Fracture toughness of a cermet and an HSLA steel

Standardized mode I fracture toughness tests used to determine J IC and KIC are currently the dominant analytical tools used to quantify material fracture resistance. However, in practice, many applications require materials to withstand combined loading. This research focuses on the mixed mode fracture behavior of a nanocrystalline WC-Co cermet and a bainitic HSLA 100 steel. The nanocrystalline WC-Co cermet followed the expected behavior of lower toughness with decreasing Co binder layer thickness, and showed no difference in toughness under mode I loading or mixed mode I/III loading. This lack of difference in toughness under combined tensile/shear loading is attributed to the tortuosity of the dislocation path through the thin cobalt layers between the angular WC grains. On the other hand, the HSLA steel showed lower toughness for certain mixed mode I/II and I/III loading conditions than for pure mode I, proving the mode I toughness to be nonconservative. This lower toughness is attributed to the enhanced localized shear damage at particle inclusions caused by the additional mode II or III loading.