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State of Stress and Mohr Circles
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Mohr Circle for stress 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 cosωWhere 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>0n 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 Stressnormal to plane
plane
Shear StressThe shear stress
s = (1-2)/2 sin2 In parametric form the equation becomes:
s = r sinω where ω = 2
s > 0 ‘+’ shear stress represents left-lateral shear
s < 0 ‘-’ shear stress represents right-lateral shear
s = (a min)at or or
s = 12(a max) at
The maximum s is 1/2 the differential stress (diameter), i.e., it is the length of the radius!
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
.Max s
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 minimumn= (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 stressGiven: 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