A model for the static fracture toughness of ductile structural steel

Fracture of ductile structural steels generally occurs after void initiation, void growth and void coalescence. In order for ductile fracture of structural steels to occur, energy must be spent to induce void initiation and void growth. Therefore, fracture toughness for ductile fracture should be contributed from void initiation and void growth. On the basis of this suggestion static fracture toughness (KIC) of ductile structural steels is decomposed into two parts: void nucleation-induced fracture toughness (denoted as KIC.n) and void growth-induced fracture toughness (KIC.g). KIC.n, defined as the stress intensity factor at which voids ahead of a crack begins to form, is calculated from crack tip strain distribution and void nucleation strain distribution. In contrast, KIC.g is determined by the void growth from the beginning of void nucleation to void coalescence. Therefore, KIC.g relates to the void sizes and void distribution. In this paper, the expression for KIC.g is given from the void sizes directly from fracture surfaces. The relationship between KIC.n, KIC.g and KIC is expressed in the form (KIC)2=(KIC.n)2+(KIC.g)2. The newly developed model was applied to the fracture toughness evaluation of three structural steels (SN490, X65 and SA440), and the theoretical calculation agrees with the experimental results.