Fracture toughness prediction of low alloy steel as a function of specimen notch root radius and size constraints

The effect of specimen notch root radius ρ and specimen size (thickness B and ligament b) on the apparent fracture toughness K_I,app is investigated. Series of fracture tests on nonstandard compact tension CT specimen of low alloy steel EN34NiCrMo6, which is equivalent to AISI 4340 low alloy steel with different values of ρ and crack length to width ratio a/W were conducted to measure the K_I,app. The experimental results show that, K_I,app depends on ρ and specimen size. There is a critical notch root radius ρ_c under which K_I,app becomes independent of ρ. Two empirical models have been proposed to predict K_I,app and can be used to transfer the data among specimens of different ρ. One additional empirical model is developed to transfer the data among specimens of different ligament, b. Finally, a model is developed by modifying the ASTM E1921 standard equation (which predicts the thickness effect only) to include the combined effect of B and b. The model predictions are in good agreement with the experimental results.