more accurately , single crystals (not ductile materials ) deforms through slipping system motion , schmid's law determines the relation between the critical resolved stress (required to initiate a plastic deformation ) and the applied shearing stress
this relation is t (crss)= (sigma) * cos(phi)*cos(lamda)
the value of the angles phi & lamda depends on the geometry of the crystal structure in question
as phi + lamda =~90
if phi & lamda is equal to or about 45
the value of the term cos(phi)*cos(lamda) will be greater as possible and hence the shearing stress required will be smaller
the deformation would be easier to occur
I agree with Ahmed El-Naml.
In any case, under uniaxial tensile test the highest shear stress is at 45 degrees.
In a polycrystalline material you will see shear banding around 45 degrees, mainly because it is the easiest direction where slipping or high density planes can occur.
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