Interpreting a negative value of equivalent plastic strain

[Doodler3D]

All,

What could result in a negative value of equivalent plastic strain??

Errors in extrapolating values from the Gauss points to the nodes or simply an elastic response within the VM yield surface? Is there a way to check this? I'm using inventor Nastran, so I can't access data at Gaussian points.

Thank you

 

[FEA way]

It’s most likely due to extrapolation, maybe you can adjust some results presentation settings in this software.

Just in case, make sure that other output variables are correct.

 

[Doodler3D]

@FEA way, I'm studying the Kalnin's twice yield method. Since the max alternating stress range is 34 ksi, while the actual yield for SA516-70 is ~38 ksi, is it appropriate to neglect these erroneous values of equivalent plastic strain and assume that the response is wholly elastic?

 

[swimfar]

The calculation of equivalent or effective plastic strain includes squaring the plastic strain (or strain rate) terms. So negative values shouldn't be mathematically possible. Maybe your software has a special meaning for negative values. If not, I would say there's some error and I wouldn't trust the results. I don't like to neglect erroneous values unless I understand where they came from and can convince myself that they don't have any effect on the rest of the answer. Maybe someone with experience using Nastran has more insight.

 

[FEA way]

Maybe mesh refinement will help, it often does in such cases. And yeah, it’s better not to ignore results like that.

 

[rb1957]

"it’s better not to ignore results like that" ... they could be a sign of other problems with the model.

another day in paradise, or is paradise one day closer ?

 

[Doodler3D]

Ok, I figured it out, Inventor Nastran can only spit out the centroidal value of equivalent plastic strain. Strangely enough, its the only parameter where the software does not store nodal values.

 

[rb1957]

why is a centroid value any different from a nodal value ?

why does -ve plastic strain make sense as a centroidal value ?

does -ve plastic strain mean compression ?

another day in paradise, or is paradise one day closer ?

 

[Doodler3D]

rb1957,

A centroidal value should be close to a corner nodal value, if a mesh is correctly refined and mesh convergence is verified. However, the codes usually call for nodal values at corners?

It's the first time I'm seeing a negative value of equivalent plastic strain, albeit it is of the order 10^-6. The two explanations for this are: Extrapolation from gauss points inward toward the centroid OR Mathematically, ε_p is positive, like a von Mises value. I suppose that a negative value indicates an elastic response where the stress state lies on/within the VM yield surface.

Does it mean compression? Probably not, as ε_p is a single positive scalar value.

 

[Doodler3D]

rb1957,

Additionally, Inventor requires the entire stress strain curve (elastic + plastic) . When it comes to evaluating plasticity, this can create problems with smoothening of the curve in the vicinity of the proportional limit.