Eddy testing for chainplates

[GordMay]

Originally Posted by Cheechako

I don't think there is any cycle fatigue in chainplates to speak of. They really aren't cycled. they are just in tension, not flexed over and over...

Except in the (not uncommon) case of misaligned chainplates/rigging.
Turnbuckles and chainplates must be angled, so that loads are in a direct line with stays and shrouds.
Toggles, which act like universal joints to allow movement in all directions, should be used with turnbuckles; but they cannot be relied on to compensate for a misaligned chainplate.
A chainplate that is not aligned has a tendency to work, until it eventually breaks.
Besides eyeballing the shroud/chainplate alignment, misalignment is sometimes indicated by damage to the surrounding gelcoat.

Chainplates Revisited ➥ https://www.practical-sailor.com/sai...ates-revisited

What’s Hiding in Your Rig?
➥ https://www.practical-sailor.com/blo...ng-in-your-rig

[Cheechako]

Originally Posted by GordMay

Except in the (not uncommon) case of misaligned chainplates/rigging.
Turnbuckles and chainplates must be angled, so that loads are in a direct line with stays and shrouds.
Toggles, which act like universal joints to allow movement in all directions, should be used with turnbuckles; but they cannot be relied on to compensate for a misaligned chainplate.
A chainplate that is not aligned has a tendency to work, until it eventually breaks.
Besides eyeballing the shroud/chainplate alignment, misalignment is sometimes indicated by damage to the surrounding gelcoat.

Chainplates Revisited ➥ https://www.practical-sailor.com/sai...ates-revisited

What’s Hiding in Your Rig?
➥ https://www.practical-sailor.com/blo...ng-in-your-rig

Yeah . Seems minimal though to my way of thinking. By comparison, Boeing cycles an airplane wing in testing up and down a number of FEET, they do this like 400,000 cycles!

[Cheechako]

Originally Posted by tsell

Thanks for the responses. The testing wasn't expensive, as the owner of the shop gave me a huge break on the cost. $150 to inspect 7 chainplates and the stemhead fitting. The plates on the 38 are a bit more complex than simple bar stock with holes drilled in them, so I suspect the cost will be significant to have them fabricated. Catalina direct sells the stem fitting for $500. I will upload some photos after I pick them up.

The question of service life and fatigue cycles has been nagging me, and it seems that there is no good way to gauge that other than replacing after x number of years. Seeing that these are likely 38 years old, replacement might be the way to go regardless of test results.

Therein lies the problem: confidence in your system and putting your mind at ease. A system which there is no easy way to determine it's condition.
Other considerations are the size of the chainplates. Some boats seem to have thin cross sections of minimal size. Some have nice thick chainplates.
-What is the "safety margin"? Is it a system built with 6 times the calculated need?
-How do they look?
-is it a shroud or the upper? An upper will likely fail a rig faster than a shroud.
-What are your cruising plans? In the E. Caribe you are never further than several hours of motoring, and often in more protected waters. Some people buy an old boat, go cruising with the rig on it for years.
-In the S. Pacific it's a bigger gamble.

[Jenna1]

The experiences that I have had with NDT suggests that you are on the right track with DP. Eddy current testing will give you indication of potential cracks, but I am surprised the shop did not offer magnaflux testing, it not only will show potential cracks, it will also show stress patterns on the metal being examined. I am envious of your boat choice. We are looking to move up from our Catalina 30 eventually.

[RaymondR]

Originally Posted by Cheechako

I don't think there is any cycle fatigue in chainplates to speak of. They really aren't cycled. they are just in tension, not flexed over and over.
I thought Eddy Current was mostly used in aluminum. ? Learn something every day I guess.
If the surface of the chainplate is clean and smooth enough, Dye penetrant should be good if done by a good reader. There is a lot of misinterpretation from DP otherwise. Any surface inclusion can be read as a defect or not.

There is a lot of cyclic stressing of a sail boat rig particularly when under sail. Wave action causes tolling and pitching motions of the hull which generate inertial loads and the action of the hull in resisting the over turning caused by wind loads on the sails can be considerable.

Stress risers are caused by sudden profile changes and consequently are most severe at thread roots which have been machined with pointed tools. Corrosion pits, being fairly round bottomed are generally not severe stress risers unless on an edge or corner.

Thickness to width ratio is important in chain plates. Too thin a material will initially fail at the hole edge and then progressively fail through the rest of the body. Where thinish material is used, which might be thought necessary to transmit loads into a lightly built hull, it should be doubled, trebled or even quadrupled.

A lot can be learned about structural engineering by closely inspecting mobile cranes.

[KP44]

Originally Posted by RaymondR

Fatigue in metals is cumulative. You could have a perfectly good test today, load it up a few times, and tomorrow have a crack.

I think aluminum has cumulative fatigue, but not stainless steel.

[RaymondR]

Originally Posted by KP44

I think aluminum has cumulative fatigue, but not stainless steel.

I vaguely recall the phenomena of metal fatigue is related to crystal boundaries and suspect that all crystalline metals would suffer from fatigue.

[KP44]

Originally Posted by RaymondR

I vaguely recall the phenomena of metal fatigue is related to crystal boundaries and suspect that all crystalline metals would suffer from fatigue.

I said no fatigue, and that is not the case. But for our work as sailors, it is close enough. I think the fatigue in stainless might take several lifetimes to have a noticeable effect on strengths; much longer than the service life of the stainless piece.