I-Beam deflection/failure

I am growing more unsure where Jeff is coming from.

There is no such thing as "no yielding"at any load in aluminum alloy materials - which are still dominant.

Brian Whatcott

You just don't see my point. Too bad..

Here's a simple example to show how the safety factors come into play.

Am aircraft part loaded in uniaxial tension is made from Aluminum

7075-T73 sheet per AMS QQ-A-250/12. The max stress in the part is 35 ksi at 70 F.

Stress, S = 35 ksi Ultimate tensile strength, Ftu = 67 ksi (A basis) at room temp Yield tensile strength, Fty = 56 ksi (A basis) at room temp

At yield (safety factor = 1.0):

Yield stress, Syd = 1*S = 35 ksi Margin of Safety = Fty/Syd - 1 = +0.60

At ultimate (safety factor = 1.5):

Ultimate stress, Sul = 1.5*S = 52.5 ksi Margin of Safety = Ftu/Sul - 1 = +0.27

This shows that ultimate is critical.

I think I finally got to your place: if you deal with the highest stength metallics, they are kinda like composites - the yield strength increases, but the ultimate increases less.

This is a fancy way of saying, stronger materials tend to be more brittle.

It's for sure, that if you deal with brittle materials, you need to know their ultimate, which as you said, is close to their yield, and keep away from that.

The contrast with traditional aero design is evident: an AN bolt may be bent double in the vise with a hammer. A high-tensile tension bolt will fail brittle, if you pull this stunt however.

THAT'S what you're talking about.

Better to avoid one hoss shays, if possible: but in missiles and RPVs nobody dies either way, if the structure fails...

Brian Whatcott Altus OK

OK but also a matter of the relative yield & ultimate safety factors.

That's very true for a given material. But not always true for different materials, say aluminum and stainless steel. Few metals have low enough percent elongation (below 5%) to actually be considered brittle.

Margins of safety are often checked at both yield and ultimate. But ultimate need only checked on ultimate cases like crash landings and the like.

Be careful: When working with alluminium alloys, plastics and some other materials different to steels it is necessary to deal with very tight fatigue constraints. The fact is that alluminium, will end up their life after millions of load cycles for these material have not got resistance after infinite number of load cycles. So will do some plastics just after a few load cycles. So fatigue and TBF, not only ultimate yield, should be one of the major design criterion from structural point of view when handling such materials. Best regards. Ignacio Simón Yarza. Mech&electronic-electrical eng.

"Jeff F>> Brian Whatcott wrote:

Be careful: When working with alluminium alloys, plastics and some other materials different to steels it is necessary to deal with very tight fatigue constraints. The fact is that alluminium, will end up their life after millions of load cycles for these material have not got resistance after infinite number of load cycles. So will do some plastics just after a few load cycles. So fatigue and TBF, not only ultimate yield, should be one of the major design criterion from structural point of view when handling such materials. Best regards. Ignacio Simón Yarza. Mech&electronic-electrical eng.

"Jeff F>> Guru wrote:

Note that Sahn who asked the original question, was talking about what he thinks is a A36 beam. And A36 is a carbon structural steel. It is not aluminum alloys or plastics.

"Ignacio Simón Yarza" wrote ..

Nice echo. You posted the exact message in another reply.