Saturday, September 24, 2016

Can Nylon Rope Melt Due to Load Cycling --- Myth Semi-Busted

The Problem. There have been reports of nylon sea anchor and ground anchor ropes failing due to heat build-up when repeatedly cycled to a high percentage of rated breaking strength. True? Let's try a few reasonableness checks. We'll start with climbing ropes, since these are tested for energy absorption and the results are well known.

In a UIAA (the governing authority for climbing gear) test fall 2.5 meters of rope arrests an 80 kg object falling about 5.6 meters, stretch included. This is an impact so severe that the rope is only expected to survive 5-15 cycles; I doubt any anchor rope would do as well, so this is very conservative. if we convert this to US units:

Energy = 176 pounds * 18.3 feet fall/8.2 feet rope = 393 ft-pounds/foot of rope

Expressed as heat. However, only energy dissipated by hysteresis (it takes more energy to stretch a rope than it returns--it is not a perfect spring) is converted into heat. After all, a metal spring does not heat, true? Hysterisis with nylon rope is about generally about 10-20%. We'll assume the worst.

0.20 x 392 ft-pounds/ft = 0.10 BTU/ft

How much heat needs to be lost? If the rope is cycling at more than 50% of this load it won't last long for many reasons, so I will assume 0.05 BTU-cycle as the limit, which corresponds to 25% of the breaking strength and 200% of the safe working load (SWL), which is normally taken as about 12% BS for nylon. 

How many cycles? Assuming we are taking about storm waves, 20 second period seems reasonable, or 180 cycles per hour.

Heat = 180 cycles/hour * 0.05 BTU/cycle = 9 BTU/hr*ft

How much strength does a hot rope lose (PA66 is nylon 6/6)?

About 18% weakening by 80C (176F). There is a reason clothes don't fall apart in the drier! Long-term, there are oxidation effects, but these take months.


How fast can a dry rope loose heat? Assuming strong winds, about 6-8 BTU/ft2-F, depending on the reference. Assuming there is some spray in the air, we will use the higher number. A wet rope will cool more quickly due to evaporation and better heat conduction within the rope.

Heat loss = 0.11ft2 area/ft * 8 BTU/ft2 * (90-80) = 9 BTU

Clearly the rope won't get that hot. In fact it will top out at about 10F above ambient. Noticeably warm, but not in any danger.

What if the rope were larger (3/4-inch is what the Dashew's reported failing), of a less efficient construction (3-strand), and operated at a higher load factor (30%?)? The surface area to core ratios is greater, the heat generation per cycle is double, and the rope generates about 20% more heat due to the construction difference. What if the boat were tied to a dock and the period was much shorter? The core temperature gain can reach about 140For 60C--still not in the danger zone. Isolated fiber bundles could get hotter, if the load is not evenly carried or if there is significant friction between the fibers in that location.

 Note: I've greatly over-simplified the engineering. Insulation from the rope fibers and the cylindrical coordinates need to be considered. However, the result was similar, about 20% higher. On the other hand, we've assumed that no spray is striking the rope (it remains dry) which seems VERY unlikely in storm conditions.

 Observation 1: Lines smaller than 1-inch do not heat significant under cyclic loading unless they are significantly undersized, in which case they would fail anyway.


Below 3/4-inch rope heating due to cycling is probably not an important factor, even in the worst hurricane docking situation; failure will be due to something else. Beginning at 1-inch moving upwards, it can be important, since the larger rope cannot cool as easily. Large Barge tow lines can heat. Thus, the myth seems plausible, but not in sea anchor applications; the period is too low. The rope would need to be ~ 2 inches in diameter to provide sufficient insulation.

So why did the rope break? First, lets look at the load. Several investigators have found the wind load to be about 1/4 the ABYC estimates (these are based on anchoring with all-chain) and the load on a nylon rode to be about 1/2 the estimate. For a 50-foot boat, that would be about 2400 pounds. The SWL of 3/4-rope is about 2000 pounds. But that is before we include weakening due to wear and water. Dynamic tests by UIAA (climbing standards group) shows as much as 50% strength loss for wet rope in impact conditions. The SWL in practice is probably closer to 1400 pounds. In short, the rope failed predictably at 20%  BS after some time in the storm (probably higher due to a larger wave) with predictably melted ends. It was simply under speced due to a misunderstanding of SWL.



Second, we should do some forensic thinking. What does nylon rope look like when it breaks under load? In fact, it always looks melted, the result of the enormous energy release at the moment of rupture. If the rope was slightly warm from cycling the effect would perhaps be slightly greater, but it would not be the cause. The larger the rope sample broken in the lab, the more noticeable the melting.

 This is very load speed break testing, yet the ends are melted. I think folks just don't understand what they are looking at when they claim mysterious heating. One sailor reported a false observation and it became internet fact.


Observation 2: Nylon ropes always appear melted when broken at high load.

What about heating under chafing gear? Yes, there can be some heating, as calculated above. Covering the rope will make it worse.
  • If the gear is waterproof, that prevents both water cooling and reduces the internal lubrication that water provides. Bad.
  • The gear provides insulation, like an over coat on the line. Thus, a 1/2-inch line is going to heat like a 3/4-inch line, and a 3/4-inch line like a 1 1/4-inch line. Bad.
  • If there is motion under load, there will be friction and some resultant heating. Permiable gear that allows the rope to stay wet will help, since that reduced friction.
  • Chafing gear should be made of low-friction materials. I like nylon tubular webbing, because nylon-on-nylon friction is very low. It has done very well both in practice and in chafe machine testing.
  • High friction chafe gear (rubber and vinyl hose) is bad. 
  • Motion at chafe points must be reduced. Chocks should be close to cleats. Using non-stretch line in chocks areas can be smart.
Thus, any chafing gear that keep the line dry will weaken a line subject to hard cycling for a long period, such as a hurricane or nor'easter.. Only permeable gear is acceptable. But that said, the weakening will be only a small percentage. Even without heating, the gear will break under the gear because it is probably over an edge. So just because we see rope broken under chafe gear with melted ends, we should not believe heating was the culprit. The rope was simply too short to absorb the energy and was under engineered. Nylon rope looks melted even when broken at very low speeds and cycles; it is an artifact of the enormous energy release at the moment of failure.

Observation 3: If the rope breaks under the chafing gear, don't leap to line heating as an explanation. The rope was just too small and too short.

Bottom Line: Lines don't heat up, but users often underestimate the load, over estimate the SWL, and sometimes use docklines and snubbers that are too short to absorb energy safely. 

__________________


An exhaustive report by the US Coast Guard goes deeply into synthetic moorings. It's a big deal for deep water ATNs. 

2 comments:

  1. Drew,

    do your calculation include heating because of friction / chafing? If not, would that make much difference?

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    Replies
    1. Good Question. No, I did not include friction for 2 reasons:

      1. If there is chafe the line is probably going to fail for that reason.

      2. The specific case studies that motivated the post were sea anchor rodes that failed away from any chocks or splices, in the middle of the line. Additionally, the failure theory proposed was that all of the heating came from hysteresis related to stretch.

      However, inside chafing gear it would be possible to have some heating from friction that is not very abrasive. I think this deserves a whole separate post, along with some other chafing gear related issues. I also have some test data regarding cover materials and coatings. I'll need to think on this.

      Thanks!

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