where σ f is the flexural stress, Y = 1.64 a geometrical factor, and a is the notch length. Work of fracture G values were obtained from the curves obtained during the 4-points bending test measuring the area below the force-displacement curve up to the maximum stress (G max ) or to a given value of strain (G 7% on typically 18 × 3 × 4mm 3 samples) divided by the remaining sample section in notched samples.