Crack Paths 2012

Figure 2. The shape, size, and types of holes made in the side of the test bars.

R E S U L T S

Initiation from subsurface inclusions

A set of 11 spherical oxides acting as fatigue initiators in the 100Cr6 bearing steel are

shown in Figure 3. A weak correlation can be seen between the size of inclusion (also

¨ Kon y-axis) and endurance. But it has been shown that, in addition to inclusion size,

properties of the metal matrix around the inclusion and configuration of multiple cracks

affect early growth of the crack. [8] An influence of adjacent grain orientations has been

demonstrated by a numerical mesoscale models. [10, 11]

Our fractography observations reveal the role of multiple crack initiation on slightly

different planes. It is worth of noting that the longest life in Figure 3 is associated with

a mediumsize inclusion less than 100 microns below the specimen surface. Figure 4

reveals how three cracks have grown on adjacent planes before coalescence. This has

led to a nonsymmetrical crack (marked by the fisheye pattern) and retarded growth, in

particular on the side where crack “3” has grown on a separate plane still when crack

“1” reached the open surface.

Crack paths from notches on surface

The paths of the small cracks that grew from the FIB holes showed a clear preference in

growth direction along the martensitic plates or laths in the prior austenitic grain

boundary. Once the crack grew through one prior austenitic grain it changed direction

and grew in the direction of the martensitic laths in the new austenitic grain or sub-grain

martensite bundle cell. This crack growth behavior is shown in Figure 5.

Also the general direction and rate of the crack growth is affected by the

microstructure besides the notch. Figure 6 shows how a crack initiated from a FIB hole

tends to grow along the direction of the martensitic laths.

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