Page 48 - Rock Mechanics For Underground Mining

STRESS AND INFINITESIMAL STRAIN

and Q under the applied load is zero, i.e. the body has been subject to a rigid-body
displacement. The problem of interest involves the case where u x = u , etc. The line
∗
x
element joining P and Q then changes length in the process of load application, and
the body is said to be in a state of strain.
In specifying the state of strain in a body, the objective is to describe the changes in
the sizes and shapes of infinitesimal elements in the loaded medium. This is done by
∗
considering the displacement components (u x , u y , u z ) of a particle P, and (u , u , u )
∗
∗
x y z
of the adjacent particle Q. Since
∂u x ∂u x ∂u x
∗
u = u x + du x , where du x = dx + dy + dz
x
∂x ∂y ∂z
and
∂u y ∂u y ∂u y
∗
u = u y + du y , where du y = dx + dy + dz
y
∂x ∂y ∂z
∂u z ∂u z ∂u z
∗
u = u z + du z , where du z = dx + dy + dz
z
∂x ∂y ∂z
the incremental displacements may be expressed by
⎡ ⎤ ⎡ ⎤ ⎡ ⎤
du x ∂u x ∂u x ∂u x dx
⎢ ⎥ ⎢ ∂x ∂y ∂z ⎥ ⎢ ⎥
⎢ ⎥ ⎢ ⎥ ⎢ ⎥
⎢ ⎥ ⎢ ⎥ ⎢ ⎥
⎢ ∂u y ∂u y
(2.25a)
⎢ ⎥ ∂u y ⎥ ⎢ ⎥
⎢ du y ⎥ = ⎢ ⎥ ⎢ dy ⎥
⎢ ∂x ∂y
⎢ ⎥ ∂z ⎥ ⎢ ⎥
⎢ ⎥ ⎢ ⎥ ⎢ ⎥
⎣ ∂u z ∂u z ∂u z
⎣ ⎦ ⎦ ⎣ ⎦
du z ∂x ∂y ∂z dz
or
[d ] = [D][dr] (2.25b)
In this expression, [dr] represents the original length of the line element PQ, while
[d ] represents the relative displacement of the ends of the line element in deforming
from the unstrained to the strained state.
The infinitesimal relative displacement defined by equation 2.25 can arise from both
deformation of the element of which PQ is the diagonal, and a rigid-body rotation
of the element. The need is to define explicitly the quantities related to deformation
of the body. Figure 2.7 shows the projection of the element, with diagonal PQ, on to
the yz plane, and subject to a rigid body rotation x about the x axis. Since the side
dimensions of the element are dy and dz, the relative displacement components of Q
relative to P are
du y =− x dz
(2.26)
du z = x dy

Considering rigid-body rotations y and z about the y and z axes, the respective
displacements are
Figure 2.7 Rigid-body rotation of
du z =− y dx
an element producing component dis- (2.27)
placements of adjacent points. du x = y dz
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