Evaluation of fracture toughness using small notched specimens

As a part of small specimen testing techniques, the notched specimen technique is studied to overcome the difficulties in introducing fatigue precrack to small fracture toughness specimens. It was noted that stress triaxiality and the plastic constraint ahead of a notch root decreased with increasing notch root radius (ρ). Considering that the applied stress at the notch tip is redistributed and relaxed due to the increased plasticity, the fracture toughness obtained from notched specimens was corrected. Fracture toughness testing was conducted using an instrumented impact and a static three-point bending tester. The specimens had notch root radii which ranged from a fatigue precrack to a Charpy notch root radius of 250 μm. Fracture toughness values corrected from the results of notched specimens were very consistent with the plane strain fracture toughness obtained from precracked specimens. In addition, a limiting notch root radius (ρ0), below which the fracture toughness was independent of ρ, was observed. To investigate this phenomenon, the fracture surface was observed with a scanning electron microscope (SEM).