PPT Slide
The shear modulus for metals is in the range 20 to 150 GPa.
Therefore, Eq. (10-10) predicts that the theoretical shear stress will be in the range (3 to 30 GPa).
However, the actual values of the shear stress required to produce plastic deformation in metal single crystals are in the range 0.5 to 10 MPa.
Since the theoretical shear strength of metal crystals is at least 100 times greater than the observed shear strength, it must be concluded that a mechanism other than bodily shearing of planes of atoms is responsible for slip.
This is the Holy Grail of materials science