Change of Plane Strain to Plane Stress

http://en.wikipedia.org/wiki/Young%27s_modulus

Thanks for the link,even I forgot simple stress-strain curve too!Ok,I have checked the cup and cone of a tensile tested specimen.I found that the inside area of cup is rough (dimples) and the edges has lips(looks like sheared).When will I get the CUP of broken specimen with ALMOST FULLY sheared manner after tensile experiment?Any formula to find it?

Send a picture of the failed zones it will help to give you indications. In the litterature you can fond on google such pictures are in several sources available to help you understand the process. Acording to what you write the failure has 2 stages one in an elastic domain still (major effect shear strains) and the second in the brittle domain when the stress for the core became too big.