Elastic-Plastic analysis - Triaxial stress limit according to ASME VIII section 2 1

[Pierrelouis]

Good afternoon,

I am currently looking for answers about material model used in Elastic-Platsic analysis - Local Strain limit used in a nonlinear analysis.

When I performed the Limit-Load analysis for protection against plastic collapse, I used an elastic perfectly plastic material model with a specific yield strength which is 1.5.S (allowable stress value for the material considered).

Then, do I use the same material model (with this specific yield strength) knowing that an elastic-plastic stress analysis could use an elastic perfectly plastic model ? Is there a paragraph in ASME VIII division 2 wich clearly mention the suitable material model for this nonlinear method for protection against local failure ?

Thanks for your assistance,

Pierre-Louis

 

[TGS4]

You must use the full elastic-plastic stress-strain curve provided in Annex 3-D for the elastic-plastic method in 5.3.

 

[Pierrelouis]

Thank you TGS4 for your answer.

I studied the elastic-plastic stress-strain curve provided in Annex 3-D. This model for the stress-strain curve shall be used when the strain hardening characteristics of the stress-strain curve are to be considered. But when protection against local failure has to be demonstrate using the local strain limit, an elastic perfectly plastic material model could be used.

What I would understand is :
- Could we use the same elastic perfectly plastic material model as the limit load method in 5.2.3 ?
- Or, could we use Yield strength defined in Annex 3-D, per 3-D.1 as Yiel strength starting point in FEA software ?

Many thanks for your answers,

Pierre-Louis

 

[TGS4]

But when protection against local failure has to be demonstrate using the local strain limit, an elastic perfectly plastic material model could be used.

No

- Could we use the same elastic perfectly plastic material model as the limit load method in 5.2.3 ?

No. The strains in a perfectly-plastic model are non-physical.

- Or, could we use Yield strength defined in Annex 3-D, per 3-D.1 as Yiel strength starting point in FEA software ?

Definitely not. The elastic-plastic stress-strain curve needs to start at the proportional limit.

 

[IdanPV]

Clause 5.3.1.2 stated that "Two analysis methodologies are provided for evaluating protection against local failure under applied design loads. When protection against plastic collapse is satisfied by the method in 5.2.3, either method listed below is
acceptable."
So I can see how someone can find it a bit confusing.

TGS4,
How would you defined proportional limit? is that the point where the plastic strain exceeds εp?

 

[TGS4]

IdanPV, the if you take a look at the 2021 Edition of VIII-2,in Annex 3-D, there is a new statement and new equation for the stress-strain curve:

When ϒ[sub]1[/sub] + ϒ[sub]2[/sub] ≤ ε[sub]p[/sub] , eq. (3-D.1) shall be reduced to

and then the new Equation 3-D.2 is ε[sub]t[/sub] = σ[sub]t[/sub]/E[sub]y[/sub]

So, the proportional limit is when the plastic strain (ϒ[sub]1[/sub] + ϒ[sub]2[/sub]) = ε[sub]p[/sub]. This codifies the Interpretation BPV VIII-2-18-09

 

[Pierrelouis]

Thanks for your informations IdanPV and TGS4.

What Y1 & Y2 mean ? And εp is the second point of stress-strain curve defining as εp(σt/Ey ; σt), right ?

Thanks

Pierre-Louis

 

[IdanPV]

Definitions can be found in 3-D.6

 

[Pierrelouis]

Ok nice !

Could someone send a screen of 3D-3 because I have 2019 Edition of VIII-2,in Annex 3-D for checking the new equation ?

Thanks a lot

 

[TGS4]

It's not Y, it's the Greek letter gamma. And my post above provides the additional words and formula. I highly recommend obtaining the current Edition of the Code.