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I'm learning how to do Finite Element calculations using Comsol Multiphysics.

In Comsol, Linear Elastic Material and Nonlinear Elastic Material are available as material models:

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Using Linear Elastic Material, you can add Plasticity, and define Initial yield stress as well as the tangent modulus:

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I'm wondering, if we have plasticity, doesn't that make our material model immediately nonlinear? Plasticity implies change in the elastic modulus, and being able to define separate Young's modulus and tangent modulus produces a nonlinear stress-strain curve.

What is the difference between defining Linear Elastic as material model and then adding plasticity, versus using a Non-linear material model?

I did a simple cantilever calculation with point load at the tip (right edge):

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Taking the stress at a cross section along the beam shows the stress gradient change around 30MPa:

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    $\begingroup$ The answer to your question: "I'm wondering, if we have plasticity, doesn't that make our material model immediately nonlinear?" is yes (assuming the stresses are high enough to cause yielding of the material) $\endgroup$ Jul 14, 2022 at 18:23

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TL;DR: as answered by @BillGreene as a comment a model with plasticity is nonlinear. Even when the stresses are not high enough the solution process is probably different from a linear one.

Regarding the other question in your title, we need to talk about types of nonlinearity. In this case, we are interested in the following:

  • Geometrical nonlinearities appear when stains are not negligible ($\epsilon \gg 1$ does not hold). This implies that the definition of strains is no longer linear and include a second-order term. You found also have that displacements are also not negligible, implying that the original and deformed configurations do not coincidence. You need to pick your measures of strain and stresses (Cauchy stress is no longer used).

  • Material nonlinearities appear when the constitutive relations (stress vs strain in this case) are nonlinear.

You have elasticity when there is a strain energy function, this implies that when you remove your loads the body returns to its original configuration. Nonlinear elasticy usually combines both type of nonlinearities described above.

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I think the OP is asking why in comsol linear electricity can't be deleted. Linear elasticity is by default the material constitutive model. The user has to add material models, nonlinear, plastic etc to override the linear constitutive relation. So whatever happens the linear model will be there but it will be overwritten when other constitutive models are imposed.

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