Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 30-11-2015, 15:59   #1
CF Adviser

Cruisers Forum Supporter

Join Date: Aug 2009
Boat: Custom Van De Stadt 47 Samoa
Posts: 3,880
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?

Click image for larger version

Name:	smaller.jpg
Views:	196
Size:	279.8 KB
ID:	114114

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)
__________________

estarzinger is offline   Reply With Quote
Old 30-11-2015, 16:42   #2
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 18,410
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.


Sent from my iPad Pro using Cruisers Sailing Forum
__________________

a64pilot is offline   Reply With Quote
Old 30-11-2015, 16:53   #3
Registered User
 
Panope's Avatar

Join Date: Sep 2012
Location: Washington State
Boat: Colvin, Saugeen Witch (Aluminum), 34'
Posts: 1,629
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
Panope is offline   Reply With Quote
Old 30-11-2015, 20:08   #4
Registered User

Join Date: Feb 2014
Location: Niagara Falls
Boat: Westsail 32
Posts: 404
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.
Seymore is offline   Reply With Quote
Old 01-12-2015, 06:04   #5
CF Adviser

Cruisers Forum Supporter

Join Date: Aug 2009
Boat: Custom Van De Stadt 47 Samoa
Posts: 3,880
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?
estarzinger is offline   Reply With Quote
Old 01-12-2015, 08:56   #6
Senior Cruiser
 
colemj's Avatar

Cruisers Forum Supporter

Join Date: Oct 2005
Location: Presently on US East Coast
Boat: Manta 40 "Reach"
Posts: 10,052
Images: 12
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
__________________
www.svreach.com

You do not need a parachute to skydive. You only need a parachute to skydive twice.
colemj is offline   Reply With Quote
Old 01-12-2015, 09:03   #7
Senior Cruiser
 
Cheechako's Avatar

Cruisers Forum Supporter

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 20,028
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!
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard











Cheechako is offline   Reply With Quote
Old 01-12-2015, 09:04   #8
Marine Service Provider

Join Date: May 2012
Location: New Orleans
Boat: We have a problem... A serious addiction issue.
Posts: 3,980
Re: metallurgy/failure question

Quote:
Originally Posted by estarzinger View Post
^^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?
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.
__________________
Greg

- If animals weren't meant to be eaten then they wouldn't be made of food.
Stumble is offline   Reply With Quote
Old 01-12-2015, 09:21   #9
CF Adviser

Cruisers Forum Supporter

Join Date: Aug 2009
Boat: Custom Van De Stadt 47 Samoa
Posts: 3,880
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.
estarzinger is offline   Reply With Quote
Old 01-12-2015, 09:28   #10
CF Adviser

Cruisers Forum Supporter

Join Date: Aug 2009
Boat: Custom Van De Stadt 47 Samoa
Posts: 3,880
Re: metallurgy/failure question

I'm going to guess that this is A36 mild steel. Galvanized.
estarzinger is offline   Reply With Quote
Old 01-12-2015, 13:41   #11
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 18,410
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.


Sent from my iPad Pro using Cruisers Sailing Forum
a64pilot is offline   Reply With Quote
Old 01-12-2015, 14:00   #12
Marine Service Provider

Join Date: May 2012
Location: New Orleans
Boat: We have a problem... A serious addiction issue.
Posts: 3,980
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.
__________________
Greg

- If animals weren't meant to be eaten then they wouldn't be made of food.
Stumble is offline   Reply With Quote
Old 01-12-2015, 14:58   #13
Senior Cruiser
 
Jim Cate's Avatar

Cruisers Forum Supporter

Join Date: May 2008
Location: cruising SW Pacific
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Posts: 12,166
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
__________________
Jim and Ann s/v Insatiable II , lying Pittwater, NSW fora while.
Jim Cate is offline   Reply With Quote
Old 01-12-2015, 15:26   #14
Registered User

Join Date: Feb 2013
Location: Lago de Izabal, Guatemala
Boat: Seafarer36c
Posts: 4,379
Re: metallurgy/failure question

Maybe it's just crap steel. You don't often see a break like that without some indication of stress.
Ecos is offline   Reply With Quote
Old 01-12-2015, 17:47   #15
Writing Full-Time Since 2014
 
thinwater's Avatar

Join Date: Nov 2008
Location: Deale, MD
Boat: PDQ Altair, 32/34
Posts: 4,808
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.
__________________

__________________
Gear Testing--Engineering--Sailing
Writing full-time since 2014
http://sail-delmarva.blogspot.com/
thinwater is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
rig failure? irwinsailor Dollars & Cents 3 18-02-2008 07:26
Another Stainless steel failure delmarrey Construction, Maintenance & Refit 15 27-07-2006 15:58
Chain plate failure, dismasting in the Southern Ocean GrayGoose Construction, Maintenance & Refit 1 27-03-2005 07:19
In the Event of Rig Failure GordMay Health, Safety & Related Gear 0 01-08-2004 04:36



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 01:30.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2018, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.