Mohr Circle In 2D space (e.g., on the 12 , 13, or 23

plane), the normal stress (n) and the shear stress (s), could be given by equations (1) and (2) in the next slides

Note: The equations are given here in the 12 plane, where 1 is greater than 2.

If we were dealing with the 23 plane, then the two principal stresses would be 2 and 3

Normal StressThe normal stress, n

n= (1+2)/2 + (1-2)/2 cos2

In parametric form the equation becomes:n = c + r cosω

Where

c = (1+2)/2 is the center, which lies on the normal

stress axis (x axis)

r = (1-2)/2 is the radius

ω= 2

Sign Conventions n is compressive when it is “+”, i.e., when n>0

n is tensile when it is “-”, i.e., when n< 0

n= (1+2)/2+(1-2)/2 cos2

NOTE:

is the angle from 1 to the normal to the plane!

n = 1 at (a maximum)

n = 2 at (a minimum )

There is no shear stress on the three principal planes (perpendicular to the principal stresses)

Resolved Normal and Shear Stress

Shear StressThe shear stress

s = (1-2)/2 sin2 In parametric form the equation becomes:

s = r sinω where ω = 2

s > 0 represents left-lateral shear

s < 0 represents right-lateral shear

s = at or or (a min)

s = 12 at (maximum shear stress)

The maximum s is 1/2 the differential stress

Construction of the Mohr Circle in 2D Plot the normal stress, n, vs. shear stress, s, on a

graph paper using arbitrary scale (e.g., mm scale!)

Calculate: Center c = (1+2)/2

Radius r = (1-2)/2

Note: Diameter is the differential stress (1-2)

The circle intersects the n (x-axis) at the two principal stresses (1 and 2)

Construction of the Mohr Circle Multiply the physical angle by 2 The angle 2 is from the c line to any point on the

circle +2 (CCW) angles are read above the x-axis -2 (CW) angles below the x-axis, from the 1 axis

The n and s of a point on the circle represent the normal and shear stresses on the plane with the given 2angle

NOTE: The axes of the Mohr circle have no geographic significance!

Mohr Circle for Stress

.

Mohr Circle in 3D

Maximum & Minimum Normal StressesThe normal stress

n= (1+2)/2 + (1-2)/2 cos2

in physical spaceis the angle from 1 to the normal to the plane

When thencos2and n=(1+2)/2 + (1-2)/2which reduces to a maximum value:

n= (1+2 + 1-2)/2 n= 21/2 n= 1

When thencos2and n= (1+2)/2 - (1-2)/2

which reduces to a minimum

n= (1+2 - 1+2)/2 n= 2/2 n=

Special States of Stress - Uniaxial Stress

Uniaxial Stress (compression or tension) One principal stress (1 or 3) is non-zero, and

the other two are equal to zero

Uniaxial compression

Compressive stress in one direction: 1 > 2=3 = 0

| a 0 0|| 0 0 0|| 0 0 0|

The Mohr circle is tangent to the ordinate at the origin (i.e., 2=3= 0) on the + (compressive) side

Special States of Stress

Uniaxial Tension

Tension in one direction:

1 = 2 > 3

|0 0 0||0 0 0||0 0-a|

The Mohr circle is tangent to the ordinate at the origin on the - (i.e., tensile) side

Special States of Stress - Axial Stress

Axial (confined) compression: 1 > 2 = 3 > 0|a 0 0||0 b 0||0 0 b|

Axial extension (extension): 1 = 2 > 3 > 0|a 0 0||0 a 0||0 0 b|

The Mohr circle for both of these cases are to the right of the origin (non-tangent)

Special States of Stress - Biaxial Stress Biaxial Stress:

Two of the principal stresses are non-zero and the other is zero

Pure Shear:

1 = -3 and is non-zero (equal in magnitude but opposite in sign)

2 = 0 (i.e., it is a biaxial state)

The normal stress on planes of maximum shear is zero (pure shear!)|a 0 0 ||0 0 0 ||0 0 -a|

The Mohr circle is symmetric w.r.t. the ordinate (center is at the origin)

