isotropic and Kinematic hardening 4

[vtmike]

Hi,

I am having difficulty in understanding the Isotropic and Kinematic hardening rules used in nonlinear FEA analysis. One of the references says that the yield surface for an isotropic material model grows uniformly with a constant centre and the yield surface along with its centre moves in a particular direction for a kinematic hardening model. But, what is the physical meaning of these statements?

Any help would be appreciated. Thanks,

Mike

 

[kellnerp]

If you load a metallic structure past yield with a monotonically increasing load both isotropic and kinematic hardening will give the same result. If you load, unload, reverse load a metallic structure then kinematic hardening gives a result in better agreement with the actual behavior of the material. In kinematic hardening yielding in the reverse load direction will occur at twice the yield stress from the point the structure was originally loaded.

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[vtmike]

Why do we have an isotropic hardening model if they both give the same result? I mean what is the advantage of using an isotropic hardening material model?

 

[GBor]

Simplicity...and it usually runs faster because it doesn't have to do a property look-up after each run.

I believe there should also be something in the description about brittle vs. ductile materials. Brittle materials are easily described by an isotropic model, but ductile materials, which stretch for a while after yield, generally follow a much less linear path to ultimate failure.

The materials guys may be able to explain this a little better, but I'm pretty sure this is on the right path.

 

[kellnerp]

They don't give the same result. Read carefully.

IF the loading is monotonically increasing, i.e., it goes from zero to some value above the yield point and stops, then they both give the same result.

This loading history does not always apply. Consider bending a paper clip back and forth. This would be reversed loading and isotropic hardening would not give the correct result, kinematic hardening would. Both would give the same result for the first bend and thereafter they would be different.

If you reverse load a structure using kinematic hardening it will begin to yield at minus two times the yield stress from the point on the stress strain curve where loading began to reverse in keeping with the way ductile metals behave.

If you reverse load a structure using isotropic hardening it will begin to yield at minus two times the stress at which loading began to reverse which is not in keeping with ductile metal behavior.

See Concepts and Applications of Finite Element Analysis, 3rd ed., Cook, Malkus and Plesha, p511. for a more thorough explanation.

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[cbrn]

May I add to this very interesting point by Kellnerp, that the EN-13445, and more "understandably" the literature which explains the EN-13445 "Design by Analysis", give explanations about this.