Ductile material fails along the principal shear plane as they are weak in shear and brittle material fails along with principal normal stress.

 

In Brittle materials under tension test undergoes brittle fracture i.e. their failure plane is 90° to the axis of load and there is no elongation in the rod that's why the diameter remains same before and after the load. Example: Cast Iron, Concrete etc. But in case of ductile materials, material first elongates and then fail, their failure plane is 45° to the axis of the load. After failure cup-cone failure is seen. Example Mild steel, High tensile steel etc.