I used an Instron 5581 Tensile Testing machine to test the tension of several bamboo samples that were sanded into dogbone shapes. They kept breaking at the grips and I am not sure why?
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$\begingroup$ A picture would help : change of diameter? Sharp change ie a stress riser? $\endgroup$– Solar MikeJun 22, 2018 at 18:59
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$\begingroup$ Just added a picture for you. $\endgroup$– Jo CaliJun 22, 2018 at 19:03
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$\begingroup$ I suspect in the way you attach some side to side force is created. $\endgroup$– paparazzoJun 22, 2018 at 19:13
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$\begingroup$ If that machine is anything like the one I used at Uni - the clamps are located with pins to reduce the chance of side force - but you do have to get things in line... $\endgroup$– Solar MikeJun 22, 2018 at 19:32
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$\begingroup$ I am going to use different grips next time and also make sure that the samples are aligned near perfect. $\endgroup$– Jo CaliJun 22, 2018 at 19:34
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Have the necked section slightly shorter and make sure the radiusing is smooth...
But, I would suggest that you should consider either natural defects (that may not be visible) causing failure or you are just hitting the tensile strength - did they break at around the same value?
How many have you tested so far - if it is only those two then you need to do more to confirm the cause.
You should do a test on a piece that does not have any reduction in csa but perhaps just a bit thinner to see as well.
answered Jun 22, 2018 at 19:12
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$\begingroup$ Thank you! I will try that or create separate grips for the next samples. I had a total of 7 samples I tested. They all broke under 10nK of Load A few of my samples that were about 1-2mm thick broke near the center, so thinner might be better. $\endgroup$– Jo CaliJun 22, 2018 at 19:32