How can the plane strain flow stress of a material be measured?

Question

Any links to a diagram of the apparatus?

Thanks.

I mean how to measure the flow stress (the stress required to plastically deform a metal) under plane strain conditions (when the speciment is free to expand in only one direction, i.e. in the tranverse or longitudinal direction while being compressed)

Alright, to clarify the question. I need to figure out a method to ensure plane strain conditions are met in a specimen when under a compressive stress. The compressive stress needs to be measured as soon as plastic yielding occurs.

Answer 1

Strain gages are normally used for this. See the following website: http://www.omega.com/literature/transactions/volume3/strain.html

Answer 2

Concur with other answer
Strain guages