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The nature of fatigue failure is pretty variable. The best that engineers can do is determine statistical fatigue failure tables where material samples of a given cross section are repeatedly loaded under controlled conditions. The result is a table which indicates the percentile of metal samples which survive past given loading cycles.
The number of test samples tested until failure is in the thousands for generating a useful table. Without a good test procedure and a lot of broken mechboxes it's hard to have more than anecdotal authority.
The reason why fatigue failure is so difficult to predict is that preexisting defects play a very large role in the growth of fatigue cracks. Mould defects like voids, initial microcracks, and internal stress strongly affect the initiation and growth of larger fatigue cracks. Without scrupulous non destructive testing it's difficult to assure the non existance of such defects so you'll have a broad statistical dist'n of failures in nearly every cast aluminum part that isn't cast under very careful conditions and post manufacturing crack detection QA.
I do note that I have replaced broken V2 mechboxes of many brands in my days of AEG repair. Some broke in cold weather play. Some broke in warm conditions. The only fatugue failure I've never seen or even heard of was a stock mechbox failing under fatigue load.
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June 25th, 2006, 01:30
