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Old 25-10-2019, 17:52   #1
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Anchor chain stretch?

I am wondering if any of all you smart people know how much anchor chain stretches under load? I am doing some calculations on dynamic loads on anchoring gear and chain stretch may play an important part in this, even if it stretches much less than the snubber being used.

Any input would be most welcome. I did e-mail a couple of chain manufacturers but have not heard back yet.
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Old 25-10-2019, 18:04   #2
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Anchor chain stretch?

I would look up and Iím no Engineer so please take that into account, but I believe itís called the modulus of elasticity of steel, Iíd look for plain ole 4130 which is a very common mild steel, but still good steel.
I think youíll find that realistically we donít have much stretch at all, pretty much zero stretch in any realistic common anchoring load
Iíd figure that 5/16 chain is 5/8Ē as it has two pieces.
That should be for the weaker BBB chain or so and not the heat treated higher grades, which of course are even stronger.

Now thatís Jim Bob way of doing it, and isnít meant to determine how much exactly, but meant to show that itís pretty much zero.

Then just as a trivia statement, steel has greater elasticity than rubber does, which surprises most people until they understand what elasticity is.
I have no idea how much stretch the zinc coating would do before it failed either.

On edit, there is some stretch of course, but I think it is minuscule as in difficult to measure.
Iíve permanently stretched chain on the farm, and it of course gets elongated and the links donít move well etc. you know itís destroyed then, but you get that when close to failure, of course the weakest link breaks, but they are all ruined before that happens, it would never run through a windlass etc.
I have never heard of that happening in anchor chain even though Iíve asked many. I just donít think chain stretch and therefore fatigue is an issue with anchor chain thatís even close to being correct.
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Old 25-10-2019, 18:27   #3
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Re: Anchor chain stretch?

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Originally Posted by a64pilot View Post
Iíd figure that 5/16 chain is 5/8Ē as it has two pieces.
More like 7/16. Twice the Area of 5/16, not twice the diameter. Otherwise, decent approach to the question.
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Old 25-10-2019, 18:32   #4
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Re: Anchor chain stretch?

PS: If you want to play, go here:

https://www.engineeringtoolbox.com/y...lus-d_417.html
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Old 25-10-2019, 19:20   #5
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Re: Anchor chain stretch?

Thanks a lot for the feedback so far. I found modulus values of 140 GPa to 200 GPa for steel. So for G4 chain (400 MPa tensile strength), that would give an elongation of 0.20 to 0.29 % at breaking load (I know the elongation is not linear with load near breaking, so I just use breaking load as a reference).



That should be a good estimate of the minimum elongation. I wonder if the links also temporarily deform a little under load to give some extra elongation?
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Old 25-10-2019, 19:40   #6
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Re: Anchor chain stretch?

The stretch due to elastic force is orders of magnitude less than the catenary sag and even the elasticity of your hull. Engineers toolbox is a good source for calculating raw numbers but chain makers can probably give you empirical data. BTW, the modulus for steel doesn’t care what steel you use. They vary only a tiny bit.
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Old 25-10-2019, 19:48   #7
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Re: Anchor chain stretch?

Quote:
Originally Posted by BjarneK View Post
I am wondering if any of all you smart people know how much anchor chain stretches under load? I am doing some calculations on dynamic loads on anchoring gear and chain stretch may play an important part in this, even if it stretches much less than the snubber being used.

Any input would be most welcome. I did e-mail a couple of chain manufacturers but have not heard back yet.
If it stretches noticeably it's knackered AFAIK. I stretched a bit of chain once but it was very windy ( dont know how windy but 90knts+) & also boat was getting flung backwards by waves.
I'm cheap but I removed that chain as I know it was weakened. Didn't have an all chain rode. I was the windlass.
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Old 25-10-2019, 20:01   #8
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Re: Anchor chain stretch?

The elastic stretch in steel anchor chain is discussed in the static case of Alain Fraysse's excellent web pages Rode - Dynamic Behavior and again later in the dynamic section of his web pages. You can play with his spreadsheets to make them match your particular cases.

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Old 25-10-2019, 20:13   #9
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Anchor chain stretch?

Quote:
Originally Posted by BjarneK View Post
Thanks a lot for the feedback so far. I found modulus values of 140 GPa to 200 GPa for steel. So for G4 chain (400 MPa tensile strength), that would give an elongation of 0.20 to 0.29 % at breaking load (I know the elongation is not linear with load near breaking, so I just use breaking load as a reference).



That should be a good estimate of the minimum elongation. I wonder if the links also temporarily deform a little under load to give some extra elongation?


Now I suck at math and am no Engineer, but I believe that roughly steel will stretch about 20ish% as it breaks or before it breaks depending on how you look at it, but that is orders of magnitude less than anything you will see in a working load.
Fatigue and cracking from millions of cycles will occur when just a tiny fraction of that kind of stretch occurs, and Iím nearly certain that anchor chainís loads are outside of the fatigue cycle, meaning that they arenít high enough to begin fatiguing the metal, because simply put there is pretty much zero stretch, if there were, then we would be replacing chain due to age and not just condition, and Iíve never heard of a fatigue type of failure in anchor chain.
That would be when the chain just parts one day in not all that bad of weather and your boats gone, and Iíve not heard of that.
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Old 25-10-2019, 21:09   #10
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Re: Anchor chain stretch?

