Crack Paths 2012

investigations by Mottitschka et al. [3] showed that these accelerations or so called

overload effects occurred steadily after overloads independent of the overload ratio.

The microstructural mechanisms of fatigue crack growth in nodular cast iron under

constant amplitude loading were extensively investigated in the literature. Di Cocco et

al. [4] studied the damage mechanisms in nodular cast iron during fatigue crack growth.

They discovered the so called onion-like mechanism, consisting of crack nucleation and

propagation in the graphite shell, as the main damaging mechanism in ferritic ductile

cast irons. Limodin et al. [5] investigated the effect of crack closure on the propagation

of the fatigue crack front in nodular cast iron. By using computer tomography and 3D

image correlation techniques they detected, that the closure process along the crack

front is non-uniform leading to inhogeneous growth and arrest, resp., of parts of the

crack front.

However, there is only little knowledge about the microstructural mechanisms in

nodular cast iron during fatigue crack growth under variable amplitude loading.

Recently, Zybell et al. [6] performed optical in-situ investigation of the microstructural

effects during overloads with a pulsed reflection microscope. They detected, that ductile

mechanisms lead to the crack growth acceleration and that the overload effects can be

regarded as beginning ductile crack initiation. Furthermore, crack branching was

observed when overloads were applied.

Within this contribution new results of optical in-situ investigations with three-point

bending specimens using the aforementioned pulsed reflection microscope are

presented. Systematic experiments under variable amplitude loading have been

performed in order to study the influence of the overload ratio on the microstructural

crack growth. Furthermore, first experiments with cruciform specimens under different

biaxial loading conditions are presented. Finally, the experimental findings are

compared to recent numerical studies of ductile failure initiation in nodular cast iron.

E X P E R I M E N T S

Material

The investigated material EN-GJS-400-18LTis a ductile cast iron with nodular graphite

particles, where the volume fraction f0 of the graphite particles is about 12%. The

mechanical properties of the material were determined by tension tests and are given in

Tab. 1. The microstructure of the material, which is depicted in Fig. 1, is described by

the graphite morphology parameters, which were determined by quantitative structural

analysis taking into account approximately 1000 graphite particles. The obtained mean

values for the particle diameter dG, the distance O between the particles, the form factor

F as a measure of the nodularity (0 < F ) [7], and the particle number Na per unit area

are listed in Tab. 2.

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