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## Homework Statement

http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/plane_stress_principal.cfm

in this notes , i couldn't understand that why there exists an angle (θp) where the shear stress (τ

*x'y'*) becomes zero , (only normal stress acting )

is there any proof on this ?

for the second part , why when we find max shear stress , there's also average normal stress acting ?

## Homework Equations

## The Attempt at a Solution

is the second diagram wrong ? when we find max shear stress , there should not be average normal stress acting ....[/B]