The fracture-energy criterion JIc was determined by the single specimen method on quenched and tempered 4130 steel used for tower anchor bars of high voltage transmission lines. A thermodynamic approach was used to rationalize JIc as a stable crack initiation criterion. Slow-bend testing was performed on three-point bend specimens of different dimensions. The effect of microstructure (martensite and bainite + ferrite) on the value of JIc is not significant. Specimen dimensions (1.5 × 3 × 15 cm; 2 × 4 × 20 cm) seem not to affect JIc values. Critical stretch zone width, WSZc, was evaluated on broken specimen halves using scanning electron microscopy. With some assumptions, relationships between WSZc, CTOD and JIc were established. Values of JIc calculated via WSZc compare favorably with JIc determined from a single specimen.

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