Cold Stabilization vs S-basis

[DonkeyPhysics]

I've got an issue I'm working now where I'm obligated to guarantee that the raw material (6061-T651) used to produce a chassis I've designed shall be supplied on an S-basis (as opposed to A or B).

In order to reliably guarantee the some faces on the part won't move due to internal stresses during machining (we have tight profile tolerance requirements), the machining vendor has the raw material "cold stabilized" prior to beginning the machining work.

The trouble is, even though no cold-working is done on the material during that cold stabilization (it's simply a temperature cycling process to relieve internal stresses, and does not affect the temper), and even though the metal still meets all of it's min yield strength requirements (based on pull tests) after the cold stabilization... there's a question as to whether or not this process invalidates the S-basis of the material.

The reason that's a problem is that I'm having a hard time validating any sort of standard that would tell me what WOULD invalidate that S-basis certification. Presumably taking the block of metal outside on a -30C or +30C day would not invalidate the spec. So... how do I found out what WOULD invalidate it?

I'm currently waiting on a response from the company that does the cold stabilizing to describe their process to me in detail, so I can address the specifics of that in a little bit, but I wanted to post my general question now to see if anybody has any insight or experience with this.

This is an aerospace part, and is considered structural, which is why I need to validate everything to some sort of national standard.

Thanks!

-Alex-

 

[DonkeyPhysics]

just a quick follow-up... this is my first time dealing with the certification basis of a metal, so I've been reading the MMPDS definitions of data basis.

I could probably benefit from a better understanding of the differences. From what I gather from the reading, S-basis defines a standard-based minimum value, where-as the A and B bases guarantee that a minimum percentage (either 90 or 99) of lots of the material will meet or exceed those values. And that, depending on the material, the S basis could be higher or lower than A or B basis.

At any rate, my part is big enough that only an S-basis exists (no A or B). But MMPDS doesn't seem to talk (that I can find) about how the basis is affected after certification, or what any temperature limits might be to maintain that cert.

 

[DonkeyPhysics]

ok, well.., further investigation has answered my own question.

Meeting S-basis simply means that the material still meets the minimum material properties. The only remaining question is how many of those properties and how much of the lot I'll have to validate in order to satisfy the customer, and only they can tell me that (or rather, I can offer my opinion, and see if they disagree).

So... long story short, nothing to see here... move along.