- Thread starter addaon
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I take the minimal values too. Testing parts, you're usually very close to the nominal values. Problem is that once a single part starts to yield the rest of the structure is affected (weakest link in the chain)

At work I use nominal values for calculations, but I suspect the tolerance may be built into the allowables somehow. Using A-basis allowables for everything

If you are working with clad aluminum, don't forget to subtract the cladding thickness from any buckling calculations you do.

Matt, can you define those terms (B-basis and A-basis)?

Answering my own question:

Answering my own question:

• A-Basis: At least 99 % of the population of values is expected to equal or exceed the A-basis mechanical property allowable with a confidence of 95 %

• B-Basis: At least 90 % of the population of values is expected to equal or exceed the B-basis mechanical property allowable with a confidence of 95 %

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I'd be asking myself: How confident am I in the loads and loadpaths for this structure? Is that already reflected in my design factor of safety? What are the consequences if this structure ends up with a 10% lower than designed factor of safety? If the part yields, is the airplane still flyable? If it buckles? If it fails outright? OR, I'll design to a little thinner than spec, and satisfy myself that the actual as-delivered sheet meets or exceeds this thickness before I start fabrication. OR, I'll design to spec, then proof test the actual design and see if passes. OR, I'll design to spec, then monitor the part in-service for cracking etc that potentially might result in some painful rework, or it might never actually be a problem.

Example: some of my employer's products use machined fittings made from 7050 plate. There are two specs for the same alloy, temper, and product form, yet one is several times more costly than the other, and has slightly better properties.

I'm not sure how often this situation occurs, but I figured it might be worth mentioning. All in all, if the AMS spec number on the material can be found in MMPDS, you're set.