What equation is this defined as?

[elephunk]

Is this a stress/strain, or elastic modulus equation? or have I got my understanding wrong?
The question asks where does this come from? I've never seen it before maybe someone can expand for me please?

http://i45.photobucket.com/albums/f98/pastilles/equation.jpg

Thanks,

 

[Mapes]

Are you referring to $\gamma=\psi_1-\psi_2$? It's not a stress-strain relation because there's no constant of proportionality that would correspond to an elastic modulus.

Gamma is often used to denote shear strain, though.

 

[elephunk]

Thank you,

I haven't run across this before. I will read some more on shear strain in some textbooks.
Once again thank you :)