The present investigation has studied the effects of notch-root radii on the Charpy apparent toughness of as-quenched and tempered 4340 steel as a function of (1) test temperature (liquid nitrogen and room temperature), (2) strain rate (high and slow strain rates corresponding to Charpy and plane-strain fracture toughness testing rates), and (3) grain size (25 and 250 μm). The toughness was evaluated by instrumented and slow-bend Charpy tests. Fracture surfaces were examined by scanning electron microscopy.

The results showed that initially the toughness increased as the notch-root radius increased, but that after a critical notch-root radius was reached, the toughness dropped. The loss in toughness was coincident with a change to an intergranular fracture mode initiation. Also, the critical root radius at which the drop in toughness occurred was temperature and strain-rate dependent. These results are not in agreement with other published results, which always show increasing toughness with increasing notch root radius beyond a limiting root radius. These results and the limitation of the current theories are presented and discussed in this paper. A physical process for the intergranular fracture mode initiation is also discussed.