Question:
If something is proof loaded to 1000#, and we want to use it in a system with a 10 to 1 design factor, how does the 200% of the proof test interact with the 10 to 1 design factor? Do we end up with a 500# WLL for the part since we proofed it at 200% of that, do we end up with a 100# WLL under the logic that the Ultimate Breaking Strength must be greater than 1000# and we are using the more conservative design factor (10 to 1 rather than 2 to 1), or do we end up with a 50# WLL, since the item's WLL is 500# from the Proof Test, and we want to build a 10 to 1 design based on that?
Answer:
So, there's a judgment call here. But if you have a shop built piece of gear that you pull to 1000# without a failure, all you know is that it has gone to 1000# without a failure. In the strictest sense you might then say 1000.0000000000001# could be the minimum failure load and so with a 10:1 design factor the WLL would be 100#.
But for a purpose built piece of gear the rule of thumb is to proof test to 200%. So if I needed something to hold 500# I would pull it to 1000#.
In terms of "confidence" you would need to evaluate the loading condition, look at the potential hazards and decide whether you need a strict interpretation of 10:1 or if the 200% proof test is sufficient.
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