Relationship between dynamic and static toughness of flake and compacted graphite cast irons

The very little plastic deformation exhibited in the fracturing of flake graphite (FG) and compacted graphite (CG) cast irons qualified them as brittle materials. The fatigue pre-cracking of these brittle material in KIC plane strain fracture toughness testing is a difficult task. Opposed to this static toughness KIC, the dynamic toughness obtained by impact testing does not have such problems. Since the stress–strain behavior of brittle material is essentially linear and the fracture appearance of the specimens after impact testing must largely be flat without shear-lip, these conditions also satisfy the linear-elastic requirement in KIC testing. Thus, it is of interest to find out if there’s any linear relationship existed between these two toughness properties of brittle material. Annealing, normalizing, and austempering heat treatment were applied to FG and CG irons to alter their matrix structures so as to obtain a range of static (KIC) and dynamic (impact) toughness values. A third toughness, calculated by integrating the area under the load versus CGD (Clip Gage Displacement) curve in KIC testing, was termed ‘calculated toughness’ and was also listed for comparison purposes. It was found, among dynamic impact toughness (X), static KIC (Y), and calculated toughness (Z), linear relationships exist for flake graphite cast iron as: Y=4.02X−0.81 Z=1.75X+2.99 and for compacted graphite cast iron as: Y=0.86X+20.2 Z=0.77X+4.77.