How does Mechanical behave at strains larger than those given in the plasticity data?
I am running a static structural model with nonlinear (plastic) material data. The material data in my model is isotropic elasticity + multilinear isotropic plasticity.
I provide piecewise points to the multilinear isotropic plasticity model, say up to a strain value of 0.50. I then apply displacement to the model until a strain in a specific location (outside surface, average over short path in region of interest) is 0.50. However, the strain in the core of the component is greater (I don't have an example to hand, but for sake of argument, say around 0.52).
I am trying to determine whether my model is still valid in this case  how does Mechanical handle the model when strain exceeds that in the plasticity model?
My thought is that strains are likely valid, since I understand the structure of the mathematical modelling to be: displacement > strain > stress > force. If that is the case, the strains throughout the model area valid. The stress at the surface, where strain = 0.50, is valid. The stress at the core of the component, where strain = 0.52 (Exceeding plasticity data model) is not valid. That would be acceptable to me, as I am not interested in the stress at the core. HOWEVER, I am interested in the force reaction at my applied displacement in the model  would the validity of this be affected?
Best Answer

peteroznewman Member
ANSYS uses the last value of stress in the table when the strain exceeds the last value of strain. This means the Tangent Modulus is zero beyond the last point in the table. The material whose strain exceeds the last value just stretches at that constant value of stress.
If the material had a tangent modulus significantly larger than zero at the last strain point, then loading that part beyond the maximum strain value will result in slightly lower values of force being carried by that section of the part. That means other sections of the part will carry more force than it would have, had higher values of stress and strain been entered in the table.
Therefore, the further past the last value of strain that is in the table, the larger the error in the model.
Answers
ANSYS uses the last value of stress in the table when the strain exceeds the last value of strain. This means the Tangent Modulus is zero beyond the last point in the table. The material whose strain exceeds the last value just stretches at that constant value of stress.
If the material had a tangent modulus significantly larger than zero at the last strain point, then loading that part beyond the maximum strain value will result in slightly lower values of force being carried by that section of the part. That means other sections of the part will carry more force than it would have, had higher values of stress and strain been entered in the table.
Therefore, the further past the last value of strain that is in the table, the larger the error in the model.
Thanks, this makes sense. I am not familiar with the "tangent modulus" property, so will take a look into that. From your description though, it essentially sounds like Ansys applies a "perfectly plastic" behaviour beyond the last datapoint for the material.
@langlinator Yes, perfectly plastic is the same as a zero tangent modulus.