Elastic-Plastic Stress Analysis Method in ASME VIII.2

[IdanPV]

Hello All,

I am trying to understand the Assessment Procedure and the Acceptance Criteria for the Elastic-Plastic Stress Analysis Method.
I've already create the Stress-Strain curve, based on ASME VIII.2 Annex 3-D and I know how to define the material in the FEA software.

What I don't understand is how to know if my design meets the requirements of the Code.
5.2.4.4 stated that: "if convergence is achieved, the component is stable..." but how do I know if convergence is achieved?

PTB-3 Example for this case only shows the von-Mises stress plot and the Equivalent Plastic Strain and stated that "convergence was achieved" so, for me, it's not very helpful.

Thank you very much.

 

[Paulettaa]

TGS4, unfortunately I am in the middle east and cannot attend your courses.
I have another basic question regarding elastic-plastic method for evaluating protection against plastic collapse.
What should be considered as the yield strength in the material model? Is it the Sy as mentioned in Sec.II tables or 1.5 times the allowable stress?

Are there any rule of thumb for preference of Limit Load analysis over elastic-plastic analysis or vice versa in different situations?

 

[IdanPV]

TGS4, Yes, it was with fully-factored load (3.5*P) according to Mandatory Appendix 46.
Another problem I have is that the stress is increasing with mesh refine, but maybe is a singularity problem and not a stress problem.

 

[TGS4]

Paulette - the rules in 5.2.3 state that the effective yield stress to be used in the Limit Load Analysis for demonstrating Protection Against Plastic Collapse is 1.5*S. (the exact reference is 5.2.3.5, Step 3.)

There are no situations where I prefer the Limit Load Analysis method. I generally prefer the Elastic-Plastic Analysis Method in all situations. It has the added bonus of being able to satisfy Protection Against Local Failure using the Elastic-Plastic analysis method, and the exact same model with the same material properties can be used for demonstrating Protection Against Failure From Buckling.

IdanPV - once you get into elastic-plastic analysis, you really need to stop looking at the stresses. I kn ow that it's difficult, because most of us are accustomed to using stress as the language for communicating. However, in elastic-plastic space, the language needs to shift to strains. And the strains at the factored loads are going to be much more theoretical than anything that you would expect, mostly because you are looking at a large factor of the loads which will put you fairly far to the right in the stress-strain curve - perhaps even to the perfectly-plastic portion of the curve after the true ultimate stress.

In your specific situation, it is likely a singularity problem, but one that will likely not affect the elastic-plastic analysis results. Don't think too hard or look too hard at the results from the factored loads. That it converged is sufficient from demonstrating Protection Against Plastic Collapse.

 

[Paulettaa]

TGS4, in the assessment procedure for elastic-plastic method it is mentioned in 5.2.4.4 that a material model that includes hardening or softening, or an elastic perfectly plastic model may be utilized. My question is that in the case of elastic perfectly plastic model what should be the yield stress?
is it the same as limit load analysis 1.5*S?

 

[TGS4]

No guidance is provided in that regard and I would not recommend using a elastic-perfectly-plastic material model for the elastic-plastic analysis.

 

[IdanPV]

TGS4, Thank you for your help.
One more question regarding the Elastic-Plastic Stress Analysis Method.
The material model for this kind of stress analysis is non-linear and required true stress-strain curve as input.
Section VIII.2 Annex 3-D gives the required model for the stress-strain curve.

This Link provide an example for the Elastic-Plastic Stress Analysis Method of Section VIII.2, 5.2.4.

The stress-strain curve in the link above use the Plastic Strain data for the stress-strain curve nor the Total Strain data.
Which data I need to use for the material model - the Plastic Strain or the Total Strain?

 

[TGS4]

That's a question specific to your software. For example, Abaqus requires the plastic strain only.