The fracture energy criterion J_Ic was determined on two carbon-manganese steels, SA-516-70 and SA-105-B, from a single specimen. A graphic method is proposed for verifying the validity of measured values of J_Ic. Static J_Ic and dynamic J_IcD experiments on precracked Charpy V-notch specimens at ambient temperature enabled the observation of the effect of loading rate on the value of J_Ic. For these two steels, the effect of side grooves was detected only in dynamic testing. For SA-516-70 steel, J_IcD > J_Ic, while J_IcD > J_Ic for SA-105-B steel, because of transition temperature fracture behavior. For SA-516-70, static testing was performed on three-point bend specimens of different dimensions to determine the effect of geometrical parameters on the value of J_Ic. A ligament 1-cm long is the lower limit for the determination of J_Ic.