Performance of Nylon Climbing Ropes
(results of a 24 factorial experiment)

by Scott Cutler and Jared Lebaron
Presented to Dr. Chris Rotz
Brigham Young University
December 9, 1992

Introduction

Nylon climbing ropes are often used in very adverse conditions. Technical rock climbers "...often have a stronger relationship with their rope then they do with their significant other--and trust it more too." (Rock and Ice) Some of the harsh conditions which ropes are subjected to while being climbed on are; abrasion from rocks and equipment, ground in dirt, repetitive falls sustained and moisture. Since any combination of these conditions could possibly affect the behavior of the rope, it would be wise to understand which combinations have the biggest effect on tensile strength. Ropes are the lifeline for climbers and a basic understanding of these conditions could mean the difference between life and death.

It is the intention of this study to better understand the behavior of nylon climbing rope under certain conditions simulating actual use. These are; abrasion, dirt, fatigue and water. The factors of the experiment were applied to the rope samples in a way that would simulate the worst possible conditions found in a climbing atmosphere. For example, the amount of abrasion applied to the samples far exceeds the amount one would ever find on a "real" climbing rope. The same goes for the rest of the factors. It would be hard to duplicate them under normal climbing use assuming that a given climber knows when to retire a worn or damaged rope. In order to cut cost and save time, 5mm rope samples were used instead of 11 mm climbing ropes. The 5mm samples that were used for this experiment could never be considered for use as a single stand climbing rope. However, since the 5mm and 11mm ropes are constructed basically the same, we can adapt the results of the experiment to predict the behavior of an 11mm rope under comparable circumstances. The brand and type of the rope is Edelrid 5mm access cord, pink/lime.

Notice the factors they have mentioned here: abrasion, dirt, fatigue, and water. Also notice the cost control of the experiment - using 5mm rope instead of 11mm rope.

Question: What method are they going to use measure abrasion, dirt, fatigue, and water saturation?

Background on Nylon 6

Ropes and cables constructed from nylon fibers are extensively used in a wide variety of applications. Among these applications are climbing ropes, fastening lines, and safety devices. In many of these applications, they are subjected to many factors that may influence and effect the strength of the rope. Such influences include, water absorption, fatigue, abrasion and fraying, abrading particles such as dirt, sunlight degradation, and aging. Therefore, it is important that the rope fibers are capable of withstanding these effects to provide safe climbing ropes and safety lines.

Nylon 6 is currently being used in many similar applications because of the good mechanical properties that it provides. The following few paragraphs will give a background on the properties of this material. Nylon 6 has outstanding physical, thermal, and chemical properties, plus good resistance to fatigue, abrasion, sunlight, and microorganisms. All organic fibers, whether natural or man-made, will degrade to various degrees from the exposure to ultraviolet light. Although heavy industrial fiber products such as nylon ropes, degrade at a much slower rate. It is important to note that for end-use where strength and elongation critical , precautions should be taken to protect fibers from prolonged exposure to sunlight.

Although they are slight, nylon does have some weaknesses for climbing and safety applications. The stress-strain properties of nylon 6 are not significantly affected by moderate humidity, however, it does show a modest decrease in tenacity with increasing humidity and water absorption. Moisture content is known to affect the mechanical properties of the hydrogen bonds in nylon 6. This is due to the polarity and hydrogen bonding of the amide groups in nylon. Water absorption results in dimensional changes and strength loss.

Temperature is another affect that influences the strength of most materials and nylon is no exception. The tensile strength of nylon is significantly reduced as temperature is increased. Nylon 6 has zero strength when temperatures exceed 245 degrees Celsius.

Rope Sample Preparation

In order to complete the 32 runs of the experiment, 96 feet of rope was acquired. This would allow for 3 feet per sample. This experiment was designed in such a way as to simulate the worst possible rope conditions. Therefore, the ropes were first abraded, ground into dirt, fatigued, and then soaked. Each of these conditions, and how they were applied will be discussed in detail.

Question: How did they decide on "32 runs" ?

Answer: As a 24 factorial experiment, the "2" means 2 levels of each factor, and the "4" means that there are 4 factors for the experiment. 24= 16 runs, each of which will be run 2 times a piece, equalling 32 total runs.

