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The stainless steel typically used for chainplates has a 'severe' lower limit of fatigue endurance. Most 300 series stainless will have a fatigue endurance limit of 30% ultimate tensile strength ... UTS at about 90,000 psi. OR a Fatigue Strength at only 30,000 psi.
What this means is that if the chainplate is strained beyond/above 30% of UTS (loading to or greater than 30,000 psi), it will begin to rapidly fatigue; if kept below 30% UTS it will be virtually indestructible vs. 'fatigue'. If possible, always 'oversize' (thickness) when possible.
For the "blue water" typical design which (should) includes an inbuilt 3:1 Safety Factor ... historical 'scantlings' will show that NEW chainplates should have a high probability of successful service for ONE circumnavigation - about 25000+ sea miles.
For the typical 'coastal' design with a 2:1 inbuilt factor of safety - about 2/3 of ONE circumnavigation or 17000± sea miles, then a high probability of failure thereafter.
Fatigue Endurance limit for 300 series stainless is: 1,000,000 'load cycles' that exceeds 30% of Ultimate Tensile Strength .... on a blue water boat this load cycle max. occurs at ~40-45° angle of heel. On a 'coastal' design ... about a ~30° heel angle.
Even with new chainplates, once you accumulate 1 million load cycles at or above 30% UTS, there is a high probability of sudden catastrophic fatigue failure.
Crevice corrosion - an ADDITIONAL failure mode that WILL shorten the service life and reduce the load bearing value. Even when the stainless is being formed in the mill, micro-cracks will be formed. These cracks, plus cracks due to additive fatigue cracks will begin to form internal corrosion of the metal - thus an additional weakening and besides any simple accumulation of developing fatigue.
Suggestion for long distance sailing:
• For any boat with an unknown in-service life ... replace the damn chainplates.
• For any boat with chainplates over ~12 years old ... replace the damn chainplates.
• For any boat with chainplates - that have a 'designed' KINK or 'bend' in them ... replace at ~8-10 years.
• For any chainplates that have 'multiple' holes for bolting to knees, hull, etc. ... pull them off and inspect them for small/teeny cracks inside the bore holes that emanate perpendicularly from the axis of strain (looks like a 'smiley face' ... | -O- | ) replace immediately.
• Immediately replace any studs or bolts that have developed 'rusting', especially 'blackish'/dark rusting. Ditto, any chainplate that has developed 'zones' of black rusting. Ditto, for any polished chainplate that has developed a zone of noticeable 'dullness'.
• Inspect 'old' chainplate studs/bolts with a torque wrench ... torque to about 60-70% of the maximum torque value for the bolt or stud. If any 'break off' at or below ~60-70% torque value ... replace ALL of them for that 'station'.
Life extension of new chainplates.
• Suggest that you use 316 SS .... if you can afford it, get your 316 with "MILL SPECIFICATIONS" for 'chemicals and physicals' (so that you're 'sure' that you're not buying 'crap').
• POLISH the entire chainplate to a mirror-like surface. If you have the extra-$ available, then have the already mirror-polished chainplates - 'electro-polished'.
hope this helps.
Last edited by RichH; 03-21-2017 at 12:56 PM.