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30-11-2015, 15:59
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#1
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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metallurgy/failure question
Below is a photo of the shank of a helix mooring that failed/broke. It had been in the water/use for 15 years. The two questions I have are #1 from the photo can anyone tell me anything about the cause of the breakage, and #2 does this mean that these sort of helix moorings have a 'life span' and should be replaced after say 10 years?
I have drawn two red lines across the photo, where there seem to be clear changes in the surface. In the upper left corner is obviously the section that broke last, all at once - it is smooth ish and unrusted. In the middle is a large section that is rusted with a granular bumpy surface, and in the lower right corner is a rough pitted section.
It is some sort of mild steel (very magnetic)
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30-11-2015, 16:42
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#2
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Moderator Emeritus
Join Date: Oct 2013
Location: Jacksonville/ out cruising
Boat: Island Packet 38
Posts: 31,351
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Re: metallurgy/failure question
Looks like classic high cycle fatigue failure. Does not look like an inclusion etc. Crack growth once it begins is often rapid meaning it's tough to catch a crack by NDI methods before failure, determining life limit and replacing before that limit is most often what is used to determine life of an aircraft part for example
Yes steel has a fatigue life, what is tough is determining fatigue cycle, by that I mean how much stress is it under and how often it is fatigued. Taking into account of course in a no wind, no current condition, I doubt it is being fatigued., but how many days per year etc is it being fatigued.
This is exactly how wing life limit is reached on aircraft, with a significant margin of safety of course. If it were an aircraft part and failed at 15 yrs a 7 to 1 safety margin is common, so replace every 2 yrs.
Now a sample of one isn't very good of course, but determine what your comfortable with and go with that as a replacement interval.
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30-11-2015, 16:53
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#3
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Registered User
Join Date: Sep 2012
Location: Washington State
Boat: Colvin, Saugeen Witch (Aluminum), 34'
Posts: 2,269
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Re: metallurgy/failure question
Does the expanding rust (that forms in a crack) exert significant pressure that would tend to open the crack further?
Steve
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30-11-2015, 20:08
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#4
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Registered User
Join Date: Feb 2014
Location: Niagara Falls
Boat: Westsail 32
Posts: 629
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Re: metallurgy/failure question
It looks like a 'cold shut'. A cold shut happens when the foundry is pouring the steel into the mold, pauses, and resumes pouring, and the bottom metal has cooled so that the new pour doesn't merge well with the old. If it's a cold shut the corrosion will go deep, say, 1/8" because it's been corroding since it was installed; tap it, dig into it with a pick, like a geologist's pick; check it out, see where it's deep.
If it's a cold shut it's a manufacturing flaw. The next one you put in will have the same flaw. I dunno, maybe count on replacing the thing after 10 years.
If you can get the bottom half up, consider cutting off the corrosion, competently welding the pieces together and putting the helical mooring back into the bottom. If it was a cold shut, then it'll last a long time.
It would be good practice to discuss this with the manufacturer. Maybe he's seeing a whole flock of these and knows exactly what's happening.
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01-12-2015, 06:04
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#5
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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Re: metallurgy/failure question
^^OK, I dug at the rust, and it is a thin surface layer only. There is clean metal right below it.
Regarding fatigue - am I right that steel in fact does not fatigue if the cyclic loads are below some limit (like below 50% of tensile)? I had thought this was one of the differences between steel and aluminum - aluminum fatigues with any amount of load cycle but steel only fatigued with a significant load cycle. But I might have mis-remembered this.
For reference - this is a 1.5" square section of mild steel - what sort (rough ballpark) of minimum load could fatigue it?
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01-12-2015, 08:56
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#6
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Registered User
Join Date: Oct 2005
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Re: metallurgy/failure question
I don't have any knowledge in this area, but I have a few questions out of curiosity.
1. Where along the shank did it break? A buried part or an exposed part?
2. What do the manufacturers of these screws state is their expected lifespan?
3. Do they make different sized ones for different size boats?
Mark
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01-12-2015, 09:03
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#7
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Registered User
Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,453
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Re: metallurgy/failure question
It's hard for me to tell, but that almost looks like cast instead of wrought structure. If so I agree with the "cold shut" idea. How big is that? Hard to believe steel would have fatigue failure on a mooring... just not loaded very high most the time.
But yeah, anything plain steel I would replace before 10 years in salt water personally!
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01-12-2015, 09:04
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#8
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Marine Service Provider
Join Date: May 2012
Location: New Orleans
Boat: We have a problem... A serious addiction issue.
Posts: 3,974
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Re: metallurgy/failure question
Quote:
Originally Posted by estarzinger
^^OK, I dug at the rust, and it is a thin surface layer only. There is clean metal right below it.
Regarding fatigue - am I right that steel in fact does not fatigue if the cyclic loads are below some limit (like below 50% of tensile)? I had thought this was one of the differences between steel and aluminum - aluminum fatigues with any amount of load cycle but steel only fatigued with a significant load cycle. But I might have mis-remembered this.
For reference - this is a 1.5" square section of mild steel - what sort (rough ballpark) of minimum load could fatigue it?
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Estar what you are referring to is the fatigue limit (the point at which the number of cycles needed to damage the part becomes exponentialy more, approaching infinity). To find this out you need to know exactly which alloy you are holding, and find the S-N curve for that alloy.
