How to define a nonlinear material in FEA?

May 1, 2018 by Cyprien 16 Comments

In this article, I will tell you the very basics you need to understand to start using nonlinear material models in your FEA simulation.

If you are just starting FEA, you may have learned that defining “Material Data” usually meant providing 2 input values into your FEA software:

The Young’s Modulus
The poisson’s ratio

And maybe other additional parameters such as the density of the material or the thermal expansion coefficient…

Why can you define your material with so few data?

That’s probably the first question you should ask yourself…

And the obvious answer is:

… You can’t ;-)

Well… at least without a lot of simplifications.

The usual material used in mechanical engineering is steel and we mostly define its characteristics using a very important curve called the “Stress-strain curve”.

Like this one:

This curve is obtained by doing testing in laboratory on small samples.

The linear material assumption

In most cases, the FEA analysis designers perform to test simply their design is a linear static analysis which ALWAYS takes as an assumption very small deformations of your material.

(You’ll have more details about linear static inside the free course here)

So… to be short and straight to the point, in linear static analysis, the material used is always also considered as a linear material.

What does it mean?

it means that we are only considering the very left portion of the stress-strain curve and we consider it to be a straight line.

That’s an assumption valid only for very small deformations (strain), of course…

How do we tell the FEA software what kind of line our material has?

We use the Young’s modulus for that.

The most basic definition of the young’s modulus is that it’s a coeffticient which represents the slope of this initial straight line.

We call this kind of material an “Elastic Material” because it behaves elastically like a spring…

You give it a certain load and the material will deform and extend (or contract) in the same proportion (that’s why it’s called a “linear” relation)

Now, a quick word about the second parameter… the poisson’s ratio.

When a load is applied to a beam and the beam extends for example, the section also decreases (because the volume stays the same, right?)

This effect is represented by the poisson’s ratio.

The astonishing thing is that we just need those 2 parameters and that’s all!

If you consider also the gravity, you will need to have the mass too, and then you need to add the density of the material (to calculate the mass)

Now, what if I want to study a material which deforms A LOT?

That’s when the hypothesis of small deformations we did previously doesn’t work anymore…

Typical examples would be:

We are studying a material closed to failure
We are studying plasticity (constant deformation)
Our material simply doesn’t have this kind of linear range
Etc…

In one of those cases, we have to find something better to represent your material…

And unfortunately that means also we have to go one level upwards in term of difficulty, because we enter now in a much more complex domain…

The domain on “Nonlinear FEA Simulation”

What is a nonlinear material?

That’s where things become tricky because a non-linear material is just a material which is “NON – Linear”…

So, except knowing that the material curve is not a line, you still don’t know much.

In fact, there are plenty of non-linear material models out there that can be used.

A material in general is a very complex arrangement of mater

… and concrete material is VERY different from Soil material, which is also very different from plastic material…

That’s probably because the reality is much more complex than any model engineers can invent to try to match the behaviour of specific materials.

So you will find models for almost every type of material:

Elasto-plastic material model
Perfect plastic material model
Visco-elastic material models
Nonlinear elastic material model
Hyper-elastic material models
Loading History dependant material models
…

The tricky thing you should remember is that once you reached the yield strain/stress, you enter into the “plasticity” domain. As you are not in the elastic domain anymore, you won’t have a direct relation between stress and strain.

You rather are working with increments of strain/stress.

The total strain increment can be decomposed into elastic strain and plastic strain increments.

In elasto-plasticity, there something called a flow rule, which is the relation used in nonlinear material models to calculate increments of stress and strains.

Said simply, elastic material always comes back to initial shape after you remove the load because it’s elastic, but as soon as you get a deformation and your part becomes plastic somewhere, your part starts to change shape for good which induces problems when you unload and load it again…

You can imagine some kind of “yield surface” which will change shape either uniformly or in certain plastic areas only.

I won’t explain about all that here in details, that would be going a bit too far… maybe I’ll talk about it more in details in another article.

Now…

How to choose the right nonlinear material model?

This is a big topic and I will only give you some clues about the most important material models you should know about and you can choose the most appropriate to your simulation in function of the material you have and the simulation you do:

Perfect elasto-plastic material model

This is probably the simplest of all nonlinear material models. It considers that once the yield stress is reached, stress becomes constant and only strain continue to increase.

That’s a very big assumption and probably only valid for steel materials near the yield stress (in reality stress continue to increase a bit until the ultimate stress limit is reached)

It’s a very easy model to set-up (you only need the yield stress value…), but don’t overuse this model just because this is the only one you can easily setup as you don’t have the required stress-strain data… you would just get wrong results.

Multilinear hardening elasto-plastic material model

This model is an improvement of the previous model. Instead of considering the stress constant equal to the yield stress in the plastic region, plastic stresses are calculated using a “hardening” curve.

It is called “multi-linear” because the hardening part (the nonlinear part of the curve) is calculated using a series of points provided by the user (you) and then approximated with a linear approximation on each segment between the points.

The nonlinear elastic material model

Some materials (especially plastics) tend to have an elastic region which is nonlinear in itself.

It means that if you applied a load with a value inferior to the yield stress and then you unload your model, the model will effectively come back to its original shape… but the stress and strain won’t follow a linear relation of proportionality.

There are much more to say, but again, I am just scratching the surface to give you an idea here.

As a side note, if you are a Salome-Meca user, you will be glad to know that it boasts to support more than 200 constitutive laws… with such awesomeness under the hood, you can hope to spend many glamorous days calculating stress in your idle time.

Now the big question:

How to define an Elasto-plastic Material?

I think you already guessed it…

The reason I talked about the “Stress-strain curve” in this whole article is because that’s the main data points we need to define a nonlinear elasto-plastic material into an FEA software.

How to get this “stress-strain” Curve?

Well, the bad news is that it comes usually from laboratory tests…

If you are using a steel material or another well-known material in engineering, chances are that you can easily find a corresponding stress-strain curve on internet for it.

(There are online material databases such as Matweb)

Such nonlinear material data is usually not provided inside the material library of existing FEA software because such data are very specific to your material.

(If FEA vendors provided nonlinear material curves inside their databases, everyone would use it wrongly and that would create a lot of mistakes… or at least much more than now)

Now if your material is non-standard, you will probably have to do some laboratory testings…

How to input this curve in your FEA software?

There are several ways to input this curve, depending on the software you use but all are pretty much similar.

You either directly input data points and the software does the job to interpolate job to reconstruct the curve.

Or… you have some kind of models implemented into your software which can help you to have a nonlinear material curve.

For example, the “perfect plastic model” is basically just a linear curve in addition with a constant portion that starts after the yield strength is reached…so it can be defined just with the yield stress value.

Also generally, in your FEA software, you will have 2 ways to input those data: Either by give it the full curve or by providing only the hardening curve. Be careful because the hardening curve doesn’t start at zero.

Ok…That’s all for today!