Mohr's circle

1. Dec 14, 2016

fonseh

1. The problem statement, all variables and given/known data
Why cant i use the equation to determine the normal stress and shear stress ? Is my concept wrong ? I use the method in the link to do this question. The question said the
stress on a plane making angle 30 degree anticlockwise from plane AD .
2. Relevant equations

3. The attempt at a solution

When it's anticlockwise , the angle is positive
http://www.dummies.com/education/science/using-mohrs-circle-to-find-principal-stresses-and-angles/
Mod note: Image of work has been deleted. OP will be including work in a subsequent post.

Last edited by a moderator: Dec 15, 2016
2. Dec 15, 2016

fonseh

$$\sigma _(max)$$ = ((25-15 )/2 ) + 20cos60 + 20sin60 = 32.3MPa

$$\sigma x_(min)$$ = ((25-15 )/2 ) - 20cos60 - 20sin60 = -22.3MPa

But the ans is 12.5 and -2.3MPa

3. Dec 15, 2016

fonseh

$r = sqrt ( ((\sigma _x -s \sigma_y) /2 )^2)+ (\tau_xy)^2) = 28$

$tan2\theta_p = 2\tau_xy / (\sigma _x -s \sigma_y) = 2(20) / 40$ , $\theta_p = 22.5$ degree ,
This is different from the $\theta_p$ = -68 degree given

Last edited by a moderator: Dec 15, 2016
4. Dec 15, 2016

fonseh

5. Dec 15, 2016

fonseh

Is the ans provided wrong ? Or is my concept wrong ?

6. Dec 16, 2016

bump