Geology Reference
In-Depth Information
M
S
P
H
J
b
A
G
E
BC
L
T
F
R
K
Fig. 6.6 Depicting a dislocation as the boundary of a slipped patch A in the slip plane EFGH,in
which the columns of atoms crossing the slip plane are displaced by the amount of the Burgers
vector b, as shown by the Burgers circuit BC
6.2 Properties of Dislocations in Crystals
6.2.1 Theoretical Shear Strength and the Concept of a Dislocation
The simple translation of one layer of atoms of a crystal over another with the
simultaneous breaking and restoration of all the interlayer bonding would, theo-
retically, require a very high shear stress parallel to the layer. This stress, known as
the theoretical shear strength, has been estimated as being around 1/15-1/10th of
the elastic shear modulus of a crystal, within perhaps a factor of two (Hirth and
Lothe
1982
, p. 5; Kelly
1966
, p. 12). In practice, plastic flow is commonly
observed at stresses that may be several orders of magnitude below this level. The
discrepancy can be explained by supposing that the breaking and restoration of
bonding is done sequentially at a ''front'' that moves over the plane on which the
translation is occurring; that is, at any given instant, bonds are only being broken
along this ''front'' and so a much smaller total force and, hence, smaller macro-
scopic stress need be applied. This ''front'' or linear crystal defect is known as a
dislocation. A dislocation can thus be regarded as the boundary between the
slipped and unslipped parts of the plane or surface on which translation is
occurring and its motion in the plane or surface represents the spreading of the
slipped patch (Fig.
6.6
). The macroscopic plastic strain results from the movement
