While testing the toughness of brittle material by izod or charpy test why do we provide v notch?
The point is to promote the condition of brittle fracture. The notch here acts as a stress riser and favours the crack propagation.
This test is designed for any material, if you already know that the material is brittle then why do you put it through the test ? The absolute values and numbers here are less important and they are not reliable. The impact engineers want to know when a material favours brittle fracture, some materials are ductile in a specific range of temperature, but they still fail like brittle material, why ? presence of stress risers, high rate of strain ... , by bringing the notch they promote the condition of brittle fracture and the notch provides three dimensional stress condition.