Recently I have been measuring the mechanical properties of my nanocomposites and I want to ask you some silly questions since nobody at my department can help me with the measures, and since it´s my first time measuring "experimentally" the mechanical properties althogh I have the "theorical" knowledge.
First of all I would like to apologize if I´m not explaining properly myself.
I have made some probets of 70 mm long x 10 mm wide x 0.9 µm thick. From that 70 mm long I used 15 mm in both sides to "grid" it.
As long as I don't have an extensometer (really is not working), to calculate the elongation at break (%) I want to divide the elongation recorded by the instron / 40 mm (70 mm lenght - 30 mm of the "grid"s). Is that correct?
I mean I have a stress-strain diagram I can see where the strain starts increasing (so L0 = 40 mm) and I see when it breaks ((Lf = 40 + X).
Apart from that I have realized that a few of my specimens got broken "into the grid" and the most part just behind the"grid" i mean maybe at 16-17 mm.
What do you think about this?
In case of strip-shaped specimens, percent elongation is given by the total extension divided by the initial gauge length (approx. distance between the grips, here 40 mm).
Breaking of a test sample within the grips results from stress concentrations caused by the clamping force around the grips or inaccurate aligning of a specimen within the jaws. Such failures are often observed while testing polymer materials that are stiff and brittle, particularly in a form of thin film. All failures at or within the grips are considered as erroneous since produce lower UTS and EAB values, and therefore should be discarded.
Hi Jon, You are probably through with your doubts by now, but your assumptions and calculations are perfectly fine. 'Breaking within the grips' is a typical problem when using rectangular specimens. One way of avoiding this could be to lower the clamping force (if possible) but make sure it is high enough to hold the samples steady, otherwise you may get some slipping near the clamps which will give you an increased strain.
Can I ask you what is the standard you prepared your specimens according to?
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