What is true stress and stain?

Answer 1

I'm assuming you mean the difference between true stress and engineering stress: Engineering stress is only accounting for the area given at the time before deformation. True stress accounts for the change in area that occurs as the material is stressed. If you stay in the elastic region, there will be almost no difference between the two.

Answer 2

Stress and strain depend on dimensional data, such as length and cross sectional area. If you are calculating average normal stress it is equal to force over the cross sectional area. Engineering stress would calculate this using the cross sectional area before the stress was applied; however true stress uses the cross sectional area at the instant of the applied stress. This cross sectional area is different from the stress free state because of Poisson's ratio which states that if I pull on something (tension) it is going to tend to shrink in cross sectional area and if I compress it it is going to tend to grow in cross sectional area. The true strain is the instantaneous measurement of strain as well and is related to engineering strain by the equation true strain is equal to the natural log of ( 1 plus engineering strain).