Chainplates especially stainless steel plates are very subject to fatigue fracture .... usually sudden/catastrophic 'brittle' failure. Fatigue occurs from the very first day of a new plate when it goes above 30% of its design stress. Replace them if you can estimate close to or more than 1000000 load cycles above 30% of design strength --- about 1 circumnavigation.

To lessen fatigue in the future, oversize the plate a bit (150%-200% of OEM thickness is good), have the plates finely surface ground then polished to a 'mirror-like' surface finish .... NO sharp edges - 'edges 'rounded' etc. including the chamfers of the boreholes. Then if you have deep enough pockets, get them 'electro-polished'.

Almost every boat chainplate I ever 'backcalculated' is UNDERSIZED versus 'fatigue' strength. The 'fatigue endurance limit' of common stainless steels is approx 30% of the ultimate tensile strength (without dynamic testing) ... and 'most' chainplates are designed only for 'ductile' failure not fatigue failure. The 'fatigue endurance limit' for common 300 series stainless is ~30,000 psi, ductile failure design is ~90,000 psi. Once you propagate fatigue which begins to form 'micro-cracks', you automatically get 'crevice corrosion' inside these cracks, and as 'early' as from when the plates are 'new' and 'first loaded' beyond 30% UTS.