Crack Paths 2012

At these locations, the damage is constrained into the process zone along the

propagating main crack front and, consequently, no river-like patterns can be formed.

Figure 3. Fracture morphology of diffusion coating in bending specimens. River-like

patterns in: (a) secondary electron image and (b) back scatter electron image. Crack

nucleation at (c) the site of the maximal bending loading and (d) the outlying site.

The local crack growth directions in the outer layer are marked by white arrows

Fracture surfaces generated by pure torsional loading are characterised by the

presence of multiple crack nucleation sites at the weakest points along the whole

specimen circumference. Serrated crystallographic facets are often observed in D A Cat

crack initiation sites, Fig. 4a, thus replacing the particle-decohesion

initiation

micromechanism observed in the pure bending experiments. On several occasions, the

factory-roof morphology generated from the surface semi-elliptical shear microcracks

[7] is present. An isolated river-like fracture pattern, similar to those found under

bending, was also indicated on a fracture surface created by torsion – see Fig. 4b, c (SE

and BSE). This pattern is obviously related to a site of an extreme concentration of

particles in D Zand it is slanted by shear. The spacing of patterns is, however, less than

that on the bending surfaces. This corresponds to a lower concentration of nucleation

capable particles on the fracture surface as documented by the related BSEimage.

Fracture surfaces generated under combined bending-torsion loading mostly

resemble those generated by pure bending, i.e. two main cracks are usually initiated at

maximumbending sites, although the topography is more rugged due to the torsion

loading component. For higher loading (LCF region), the initiation sites in coated

specimens are also characterized by the presence of river-like fracture patterns starting

at secondary phase particles in DZ, Fig. 4d. River-like patterns are somewhat slanted

due to the presence of shear.

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