How to convert Durometer Reading to a Stiffness for modelling in Nastran

[MrIcu]

Hello,

Hoping someone can help me out here.

First the scenario:
We have a test set-up that I have to model in Nastran. As an oversimplification of the set-up, think of a simply supported beam on both ends. The supports are made of steel blocks topped with a 1/4" thick silicon rubber so that the "beam" does not get localized damage when a force is applied at the center.

I would like to model this in Nastran, but would like to include the stiffness of the rubber material so that I can get more realistic deflections and internal loading.

My plan is to introduce a GAP element between the beam, and the support/constraint with an initial GAP of 1/8" (since the rubber will never be fully compressed during loading. However, I have to give this GAP a open stiffness of the rubber material. The closed stiffness will be something high like 10e7.

The only information I have of this rubber material is that it has a durometer hardness of 67A. Can someone tell me (hopefully with insudtry references) how I can convert this durometer reading to an equivalent stiffness that I can use in my models? It doesn't have to be highly precise, but perhaps an average stiffness will work since I know rubber stiffness changes as the amount of force is being applied.

Keep in mind that I'm doing this to reduce the conservatism in my analysis. I could forego modelling the rubber altogether and get high stresses and larger deflections if I assume a hard constraint.

Thanks in advance for your insight and advice.

 

[GregLocock]

Well I'm sure you can google as well as I can. is the rubber bonded to the steel plate or just lying on top? The former is easier to model.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[3DDave]

More important than the elasticity is the fact that such materials are essentially incompressible. The elasticity is highly non-linear.

My experience is that bonded rubber items produce very high shear loads when compressed if they cannot bend or otherwise distort.

Google result:

https://www.3dvision.com/blog/entry/2011/07/14/convert-durometer-to-youngs-modulus.html

and

https://www.3dvision.com/blog/entry/2016/04/01/shore-durometer-and-finite-element-analysis.html

 

[MrIcu]

Ok maybe I'm just a really green engineer by asking this ... but I have stumbled onto those google searches and all I can calculate is the Young's Modulus (E) which has a unit of pressure.

Nastran stiffness is not given in units of pressures. It is in units of lbs/in or N/mm. So my follow-up question would be if I only had the durometer hardness of 67A ... I can calculate the Young's Modulus using the equations that google can give me ... but how do I convert that to a stiffness?

The rubber is essentially laying on top of a steel block ... all being held together with good ole duct tape all around.

 

[BAretired]

If you have Young's Modulus for the material E, you can calculate the unit strain e (dimensionless because its units are in inches per inch) for a given stress σ using the relationship E = σ/e. If the thickness t of the neoprene pad is known and the bearing pressure σ on the pad is known, the deformation is e.t which is equal to σ.t/E.

BA

 

[Tmoose]

http://www.moldeddimensions.com/images/urethanecompfig2.png

It is from From this page, which is pretty interesting but does not mention silicon rubber that I coul see.

http://www.moldeddimensions.com/urethanes-in-compression

 

[3DDave]

Tmoose - very interesting info. Thanks.