AbstractIn this research, size effects on the mode I notch fracture toughness of polymeric specimens weakened by a rounded-tip V-shaped (RV) notch are investigated experimentally and theoretically. For this purpose, multiple fracture tests are performed on the RV-notched Brazilian disk (RVNBD) and RV-notched semicircular bend (RVNSCB) polymeric samples with four different scales. Then, the fracture loads measured from the experiments are converted to the notch fracture toughness using finite element analysis. The experimental results show that the mode I notch fracture toughness is significantly dependent on the nominal dimension (radius) of RVNBD and RVNSCB samples, so by increasing the sample radius, the mode I notch fracture toughness increases. In order to justify the size dependency of notch fracture toughness, a modified form of the point stress criterion is proposed. The value of critical distance rc in the modified point stress (MPS) criterion is considered to be size-dependent. Subsequently, a semiempirical relation is used for describing this size dependency. It is revealed that the MPS criterion is able to predict the size-dependent value of notch fracture toughness for both RVNBD and RVNSCB samples made of polymeric materials well.