FEA OF RUBBER LIKE MATERIALS 10

[Bhat165]

Hi, I am trying to do a finite element analysis of neoprene rubber. My problem is like this: I have a layerd elastic system where one of the layer is a rubber layer of neoprene with 10 MPa modulus (very low!). A surface load from a car tire is acting on it with 690 kPa pressure. What kind of analysis techniques I have to consider to model a rubber like material? Does a simple elastic analysis works? I tried a simple elastic analysis with ALGOR and the result shows badly deformed elements.

Thanks.

Sudip

 

[brad]

Rubber doesn't have "Young's modulus". Young's modulus is fundamentally based on linear elasticity. Rubbers are modeled (in the most basic sense) as hyperelastic--i.e. nonlinear elasticity. A linear elastic model for rubber analysis will be incredibly limited in its capability as it fundamentally cannot deal with even the most basic capabilities of the material.

You need at a minimum a code which can handle hyperelasticity. Your current approach is inappropriate, hence the poor results.

Brad

 

[corus]

To add to Brad's comments, I would also include the effect of large displacements in the analysis rather than rely on small displacement theory.

 

[brad]

Good point Corus. I had failed to add that very important detail.
Brad

 

[Bhat165]

Hi Bradh and corus

As follow up to your very helpful replies, I am writing this. May be this is a stupid question, but I am just wondering what kind of modulus we get when we test a rubber sample in UTM. We get a linear stress/strain curve from which we can get a modulus. What does this mean, or mean anything at all. Can we use this "modulus" for any purpose? Frankly speaking, I have never worked with rubber.

Thanks.

 

[brad]

I agree with the above. I cannot imagine how a linear stress-strain curve would come out of any reasonable rubber test.
Brad

 

[corus]

Rubber is a hyperelastic material for which the modulus has no relevance other than providing the initial part of the stress-strain curve for the elastic-plastic properties to be defined in your FE model. Plastic behaviour is represented by a piecewise linear stress-strain curve but I'd check out other models that can be used, such as the Mooney-Rivlin model for hyperelasticity, what ever that is. I also note that in Abaqus you can supply material test data as input, but whether Algor can do that is another matter.

 

[brad]

Note to Corus--I don't think you intended to mean this, but your post sounded this way. I just want to clarify.

Modulus has NO relevance to hyperelastic modeling. The most common hyperelastic models amount to curve-fitting test data with polynomial functions. The constants utilized for these polynomial fits take the place of any kind of "modulus", in the sense that the term is applied to linear elasticity.

Brad

 

[corus]

Thanks Brad, but if you are using elastic plastic properties and using abaqus then the Modulus provides the first (approximately linear) part of the stress-strain curve after which the curve is defined by the yield stress (zero strain) followed by the stress and corresponding residual plastic strain. The series of points provide a piecewise linear fit to the stress-strain data. I should have said the Modulus has no physical relevance but is relevant to the data required by the program. The cases I've come across for plastic materials, the input data has been prepared manually from the test data, initially calculating a 'modulus' as the first part of the curve, even though this part of the curve was relatively small. The other curve fitting methods I can't comment on as I've never used them but these would obviously be better if Bhat165 had the appropriate software.

In Abaqus an elastic modulus is defined for hyper-elastic material, athought this refers to instantaneous and after long term effects.

I would have thought that the rate at which the strain is applied would be significant for rubber and as such strain rate dependency would be a major factor in the material property definition. A simple elastic-plastic approach might not be appropriate therefore.

 

[brad]

Corus,
Now I understand the point that you were making. However, elastic-plastic properties are completely inappropriate for general modeling of hyperelasticity. They will give errant results if there is any cycling of the material (as the return curve will be nonconservative for elastic-plastic, whereas it is conservative for hyperelastic). There are additional fundamental formulational differences also, but they are beyond a basic discussion.

The "modulus" defined in ABAQUS regarding "long-term" or "instantaneous" is simply the definition utilized when the hyperelastic material is extended into a viscoelastic assumption. No modulus "value" is utilized for hyperelastic (in fact, the data lines are identical independent of such a definition).

This simply indicates to the program how to incorporate the prescribed hyperelastic properties into the viscoelastic equation being employed.

Brad

 

[koverend]

Checkout this web site from MSC for some good reference papers on nonlinear FEA of elastomters.

http://www.marc.com/Support/Library/brochures_and_white_papers.cfm

 

[chrbiss]

I design rubber parts bonded on metal inserts and I'm looking for a FEA software. I often work with neoprene like bhat165 does in our present case.
Refering to thread727-37632, which software would you recommend for this particular application ?
It seems like ALGOR encounter some problems (bhat165).
How good are the results on non linear materials with ABAQUS ?
I heard that Mooney-Rivlin model (as mentioned by corus) or some equivalence should be used for non linear materials but almost every FEA companies claim to perform well with non linearities (even ALGOR).
Is there a subtlety I don't catch ?
Does anybody ever heard of SAMTECH ?

