PSI - Issue 74

Marta Kianicová et al. / Procedia Structural Integrity 74 (2025) 38–43 Marta Kianicová / Structural Integrity Procedia 00 (2025) 000 – 000

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3.1. Macromorphology of fracture surfaces The initiation of fatigue fracture during the CTPB tests was localized at sites of the highest tensile stress on the fiber surface. These surface sites were color-coded to facilitate their identification in the confocal microscope. An example of fatigue fracture morphology is displayed in grayscales in Fig. 1, where the crack initiatio n site is located on the top of the semi-elliptical part of the fracture surface. Lighter areas represent higher parts and darker areas lower parts of the fracture surface and the scale on the right quantifies the topological height range from 583 to 44676 nm. A fragmented fracture morphology partially following the grain structure can be observed. Fracture patterns are composed of transgranular facets connecting cavities and smooth intergranular facets, which are seen as lighter areas on the fracture surface.

Fig. 1 An example of fatigue fracture morphology of porous fibers partially reflecting the grain structure and consisting of both intergranular and transgranular facets.

Colored topological height maps of fatigue fracture surfaces of compact and porous filaments fractured after approximately five thousand cycles are depicted in Fig. 2a) and 2b), respectively. The surface elevations are marked red and the valleys in blue. One can see that the fracture surface of the compound fiber is less rough than that of the porous fiber, which is reflected by more colored surface and highest absolute numbers at scale limits. This nearly applies to all fractured fibers, as will be shown in the next subsection.