The Effect of Notch Sharpness on the Fracture Toughness Determined From SENT Specimens

Fracture toughness test standards require specimens to be fatigue precracked to generate conservative values of fracture toughness. Nonetheless, it is believed that for ductile steels on the upper-shelf, whether or not the electro-discharge machined (EDM) notch is subsequently fatigue precracked does not affect the value of fracture toughness obtained. Avoiding fatigue precracking during single edge notched tension (SENT) specimen preparation would reduce the testing time, and improve notch placement accuracy and straightness. However, there are circumstances when using EDM notches causes the fracture toughness to be overestimated. It is important to know when fatigue precracking affects the fracture toughness and when it does not. In the work presented here, SENT testing was performed on EDM notched specimens, and on specimens which were subsequently fatigue precracked. Tests were conducted at +20°C, −20°C and −80°C to compare ductile and brittle behaviour. The full tearing resistance curves (R-curves) were reasonably independent of the fatigue precracking, but the initiation value of δ_0.2 was higher when EDM notches were used. At lower test temperatures, the difference in fracture behaviour between both notch types was more significant. EDM notches can therefore be most justified for the assessment of fracture toughness determined from the maximum load in the load-displacement curve, i.e. upper shelf behaviour. The upper shelf can be determined from standard fracture toughness testing or estimated using Charpy data, without performing additional fracture toughness tests. Several Charpy-based criteria for determining the temperature of the upper shelf were also evaluated in this work.