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Split Hopkinson Pressure Bar

Hello I am an undergraduate researcher working with a Split Hopkinson Pressure Bar and I'm new to this site. We recently changed from a set of 3/4" aluminum bars to a set of 1/2" aluminum bars and an interesting result occured. Our data collection system consists of a pair of strain gages midway along each bar. Data is recorded and presented as the initial impulse wave passes the incident bar strain gages, a reflected pulse from when the wave hit the end of the bar and reflected back to the incident bar strain gage and finally a transmitted pulse which is what remains of the wave after it has passed through a sample. In all cases the voltage reading from the strain gages should return to zero once the wave has passed. However with the new bar the signal lingers after the wave has passed a gage. Does anyone know a reason this might be occuring? The lingering effect makes data analysis challenging since it is unclear when the pulse can be considered ended.

I have included our most recent data. The first blue pulse is the incident pulse, the second blue pulse is the relfected and the first red pulse si the transmitted pulse. Voltage from the strain gages is read by a Vishay analog strain amplifier and then fed into a picoscope.

Thanks for any assistance!

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Hi!

 

Assuming that the diameter of the bars is the only parameter changed then it could be a number of issues I’m afraid. one stab at it - there might be a slight chance of misalignment when the bars were changed. Misalignment of the bars have been shown theoretically to produce a similar effects ( albeit for polymer bars) -H.Zhao et al "On the use of viscoelastic split hopkinson pressure bars".  It is very difficult to keep all parameters the same in a complete change-out like changing the bars so I'd probably go on guessing if I were to make further suggestions.

 

If you can't get rid of this 'lingering effect' then I would suggest you apply wave separation techniques within your data analysis. They are quite straightforward algorithms for small diameter 'elastic bars'.

 

B.t.w. are you using a pulse-shaper??? Your bar signal appears as if you are using one.

 

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