Allowable Stress % of yield 2

[uzairsahmed]

Can anybody give a detailed answer as to why the allowable stress of carbon steel (A 106) is 50% of its yield strength and the allowable stress of stainless steel is 70% of its yield strength?

B 31.3 also specifies for flanges that when we are dealing with stainless steel the allowable can be set high. By the look of it i assume it has to do with SS having greater ductility but would appreciate somebody elaborating on this.

 

[bernoullies123]

Detailed explaination given in:
ASME BPVC Section II Part D MANDATORY APPENDIX 1 BASIS FOR ESTABLISHING STRESS VALUES IN TABLES 1A AND 1B.

as referred to in B31.3:2008 302.3.2 Bases for Design Stresses footnote 2.

 

[BigInch]

The ultimate strength is 33% higher for SS at about 80 ksi vs. 60 ksi for CS. Using the same ratio to calculate the allowable stress level from that of CS, 50% * 1.33 = 67% for SS. That's close to 70%.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

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[uzairsahmed]

big inch...

the allowable stress is based on the yield right? our use of allowable stress is to ensure that the material doesnt yield.I understand the fact that utlimate tensile is greater for SS but if allowable for SS is kept high, the room between allowable and yield is less for SS which makes it more likely to locally yield at a point?

 

[BigInch]

Yes, but yield, though often undesireable, is not necessairily bad, in fact some localized high stress will be residtributed as the material yields. The Ultimate (shear) strength is where things come apart, so while yield stress is where deformation begins, a safty factor can be established with consideration of both yield and the reserve strength that is provided by the margin of the material's ultimate strength.

At least that's my idea. I don't have a copy of the BPV reference above, so it would be interesting to compare my understanding of things with what that says.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[TGS4]

The allowable stress is the LOWER of 2/3 yield or 1/3 (or some other fraction, depending on your Code of Construction) of ultimate. It's not one or the other, it's the lesser of BOTH.

Sometimes, the 2/3 yield governs, and sometimes the 1/3 ultimate governs.