Hi all,
i was looking through a standard for FRP tensile test, it was stated that the extensometers or LVDTs used should be capable of recording specimen elongation during testing with an accuracy of not less than 0.002% of
the gauge length.
If my sample gauge lenght is 170mm, 0.002% of 170mm is 0.0034mm. I have checked that my extensometer is able to measure up to an accuracy of 0.001mm. Is it acceptable? Or does it have to measure up to 0.0001mm?
Thank you :))
Another very accurate alternative to extensometer is to use digital image correlation technique. For that you would need a camera, acquisition and DIC softwares, and would need to speckle your sample prior to the measurements. The results obtained are quite accurate.
Bests,
Ali
ASTM std has round robin test results for different stiffness materials and specified variations. Your method is sufficient to meet your requirement. Strain gunge is much better.
The problem with testing high modulus materials is the pressure you apply to sample in holding jigs and sample preparation. If holding pressure is different for the upper and lower jaws, chances are you will get much variation in % strain. Does your std. deviation falls below .002%? if not, whatever tool you are using to measure strain rate is irrelevant.
Also check for application and see what variation is acceptable.
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