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Rock Mechanics.pdf - Mining and Blasting

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Figure 2.11 Cylindrical polar coordinate<br />

axes, <strong>and</strong> associated free-body<br />

diagram.<br />

STRESS AND INFINITESIMAL STRAIN<br />

<strong>and</strong><br />

The co-ordinate transformation is defined by the equations.<br />

r = (x 2 + y 2 ) 1/2<br />

<br />

y<br />

<br />

= arctan<br />

x<br />

x = r cos <br />

y = r sin <br />

If R, , Z are the polar components of body force, the differential equations of equilibrium,<br />

obtained by considering the condition for static equilibrium of the element<br />

shown in Figure 2.11, are<br />

∂rr<br />

∂r<br />

1 ∂r ∂rz<br />

+ +<br />

r ∂ ∂z + rr − <br />

+ R = 0<br />

r<br />

∂r 1 ∂ ∂z 2r<br />

+ + + + = 0<br />

∂r r ∂ ∂z r<br />

∂rz<br />

∂r<br />

1 ∂z ∂zz zz<br />

+ + + + Z = 0<br />

r ∂ ∂z r<br />

For axisymmetric problems, the tangential shear stress components, r <strong>and</strong> z,<br />

<strong>and</strong> the tangential component of body force, , vanish. The equilibrium equations<br />

reduce to<br />

∂rr<br />

∂r<br />

∂rz<br />

+<br />

∂z + rr − <br />

+ R = 0<br />

r<br />

∂rz<br />

∂r<br />

+ ∂zz<br />

∂z<br />

+ rz<br />

r<br />

+ Z = 0<br />

For the particular case where r, ,zare principal stress directions, i.e. the shear stress<br />

component rz vanishes, the equations become<br />

∂rr<br />

∂r + rr − <br />

+ R = 0<br />

r<br />

∂zz<br />

+ Z = 0<br />

∂z<br />

Displacement components in the polar system are described by ur, u, uz. The<br />

elements of the strain matrix are defined by<br />

38<br />

εrr = ∂ur<br />

∂r<br />

ε = 1 ∂u ur<br />

+<br />

r ∂ r<br />

εzz = ∂uz<br />

∂z

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