How can I obtain yield stress and module of elastic of polymer in tensile test?
I read astm D638 but I don't know how I can determine them if my polymer have a few non-linearity.
When you do the test you will obtain a stress-strain curve. The modulus of elasticity or Young's modulus will be the slope of the initial linear part of the graph and can be obtained simply by dividing any stress in that region b its corresponding strain. See definition here http://en.wikipedia.org/wiki/Young's_modulus. The yield strength will be calculated as the stress corresponding to 2% remaining strain when you unload after the elastic/proportionality limit as explained here http://en.wikipedia.org/wiki/Yield_(engineering) where yield corresponds to point 4 of the curve. For PMMA though your curve should look like the ones on page 12 of http://www.cmse.ed.ac.uk/MSE2/Tensile%20testing%20of%20polymers_lab.pdf and it is sensitive to temperature and strain rate as seen on this presentation and in this paper http://link.springer.com/chapter/10.1007%2F978-1-4614-0216-9_18#page-1
because our equipment is limited i only use the tensile machine
i want fracture test rubber toughened PMMA and i need yield and mudule of elastic that i want determine from tensile test
i don't think that have large non linearity
you advise me i use astm D638?
When you do the test you will obtain a stress-strain curve. The modulus of elasticity or Young's modulus will be the slope of the initial linear part of the graph and can be obtained simply by dividing any stress in that region b its corresponding strain. See definition here http://en.wikipedia.org/wiki/Young's_modulus. The yield strength will be calculated as the stress corresponding to 2% remaining strain when you unload after the elastic/proportionality limit as explained here http://en.wikipedia.org/wiki/Yield_(engineering) where yield corresponds to point 4 of the curve. For PMMA though your curve should look like the ones on page 12 of http://www.cmse.ed.ac.uk/MSE2/Tensile%20testing%20of%20polymers_lab.pdf and it is sensitive to temperature and strain rate as seen on this presentation and in this paper http://link.springer.com/chapter/10.1007%2F978-1-4614-0216-9_18#page-1
For some polymer and polymer composites, they do not have an absolute initial linear part but have a visco-elastic part. Therefore, it is hard to measure the initial slope as the Young's modulus. Under this condition, you can choose a fix strain to measure the modulus such as strain=0.02 (this small strain depends on your materials and your test conditions, choose the same strain for the test conducted under different conditions) For the yield stress, you can take the highest point when the materials have a significant post strain softening. If not, take the point of intersection for the initial slope and post yield slope (strain hardening part).
my material is PMMA that rubber toughened may be 10 percent
is that non linear in your mind?
You can contact 'Central Glass & Ceramic Research Institute, Kolkata - 700032'.
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