Crack Paths 2012

The results currently obtained by the P R Mare also in agreement with former findings

[6]. Whenan overload is applied, there is crack branching, micro crack initiation in

front of the main crack tip and micro crack coalescence. These mechanisms can be

regarded as early stages of ductile fracture initiation. However, it is still not clear what

exactly happens inside the SENB3specimen when an overload is applied. As already

stated in [6], it is expected, that the ductile effects are amplified inside the specimen due

to increased triaxiality.

The experimental observations of crack branching are in agreement with current results

of numerical investigations of ductile fracture initiation in metallic materials with voids

by Hütter et al. [11]. In this study detailed finite element simulations of ductile fracture

initiation are performed. Around the crack tip a refined microstructure is modelled,

realized by different void arrangements. Since the graphite particles in nodular cast iron

debond easily from the metallic matrix under tension load, finite element models

containing voids are usually used as approximation for nodular cast iron. In the

simulations in [11] elastic-plastic

behaviour is employed for the matrix material,

resulting in growth of the initially spherical voids. Finally, crack growth takes place by

plastic collapse of the intervoid ligaments.

Figure 6. Crack branching observed in finite element simulations

of ductile crack initiation in materials with voids [11]:

diagonal cubic primitive (top) and hexagonal void arrangement (bottom).

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