The effects of notch root radius on fracture toughness and crack initiation sites have been investigated in this paper using three different classes of materials. Data on alumina which represent ceramics, mild steel from the metals ffeily and polycarbonate representing plastics were obtained and analysed. The locations of crack initiation sites have been pinpointed by scanning electron microscopy. These identified sites more or less are located within the critical process zone or the theoretical plastic zone. The critical process zone size (Dc ) or the theoretical plastic zone size (RYF ) are independent of the notch root radius unlike the plain‐strain fracture toughness of notched specimens KIC (ρ). The authors emphasize why the parameters Dc and RYF are useful for a quantitative evaluation of the reliability of structural materials.