Hello Fariborz:

Most probably it requires a more thoughtful analysis, however this paper shows a good approach vs the traditional Timoshenko´s (which I´ve used by the way in my PhD thesis long time ago), measuring the strain from fibre-metal laminates samples.

Regards,

Fernando Guillén

The electrical tensometry method and corresponding strain-gauges are seemed a suitable means of strain measuring in your situation. The electrical resistance of a strain gauge is directly related to its sensitivity. For a given wire gauge of a material, higher resistance gives higher sensitivity. Enough large strain of specimen in form of composite tube allows you to choose simple and cheap wire or foil strain-gauges having resistance 100 or 200 Ω.

All the best 

Dear J. Fernando Guillen G,

Thank you for your answer and your help. However, I could not find the paper that you have mentioned.

Best Regards,

Fariborz

Dear Serge V. Shil’ko,

Thank you very much for your answer and your useful description. I will more focus on the strain gages with the resistance of 100 and 200 ohm. 

Sincerely Yours, 

Fariborz

Dear Fariborz Hashemi, once again, taking into account large sizes of your specimen, you may use gauge with long base (active part), at least, more than 20 mm length. That will increase the accuracy of strain measuring. Good luck! 

Dear Serge V. Shil’ko,

Thank you so much for mentioning this. Yes, the length of the sample would be 10 cm, and I am going to measure the axial compression strain on the sample. As you have mentioned, I will choose the longer strain gage.

Thank you, 

Best Regards,

Fariborz

Dear Fariborz Hashemi, have you any sucsess on tensometry of your composite specimens? Take into account the nonlinear strain behaviour and viscoelasticity of polymer composites. In some cases (especially, for thermoplastics), the role of these deformational peculiarities is essential. But, I think, you are testing tubes made of thermoreactive plastics (probably, based on epoxide resin and glass fibers) having elastic properties. Good luck!

Dear Prof. Serge V. Shil’ko,

Thank you very much for your enquiries.

I have not done the tensometry for compression test yet. 

The target of my study is to evaluate the compression modulus and strength, therefore I am going to neglect the  nonlinear part of the strain. May I know what your idea is? 

I am going to do the test two weeks later. Four strain gages will be mount on four sides of the composites. The hybrid composites (natural fibers, glass fibers and polyester mat) has made by pultrusion technique and the matrix that we have used is polyester. 

If we use the thermoplastic matrix, the nonlinear strain is still important for evaluation of the compression modulus and strength?

Thank you so much for your consideration,

Sincerely Yours,

Fariborz

Dear Fariborz Hashemi, if I understand your goal correctly, you are gathering to test specimens under axial (longitudional) compression. If the compressive strains is not very large, so one can agree with your assumption about their elastic linearity. But if you allow the loss of stability of specimen, the strain will be change dramatically and their magnitude become great.  

Sincerely Yours, Serge