existing in a previously-untensioned rope. It would be more useful to take

elongation measurements within the stable portion of loadelongation curves for the purpose of determining the

characteristic stiffness. According to the test results,

elongation measurements at 40 and 80 percent of actual breaking

load (or 50 and 100 percent of specified breaking load) should

yield reasonably accurate estimates of the characteristic

stiffness.

6. RECOMMENDATIONS

The tensile tests of pleated nylon rope provided a ceratin amount

of basic information on strength and stiffness properties which

will be useful in upgrading current specifications and in

developing the scope of supplementary tests. The following are

suggested areas of research.

Selective field tests should be carried out to increase the data

base on stiffness properties of rope under pulse-type loads. The

results can be used to better correlate the stiffness obtained by

static load tests with that expected to occur in recovery

operations.

One question not addressed by this study is the maximum load the

rope can be tensioned to without causing strength degradation.

This can be achieved by cyclic tests followed by loading to

rupture. The test variables would be the number of cycles and

the amplitude of load. Another useful product of these tests

would be the stabilization of the ropes as a result of cyclic

loading. The stiffness of stabilized rope can then be compared

16
and correlated with the characteristic stiffness obtained from

static load test.

Frictional damage to surface fibers, debris penetration, and high

moisture content are probable occurrences in the field. The

effect of these factors on strength, stiffness and

energy-absorption properties of pleated synthetic rope should be

studied through controlled environment tests, or field tests, or

preferably both. These investigations will provide information

which can be used to develop criteria for rejection or

replacement

7 . ACKNOWLEDGEMENTS

The assistance of the following persons or group is acknowledged.

R. Sanders and K. North of the D.S. Army supplied information on

the product and field test data, N. J. Carino developed the

computer plots in this report, several CBT technicians provided

assistance in conducting the tests.

8. REFERENCES

1. Kirstein, Arthur, F., Universal Testing Machine of 12-Million

-lbf Capacity at the National Bureau of Standards, NBS

Special Publication 355, National Bureau of Standards,

Washington, D.C., September, 1971.

2. Strength and Elongation, Breaking of Cordage; Spliced

Specimen Method 6015, Federal Standard 191a, July, 1978.

3. Shaieb, Mark, Tensile Loads Applied to the BFV During a Kinetic Energy Recovery, Technical Report 4143, Ordinance

Division (Engineering), FMC Corporation, San Jose,

California, July, 1985.

17
T I E PLATE

Figure 2.1 Drawing of 12,000,000 lbf capacity universal testing machine

18
Figure 2.2 Special fixtures for attachment of rope to

built-in clevises.

19

MATERIAL:

AS

TM

A

291-

*.4

,

CLASS
ROOM

TEMPERATURE

(*d|>o oven

20

PERCENT

ELONGATION
ROOM

TEMPERATURE

o

CO

<

o o o o O o o o

m o m o m o in

•o »o CM CM •— *—

(sdi>o oven

21
HEATED

64

(*di>o avon

22

PERCENT

ELONGATION

+

#10
HEATED

80

o

CO

(sdpt) oven

23

PERCENT

ELONGATION
COLD

64

(cdiJi) QVOI

24

PERCENT

ELONGATION

#16(PTP)

O

#19(PTP)
COLD

80mm

PIN-TO-PIN

(•dpi) ayol

25

PERCENT

ELONGATION
26

Figure

3.7

Failure

of

64-iran

untreated

ropes.
27

Figure

3.8

Failure

of

80-mm

untreated

ropes.
28

heat-treated

ropes.
29

Figure

3.10

Failure

of

80-mm

heat-treated

ropes.
30

Figure

3.11

Failure

of

64-mm

cooled

ropes.
31

Figure

3.12

Failure

of

80-mm

cooled

ropes.
Figure 3.13 General view of 12M lbf universal testing facility.

32
33

Figure

3.14

Specimen

//

17

being

removed

from

cooling

box.
Figure 3.15 Specimen #17 being installed in the lower clevis.

34
35

Figure

3.16

Specimen

//

17

under

load.
36

Figure

3.17

Specimen

fl\7

after

rupture.
NBS-114A (rev. 2 - 6>c) U.S. DEPT. OP COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 86-3375

2. Performing Organ. Report No 3. Publication Date

SEPTEMBER 1986

4. TITLE AND SUBTITLE

Tensile Properties of Pleated Synthetic Rope

5. AUTHOR(S)

S. George Fattal

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

United States Army Aberdeen Proving Ground

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

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

bi bliography or literature survey, mention it here)

Pleated nylon ropes of two sizes and approximately the same length were tensioned

to rupture in a universal testing machine. Several of the ropes were tested at room temperature. The others were subjected to. specified high and low temperatures

before testing. Deformation measurements of all the specimens were recorded while

testing was in progress. The results were used to evaluate the breaking strength,

ultimate elongation, and loajd-defQrmatidn properties, and to develop criteria for

possible application in the recovery of mired vehicles.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) breaking strength; load-deformation; nylon rope; pleated rope; pulse loads;

specified strength; stiffness; synthesis; ultimate elongation

13. AVAILABILITY

Unlimited

| | For Official Distribution. Do Not Release to NTIS

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[V] Order From National Technical Information Service (NTIS). Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

43

15. Price

$9.95

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