Negative strain

[CT1986]

The tube is relative small, with 6 cm height and 3 cm diameter. The tube thickness is about 5 mm. We want to measure pressure inside the tube after explosion by measuring the strain on the tube. The expected overpressure is about 500-700atm, but the measured peak maximum is very high (about 1400 atm) and there is negative zone after explosion. This means that after explosion the bottom face and the side face of the tube were in compression state, but not in tension state due to pressure inside the tube. I guess that there was an uneven expansion due to temperature inside and outside the tube. But the negative strain zone on the graph was very long (greater then 30 seconds and the process not stopped). This is very strange.

[billko]

If the tube deformed, would that explain a negative strain?

[CT1986]

The maximum peak strain was about 1.5e-3, so after explosion the tube was still in elastic zone. When the heat balance has happened, the bottom face and side face must be in tension state due to high pressure inside the tube. But the measured data shows negative strain for a long time after explosion.

[AeroSoul]

This sounds like a question for some other forum, since your measurement tool is working and there's seemingly nothing wrong with your VI...

[santo_13]

Going along the lines of @AeroSoul, essentially the VI works, instrumentation works, just that you don't get the measurement you expect to get.

Definitely, the explosion did not affect the instrument or the VI operation, this leaves you with further investigation of why the strain gages are giving incorrect readings (compared to your expectation) which is challenging for anyone to debug given they are limited to the graphs and images of setup. Not trying to stop any progress but trying to steer back to the goal of this forum.