Ireneo | 29 Jul 11:52 2009
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Material with softening behavior

 

Hi everybody,

We're trying to simulate in Abaqus/Standard as well as in Abaqus/Explicit a deformable aluminum tube with an inner pressure. In both cases, we find the following message at the data file:

***WARNING: MATERIAL ALUMINUM HAS STRAIN-RATE SOFTENING BEHAVIOR. THIS MATERIAL MAY BECOME UNSTABLE AND CAUSE CONVERGENCE PROBLEMS. WE RECOMMEND THAT THE RATE DEPENDENT DATA BE MODIFIED SUCH THAT THE YIELD STRESS INCREASES WITH INCREASING STRAIN RATE

By explicit formulation there is no problem to solve the calculus but we can't solve it by implicit formulation. Is there any way to introduce in Abaqus a material with softening behavior and with no problems to simulate it?

Thanks a lot,
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Ireneo Torca

Mechanical and Manufacturing Department
Mondragon Unibertsitatea – Engineering Faculty http://www.mondragon.edu/eps
Loramendi, 4; 20500 Arrasate-Mondragón (Spain)
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Fernando | 31 Jul 11:08 2009
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Re: Material with softening behavior

 

Hi Ireneo,

> By explicit formulation there is no problem to solve the calculus
> but we can't solve it by implicit formulation. Is there any way to
> introduce in Abaqus a material with softening behavior and with no
> problems to simulate it?

The "normal" solution method in /Standard increments the strain or displacements and computes the forces to maintain equilibrium, assuming that the stiffness matrix is positive definite. In the softening part of the curve, this condition is not fulfilled, and ABAQUS cannot find a solution. In order to overcome the softening problems in /Standard you should use the RIKS method, which addresses these situations (read the manual to know more about the limitations).

Hope this helps,
Fernando

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