# Mohr's circle

1. Jul 29, 2016

1. The problem statement, all variables and given/known data
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 (τx'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 ?
2. Relevant equations

3. The attempt at a solution
is the second diagram wrong ? when we find max shear stress , there should not be average normal stress acting ....

2. Jul 29, 2016

### Staff: Mentor

Are you familiar with the Cauchy stress relationship?

3. Jul 29, 2016

no , i have never heard of that

4. Jul 29, 2016

### Staff: Mentor

Then how can you possibly determine the components of the stress (traction) vector on a surface of arbitrary orientation?

The Cauchy stress relationship says that $$\tau=\sigma n$$
where n is a unit column vector normal to a surface of interest, $\sigma$ is the matrix of stress tensor components, and $\tau$ is the stress (traction) vector acting on the surface. Have you ever heard of anything like this?