If a standard sample has been put under compression till the grips are met, how can we prove that the stress strain curve at the last stage is for the sample not for the grips?
Can the grips when become close from each other affect the reading of force-extension for the sample?
Of course, when the grips come into contact, there will be a load transfer through the contact surfaces of the grips to the load cell (hence affect the load-extension reading). The more serious thing is that you may permanently damage the grip/connecting studs or even load cell of the machine. This may even spoil the alignment of the entire load train. Therefore, be careful when you see the grips are coming into contact. Best way to check is to insert a single sheet of paper all around. If you can move the paper around easily you are safe !.
Coming to your actual question (though I do not understand what you mean my "standard specimen".....assuming a cylindrical or cuboidal piece of material perfectly machined to do a compression test). Technically, the stress-strain curve in tension and compression up to max load should be similar in shape. Beyond max load you will see a load drop in tension (due to strain localisation) and you will not see such a load drop in compression (since there is no concept of necking in compression). Said that, if you see an unusual increase after the max load when you perform a compression test (a sudden increase in load after max load) and you cannot insert the paper between the grip faces, you can be rest assured that the sudden increase in load is due to grip contact.
But what I could not understand is why you have to end up with such a situation? You are not supposed to continue the compression test if you see bulging/barrelling of the specimen. If your specimen is sufficiently thick, this condition will reach much before the grip faces come into contact. Are you using a sheet having a thickness not sufficient enough to perform a compression test?
Dear Sivaprasad
Thank you so much for your answer.
About your question, No, the sample is a cylindrical standard sample. However its a small sample, within mms.
Compression samples will of "mm" size only. What L/D (length to diameter ratio) you are using? I do not see any reason why your compression platens should come into contact. Nevertheless, be careful not to damage the test frame/fixture.
Sadeem, for miniature samples, the standard compression test set-up is not always the most appropriate/suitable. If you're only interested in measuring loads in compression, you can still use compression platens, yet, you would still need to make sure the compression platens do not touch one another towards the end of the tests. However, if you're interested in measuring the deflection of miniature samples (you stated extension in your question, we refer to extension in compressive direction as deflection), then using compression platens-only will skew your results. You would need a more sophisticated set-up, similar to the fixture required per ASTM D695 (see attached picture), with a displacement transducer to measure deflection and a cage to hold the miniature samples in place. The link below includes videos too and can give you a better idea. Hope this helps!
http://www.admet.com/testing-applications/testing-standards/astm-d695-rigid-plastic-compression-testing/
What is the rate of compression we have to give mm/min? Till the fracture, do we have to give compression, or start of the compression alone is required?
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