Geometry during slip
Two conditions restrict the geometry of a crystal as slip proceeds:
- the spacing of the planes remains constant;
- the number of planes in the specimen is conserved.
These give rise to two important relationships that describe the way that the orientation of slip planes and slip directions changes as slip proceeds:
- l cos φ is constant, so that as the specimen length l increases, the angle between the slip plane normal and the tensile axis approaches 90°
- l sin λ is constant, so that as l increases, the angle between the slip direction and the tensile axis approaches zero.
If a crystal is extended from length l0 to length l1, then the angles φ and λ are related as follows:
l0 cos φ0 = l1 cos φ1