stress calculation from strain gauge

[meher634]

Hello,
I am trying to validate the stress results from FEA with mechanical testing.I have microstrain values from the strain gauge, how can i get the von mises stress values from the mechanical testing data that i have?.Do i need to take modulus of elasticity (E=stress/strain) coupled with strain to get the stress values?
thank you.

 

[nazariosr]

It will be easier if you compare the Strain results with the Experimental Strain values. The strain gauge only measures plane strain which assumes that the strain normal to the surface is approx zero. A rossette strain gauge will give you simultaneous readings in two direction but still assumes the the strain normal to the surface is zero. I do not think you can calculate the Von Mises strain with only one value.

 

[EdR]

meher634:

If the stress state is other than 0ne dimensional you need a strain rosette in order to determine orthogonal strain values....Once you have orthogonal strain values from the rosette use the 3-D Hookes law equations to convert to stresses...(Yes you need both Youngs modulus and Poissons ratio)...Once you have the stress valuues then calculate the Von Mises stress...

Note that naxariosr is incorrect when he states strain gages measure in a plane strain state (epsz=0)....Actually they measure in a plane stress state...i.e. sigmaz=0.....

Hope this helps....

Ed.R.

 

[bjpil]

Meher634,

The information you are getting from the strain gage is the resistance in an electrical circuit. This resistance correlates to a strain. Stress is then calculated from the strain. As mentioned above you most likely need a three gage Rosette to get a von Mises Stress. I believe it can be done with two gages if they are on the principle axis. There are different equations for the different Rossetes. What kind are you using?

BJP

 

[DonkeyDude]

Why go after VonMises if you have a strain gage ? Why not align your gage in the direction in which FEA normal stresse/strains are available? and compare the strains with normal strains from FEA as "nazariosr" advised?

 

[bjpil]

You can't go after von Mises stress if you only have a single strain gage. If you are measuring strains for a single load then you must have two or more gages and then you can get a von Mises strain/stress. Why go after von Mises? Because it is a good learning exercise and it will give you more confidence in your results. Measuring only one direction may not give you all the necessary information and confidence that your model is correct and you will never know it if you only have a single gage.

BJP