The fracture-energy criterion J_Ic was determined by the single-specimen method on precracked Charpy V-notch (CVN) specimens as a function of loading rate and temperature. The material for this study was quenched and tempered 4130 steel used for tower anchor bars of high-voltage transmission lines. The experimental temperature range was between −40 and 80°C, and the loading rate range varied from that characterized by slow bending (5 × 10⁻⁶ to 5 × 10⁻⁴ m/s) to that used for impact testing (1.8 to 5.2 m/s). Crack onset detection in slow bend testing is measured by the four-point resistivity method, and in the case of impact testing, maximum load is considered to be the crack onset load. The loading rate effect on the critical value of the J-integral is importa:it over the whole range from slow bend testing to impact testing; however, it is imperceptible in narrow ranges of loading rates. J_Ic and J_Icd present a temperature transition zone. J_Icd is greater than J_Ic on the upper shelf and in the transition zone.