Material Strength Confidence 1

[Sparweb]

Occasionally, I encounter something on a customer's drawing that's just called out as "1020", without any further specification, so I get whatever seems appropriate. I've gone to suppliers in the past and got material test reports that tell me the UTS, (ignoring comments like "everybody knows it's good for 85 ksi&quot, but what is the basis of their claim?

Aircraft materials have to beat the allowable strength 95% of the time, and the test data must show this with a 98% confidence. Do the common ASTM grades get this treatment, too?

For example, the 2024-T3 aluminum sheet used in aircraft usually tests for 72 ksi from the mill, but we always use 60 ksi for the Ultimate Tensile Strength, because our regulations require use to use the "allowable".

Given that some of these materials have been produced for over a century, in staggering quantities, and modern quality control being what it is, does the question even come up when choosing your materials?


STF

 

[Metalguy]

1020 steel for an aerospace application? Must be for a part that stays on the ground!

Critical parts should always have the details of the requirements given and verified.

 

[IFRs]

Aluminum parts, especially tempered or hardened ones have "typical mechanical" properties that are higher than the "minimum mechanical" properties. The Aluminum Association publishes this data. Design is always to "minimum mechanical" properties. A good reference is "Aluminum Structures: A Guide to Their Specifications and Design", 2nd Edition by J. Randolph Kissell and Robert L. Ferry ISBN: 0-471-01965-8

 

[lilliput1]

You always must have safety factor built in. It is not possible to forsee all forces that may happen in actual application. More so in applications where human life can be at stake. Most engineering formulas are based on assumptions like, plane sections remain plane etc., when in real life things are not so. Wind can cause flutter setting up damaging vibrations, shock and impact amplify forces, cracks in material during manufacture. In real life it would be difficult to account for all factors. Even if you do you will still want to have some factor of safety left for your peace of mind.

 

[Sparweb]

Metalguy:
You'd be surprised what helicopter and aircraft operators will hammer together to be bolted onto their aircraft. Usually a load test beyond the aircraft design loads is sufficient, no matter what the material, as long it serves the purpose and is stronger than the underlying structure. Well, it's a longer story than that - explaining how the aircraft mod & repair industry works is whole other thread.

Getting back to the original question: What is the statistical/analytical basis that guarantees the "minimum mechanical properties" for non-aerospace materials? The Kissel and Ferry Textbook may have the answer - for aluminum. It looks like a good book anyway (checked it out on Amazon). But Steel and Aluminum are handled by independent and competing bodies. I expect they treat their allowables differently. Especially for yield strength, as most aluminums I know of don't have a yield strength.

Any quick answers for the guy who's too cheap to buy the entire AMS specification reference (several thousand bucks)?


STF

 

[EnglishMuffin]

Metalguy :
Educate me - is there actually a strength specification that things like 1020 must meet ? I has always assumed that the only official requirement was metallurgical.