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Old 12-17-2010
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Aluminum7075 T6 in contact with halides (NaCl > 0.5% wt.) similar to seawater has a very low nominal "Fatigue Endurance Limit" .... about 15,000 psi. Chainplates and other components that are subject to repetitive cyclic stress loads will typically fail in fatigue/brittle failure mode and not in 'ductile failure'.

The nominal Fatigue Endurance Limit FEL for 7075 T6, depending on post treatment, can be as high as 25-30,000 psi; but, when in contact with chlorides, etc. can be 1/2 that value 12-15ksi. Assuming a design Safety Factor for this boat at FS=3 and a UTS of 60ksi, UYS of 53ksi, FEL of 15ksi ... ... ... ... I'd be selecting stainless 316 for the plates and would be mechanical mirror polishing (followed by electropolishing if possible) the 316 to retard both fatigue and crevice corrosion potential and would be selecting 30ksi as the FEL for that material.
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