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Interpreting a negative value of equivalent plastic strain

Interpreting a negative value of equivalent plastic strain

Interpreting a negative value of equivalent plastic strain

(OP)
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

RE: Interpreting a negative value of equivalent plastic strain

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.

RE: Interpreting a negative value of equivalent plastic strain

(OP)
@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?

RE: Interpreting a negative value of equivalent plastic strain

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.

RE: Interpreting a negative value of equivalent plastic strain

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

RE: Interpreting a negative value of equivalent plastic strain

"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 ?

RE: Interpreting a negative value of equivalent plastic strain

(OP)
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.

RE: Interpreting a negative value of equivalent plastic strain

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 ?

RE: Interpreting a negative value of equivalent plastic strain

(OP)
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, εp 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 εp is a single positive scalar value.

RE: Interpreting a negative value of equivalent plastic strain

(OP)
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.

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