Mohr’s circle is a graphical representation of the stress state of a point inside a body under plane stress loading. For three dimensional analysis we may use the extended Mohr circle for three dimensions (Mohr’s circle 3d).

Consider a body in equilibrium under plane stress loading (cf. Fig. 1). The stress tensor for this case is given by:

(1) |

In this article we will use the following sign convention: Compression stresses are positive and shear stresses are positive when they have a direction as shown in Fig. 1.

Consider now an arbitrary plane inside this body. The normal vector of this plane angle with respect to the horizontal axis . The stress vector that acts on this plane may be calculated as (see also Calculation of normal and shear stress on a plane):

(2) |

where are the direction cosines of the plane. The normal stress that acts on this plane is given by:

(3) |

Expanding the above equation (3) and taking under consideration:

(3) |

leads to:

(5) |

The shear stress on the plane may be derived from the Pythagorean theorem:

(6) |

Expanding the above equation (6) and also taking under consideration equations (3) leads to:

(7) |

or

(8) |

Equations (5) and (8) are the parametric equations of a circle. To prove the latter we rewrite equation (5) as follows:

(9) |

If equations (8) and (9) are squared and added by parts, then after some calculations lead to:

(10) |

If we set

(11) |

and

(12) |

then equation (10) becomes:

(13) |

which is the equation of a circle with center and radius on a coordinate system with the abscissa representing the normal stress and the ordinate the shear stress (cf. Fig. 2).

In order to construct Mohr’s circle for the stress tensor (1) we must firstly define a special rule. The shear stresses that yield a clockwise moment about the center point of the element (cf. Fig. 1) will be taken positive. The shear stresses that tend to rotate the element counterclockwise will be considered negative. Thus, for the case of Fig. 1 we firstly draw the two points and as shown in Fig. 2. Next, we connect these to points by a straight line. This line is the diameter of the circle. The interception point between this line and the horizontal axis is the center of the circle. The distance between the center and or is equal to the radius. Hence, we may now draw Mohr’s circle.

According to equations (5) and (8) a plane with angle as shown in Fig. 1, angle on Mohr’s circle as shown in Fig. 2. Hence, for a given plane it is suffice to measure angle on Mohr’s circle and the coordinates of the point on the circle are the normal and shear stress that act on that particular plane. It should be noted that the angle is measured from the straight line connecting the center of the circle and with counterclockwise direction (cf. Fig. 2).

Subsequently, we may derive the principal stresses and their directions. If we set:

(14) |

then from equation (8) we derive:

(15) |

Equation (15) has two solutions:

(16) |

The two angles and are called principal directions and represent the direction of the two principal stresses. Substituting angles from equation (15) into equation (5) and taking under consideration that:

(17) |

leads to:

(18) |

where and are the two principal stresses. In Fig. 2 the two principal stresses are the intersection points between Mohr’s circle and the horizontal axis.

Maximum shear stress occurs when the derivative of shear stress with respect to equals to zero (see equation (8)). The maximum shear stress angle with respect to the principal directions. Its absolute value is equal to the radius of Mohr’s circle:

(19) |

It must be noted that the above equation (19) holds true for planes parallel to z-axis. If inclined planes with respect to z-axis are considered then higher shear stresses may occur.

**Pole points**

So far we have discussed two methods to calculate the normal and shear stress on an arbitrary plane. The first method is to use equations (5) and (8) and the second method is to measure angle on Mohr’s circle. An alternative graphical method to calculate the normal and shear stress is to use the pole point on Mohr’s circle.

Two pole points can be established on Mohr’s circle. The first pole point is related with the directions of the stresses and the second is related with the planes on which the stresses act. Consider the case of Fig. 1. We construct Mohr’s circle as shown in Fig. 2. Then, the *Pole for stresses* may be found if we draw a horizontal line from point parallel to direction, or by drawing a vertical line from point parallel to direction (cf. Fig. 3). Both lines intersect Mohr’s circle to the same point which is the pole for stresses (). *Pole for planes* may be found if we draw a vertical line from point which is parallel to the plane where acts. Similarly we may draw a horizontal line from point which is parallel to the plane where acts. Both lines intersect Mohr’s circle to the same point, the pole for planes ().