Abrasion of rope

Since half of the samples required abrasion (refer to table 1), the 96 feet of rope was halved and thoroughly abraded. The abrasion was applied to the ropes by fixing a wood bastard file in a vise and then running the full length of the rope over the file until the rope received equal abrasion. In the end, the rope was quite frayed. This condition simulates a climbing rope running over very sharp or jagged rocks or equipment. At any rate, the rope was abraded past the point that a regular climbing rope would endure. Again, the idea was to simulate the worst possible conditions.

"Dirtying" the rope

Since half of the sample required dirt, half of the abraded ropes and half of the non-abraded ropes were taken and subjected to brutal treatment. Rock Canyon dirt was used in the preparation of the ropes. The two 24 feet lengths of rope were combined and jointly ground into dirt containing small rocks, sand and just plain dirt. This was simply done by actually stomping on the samples with rugged boots for over 15 minutes. In the end, the samples were both equally dirty (and very disgusting). This simulates the severe abuse that climbing ropes could receive, but usually don't since most climbers avoid stepping on their rope.

Fatiguing the rope

The UIAA has a standard test for single climbing ropes (single ropes are those used as a single strand, generally 9.8 to more than 11mm). A single rope must hold 5 fall supporting a 80kg weight dropped 5 meters onto 2.8 meters of rope. This is a very severe fall due to the rigidity of the setup etc. Some ropes only held the minimum of 5 falls. With the 5mm samples we wanted to fatigue the ropes, but not to the point of failure. Therefore, 25 pounds was dropped on 6 feet of rope and repeated 6 times (see Figure 1). The setup used was rigid. No slack in the knots or carrying device was present. This was done on a cool day at 38F. The UIAA test applies a K. of 3920 joules on the rope. Dropping 25 pounds on the 5mm rope from 6 feet applies 203 joules on the rope. The ratios of the areas for 11mm compared to 5mm is 4.84. And the ratio of energy is 19.31. The difference seems to be large, but when the ropes were tested, they appeared as if they would not be able to hold the fall. However, the samples turned out to be tougher than thought. Interestingly enough, some climbers will use a dirty and abraded rope without questioning it, but will be hesitant to use it if they know it has taken several good falls. Therefore, the amount of fatigue that was applied to the 5mm ropes should be an adequate amount to simulate actual use. Of the previous samples, half were taken and fatigued.

Figure 1. Weight used to fatigue rope

Soaking the rope

The last step in the preparation of the samples is soaking. 46 feet of rope (half of each of the previous steps) were immersed in water for 8 hours to allow for total wetting of the fibers and partial saturation.

Factor High Level Low Level
Abrasion Abraded on a file no abrasion
Dirt Stomped in dirt no dirt
Fatigue Heavy testing no weights
Water Immersed in H20 for 8 hours no water

Each of these 2 levels for the four factors is used to establish interactions between the factors. As seen in Table 1 below, each of the 32 runs has a different combination of factors.

Experimental Procedure

A 24 full factorial experiment consists of 16 runs comprised of 4 factors set at all possible combinations. Two replications were done in order to entrance the accuracy of the results. The first step performed before pulling the ropes was to measure and weigh the rope samples to find the percent of water absorbed by the soaked samples (wet samples were dried slightly to remove excess water). While the ropes requiring soaking were still wet, all of the samples were pulled to failure. The 32 runs of the experiment were randomized to minimize the effect of experimental error. The ropes were tied at each end in an identical manner so as to not make the knot tying a lurking variable (see figure 2). The length of the tied samples was 12 inches from center of loops.

The samples were pulled on the Instron machine by inserting the connecting pins through the loops at either end of the rope. In order to do this, the jaws on the machine were removed. The pins are of equal diameter, thus avoiding the pitfall of unequal stress concentrations (we were pleased to see that the majority of the samples broke at the midsection between the knots). The settings of the machine were as follows: load range 5%, speed 20 in/min.

Once the samples were placed in the pins, the crosshead was jogged up until the rope became taut, and the gauge length reset, etc. Then each sample was pulled to failure and the readings taken. Unfortunately, the data acquisition unit was not used to record the stress/strain data. At any rate, the most important result from the experiment is the tensile strength.

