Negative strain

[CT1986]

Hi guys!

I'm new here and I hope someone may be able to offer me some advice.

I have 2 strain gauge attached on a closed steel tube (one at the bottom of the tube and one on the side face of the tube). All sides of the tube are tightly closed. Inside this tube, there is some kind of dynamite and I have a mechanism to ignite it. I use NI9236 with LabView program to measure the strain. The results are very strange. Strains go from zero to maximum peak, and then go down to negative values, despite the fact that after explosion the pressure inside the tube was very high. I have attached raw strains and filtered strains below.

[chuggins1434]

Just throwing this out there.... is there a chance that you damaged the gauge?  I've only messed with strain gauges once in college, but IIRC they were pretty small and seemed to be delicate... and with "dynamite" as the catalyst for this experiment things will happen extremely fast... might not have caught the actual peak and have exceeded the limits of the gauge??

[CT1986]

@chuggins1434

I have set the sampling frequency to 10000Hz, so the device could catch event to 0.1 ms. The type of the strain gauge is Showa N11-FA-5-350-23 with resistance 350 Ohm, gage length 5 mm. Maximum strain peak was about 1.5e-3, so material was in elastic zone and I don't think it has exceeded the limits of the gauge.

[paul_cardinale]

My wild guess is that the explosion causes deformation of the pipe in such a manner that it throws off your zero-point.

[altenbach]

How are the gauges attached? Could it be there is some slippage during the explosion.

Does the max strain agree with the expected values?

Do both gauges show the same pattern?

[CT1986]

Yes. Two strain gauges have the same pattern. I have attached the results below.

[Yamaeda]

Is it the same behaviour if you use a smaller charge?

[James_W]

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@Yamaeda wrote:

Is it the same behaviour if you use a smaller charge?

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I'd like to know about a bigger charge - and when the police turn up😉

[altenbach]

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@James_W wrote:

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@Yamaeda wrote:

Is it the same behaviour if you use a smaller charge?

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I'd like to know about a bigger charge - and when the police turn up😉

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Already tried. 😮

[Yamaeda]

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@James_W wrote:

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@Yamaeda wrote:

Is it the same behaviour if you use a smaller charge?

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I'd like to know about a bigger charge - and when the police turn up😉

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That got me thinking about this classic!

https://youtu.be/lIbl_3g5FRA