I have done uniaxial tensile tests on a machine with and without using an extensometer. And I found that it used different methods for the determination of Young's modulus for different cases. The difference is the way how it calculates the strain:
With an extensometer: strain = Extensometer travel/Gauge length (25 mm in my case)
Without an extensometer: strain = Grip travel/Initial grip-to-grip distance
I'm wondering whether both of them are correct. In my case, an extensometer is not suggested since my samples are very thin and flexible, the extensometer will put extra load on the samples. So I want to know how accurate the results will be when no extensometer is used.
It is a matter of relative stiffness between sample and machine. Without extensometer, your axial strain also includes the deformation of the machine, and it is inaccurate for stiff specimens (steel)
Hi,
Without the use of an extensometer, the results will be inaccurate, perhaps by an order of magnitude. In clamping heads all displacements will be included in the total extension. You must use an extensometer and attach it so that it does not slip... Or use a camera system to measure the extension (DIC)...
Vladimír Chmelko
Slovak University of Technology in Bratislava
Hi Han Lu ,
I agree with Denis Benasciutti and Vladimir Chmelko .
If you still want to use your tensile device without an extensometer to "estimate" elastic modulus, you could first strain a material where you know the elastic modulus precisely and then calculate the machine stiffness from it. You will most likely still have a huge error but it is better than nothing. You can at least compare different materials measured with the same setup and sample geometry.
If you want to do it precisely you should use a laser speckle extensometer or a digital image correlation, especially if your samples are thin. Even with usage of these techniques it can be hard to determine an elastic modulus precisely e.g. for metal foils. If you need information how to deal with such issues I recommend you to read
https://www.researchgate.net/publication/344267551_A_new_approach_to_evaluate_the_elastic_modulus_of_metallic_foils
Best regards,
Claus
I agree with Denis Benasciutti, Vladimir Chmelko, and Claus O. W. Trost
In addition to problems with the deformation of the tension machine itself, slips at grips offer a potential (significant) contribution to false deformation data. Since Han Lu's materials are flexible as described, it's quite likely that the latter contribute more false deformation than the former. Therefore, non-contact deformation measurements, such as those based on laser speckle or camera/image processing appear to be the only choice.
Good luck
I would advise you to use Digital Image Correlation DIC, as it non-contact and full-field technique which will provide you with the whole field deformation. Having such information will help determine not only the Young modulus but also the regions where the strains concentrate or yield regions in the specimen.
Strongly suggest to use Digital Image Correlation (DIC) which can measure stress and strain on almost any material without contact and then to calculate the Poisson’s ratio and Young’s modulus.
You would have to have a perfect fit in the clamps and a sample of constant cross section for the Young's modulus to be measured correctly. You'd probably be the first in the world to do this, because perfect clamping in the jaws doesn't exist (the displacement sensor is probably coupled in the jaws and therefore also measures deformations and displacements in the sample head clamping). In general, for soft materials with a low Young's modulus value, the differences will be multiples, for hard materials (metals) the differences will be orders of magnitude. Such cases are solved by non-contact elongation measurement, e.g. optically (DIC), but you have to measure very precisely: e.g. for steel with a sensitivity of 10 to the minus 5
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