Allowable combined stresses for local load analysis of SS vessels 1

[Antonin]

Hallo!
I encountered the next problem:
According to ASME VIII, div 2, the maximum range of stress intensity for combined (primary and secondary) stresses is limited to 3 x Sm. Some stainless steel materials (e.g. 304L) have allowable stresses equal to 90% of yield stress at higher temperature. Thus the maximum stress intensity allowed is 2.7 x yield stress.
In technical literature however the maximum stress limit = 2 x yield stress (the maximum stress range for elastic shake down to be possible).

Which limit should I apply?

As the ASME VIII approach seems to be unconservative, can anybody advise on the interpretation of ASME or theory behind this?

Thanks for any help.
Antonin

 

[prex]

Of course you should apply the limit per code.
As to what this is unconservative, I would be prudent about that. The theory in technical literature you refer to is for ideal materials with a constant post yield stress. Now not only most materials display some strain hardening (that in the particular condition you refer to behaves like an additional safety margin), but austenitic steels (and some non ferrous materials) display a non sharp transition from elasticity to yield and hence a non zero tangent modulus at yield point.
The result is that austenitics will display a higher plastic deformation, but this is typically what those materials can accept.

prex

http://www.xcalcs.com

Online tools for structural design

 

[JoeTank]

Antonin,
I don't have my ASME Code in-hand so I may be a little off here. Eventhough one can approach 90% of yield, I believe that ASME also requires a limit of 30% to 40% of tensile. This keep the S value much lower that the 90% yield value. So, it may be that the 3S value ends up closer to twice yield than expected.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[Antonin]

Thanks prex and Steve for your replies.

 

[bvi]

The 90% yield only applies at higher temperatures, because an additional criteria in setting the allowable stress is 2/3 of the specified minimum yield strength (which is at room temperature).

The yield strength is the 0.2% offset yield strength (draw a line parallel to the linear elastic portion of the stress strain curve, offset 0.2%; the intercept with the stress strain curve is defined as the yield strength). Stainless steel has considerable strength beyond this value of yield strength, although there will be some accompanying plastic deformation.