Special States of Stress

Special States of Stress - Triaxial Stress

Triaxial Stress: 1, 2, and 3 have non-zero values

1 > 2 > 3 and can be tensile or compressive

Is the most general state in nature|a 0 0 ||0 b 0 ||0 0 c |

The Mohr circle has three distinct circles

Triaxial Stress

Two-dimensional cases: General Stress

General Compression Both principal stresses are compressive

is common in earth)

General Tension Both principal stresses are tensile Possible at shallow depths in earth

Isotropic Stress The 3D, isotropic stresses are equal in magnitude

in all directions (as radii of a sphere)

Magnitude = the mean of the principal stressesm= (1+2+3)/3 = (11+22+33 )/3

P = 1= 2= 3 when principal stresses are equal

i.e., it is an invariant (does not depend on a specific coordinate system). No need to know the principal stress; we can use any!

Leads to dilation (+ev & -ev); but no shape change

ev=(v´-vo)/vo= v/vo [no dimension]

v´ and vo are final and original volumes

Stress in Liquids

Fluids (liquids/gases) are stressed equally in all directions (e.g. magma); e.g.:

Hydrostatic, Lithostatic, Atmospheric pressure

All of these are pressure due to the column of water, rock, or air, respectively:

P = gz z is thickness is density g is the acceleration due to gravity

Hydrostatic Pressure- Hydrostatic Tension

Hydrostatic Pressure: 1 = 2 = 3 = P|P 0 0||0 P 0||0 0 P|

All principal stresses are compressive and equal (P) No shear stress exists on any plane All orthogonal coordinate systems are principal

coordinates Mohr circle reduces to a point on the n axis

Hydrostatic Tension The stress across all planes is tensile and equal There are no shearing stresses Is an unlikely case of stress in the earth

Deviatoric Stress A total stress can be divided into its components:

isotropic (Pressure) or mean stress (m) Pressure is the mean of the principal stresses (may be

neglected in most problems). Only causes volume change.

deviatoric (d) that deviates from the mean Deviator’s components are calculated by subtracting the

mean stress (pressure) from each of the normal stresses of the general stress tensor (not the shear stresses!). Causes shape change and that it the part which we are most interested in.

T=m+d or d=T-m

Confining Pressure

In experimental rock deformation, pressure is called confining pressure, and is taken to be equal to the 2 and 3 (uniaxial loading)

This is the pressure that is hydraulically applied around the rock specimen

In the Earth, at any point z, the confining pressure is isotropic (lithostatic) pressure:

P = gz

Decomposition of Matrix

• Decomposition of the total stress matrix into the mean and deviatoric matrices

• The deviatoric part of total stress leads to change in shape

Example - Deviatoric & Mean stress

Given: 1 = 8 Mpa, 2 = 5 Mpa, and 3 = 2 Mpa Find the mean and the diviatoric stresses

The mean stress (m):

m = (8 + 5 + 2) / 3 = 5 MPa

The deviatoric stresses (n ):

1

= 8-5 = 3 Mpa (compressive)

2 = 5-5 = 0 Mpa

3 = 2-5 = -3 Mpa (tensile)

Differential Stress The difference between the maximum and the

minimum principal stresses (1-2) Is always positive Its value is:

twice the radius of the largest Mohr circle It is twice the maximum shear stresses

Note: s = (1-2)/2 sin2s = 12 at

(a maximum) The maximum s is 1/2 the differential stress Is an invariant of the stress tensor

Effective Stress Its components are calculated by subtracting the

internal pore fluid pressure (Pf) from each of the normal stresses of the external stress tensor

This means that the pore fluid pressures opposes the external stress, decreasing the effective confining pressure

The pore fluid pressure shifts the Mohr circle toward lower normal stresses. This changes the applied stress into an effective stress

Effective Stress (applied stress - pore fluid pressure)= effective stress

|11 12 13 | | Pf 0 0 | |11- Pf 12 13 |

21 22 23 | - | 0 Pf 0 |=| 21 22 – Pf 23 |

|31 32 33 | | 0 0 Pf | | 31 32 33- Pf |

Mechanical behavior of a brittle material depends on the effective stress, not on the applied stress

Pore Fluid Pressure