Geometrical effects on lamellar grey cast iron fracture toughness

Lamellar grey cast iron presents a non-lineal load-displacement record. This macroscopic behaviour is originated in the complex microstructure of these materials that are formed by two phases (one metallic matrix with distributed graphite lamellaes) that induce a non-uniform distribution of stresses. The graphite lamellaes act as stress concentrators that produce microcracking of the lamellaes or interface debonding at very low stress level. Fracture mechanics properties in lamellar grey cast iron have traditionally been evaluated from parameters calculated at maximum load values from load versus crack mouth opening displacement records by testing notched specimens. Geometrical effects on different fracture toughness parameters were analysed in the present work. The tested material was a lamellar grey cast iron with A-type graphite, size 4 and ferritic-pearlitic matrix. The chemical composition was C, 3.5 wt.%; Si, 2.5 wt.%; Mn, 0.2 wt.%; P, 0.09 wt.% and S, 0.02 wt.%. Two different geometries were tested: SE(B) and C(T). The typical dimensions were approximately: B = 12.5 mm; W = 25 mm; L = 100 mm, a/W = 0.5. The specimens were not fatigue precracked. The effect of the a/W ratio on the fracture toughness was analysed by testing SE(B) with a/W ranging from 0.45 to 0.649. SE(B) specimens with B/W = 0.52, 0.4 and 0.24 were also tested in order to determine the thickness effect. SE(B) specimens with different notch radius (0.2–3.0 mm) were also tested and no effect was measured.