Load until plastic collapse
Load until plastic collapse
(OP)
Hi,
I was going through some old FEA reports by a consultant as a part of my FEA training. Throughout the report, they mention "The plastic failure occurs at the load where the solver fails to converge". The structure in question is a rectangular steel plenum for an air cleaning process that is to be checked for a rapid over-pressurization event. (You can assume this to be a steel tank)
The confusion is that they describe their conclusions based on plots that have Von Mises contours limited to 30,000 psi (steel) and mention that the solver has failed, while the same plots indicate maximum stresses/stress concentrations/singularities that exceed 150,000 psi.
Question:
1. How do you determine whether plastic collapse has occurred if several sections within your model have already exceeded the proportional limit?
2. How can you 'load' a model till the solver fails to converge?
Thank you.
I was going through some old FEA reports by a consultant as a part of my FEA training. Throughout the report, they mention "The plastic failure occurs at the load where the solver fails to converge". The structure in question is a rectangular steel plenum for an air cleaning process that is to be checked for a rapid over-pressurization event. (You can assume this to be a steel tank)
The confusion is that they describe their conclusions based on plots that have Von Mises contours limited to 30,000 psi (steel) and mention that the solver has failed, while the same plots indicate maximum stresses/stress concentrations/singularities that exceed 150,000 psi.
Question:
1. How do you determine whether plastic collapse has occurred if several sections within your model have already exceeded the proportional limit?
2. How can you 'load' a model till the solver fails to converge?
Thank you.
Talk To Other Members
RE: Load until plastic collapse
2) Depends on the solver. LS Dyna for example will cope with shattered fragments. I imagine a Mickey Mouse program won't understand things breaking.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Load until plastic collapse
another day in paradise, or is paradise one day closer ?
RE: Load until plastic collapse
"The plastic collapse is said to occur when the solver cannot reach convergence". Confusing. They were using abaqus, non linear static analysis.
RE: Load until plastic collapse
There are two methods; limit analysis and plastic analysis (also elastic analysis).
Both use non-linear material models. Limit analysis uses an elastic perfectly plastic material model (bi-linear) with small displacement theory. Elastic plastic analysis uses a material curve with strain hardening and large displacements.
In both methods the loading is increased until the model fails to converge. As in, the loading is increased until there is structural instability in the model causing an error. The last successfully converged load step is the plastic collapse load.
This can be thought of as when all the material in the load path's critical cross section has reached the maximum strain from the material curve and any additional loading would cause infinitely more strain.
RE: Load until plastic collapse
this is under pressure, yes? I wonder what's happening to the seams of the box ... how much pressure is venting in practice ?
If you're trying to protect from a bomb-like overpressure, why not have a blow-out door ? or intentionally leaky seams (once the load is well beyond service levels ??
another day in paradise, or is paradise one day closer ?
RE: Load until plastic collapse
I think I get what you mention. I allow the solution to progress till 1% strain on the ASME BPVC VIII Div 2 Annex 3-D stress-strain curve.
OR
Alternatively, could I run an iterative nonlinear simulation and observe the residuals at the different load steps. When I exceed elastic limit for the general stress away from discontinuities (defined by 5.2.2.2 of the BPVC VIII Div 2 code), the design has failed?
Also, I am using Elastic-plastic analysis.
@rb1957
Yes, vent door had been sized per NFPA 68. The design was complicated and Roark's/Blodgett/Troitsky etc. were not very helpful.Also, the design was evaluated with ASME FFS-1.
RE: Load until plastic collapse
The report you referenced was using either Limit-Load or Elastic Plastic. 5.2.2 is the elastic (linear) method so not relevant.
With either limit-load or elastic-plastic you add a load case which is beyond the capacity of the design and then let the FE solver attempt to get a solution. It will iterate a solution, gradually increasing the load until it can't get a solution. The last successfully solved load step is the plastic collapse load (strain will be a lot more than 1%).
As this is just a training exercise have a look at the ASME example problem manual (ASME PTB-3) and attempt to replicate the results.
RE: Load until plastic collapse
RE: Load until plastic collapse
another day in paradise, or is paradise one day closer ?
RE: Load until plastic collapse
The vent sizing was done correctly, but the customer requested last min addons after the dwgs were signed off and metal was cut. So they decided to do an FEA check and accommodate the modifications. I was not a part of the organization at that time, but it was interesting to go through the goofy stuff in the FEA report as a training exercise - like, they refer to 2004 ASME code numbers instead of 2013.
RE: Load until plastic collapse
RE: Load until plastic collapse
RE: Load until plastic collapse
RE: Load until plastic collapse
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik