Von misses yield criteria for Bilinear kinematic hardening material

[mon1299]

HI Everyone,
I am modeling a steel beam with SHELL93 element. I am trying to find the ultimate load capacity of the beam considering geometric nonlinearity, stress stiffening on, and also material nonlinearity. I used Bilinear kinemetic hrdening material model. Yield stress=50ksi, E=29500ksi. As I understand, the element von misses yield stress should be lower or equal to yield stress. But I am wondering why I am getting von misses stress higher than yield stress as specified in some location? Is there any one who can make this clear for me? I realy want to know wheather von misses stress can be higher than yield stress or not? If it cant be higher than yield stress, then why I m getting this? How can I overcome this problem? Please help me. I am stucked with this problem and finding no one to help me.
Thanks
Amy

 

[SKJoe]

Hi,

by entering von misses yield stress you only tell ansys when onset of plasticity occur - it doesnt necessarily mean collaps of beam. Progress of plasticity continous because of your tangent modulus is higher than zero in hardering model. So you can observe that these stresses are higher than you entered. In this phase it is up to you how criteria you choose to be your limit state : 1. onset of yielding or 2. total collaps of the beam.

Regards,
Lubo

 

[mon1299]

Hi Lubo,
thanx for ur reply. I understand ur point. For the steel beam, in some location yielding may start but it doesnt mean that the beam will collapse at the initation of yielding.I want to find the failure load but at the same time stress of any part of the beam will not exceed yield strength.What I understood from ur mail is that if my tangent modulus equal to zero, than that condition will be satisfied.In ansys can I define tangent modulus = 0 for bilinear kinematic modeling? I found that Et shoulb be greater than zero and less than or equal to E. Do you have any suggestion?
Regards
papia

 

[TGS4]

In ANSYS, you always have to input the tangent modulus. You are correct in saying that for a perfectly-plastic material, the tangent modulus is zero and you should not have stresses in excess of yield.

 

[mon1299]

Hi TGS4,
thanx for ur reply. I think I can put a very small value for tangent modulus. m i right?
regards
papia

 

[TGS4]

Two things:
1) You can put any value for tangent modulus that you want, including zero.
2) Please see the FAQ about using internet abbreviations - it's unprofessional and is not likely to get you any responses in the future - see

http://www.eng-tips.com/faqs.cfm?fid=1083