Torsion in plastic material

[ZMan00]

I have a round shaft that will be made out of plastic material and cannot find a consistent property to use for determination of material. For example, my part will have a max shear stress of 6,800 psi and I am looking at a nylon material that shows a break tensile of 16,000 psi. Any input would be greatly appreciated. Thanks.

 

[Pud]

You will need much more information than what you are requesting, which is the short term properties.

Long-term properties are very critical to parts subjected to continuous or repeated stress.

Much too long to reply as - Google "creep in plastics" will start you on a very long and sometimes pointless "yellow brick road".

H

(sorry for short and seemingly curt reply, but the answer you are after is very long, and very complex, and life is much too short...)

http://www.tynevalleyplastics.co.uk

 

[Pud]

Sorry, forgot to add - try EMS Chemie - Grivory. Al alloy replacement stuff, imho the best there is but there are others.

H

http://www.tynevalleyplastics.co.uk

 

[ZMan00]

I am trying to identify a failure on a part that was designed by others and we see intermitent shearing failures during a torque test that we perform in our assembly process. One shipment will have 0.5% failures and the next months shipment will show 11%. I am trying to determine if we should change to a better material like the Grivory. We are currently using a 10% GF nylon 6 but, it is not a name brand.

 

[TVP]

When dealing with failure of plastic materials you will need to understand how the finished, molded part has been produced in order to determine a proper root cause, make an effective decision regarding material selection, etc. Conditions like underfill, flashing, molded-in-stresses, fiber orientation due to gate location and filling, etc. will all have an effect on the failure mode and its frequency. To address your specific question, there is no standard test method (ASTM, ISO, etc.) for torsion testing of plastic materials in order to determine a strength-based or fracture-based property.

 

[Pud]

ZMANOO: Your figure of break stress at 16,000psi is based on a carefully moulded (i.e. ideal conditions) of a "dog bone" test bar with the glass fibre orientation parallel to the tensile stress. This gives the absolute maximum value.

If your part has a weld line*, for instance, you can derate the specification by at least 50% for ideal moulding conditions.

* btw, where is this "tube" gated?

Cheers

Harry

http://www.tynevalleyplastics.co.uk