lec.23.pptx dislocations
DESCRIPTION
A Dislocations and other defects in solidsB Significance of dislocationsC Planar dislocationsD Displacement and stress fields for a screw dislocation (mode III)TRANSCRIPT
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23. Disloca/ons
I Main Topics
A Disloca/ons and other defects in solids B Significance of disloca/ons C Planar disloca/ons
D Displacement and stress fields for a screw disloca/on (mode III)
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23. Disloca/ons
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hHp://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/fissure4_large.JPG
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23. Disloca/ons
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hHp://upload.wikimedia.org/wikipedia/commons/4/43/Mackenzie_dike_swarm.png
23. Disloca/ons
II Disloca/ons and other defects in solids A Disloca/ons
1 Originally, extra (or missing) planes or par/al planes of material (e.g., atoms)
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hHp://www.T.uni-‐kiel.de/matwis/amat/def_ge/kap_5/backbone/r5_2_2.html
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23. Disloca/ons
II Disloca/ons and other defects in solids A Disloca/ons (cont.)
2 Evidence for disloca/ons from electron microscopy
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hHp://en.wikipedia.org/wiki/Disloca/on
Transmission Electron Micrograph Of Disloca/ons
23. Disloca/ons
II Disloca/ons and other defects in solids A Disloca/ons (cont.)
3 Con/nuum mechanics usage: surfaces across which displacements are discon/nuous
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hHp://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/fault2_large.jpg
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23. Disloca/ons
II Disloca/ons and other defects in solids (cont.) B Point defects 1 Originally, extra (or missing) volumes (e.g., atoms)
2 Displacements are discon/nuous across point defects
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hHp://www.ndt-‐ed.org/Educa/onResources/CommunityCollege/Materials/Graphics/Chrystal-‐Defects.jpg
23. Disloca/ons
III Significance of disloca/ons
A Account for permanent plas/c deforma/on in crystals
B Account for the low observed strength of crystals rela/ve to theore/cal predic/ons
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hHp://www.ndt-‐ed.org/Educa/onResources/CommunityCollege/Materials/Graphics/Chrystal-‐Defects.jpg
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23. Disloca/ons
III Significance of disloca/ons C They provide useful
quan/ta/ve descrip/on of rela/ve mo/ons across surfaces across a broad range of scale (crystals [10-‐6 m] to plate boundaries [106 m]) – ~12 orders of magnitude!
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hHp://en.wikipedia.org/wiki/Disloca/on
Disloca/on model of subduc/on zone
From Bevis and Martel, 2001
Transmission Electron Micrograph
23. Disloca/ons
III Significance of disloca/ons D Disloca/ons induce tremendous stress concentra/ons and account for “large” deforma/ons under small “average” stresses
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hHp://pangea.stanford.edu/research/geomech/Faculty/crack.html
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23. Disloca/ons
IV Planar disloca/ons A Represented
mathema/cally as infinitely long cut with a straight edge
B Rela/ve displacement (of one side of the disloca/on rela/ve to the other) across a disloca/on is called the Burger’s vector b.
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23. Disloca/ons
D Edge disloca/on 1 Accommodate opening
or sliding mo/ons 2 Displacement is
exclusively perpendicular to the disloca/on edge
3 Displacement can be parallel or perpendicular to the disloca/on plane
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hHp://www.T.uni-‐kiel.de/matwis/amat/def_ge/kap_5/backbone/r5_2_2.html
View along edge of edge disloca/on
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23. Disloca/ons
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blog.faTreevegan.com hHp://www.T.uni-‐kiel.de/matwis/amat/def_ge/kap_5/backbone/r5_2_2.html
Extra half-‐plane of atoms in lajce
Extra row of kernels in an ear of corn D Edge disloca/on (cont.) 4 Analogy: an extra row of corn kernels on
a cob of corn
23. Disloca/ons
D Edge disloca/on (cont.)
5 Macroscopic geologic use: modeling dikes or faults
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hHp://www.experimentalstress.com/topic/reflect.jpg hHp://hvo.wr.usgs.gov/gallery/kilauea/erupt/24ds182_cap/on.html
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23. Disloca/ons
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C Screw disloca/on
1 Accommodate a tearing mo/on
2 Displacement is exclusively parallel to the disloca/on edge
3 Analogy: a lock washer or a 360° spiral staircase
Disloca/on edge
hHp://www.T.uni-‐kiel.de/matwis/amat/def_ge/kap_5/backbone/r5_2_2.html
hHp:///www.boltsplus.com
23. Disloca/ons
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hHp://volcanoes.usgs.gov/Imgs/Jpg/Photoglossary/fault2_large.jpg
hHp://www.T.uni-‐kiel.de/matwis/amat/def_ge/kap_5/backbone/r5_2_2.html
C Screw disloca/on 4 Macroscopic geologic use: modeling faults
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23. Disloca/ons V Displacement and stress fields for a
screw disloca/on (mode III) A Displacement parallel to the
disloca/on edge (w) increases uniformly along a spiral-‐like circuit from one side of the disloca/on to the other (for a right-‐handed screw disloca/on, point your right thumb along the disloca/on edge; displacement parallel to the edge increases in the direc/on your fingers curl.
