Steel VS Dyneema: An engineer's wild speculation
Im rebuilding an old Ramsey REP8000 that I had laying around to put on my 97. I will need to get a new winch line for this thing and im trying decide which way I want to go. From what I can tell I can save around 30 lbs between the wire and roller fairlead and the synthetic and hawse fairlead. Would this give me any noticable difference in MPGs?
What are the pros/cons of each? The wire cable is definatly cheaper.
SO I need this question answered too. I am in the process of wiring up my lights and winch to my new SOS off road concept bumper before install. I got the aluminum bumper so weight of the cable really does apply for me. My dad told me stories of aircraft recovery cables breaking on his aircraft carrier so I have concerns. But then I think about synthetic fiber and the elongation curve and increased incidence of failure relative to steel due to maintenance, but then ther is P=MV, and steel is nearly 8 times as dense as synthetic. etc. So I watched youtube vids of cable and rope breaks and it was nothing. No cables flew anywhere but the ropes did shoot back like a rubber band snapping off your finger and hitting you in the eye. So luckily, I researched carbon fiber in engineering school so I'm going to drop some knowledge and compare products.
The products I will be comparing are 3/8" diameter 7x19 galvanized steel vs. 3/8 Dyneema SK75 12 strand braided rope.
I picked these two products because this is the steel rope that came on my 1200 winch and 3/8 Dyneema is widely attached to other winches (according to some googling).
Cable Strength rating:
Steel = 12000#
Dyneema=17500#
Ultimate Tensile Strength of Material
Steel=75,000 psi
Dyneema=478,625psi
Yield Strength of material
Steel=36,000 PSI
Dyneema=none
Carbon fiber=none
Modules of elasticity:
Steel=29,000,000 psi
Dyneema=16,820,000*
Carbon fiber=26,250,000
Elongation at break
Steel=.125-0.26%
Dyneema=3-4%
Max operating Temperature
Steel=572F
Dyneema=160F
Specific gravity
Steel=7.8
Dyneema=0.977
Water=1.0
UV resistance
Steel=Immortal
Dyneema=50% fiber retention strength at 1630hrs
Strength reduction due to time and use
Steel=years
Dyneema=60% of original strength at 24 months
There are all the specs. I had to scower the internet for this stuff. I went to manufactures specs, yach rope sites, American Institute of Steel Construction, and more. Here are the results from easy to complicated.
Specific gravity:
This is a ratio of the density of the product to water. So comparing steel to Dyneema, we can see it is 7.98 times heavier for the same length of rope. 100' of steel is 25# and Dyneema would be about 4#. So we are talking about 21# difference.
Cable strength rating:
Dynemma is 45% stronger given the same size. But a lot can be said here. Most importantly, is the effect of UV. The manufacturers technical bulletin (which is super biased to their own product) reported a 50% reduction in strength at 1630hrs of UV. That like 9 months of indirect sunlight then the Dynemma is rated at only 73% of the steels strength. I have also come across a lot of reading material talking about abrasion. Looks like raw, un sheathed 12 strand braded dyneema is like a glue trap for dirt. So you need a sheath to protect against abrasion and UV. well if we increase the diameter of the Dyneema for a sheathing, a similar increase in steel diameter results in superior strength for steel. Like a 1/16" increase in diameter for steel provides a strength of 17,600#. So strength per weight, dyneema wins. But strength per diameter and therefor length of rope on the spool, steel wins.
Strength reduction due to time and use:
It is important to note that manufacturers spec sheets are made as an advertising aid supporting their product. But Dyneemma still states that with continued use and exposure to the elements, you should expect to lose 40% of the original strength within 2 years. A similar metric for steel could not be found.
Max operating temperature:
I don’t know a lot about winches but I hare they can put off some heat. According to manufacturer’s specs, Dyneema begins to experiance plastic deformation at reduced loads around 150 degrees F. Yall chime in if you know how hot a winch cylinder can get. What that means is that you will induce a permanent stretch in the cable called plastic deformation. It reduces the cross section of the cable while lengthening it. This results in a permanent strength reduction. I can see how a few cycles of this could be a failure mechanism. Brain wave, the radiator that it is mounted next to the rope puts off a lot of heat. Actually, that is really important to discuss. If you like to provide a little tension to your rope when you spool all the way up to keep the clevis from vibrating free on the road, you are loading the rope for very extended periods of time next to your radiator. I suspect this is bad.
However, this stuff is amazing in the cold, far superior to carbon steel. Dyneema apparently does not experience the temperature induced brittleness many crystallin materials do. So if you live in real cold, this product is a solid win. Anyways, almost nothing sounds as bad to me as manipulating a wet steel cable in below freezing conditions.
OK lets get to the fun stuff, breaking under load. In this section I am going to combine the first 6 categories.
So the most significant mechanical difference between steel and Dyneema is that, at standard operating temperatures steel experiences plastic fatigue prior to failure and Dyneema does not. It is important to remember that what makes a failure dangerous is the amount of stored energy. In this case we are dealing with strain energy (i.e. potential energy). As you load the two ropes they stretch relative to their modulus of elasticity ( E ). The larger the number, the stiffer the material and smaller the stretch. Something with a low E like a rubber band give you a lot of notice that you have loaded it. You know, all the stretching? But it is not so easy to tell with a material with a high E. Think about a steel cable loaded with 1000# vs 40,000#. You cant tell be looking at it or plucking it. But when 40,000# breaks, of god. Dyneema’s E is nearly half that of steel. That means twice the stretching for the Dyneema. And that is not considering the stretch cause by the rope braid collapsing under load. Ropes do that. Steel cables don’t. Steel ropes go through a process called construction stretching that is permanent after the first use.
Importantly, steel has this cool safety property called plasticity and engineers love it because it allows ductile failure. What that looks like is a phenomenon called necking. The material elongates plasticly under no additional load. In regards to strain energy, it actually releases energy prior to failure. This is what it looks like.
https://youtu.be/W5A8gU37wGg
Dybeema, much like carbon fiber has no plastic phase. It is all elastic till failure. This means it continues to build load as your winch is turning till it fails suddenly at peak load. This is what it looks like
https://youtu.be/aH9vcV7jzG0
Say both a steel and Dyneema rope were damaged and were going to break at 10,000#. The Dyneema with the lower E value and no plastic phase would have a much larger stretch than steel. The low mass relative to the steel would also allow the light rope to whip more. Steel will experience necking and may only have partial strand failure. This could save you from going over on a slope. Here is an example of the two failure methods.
https://youtu.be/cYEMT5avIUk
Conclusion: Steel is more durable and reliable hands down. I don’t thing you can argue the UV, abrasion, and temperature vulnerability of Dyneema. The claim “dyneema is stronger than steel” is only partially true. Dyneema’s Strength to weigh ratio is herculean coming in at 9 times the strength of steel given the same weight. But it’s strength to area ratio when braided and sheathed is less than steel but not by much. Given the same diameter ropes of the same length on the same winch, the steel rope would have a 1% advantage. That’s negligible in my book. Dyneema is lighter, more flexible, spools easier, and does not stab you in the hand like steel does. That a huge plus. But it should have higher incidences of failure than steel, and dyneema’s failure is always complete where steel has the potential for partial failure. I hear dyneema can be spliced in the field. If that is true, that is an awesome selling point. I think I am going to stick with my steel till the broken fibers stab me in the hand one to many times. Then I may try Dyneema.
