Effective length of circular specimen

[gfbotha]

Hallo everybody

Sometimes a circular tensile specimen is used in the case of relatively thin-walled tubular products, where one need to determine mechanical properties in the hoop direction. Two internal semi-circular jaws are used in the tensile test machine.

Aparently one test standard recommends an effective gauge length of equal to one diameter. Could somebody perhaps tell me what the effective gauge length should be in the case of a plastic material with an expected true max elongation of typically 2 to 10%. I realise it will also depend on friction, but I just like to have an indication to start with.

Thanks

 

[Compositepro]

The gage length is only important if you are measuring modulus in addition to strength. If modulus needs to be measured acurately then you have to use an extensometer or strain gage. With your grip and specimen geometry you have a very long grip length and short gage length, so you cannot get a very accurate estimate of the true gage length.
The same situation exists with wedge grips and straight specimen but the percent uncertainty in gage length can be made much smaller.

 

[gfbotha]

C-pro, thanks for your response. I agree with what you are saying but would like to comment like following:
Elongation at rupture is of course the other property requiring an effective gauge length,
and when monitoring production or lot variation the not-so-accurate results are still practical and usefull.

Could it be that you are a bit over pessimistic when referring to "a very long grip length and short gage length"? There is considerable slippage around the near tangential area; from there one standard's recommending 1 diameter...

Correction: the expected true max elongation of my material is rather 2 to 30% (forgotten about higher temperatures).