How can I improve the accuracy of neoprene tear testing?

[wxrocks]

Hello -

I am working on a test for different neoprene mixtures. The set up is two jaws that clamp across the width of a strip of neoprene. One jaw is attached to a stationary post, the other is attached to a cylinder able to apply a variable pulling force.

The problem I am having is when clamping the neoprene, it forms a weak point where the material is being compressed. I can of course score the material or cut a notch somewhere, but I am concerned it is then causing a directional stress and does not give me a true cross sectional strength of the material.

Can anyone help provide some insight on how to best test the overall strength and tear resilience of materials such as neoprene? Is there a way to curtail the stress point where it is clamped? Is there a good way to perhaps cut the material such that it is wider on the ends so that the stress in the pinch points doesn't become a factor?

Thanks in advance!

 

[PerennialII]

yeah, I would work on the clamping as well rather than introduce notches etc. for uniaxial properties. Most common tensile test specimen geometries, even with material where clamping isn't really a problem, are wider at the clamped ends compared to the actual section being characterized (I think even with metals the clamped section with most of our tests is 20-200% wider than the actual tested section, a small curvature between these 2 then). Suppose could also try to lessen the problems associated with the clamping itself if there was a more 'delicate' way to do it & lessen the 'strain' on the material, btw have you looked at standards what sort of configurations they recommend for testing materials like elastomers? Would think for example ASTM has had to work on a similar problem at some point (can take a look if you like unless someone comes up with something ingenious :tongue2: ).

 

[wxrocks]

Thanks for the response --

I did find one website talking about testing methods -- they of course offer to do the test for a fee. It seems they pull on larger pieces and must extrapolate to whatever width I might be using. I think it was the ASTM way of doing things.

The problems become: 1) We have a lot of different forcing equipment, but nothing that will measure what is needed to pull a wide piece like that. 2) Paying the money to do the testing.

Interesting point -- after testing narrow strips of 4 different blends... 2 materials streched farther before breaking, yet those same materials broke sooner once a tear was introduced.

I guess I'll keep trying for awhile unless anyone else has a brilliant idea!

 

[PerennialII]

Yeah, at higher load levels the test is probably more like a "crack arrest test" while at lower a "crack propagation test".

The ASTM way of doing tests is probably standard D624 - which is a tear test standard. I'll take a peek if it says anything about fixtures.