2.8 Slip surfaces
Figure 2.9 represents homogeneous straining where there are no discontinuities, or slip
surfaces, like those that appear during rigid body deformation and the double lines
show the directions of zero strain. Figure 2.10(a) shows material that is deforming
by intense shearing in a very thin zone AB and Fig. 2.10(b) shows a detail of the slip
zone. This thin zone of shearing material has a small but finite thickness which is
usually too small to see; in soils it is probably of the order of ten grains thick. Shear
zones usually appear to have no thickness and so they are called slip planes or slip
surfaces.
Since the length of AB in Fig. 2.10(a) remains constant, because the material on
either side is rigid, it is a zero extension line and its direction is given by α = 45◦+ 12
ψ,
as in Fig. 2.9. From Fig. 2.10(b),