Crack Paths 2012

strength of D A Cspecimens seems to be comparable to that of virgin specimens, despite

a scatter of available experimental data.

Figure 2. S-N curves of bending, torsion, and, combined bending-torsion fatigue

Considering the pure bending experiments, two fatigue cracks are usually initiated on

the specimen’s surface at two opposite sites that are subjected to the maximumbending

loading. Both these cracks propagate nearly perpendicularly to the specimen axis and, in

general, the macroscopic appearance of fracture surface is almost flat. In the case of

uncoated specimens, the crack is mostly initiated at the surface, although the nucleation

on the casting defects close to the surface was also observed on several occasions. In the

case of coated specimens, the initiation sites are located both at the surface and in the

coating in dependence of the applied stress level. Fig. 3a, b shows, in the secondary

electron (SE) and the back scatter electron (BSE) images, an example of the river-like

fracture patterns starting in D Z at secondary phase particles that are clearly seen as

small bright dots in the BSEimage. This initiation mode is typical for the LCF bending

domain. The river-like morphology is located only at two opposite sites that are

subjected to the maximal bending loading and is not present at other locations along the

specimen circumference. Most likely, the fatigue microcracks are nucleated on particles

in the upper part of D Z by decohesion at the matrix/particle interface. Further cycling

leads to mutual interconnections of these microcracks along D Z and their gradual

propagation both into the specimen bulk and towards the free surface, Fig. 3c. In the

latter case, most of these microcracks meet, inside OL, with those nucleated on the

surface, thus finalizing the fracture of DAC.The local crack growth direction in O L

(white arrows) can be deduced from the convex shape of striations (imprints of

subsequent crack front positions) which is caused by slower propagation at grain

boundaries. The observed river-like morphology is generated due to the variance in the

height levels of individual particles and because of local deflection of the crack front

when passing through larger particles on its way into the specimen interior. At locations

far from the initiation sites, the microcracks are nucleated ahead of the main crack front

on the secondary particles within D A Cas well as at the surface imperfections, Fig. 3d.

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