Crack Paths 2012

Fig. 6 shows the numerically predicted crack bifurcation for the diagonal cubic

primitive and the diagonal hexagonal void arrangement. Keeping in mind the

inhomogeneous microstructure of nodular cast iron, the presented two idealized

microstructures showing crack branching are closer to reality than other straight void

arrangements usually investigated in the literature.

Regarding the possible failure of single ligaments in front of the crack tip, the ductile

failure mechanisms in nodular cast iron are of importance. An interesting result was

found by Liu et al. [12], who analysed the development of microvoids under strain

loading. For neighboring graphite particles they found, that there is direct void

coalescence by necking of the matrix material. However, regarding widely spaced

particles, there is nucleation of small secondary voids in the matrix material between the

graphite particles. Under hydrostatic tension load these secondary voids grow until the

ligament between the graphite particles is fully damaged. So, there are remaining

ligaments on the fracture surface for widely spaced graphite particles, see Fig. 7, due to

limited micro-ductility of the matrix material.

The ductile failure behaviour of nodular cast iron due to secondary voids has been

recently investigated by Zybell et al. [13]. In this numerical study 3D cell model

simulations have been performed, see Fig. 7. In contrast to previous studies by Kuna

and Sun [14] a nonlocal Gurson model is used to describe the damage evolution in the

ferritic matrix. Due to the internal length scale incorporated in the nonlocal model, it is

possible to describe the size of secondary voids in the matrix material. The simulations

have been performed for different void arrangements, namely cubic primitive and body

centered cubic. The results show, that the two ligament failure mechanisms discovered

by Liu et al. [12] are caused by the size of the secondary voids compared to the graphite

particles and that these mechanisms can be captured by the model.

V V F

Figure 7. Failure mechanisms in nodular cast iron due to secondary voids [12] (left) and

cell model simulation of ductile failure in nodular cast iron [13] (right).

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