Dimensions of my Natural fibre composite specimen is 250*25* 5..I have done testing at 2mm/min..But i got lesser values..So if i decrease the rate of loading..Is any change can i see..please suggest the rate of loading based on ASTM 3039.
generally for metals...strain rate inc will lead to inc in strength...this may also hold good for composites...
Surely you are testing the specimens per some standard (ISO, ASTM, etc.). What loading rates are acceptable per the standard and what effects are noted in the standard.
Testing rate will affect for sure the material properties. You have to decide which rate makes more sense for your application and have to definitely give the testing rate in your report. If you strictly adhere to standards, then you should specify the standard.
Dear Dr Linto / all peers,
I beg to differ from the above comments.
In metals the strain rate sensitivity is due to the accumulation / flow of defects or dislocations at grain boundaries / hardened particles in the structure.
In the case of composites there is only load transfer between the reinforcement and the matrix, there are no dense particles or grain boundaries to lock the movement of molecules. So the failure modes in metals and composites are different.
I think that if you lower the strain rate, there should not be a appreciable difference unless there is impact loading.
Whether in metals, polymers or composites, an increase in strain rate, like a decrease in temperature, will generally serve to elevate the strength and perhaps diminish ductility. The effect is often far greater in polymeric materials which display viscoelastic/plastic phenomena in their deformation characteristics. However, there is no right strain rate - it is generally what is relevant to the application that your material might be used for. If this is not known, and/or if you are seeing effects of displacement or strain rate, this might be a pertinent line of research to follow.
In our institute TsAGI it was shown that tensile strength properties of composite materials during UTM testing are exponential function of the rate of loading for glass fiber reinforced plastics and for some carbon fiber reinforced plastics
Dear Dr Linto:
The crosshead speed of the testing machine is directly related to the strain rate in test specimens. The strain rate and the testing temperature generally affect the tensile properties of test materials. The maximum strain rate in the natural fiber composite specimen of this size obtained by the Universal Testing Machine is quite limited to less than 0.01/s. Therefore, the tensile properties of the present composite material are not probably affected by the strain rate.
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