With your help in getting to know the relevant terms, I was able to find a datasheet which actually contains information on the effective modulus of chain: https://ramnas.com/wp-content/upload...-Broschure.pdf

The value is 64 GPa for studded chain. I also found a reference indicating that the value is 54.4 GPa for studless chain: https://www.orcina.com/webhelp/OrcaF...properties.htm

This seems to be just what I need for my calculations. As I suspected, chain is more stretchy than raw steel, which must be due to temporary deformation of the links.

The reason for me doing this is to get a better impression of the relative effects of snubbers, chain elasticity and catenary under static and dynamic loads. I know these are very sensitive topics on Cruisers Forum and figured that injecting a bit of science to the discussion might be helpful. One of the coming days, I will start another thread with some of my conclusions.
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Old 25-10-2019, 21:46   #11
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Re: Anchor chain stretch?

For your science you might find this DnV standard useful.

They specify asking the manufacturer for effective modulus values but specify defaults from 54 to 60 GPa for studless chain depending on type of steel.

Note that one reason these values are so low is that they intend the use of one wire diameter/area in subsequent equations.
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Old 25-10-2019, 22:58   #12
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Re: Anchor chain stretch?

Quote:
Originally Posted by a64pilot View Post
I would look up and Iím no Engineer so please take that into account, but I believe itís called the modulus of elasticity of steel, Iíd look for plain ole 4130 which is a very common mild steel, but still good steel.
I think youíll find that realistically we donít have much stretch at all, pretty much zero stretch in any realistic common anchoring load
Iíd figure that 5/16 chain is 5/8Ē as it has two pieces.
That should be for the weaker BBB chain or so and not the heat treated higher grades, which of course are even stronger.

Now thatís Jim Bob way of doing it, and isnít meant to determine how much exactly, but meant to show that itís pretty much zero.

Then just as a trivia statement, steel has greater elasticity than rubber does, which surprises most people until they understand what elasticity is.
I have no idea how much stretch the zinc coating would do before it failed either.

On edit, there is some stretch of course, but I think it is minuscule as in difficult to measure.
Iíve permanently stretched chain on the farm, and it of course gets elongated and the links donít move well etc. you know itís destroyed then, but you get that when close to failure, of course the weakest link breaks, but they are all ruined before that happens, it would never run through a windlass etc.
I have never heard of that happening in anchor chain even though Iíve asked many. I just donít think chain stretch and therefore fatigue is an issue with anchor chain thatís even close to being correct.

My anchor stretched noticeably in Irma.
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Old 25-10-2019, 22:59   #13
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Re: Anchor chain stretch?

How are you planning to model soil deformation around the anchor? Soil
moduli are many orders of magnitude lower than your chain steel. The soil is also highly likely to be deforming plastically. Finally, it isn’t hard to cause shear failure in seabed soils, so anchor drag/creep, even if small and slow, will be a very large portion of deformations,
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Old 26-10-2019, 03:45   #14
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Re: Anchor chain stretch?

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Originally Posted by Compass790 View Post
If it stretches noticeably it's knackered AFAIK...
Indeed.
For alloy hoisting chain, ANY amount of stretch indicates overloading, and the chain should - MUST- be removed from service. As such, ASME B30.9 “Slings” lists stretched chain links or components as one of the reasons a chain sling shall be removed from service. The word “shall” in a standard resolves any doubt. No stretch or elongation is allowable.
Note that a significant degree of stretch, in a few individual links, may be hidden by the apparent acceptable length gauge of the overall chain. This highlights the importance of link-by-link inspection.
Also note that chain wear and stretch are two different things, and both will make the sling length increase. So, it is sometimes difficult to differentiate between the two. Wear will show itself at the bearing points of the links, and can exhibit itself in the bearing points of the chain as a groove. A certain amount of wear is permissible (usually given as 5% - 10% of diameter).

See also ➥ Has Anyone Broken their Anchor Chain or Stretch Damaged It?
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Old 26-10-2019, 03:49   #15
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Re: Anchor chain stretch?

Quote:
Originally Posted by Nicholson58 View Post
The stretch due to elastic force is orders of magnitude less than the catenary sag and even the elasticity of your hull. Engineers toolbox is a good source for calculating raw numbers but chain makers can probably give you empirical data. BTW, the modulus for steel doesnít care what steel you use. They vary only a tiny bit.

Actually stretch of the chain surpasses the chain catenary in energy absorbing capacity at a certain point. Someone on here demonstrated it -- showing that the idea of a "bar-tight chain" is almost literally applicable to anchoring on chain. The OP is absolutely correct to be taking this into consideration.



Can't remember what discussion that was; if I find it I will post a link.
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