Durometer versus Stiffness of Natural Rubber

[wzwqx2]

Is it possible for a rubber with a higher durometer to be less stiff than another rubber with a lower durometer? I thought that durometer was a direct indicator of a rubbers hardness and stiffness

 

[tr1ntx]

Are you talking about the same scale, i.e., Shore A or D?

 

[wzwqx2]

Yes using the same durometer scale.

 

[GregLocock]

For a given scale the the higher the elastic modulus of the rubber, the higher the Shore reading.

http://en.wikipedia.org/wiki/Shore_durometer

Cheers

Greg Locock


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

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[btrueblood]

"Is it possible for a rubber with a higher durometer to be less stiff than another rubber with a lower durometer?"

Greg's reply is generally correct, i.e. the answer is generally no.

There are a couple of oddball conditions where the above could be answered yes - one is if the rubber was poorly cured, from the o.d. in, and had a relatively soft core, with a harder (more fully cured) outer surface. Can also be done deliberately, i.e. if the core is foamed with a harder outer skin. Since a durometer test measures local stiffness only, the skin may have a higher durometer than the bulk of the material. A second condition is if you are measuring very high elongation in tensile testing relative to the (relatively small) deflections imposed by a Shore A or D penetrator. At high deflections the rubber may show a softer average stiffness than you might expect due to nonlinear elasticity of the rubber, and/or many or most rubbers will undergo strain softening (Mullins effect) with repeated loading, and you will see a decrease in apparent modulus with each load/unload cycle.

 

[GregLocock]

Also, using a handheld durometer requires a bit more skill than may be apparent. Even the lab machine is a pretty crude test, or more accurately, it is measuring several different things at once, with the hope that they are all directly proportional. (For instance, poisson's ratio will probably have an effect, for a measure of E it shouldn't).

Cheers

Greg Locock


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

http://eng-tips.com/market.cfm?

 

[ione]

Besides the operator’s sensitivity, I would include that the A scale is used for soft rubbers and elastomers, whilst yype D is used for hard rubbers and plastics.
As it happens for metals, applying a certain hardness measurement procedure to a category of materials which does not belong to the range the procedure has been conceived for, can lead to misleading results.

 

[wzwqx2]

OK. Thank you for the replies.

Let me ask another question on this subject. What if the rubber was different - say Nitrile versus Vamac?

 

[ione]

Nitrile hardness is listed to be in the range 20 – 95 Shore A, while Vamac® (Ethylene Acrylic Rubber) is listed to be in the range 35 – 95 Shore A. Shore scale D usually applies for materials exhibiting hardness value above 90 degrees (i.e plastics) and so the use of scale D for the materials you’ve quoted in your post should not be the most appropriate.

 

[ione]

To bring evidence of the lack of an unequivocal matching between shore A and shore you can take a glance at the link below

http://www.matweb.com/reference/shore-hardness.aspx

As you will see for a given value of shore A there is more then one value of shore D and vice versa

 

[btrueblood]

I would not expect shore hardness, even using the same Shore penetrator, to predict the tensile modulus of two different rubber materials very well at all, i.e. you'd be lucky to get within, say 25%.