The critical stress sc has been estimated here by using the average value of the stresses obtained from the 2 mm radius notched specimens (the largest available notch radius). The presence of a notch ensures that all fracture energy is released within the notched section and not all along the specimen as in the case of the plain un-notched specimens. For this reason, it is preferable to obtain the critical stress from notched specimens with large notch tip radius instead of plain round specimens. The critical stress has been obtained here by considering the average stress occurring locally at the notch tip at maximum torque and, being consistent with the application of the SED criterion employed here, it has been obtained by assuming linear elastic behaviour. sc has been found to be equal to 298 MPa which is higher than the critical stress obtained experimentally from plain specimens, that reaches an average value of 262.65 MPa, since the actual behaviour exhibits a clear nonlinearity, as shown in Fig. 5.