Necessity of a poisson ratio when using strain gage (wheatstone) measurement

[Strain_Life]

Hi,

I am building this labview interface to eventualy make strain gages measurements on composites samples. I have been testing what I have done so far in half bridge config. and so far so good. I have read about the need of disposing my gages on the material whether in an half or in full bridge ( http://www.ni.com/white-paper/4172/en/#toc1 ). My question :

Does Labview need a precise material poisson ratio value(s) when using the DAQmx tools (one of 'strain gage information' asked as input for DAQmx) for getting good accurate values of strain?

1) In other words, if I am using 2 gages (half bridge: one in axial direction of the sample, one at 90° of the axial direction on the sample), will Labview DAQmx only use the 'equilibrium purposes' (precision/temperature/offset) of a half/full-wheatstone-bridge-config with only  a relevant poisson r. value?

2) If yes: that would mean I need to find the material's poisson ratio for each material type (composite) first. Then, what would be the best way to put my gages (half of full bridge) on the material to find out a 'precise experimental value' of the poisson ratio without having it? (seems like using bending modes are the only way out)

(please consider  http://www.ni.com/white-paper/4172/en/#toc1   as a reference)

TY,

Daniel

[Strain_Life]

If my first question is not clear enough again : I wanted to know if LabVIEW's DAQmx was using a half/full bridge configuration not only for precision and equilibrium purposes BUT ALSO simultaneously for finding out a more real precise poisson ratio (since we have 2 gage, and -engineer speaking- 2 gages means 2 equations whatsoever, and 2 equation means 2 solutions = say strain & poisson ratio, i guess...).

Maybe I am trying too much 😉

[AaronTheAE · Accepted solution]

Strain_Life,

From what I've been able to dig up, LabVIEW will not calculate a real precise poisson ratio since it is purely an input and just uses that input for calulations.  The half and full bridge configurations are just for 'equilibrium purposes'. This limits you to calulating your own poisson ratio for the composite, which at best would be an average of the materials you are using.  This is a pretty good website for known values of poisson ratios to start from: http://www.engineeringtoolbox.com/poissons-ratio-d_1224.html