Figure 2. Method of tying knots

Results

Below is Table I containing the random order of the experiment and the response of each experimental condition, in pounds.

Random order A (A) D (B) F (C) S (D) Response lbs.
A   Dirt   Soaked 515
B         837
C Abrasion Dirt     655
D Abrasion   Fatigue   695
E     Fatigue Soaked 621
F Abrasion Dirt Fatigue Soaked 524
G Abrasion Dirt Fatigue   624
H   Dirt Fatigue   699
I Abrasion       769
J Abrasion   Fatigue Soaked 637
K     Fatigue   857
L       Soaked 664
M   Dirt     722
N Abrasion     Soaked 609
O   Dirt Fatigue Soaked 417
P Abrasion Dirt   Soaked 486
Q   Dirt   Soaked 520
R         831
S Abrasion Dirt     625
T Abrasion   Fatigue   698
U     Fatigue Soaked 615
V Abrasion Dirt Fatigue Soaked 520
W Abrasion Dirt Fatigue   620
X   Dirt Fatigue   702
Y Abrasion       777
Z Abrasion   Fatigue Soaked 640
AA     Fatigue   845
BB       Soaked 660
CC   Dirt     721
DD Abrasion     Soaked 600
EE   Dirt Fatigue Soaked 415
FF Abrasion Dirt   Soaked 486

Table 1. Conditions and data.

Figure 3 shows the time order of the experiment. The data appears to be fairly random, which is a good indication that time ordered variation is minute.

Figure 3. Time ordered plot

Table 2. Statistical Data

Conditions Rep 1 Rep 2 Std. Dev. AVG (std dev)2
DS 515 520 2.89 516.67 8.33
  837 831 3.46 835 12
AD 655 625 17.32 645 300
AF 695 698 1.73 696 3
FS 621 615 3.46 619 12
ADFS 524 520 2.31 522.67 5.33
ADF 624 620 2.31 622.67 5.33
DF 699 702 1.73 700 3
A 769 777 4.62 771.67 21.33
AFS 637 640 1.73 638 3
F 857 845 6.93 853 48
S 664 660 2.31 662.67 5.33
D 722 721 0.58 721.67 0.33
AS 609 600 5.2 606 27
DFS 417 415 1.15 416.33 1.33
ADS 486 486 0 486 0

Table 2 shows the calculated means and standard deviation for each cell. The square of the standard deviation is used to calculate the pooled standard deviation, which indicates the experimental variability. The pooled standard deviation is 5.33.


Figure 4. Factor effects.

Figure 4 illustrates the calculated factor effects ordered from largest to smallest. This allows us to readily see which factors had the largest effect on the ropes.

Fa=r Effects 180 160 t3 140 120 ioo so 60 40 10 0

The next chart shows the results of the most important two factor interaction. It is a two way interaction plot of abraded and soaked ropes.

Figure 5. Two way interaction plot.

Discussion of Results

From Figure 3 we see that the experiment has little time variation. This is a good sign. It basically means that no lurking variables such as equipment wear, experimental procedure etc., came into play.

From Table 1 it is hard to see which factors cause the rope to break at a premature strength. The values appear to be fairly random at first glance. But, after calculating the effects it is clear which factors had the biggest impact on the rope strength. From Figure 4 we readily see that soaking the ropes caused the ropes to be weaker more than any other single factor (factor effect calculations are found in the appendix). The next largest factor is dirt. Dirty ropes have lower strength than clean ones, but not as low as wet ones. The largest two-way effect is that of abrasion and soaking. The other two, three and four way effects having a smaller impact on the breaking strength.

From table 2 we can see that the weakest ropes are dirty, fatigued and soaked. They are nearly half as weak as rope with no conditions applied.

Effects of water on the rope

Possible reasons why soaking the ropes contributed the most to premature breakage is the fact that the samples are not the type of rope that receive a "dry" treatment of silicon or teflon. These treatments would help the nylon not to absorb so much water. For this reason, many standard climbing ropes come pretreated to protect them from water. The dry ropes in the samples averaged .01 oz/in. The wet ropes (includes other factor settings) averaged .025 oz/in. This is an increase in weight of 253.4%. With the ropes retaining this much water, the capillary action of the fibers made the nylon much weaker.