Now here is the tricky bit... The S-N curve for steel is subject to change if it is submerged. Generally you see a reduction in fatigue resistance, but some alloys loose all fatigue resistance, so an alloy that in air has a reasonable fatigue limit may no longer have one when submerged. Stainless alloys are notorious for this btw.
The largest player in this sub-surface change is inter granular galvanic corrosion, and so adding an anode protection (or galvanizing the part) can restore the fatigue limit, at least temporarily.
In a general sense however the fatigue limit for mild steel without an alloy number I would put at 280MPa or so. Say a little below the UYS. But it is critically important to know the alloy number. Even two bars of identacle chemical composition can have radically different physical properties if one is cold rolled vs hot rolled vs cast.
The other thing to keep in mind is that cycle fatigue only matters if the cycles are close to the same amplitude over the lifetime of the part. One or two massive stresses can destroy the service life of the part in one go. If I had to guess, and keep in mind I am at best an interested amateur in this field, I would guess the anchor went it correctly, took a one time shock load high enough to cross the UYS, and this caused accelerated cycle fatigue.
If you want a real guess find a local civil engineering school and call up the material properties professor and ask for a quick once over of the part. When I sold titanium a local professor was happy to look over things for me for the cost of a cup of coffee and the story behind the break. If I wanted a full failure Assesment it was a couple of thousand dollars, but a couple minute look was pretty much free.
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- If animals weren't meant to be eaten then they wouldn't be made of food.
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01-12-2015, 09:21
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#9
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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Re: metallurgy/failure question
It broke at a spot about 2' buried. Softish bottom.
No, there is no "expected lifespan"
There are two different shaft sizes 1.5 and 1.75 sq". The 1.5 seems to be picked for pretty much all pleasure boat applications. There are different lengths that seem to be picked based on soil firmness.
As I said in my OP, one question is "should there be a replacement (or inspection) life span". Right now these are pretty much screwed into the bottom and just left there forever without removal or inspection (removal is difficult/labor intensive). These screw style moorings are a relatively recent development (15 or 20 years?) so there is not a lot of long term historical experience with them.
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01-12-2015, 09:28
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#10
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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Re: metallurgy/failure question
I'm going to guess that this is A36 mild steel. Galvanized.
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01-12-2015, 13:41
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#11
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Moderator Emeritus
Join Date: Oct 2013
Location: Jacksonville/ out cruising
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Re: metallurgy/failure question
You are correct about fatigue the part about being below the fatigue point. You can bend and pull on a railroad rail till your blue in the face and it will never fatigue, because a man does not have the physical strength to fatigue it. A coat hanger wire though is another story.
Then there are stress risers, think of a scratch for example or other damage that can focus fatigue on a point. The little cuts in a candy bar wrapper so you can tear it easily are a stress riser for example.
I think that a replacement interval of about ten years would be prudent, but that is an opinion, not a fact.
Truth is I do not think you would ever get a fatigue life determined, except by having a representative sample of them break, if you get say ten to break, then you can begin maybe to determine a life limit. Of course that maybe isn't realistic, then you are pretty much left with just picking an interval that you think conservative and replacing them at that interval.
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01-12-2015, 14:00
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#12
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Marine Service Provider
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Re: metallurgy/failure question
I think A64 is on to something here. The reality is that in a low engineered issue like this (as opposed to an aircraft) best practice may just to figure out a reasonable replacement interval and be done with it. Otherwise you would need to do long term stress logging to identify what the cycle loads are, frequency, etc. it may take years, cost a fortune and not provide much useful data.
The other option is to either switch size, or material to provide a larger cushion. I know I harp on it, but a titanium rod would last forever. It's flex reduces the shock loads, and since it is non-corrosive the being buried wouldn't effect it at all. Purchase price may go up, but lifecycle cost may be a fraction of mild steel.
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- If animals weren't meant to be eaten then they wouldn't be made of food.
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01-12-2015, 14:58
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#13
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Moderator
Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
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Re: metallurgy/failure question
Another factor that might impinge upon lifetime is the chemical nature of the seabed. In industrialized areas there are often local anomalies in chemistry that could be damaging to mild steel. EG, the anchorage off the Suva YC in Fiji is also just off a large rubbish tip (or it used to be). The seabed was so corrosive there that it stripped the galvanizing off our anchor chain in just a couple of days... two years in a row (stupid me!).
Evans, I'm curious as to the cost differential between the two sizes of screws. Considering the cost of installing and/or removing for testing, perhaps using the larger one would be cost effective in the future.
Jim
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01-12-2015, 15:26
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#14
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Registered User
Join Date: Feb 2013
Location: Oregon
Boat: Seafarer36c
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Re: metallurgy/failure question
Maybe it's just crap steel. You don't often see a break like that without some indication of stress.
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01-12-2015, 17:47
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#15
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Writing Full-Time Since 2014
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Boat: PDQ Altair, 32/34
Posts: 9,559
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Re: metallurgy/failure question
It would be good to know more about what was attached to it.
- Location, including fetch to major winds.
- Depth and length of chain.
- Chain spec.
- Nature of buoy.
- Boat (or type if several).
- How much broke off (was it fully driven?).
My first thought is that it is almost unlikely that the chain and swivel could have fatigued the metal without failing itself (but we need to know more). If that is the case, then this goes in the direction of flaw, and those aren't easy to do statistics with. It suggests a different manufacturing method may be needed, or at least some significant changes.
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