Consider now an arbitrary plane with normal vector that angle as shown in Fig. 3. We can use any of the two poles to calculate the normal and shear stresses that act on this plane. If we use the pole for stresses, we draw a straight line from the pole point perpendicular to the plane (parallel to normal vector ). The intersection point between the line and Mohr’s circle gives the normal and shear stresses that act on this plane. Alternatively we may draw a straight line with angle from and find the intersection point. is measured from the horizontal axis counterclockwise as shown in Fig. 3. If we use the pole for planes, we draw a straight line from the pole point parallel to the plane of interest. The intersection point between the straight line and Mohr’s circle gives the normal and shear stresses that act on this plane.

Both pole points can be used to calculate the directions of the principal stresses. We will use pole for stresses . From point we draw straight lines to the two principal stresses and on Mohr’s circle as shown in Fig. 4. The angle between and the horizontal stress is measured counterclockwise from the horizontal axis on Mohr’s diagram as shown in Fig. 4.

**Important note**

The sign of shear stress calculated by equation (8) gives the direction of the shear stress for a local right handed coordinate system attached on the plane of interest. The positive direction of the horizontal axis on this coordinate system is the direction of the normal vector . The sign of shear stress calculated by Mohr’s circle gives the direction of the shear stress according to the special rule that we defined previously. Shear stresses that tend to rotate the element clockwise are positive and negative otherwise. Thus, a positive shear stress calculated by equation (8) is represented as negative on Mohr’s circle and the other way around.

**Example**

Consider a solid body under plane stress loading as shown in Fig. 1. The stresses acting on this body are given by the following stress tensor:

(20) |

Consider also an inclined plane inside this body. The normal vector of the plane angle with respect to horizontal axis. Calculate the normal and shear stress on this plane by using equations (5) and (8) and by using the pole on Mohr’s circle. Also, calculate the principal stresses and the principal directions.

Solution:

By using equation (5) we calculate the normal stress:

(21) |

And for the shear stress we use equation (8):

(22) |

We use equation (18) to calculate the principal stresses:

(23) |

(24) |

and the directions of the principal stresses are calculated by equation (15):

(25) |

To construct Mohr’s circle we draw firstly the two points from stress tensor (20). Note that the shear stress on the vertical sides of the element tend to rotate it counterclockwise thus according to the special rule we will consider them negative. Then we connect these two points and find the center of the circle (cf. Fig. 5). With this center and radius equal to the distance between the center and any of these two points, we draw the circle.

Next we draw a horizontal line from point parallel to direction. The intersection point between the line and the circle is the pole for stresses. The shear and normal stress on the plane of interest can be found by either measuring angle from the pole point or by measuring angle from the center. The intersection points of the circle with the horizontal axis are the principal stresses and . We connect the pole point with these points and we measure the angle of the principal stresses (principal directions) with respect to coordinate system of the stress tensor (20). Note that the shear stress on Mohr’s circle is positive which means that it tends to rotate the element clockwise. Shear stress from equation (22) is negative which means that its direction is opposite to the positive direction of the local right handed coordinate system attached on the plane. Both methods show the same direction for the shear stress. In Fig. 5 we have drawn on the plane the directions of and .

**Suggested Bibliography**

R.H.G. Parry. Mohr Circles, Stress Paths and Geotechnics. Taylor & Francis, London and New York, 2nd ed., 2004.

Y.C. Fung. A First Course in Continuum Mechanics. Prentice Hall, Englewood Cliffs, New Jersey, 3rd ed., 1994.

L.E. Malvern. Introduction to the Mechanics of a Continuous Medium. Prentice Hall, Englewood Cliffs, New Jersey, 1969.

It was a great assistance. Thanks!

I’m glad that I have helped you.

Thanks, I have been trying to understand the Poles Method, and this was extremely informative and easy to understand.

Thank you for your kind words MacEng.

help me (what is deviatoric ????)

To learn what is deviatoric stress please read: Deviatoric stress and invariants

Thanks & regards to the author

Sivakumar

wao good

Good recap after a long time.thanks for making it available to all.