Tensile Test Specimen Point of Failure

[Ben_P_1992]

I am a part C Mechanical Engineering student and have been undertaking a project investigating the strain sensitivity of a particular glass filled composite.

From my quasi-static results, the stress-strain curves seem to be reasonable and match that of the mechanical material properties supplied by the manufacturer, but during quasi-static tensile test it can be seen that the samples failed just above the gauge section (please see attached).

If my stress-strain curves produce the expected results and I feel that the sample was correctly aligned within the grippers of the Instron testing machine, then what could be an explanation for the point of failure? Could it be the slight misalignment of the specimen in the machine, or is there another explanation?

Thanks,

Ben

 

[Ben_P_1992]

Also, it can be seen in the image, but the grip marks on the specimen seem to be pretty linear in terms of direction, and so I can assume that the sample was properly aligned.

 

[billy_joule]

Where they cut from the same sheet? Put in in the same orientation? Break near the same jaw?

 

[Ben_P_1992]

I could not tell you whether they were cut from the same sheet or not, as the samples were already prepared for me by a PhD student who had these leftover. They were all put in the same orientation, however they did not all break near the same jaw, although most of them broke near the top one. I tested 9 specimens, all of which fractured in this manner, however, as I said it wasn't always near the top jaw.

 

[insightful]

It looks like the details of the glass fill pattern at the failure points is the cause. A small difference at that point would do it.

 

[Chris Thomas]

When a sample breaks at the grip jaw face the sample should be discarded and the results should be ignored. This is a failed test which is why the stress-strain curve is reporting data that is slightly lower than anticipated. The sample must break somewhere in the middle of the gauge length, away from the grip face. There are several potential causes of your failed test.

I am assuming you are using some sort of wedge grip, this is the correct grip, however there may be other attachments that you need to use. I would heavily recommend using a universal swivel joint. This will allow the apparatus to self align as you are performing the test. You also must make sure that the sample is properly aligned in the grip, prior to testing. This can be done with a sample aligner which is a small piece of metal that is attached directly to the grip. It behaves similarly to a T-square.

If you have any similar questions, we would be happy to answer them at The Universal Grip Company

- CT