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Mohr Circle for stress In 2D space (e.g., on the 1 2, 1 3, or 2 3 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 1 2 plane, where 1 is greater than 2. If we were dealing with the 2 3 plane, then the two principal stresses would be 2 and 3

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Normal Stress The 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

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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)

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Resolved Normal and Shear Stress normal to plane plane

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Shear Stress The 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 = 1 2 (a max) at The maximum s is 1/2 the differential stress (diameter), i.e., it is the length of the radius!

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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 )

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

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Mohr Circle for Stress. Max s

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Mohr Circle in 3D

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Maximum & Minimum Normal Stresses The normal stress n = ( 1 + 2 )/2 + ( 1 - 2 )/2 cos2 in physical space is the angle from 1 to the normal to the plane When then cos2 and n =( 1 + 2 )/2 + ( 1 - 2 )/2 which reduces to a maximum value: n = ( 1 + 2 + 1 - 2 )/2 n = 2 1 /2 n = 1 When then cos2 and n = ( 1 + 2 )/2 - ( 1 - 2 )/2 which reduces to a minimum n = ( 1 + 2 - 1 + 2 )/2 n = 2 /2 n =

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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 00| | 000| The Mohr circle is tangent to the ordinate at the origin (i.e., 2 = 3 = 0) on the + (compressive) side

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Special States of Stress

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Uniaxial Tension Tension in one direction: 1 = 2 > 3 |00 0| |00-a| The Mohr circle is tangent to the ordinate at the origin on the - (i.e., tensile) side

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Special States of Stress - Axial Stress Axial (confined) compression: 1 > 2 = 3 > 0 |a00| |0b0| |00b| Axial extension (extension): 1 = 2 > 3 > 0 |a00| |0a0| |00b| The Mohr circle for both of these cases are to the right of the origin (non-tangent)

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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!) |a00 | |000 | |00-a| The Mohr circle is symmetric w.r.t. the ordinate (center is at the origin)

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Special States of Stress

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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 |a00 | |0b0 | |00c | The Mohr circle has three distinct circles

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Triaxial Stress

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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

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Isotropic Stress The 3D, isotropic stresses are equal in magnitude in all directions (as radii of a sphere) Magnitude = the mean of the principal stresses m = ( 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 (+e v & -e v ); but no shape change e v =(v´-v o )/v o = v/v o [no dimension] v´ and v o are final and original volumes

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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

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Hydrostatic Pressure- Hydrostatic Tension Hydrostatic Pressure: 1 = 2 = 3 = P |P00| |0P0| |00P| 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

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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

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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

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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

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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 = ( ) / 3 = 5 MPa The deviatoric stresses ( n ): 1 = 8-5 = 3 Mpa (compressive) 2 = 5-5 = 0 Mpa 3 = 2-5 = -3 Mpa(tensile)

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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 sin2 s = 1 2 at (a maximum) The maximum s is 1/2 the differential stress Is an invariant of the stress tensor

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Effective Stress Its components are calculated by subtracting the internal pore fluid pressure (P f ) 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

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Effective Stress (applied stress - pore fluid pressure)= effective stress | 11 12 13 | | P f 0 0 | | 11 - P f 12 13 | 21 22 23 | - | 0 P f 0 |=| 21 22 – P f 23 | | 31 32 33 | | 0 0 P f | | 31 32 33 - P f | Mechanical behavior of a brittle material depends on the effective stress, not on the applied stress

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Pore Fluid Pressure

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