B Angular posi/on: θ = tan-‐1(y/x) C w = bθ/2π D An/-‐plane strain (u,v = 0)
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23. Disloca/ons
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Normal Strains
Shear Strains
Cylindrical Reference Frame ur = 0, uθ = 0, w = bθ/2π
εrr =12
∂ur∂r
+∂ur∂r
⎛⎝⎜
⎞⎠⎟
εrr = 0
εrθ =121r∂ur∂θ
+∂uθ∂r
−uθr
⎛⎝⎜
⎞⎠⎟
εrθ = εθr = 0
εθθ =urr+1r∂uθ∂θ
εθθ = 0
εθz =12
∂uθ∂z
+1r∂w∂θ
⎛⎝⎜
⎞⎠⎟
εθz = ε zθ =b4πr
ε zz =12
∂w∂z
+∂w∂z
⎛⎝⎜
⎞⎠⎟
ε zz = 0
ε zr =12
∂w∂r
+∂ur∂z
⎛⎝⎜
⎞⎠⎟
ε zr = εrz = 0
* The deriva/ves are very easy to do. Check them to verify the solu/ons.
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23. Disloca/ons
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Normal Strains
Shear Strains
Cartesian Reference Frame u = 0, v = 0, w = b[tan-‐1(y/x)]/2π
ε xx =12
∂u∂x
+∂u∂x
⎛⎝⎜
⎞⎠⎟
ε xx = 0
ε xy =12
∂u∂y
+∂v∂x
⎛⎝⎜
⎞⎠⎟
ε xy = 0
ε yy =12
∂v∂y
+∂y∂y
⎛⎝⎜
⎞⎠⎟
ε yy = 0
ε yz =12
∂v∂z
+∂w∂y
⎛⎝⎜
⎞⎠⎟
ε yz =b4π
xx2 + y2
ε yz =b4π
xr2
ε zz =12
∂w∂z
+∂w∂z
⎛⎝⎜
⎞⎠⎟
ε zz = 0
ε xz =12
∂u∂z
+∂w∂x
⎛⎝⎜
⎞⎠⎟
ε xz =−b4π
yx2 + y2
ε xz =−b4π
yr2
* The deriva/ves are s/ll rather easy to do. Check them to verify the solu/ons.
23. Disloca/ons
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σ rθ = σθr = 2Gεrθ = 0
σθz = σ zθ = 2Gεθz =Gb2πr
σ rθ = σθr = 2Gεrθ = 0εrr = 0εθθ = 0ε zz = 0
Strains Stresses
σ rr = 0σθθ = 0σ zz = 0
εrθ = εθr = 0
εθz = ε zθ =Gb4πr
εrθ = εθr = 0
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23. Disloca/ons
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ε xy = ε yx = 0 σ xy = σ yx = 2Gε xy = 0
ε yz =b4π
xx2 + y2
σ yz = σ zy = 2Gε yz =Gb2π
xx2 + y2
=Gb2π
xr2
ε xz =−b4π
yx2 + y2
σ xz = σ zx = 2Gε xz =−Gb2π
yx2 + y2
=−Gb2π
yr2
ε xx = 0 σ xx = 0
ε yy = 0 σ yy = 0
ε zz = 0 σ zz = 0
Strains Stresses
23. Disloca/ons Key points a Only the shear stresses
ac/ng on a plane normal to the z-‐direc/on or in the z-‐ direc/on are non-‐zero
b The stresses are singular (i.e., go to infinity) near the disloca/on edge: a powerful stress concentra/on exists there.
c This theore/cal singular stress concentra/on exists no maHer how small G or the rela/ve displacement b is, provided G > 0 and b > 0.
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σθz = σ zθ =Gb2πr
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23. Disloca/ons SUPERPOSTION OF TWO (INFINITE) SCREW DISLOCATIONS (A,B)
TO FORM A FINITE DISPLACEMENT DISCONTINTUITY (C)
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