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About estimating limit load using elastic-perfectly plastic material

Hello everyone. I have gotten this question for a long time.

As we all know, people are using elastic–perfectly plastic material to estimate the limit load for a specimen (structure). If the plastic zone is throughout the ligament as the load is increased, we say that the load is now reaching the limit load.

If we define the limit load as a collapse load, the material collapses only when the flow stress is reached. In this way, we have to set the inflection point of the elastic–perfectly plastic material on the flow stress, which sometimes is asumed to be (1/2)*(Sig_y +Sig_m). [Sig_y: yield stress, Sig_m: tensile strength]

If we use the R6 (FAD), we have to set the inflection point of the elastic-perfectly plastic material on the Sig_y, because the maximal Lr exceeds 1.

The  above mentioned two paragraphs are my understanding. I really appreciate your correction.

I take it as approximated methode to calculate the limit load. Sometimes, if we try to use another material, e.g. Ramberg-Osgood power-law, we get a quite different result.

So my question is the following:

How accurate ist the estimation using elastic–perfectly plastic material. Did anyone a test?

Thank you for your time. 

Jayadeep U. B.'s picture

Elastic-Perfectly plastic is "just" a material "model", adopted for simplifying many design problems.  In that case, the material will not develop (equivalent) stress higher than the yield strength.  In other words, flow stress as defined by you becomes the same as the yield strength, and I don't think it is a "useful" idea to use with Elastic-Perfectly plastic material model.  This model is a good approximation for materials, which do not show appreciable strain hardening.


According to ASME rules, the limit load for pressure vessels is the load obtained from the following case.

Use small displacement and small strain theory and the material model has to be elastic-perfectly plastic,  run the FEA model until the solution diverges. The load right before the divergence is the limit load.

So I guess there must be some accuracy people can trust.

Thank you, Mr. Jayadeep and Mr. Wang.

to Mr. Wang: Can you give me the reference of the ASME rules you mentioned.  

ASME Section VIII, Division 2 Part 5 Design by analysis requirements

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