Material testing and specimen geometry

[Drej]

A series of very low cycle and high strain fatigue-type tests are being carried out on a carbon steel material. The testing is not typically "fatigue" as such, since the specimen will be cycled approx 20 times under total strain limits of 10% (ie 10% strain range). The only material that is available is in billet form (300mm x 50mm x 150mm - length/thickness/width) and current testing of this type has always been carried out (uniaxially) on cylindrical specimens with a gauge length to gauge diameter ratio < 1 (to avoid specimen buckling problems). Hence, I am trying to understand if using flat (dog bone) specimens would be appropriate.

Any insight appreciated.


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[Ron]

If your cycle includes a compression cycle, then I would be concerned about a dogbone specimen. I would also be concerned about cyclic stress distribution and strain difference at the corners. In short, I would stick with a round specimen. I would also look at doing a short specimen for compression cycling and a long gage length specimen for tension cycling.

If full reversal is necessary, then obviously the shorter gage length would be appropriate. I would also measure lateral strain and check against expected Poisson's ratio.

 

[Drej]

Thanks, Ron.

I had similar reservations also - given previous literature - but was being 'pushed' into using dog bone specimens for this job. Not being a seasoned tester, it's good to get a second opinion on these issues to confirm said reservations. Just to clarify, the cycles are fully reversed over the strain range for most tests, and some tests are expected to be load controlled and would expect some ratchetting effect to be present for this material. Thanks for the insight on gauge length for tension cycling.

Cheers.


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