Mohr’s Circle is used to graphically determine the principal normal and shear stresses.

First, some definitions:

- Tensile stresses are positive.
- Compressive stresses are negative.
- Clockwise shear stresses are positive.
- Counterclockwise shear stresses are negative.

To draw Mohr’s circle, and find the principal normal and shear stresses, you must know the stresses applied to the object, σ_{x}, σ_{y}, τ_{xy.}

- Draw a set of σ-τ axes
- Using the following formula, locate the center of the circle, point c.

$latex \sigma_{c} = \frac{1}{2}(\sigma_{x} + \sigma_{y})$ - Using the following formula, locate the point p
_{1}

$latex p_{1} = (\sigma_{x}, -\tau_{xy})$ - Using point c as the center, and p
_{1}on the circle, draw the rest of the circle. - Draw a line straight across the circle from point p
_{1}, through the center c, and to the other opposite point. Call this point p’_{1}. - The point at which the circle overlaps the σ axis are points p
_{2}and p_{3}, the smaller and larger principle stresses, respectively. - Determine the angle θ, which is half of the angle 2θ on the circle.
- The top and bottom of the circle are the largest and smallest shear stresses, respectively.

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