And about the discussion on rubber properties, I would like to know what is the "material test data as input" (corus) in ABAQUS.
Since the rubber properties are highly shape-dependant and frequency-dependant, I assume that you need to have a pretty good idea of the final shape of the designed part to provide the software with the right information.
thank you

 

[brad]

The means to describe rubber is via some standardized tests.
Three are the most common (I hope my recollection is right):
Uniaxial
Equibiaxial
Volumetric

One, two, or all of these can be run and input into whatever constitutive hyperelastic model one wants to utilize (and there are many). Essentially, the various hyperelastic models amount to a curve-fitting (with some constitutive basis) to the input data.

ABAQUS, for instance, will automatically take such calibrated test data and curve-fit for the appropriate constitutive model parameters.

Many codes claim to be good at this. The two dominant codes, by far, are ABAQUS and Marc. I will note that most of the more recent constitutive models were originally modeled by researchers utilizing ABAQUS. I think that speaks for the legitimacy of the code for such modeling (but I acknowledge that I am an ABAQUS bigot).

Axel Products at axelproducts.com runs classes for modeling and inputting hyperelastic test data into both ABAQUS and Marc.

Another company, Datapoint Labs, will run appropriate tests to derive hyperelastic properties for the various codes.
Brad

 

[brad]

Correction to above:
There are four tests commonly used. The three mentioned above plus shear (or "planar&quot test data. Shear test data, however, MUST be used in conjunction with another set of test data as it cannot by itself describe the behavior of hyperelastic materials.
Brad

 

[chrbiss]

thanks to koverend for the reference papers, it was really helpfull.
And I also want to thank Brad for his reference to Axel Products.
Right now I'm putting ABAQUS on the top of my list (even though I don't have a clue about the price).
Anybody else wants to preach for is favorite software ?

I'm a new member and I'm really impressed by the efficiency of this site.
thank you

 

[Bhat165]

Hi everyone,

I am now using ABAQUS for hyperelastic modeling of rubber. But still I have one doubt. My problem consists of a layer of rubber sandwiched between two different materials of different elastic modulus and the whole system is under surface traction, which is compression in nature. So the rubber layer is in compression. The test data I have is the standard ASTM D412 test on rubber material which is uniaxial tension. Can I use this data in ABAQUS for modeling my problem?

My rubber is Neoprene D60. Have any one used Neoprene before?

Thanks.

Sudip

 

[Bhat165]

Hi Everyone,

I just wanted to inform that Abaqus ran excellent with hyperelastic model. The only doubt I have till now is that: Abaqus is not asking for a Poisson's ratio for hyperelastic model. Is it calculating the Poisson's ratio from input data or assuming something close to 0.5 (say 0.4999999)? If it calculates Poisson's ratio then it must need biaxial test data with uniaxial data. But I have only uniaxial data. So it can not calculate Poisson ration from that. So what it is doing? One more thing I want to mention is that all rubber does not have Poisson ratio close to 0.5.

One mroe question to bother you. I have already posted this before but did not get any reply. My material is in compression. Should I have to use uniaxial tension data or compression data. Thanks.

Sudip

 

[brad]

The assumption of hyperelasticity (and specifically *hyperelastic) is near-incompressibility, hence Poisson's is assumed as nearly 0.5.

Other models are available for compressible-hyperelasticity.

Regarding the question of compression--the best tests for hyperelasticity are those which simulate the load-conditions. So ideally one would want (for your case) compression data in the range that your rubber component will operate in. If, for instance, your expected strain range is 0 to -15%, you are better to describe a curve from 0 to -15% than one from 0 to -200% (because the averaging functions will average the whole range, rather than focusing on the range of interest--this results in less accuracy in the range of interest).

Brad

 

[elogesh]

Hai,

Bradh: I have a doubt in this regard.I don't know whether I am completely in the wrong thinking?

For me incompressible means means,

"During the application of conpressive loads, rubber never undergoes permenant deformation,but undergoes non-linear elastic deformation and hence returns back to original position(shape and size),hence the term coined as incompressible"That is the reason it is termed as "non-linear elastic-hyperelasticity" as you have pointed out.

Earlier I was thinking that rubber is compressible, this is based on elastic compressive deformation during the application of compressive load. So when I have seen that rubber as incompressible being mentioned in this thread, I got confused.

Please clarify whether my interpretation is correct.If not please correct me.

Eventhough, until now I haven't dealt with rubber,out of curiosity I am asking this question?

Regards,
Logesh.E

 

[brad]

Logesh,
Your definition is not correct. Effectively, your definition is that of elasticity--whether linear or nonlinear. Incompressibility basically means that the volume remains constant, which is the typical assumption for rubbers and metals in the plastic region.

There are examples of hyperelastic materials--foamed polyurethane is the one that comes to my mind--which exhibit essentially elastic behavior but which also exhibit very significant change in volume.

I think I've basically gotten my facts right. If I've said something stupid, somebody feel free to correct me.

Regards,
Brad