ASME Sec VIII Div-2 Design by analysis (Part-5)

[Alpete1430]

Hi,
I am working on recreating a "Pressure-Tension-Bending moment chart" similar to API 6AF charts for a non standard pressure of 6500PSI.
I had a doubt regarding one of the statements made in ASME Sec VIII Div-2 Design by analysis (part-5)
I'm referring to 2010 edition.
While considering the load cases for Elastic or elastic-plastic analysis under the Loading Conditions section of clause 5.1.3.2, the code quotes as follows:
'In evaluating load cases involving term P, the effects of pressure being equal to zero shall be considered'
So my question is, does adding a 'zero pressure' loadstep - LS-3 (see load steps defined below) even for Elastic analysis make a difference? As in Elastic analysis, all the stresses arising from test pressure (not working pressure) will be in elastic region.
And for that matter for Elastic-Plastic analysis, if I come to know that the stresses in the component I'm analyzing are under elastic zone itself even after applying maximum test pressure, is it really required to add a loadstep of 'zero pressure' to consider the effects of plasticity? The loadsteps (LS) generally considered are as follows:

LS-1 : Apply Preload (stays throughout the analysis)
LS-2 : Apply test pressure
LS-3 : Reduce pressure to zero
LS-4 : Apply Design pressure
LS-5 : Apply Tension
LS-6 : Apply Bending Moment

PS: I'm aware of how the selection of the type of analysis (either elastic or elastic-plastic) is done.

Hope I'm not breaching any rules of the community. If so, please correct me.

Thank you all in advance.
Alpete1430

 

[TGS4]

As in Elastic analysis, all the stresses arising from test pressure (not working pressure) will be in elastic region.

This is not a correct assumption on your part.

And in the elastic-plastic analysis, due to the factored nature of the loads, you are most assuredly in the plastic region.

For subsea equipment, the internal pressure = 0 is very important. This has the full external pressure applied. If you are relying on pressure-stiffening, then removing the internal pressure as a case is critical.

In summary, yes, it is very important.

 

[Alpete1430]

Thank you TGS4.
Is it also important for elastic analysis also?
Because the stresses and deformations will still stay in the elastic region itself right?

Alpete1430

 

[TGS4]

It is indeed important. The solution in an elastic analysis is no longer path-dependent, but the zero-pressure case is, nevertheless, critical to evaluate for all failure modes.

It should be noted that in an elastic analysis, all of the stress should be considered to be pseudo-elastic, because it is very likely (in fact, it is allowed) that there will be stresses in excess of the engineering yield stress. (I use the term engineering yield stress very specifically, and in contrast to the proportional limit, because the former is based on the 0.2% offset method, and the latter is calculated based on the total plastic strain limit as defined in Table 3-D.1.)

And please join us in the present and use the 2017 Edition. There were issues with the older Editions that have been corrected. You're dealing with obsolete technology.

 

[Alpete1430]

Thank you TGS4 for the advice.
I will update the standard to the current one.

Regards,
Alpete1430