Effect of dirt on the rope

With dirt having such a large effect on the ropes, one may wonder how this happens. Dirt is basically made up of "ground-up rock particles than can externally and internally abrade your rope and shorten its life." (Rock and Ice) In the preparation of the samples, dirt was thoroughly ground into the ropes. The ropes consist of a core and sheath. The dirt penetrated the protective sheath and was ground into the core. The particles which look harmless to the eye actually have very sharp and abrasive microscopic edges. The dirt had time to do plenty of damage to the core and the sheath by damaging the individual fibers in a way that could have left them with surface defects which would cause each fiber to have a lower strength than before.

Effect of abrasion and soaking of the rope

When the ropes were abraded and soaked, the ropes are weaker than the other two way effects. Figure 5 shows us the interaction between the two. Non abraded and dry ropes obviously have the highest strength. An interesting result is when the rope is wet, the data shows the abraded rope to be the stronger of the two. It would seem logical for the reverse to be true.

Recommendations

From the results, we clearly, see that a rope that performs well in wet environments is clearly more desirable than one that does not. Rope manufacturers produce many ropes with the option of dry treatment. This is a large advantage over standard ropes. However, the treatments only prevent water from saturating the rope immediately, they do not make them water proof. In addition, dry treatments wear off. Some treatments wear a lot faster than others. The world would be a nicer place for climbers if they could stop worrying about their ropes and enjoy climbing more.

One possible solution would be to combine kevlar fibers with the nylon fibers. The core of the rope could not be strictly kevlar due to an elongation of only 2.2% (nylon has a 20% elongation). This would not give the necessary "spring" required to arrest a falling climber. However, if kevlar fibers (which are roughly 3.3 times as strong as nylon fibers[DuPont]), were added to the core in a way that allowed for elongation, then the ropes would be a lot tougher than before. In fact, water absorption probably wouldn't be a concern any more due to the increased tensile strength. If the kevlar yarn was wrapped around the nylon core of the ropes in a helical fashion and then covered with the sheath, it is quite possible that the core and mantel would slip(some slippage occurs in a standard rope[Rock and Ice]) and the kevlar would be allowed to extend until stressed. This would increase the strength of the rope, but would cause it to experience torsion during extension. Possible draw backs would be increased cost and a slightly increased linear density. These may be offset by a demand for a more reliable and rugged rope.

Conclusions

From this experiment, we were able to confirm the findings of role manufactures. Furthermore, we were able to detect the important factors and their interactions. This is valuable information to one who uses ropes extensively. In certain circumstance, it may be to one's advantage to know if a rope is in greater danger of failure than normal conditions. An example of this would be using a wet rope over jagged rocks, which would be worse than using a simply dirty rope.

To remedy the affects of water absorption on tensile strength, a new rope design consisting of a kevlar wound nylon core, covered with a nylon sheath would eliminate the threat of breakage due to water weakened nylon.

References

Faxed technical documents from Du Pont company, sent by Michael T. Downes of Du Pont.

Rock and Ice, Issue 50 p. 79-84 Textile Research Journal, Anu Verma, B. L. Deopura, and A.K. Sengupta, February 1984 p. 92-97.

Textile Research Journal, V. Sampathkumar and P. Schwartz, February 1989 p. 94-97.

Appendix

  Abrade Dirt Fatigue Soak                      
  A B C D AB AC AD BC BD CD ABC ABD ACD BCD ABCD
SUM+ 9965 9251 10129 8929 10470 10428 10716 10268 10192 10326 10459 10359 10609 10227 10266
SUM- 10641 11355 10477 11677 10136 10178 9890 10338 10414 10280 10147 10247 9997 10379 10340
(+)+(-) 20606 20606 20606 20606 20606 20606 20606 20606 20606 20606 20606 20606 20606 20606 20606
(+)-(-) -676 -2104 -348 -2748 334 250 826 -70 -222 46 312 112 612 -152 -74
((+)-(-))/16 -42.25 -131.5 -21.75 -171.75 20.88 15.63 51.63 -4.38 -13.88 2.88 19.5 7 38.25 -9.5 -4.63

Table A. Calculation of effects.

Figure A. Standard deviation vs. average