PSI - Issue 33

Jesús Toribio et al. / Procedia Structural Integrity 33 (2021) 1193–1196 Jesús Toribio / Procedia Structural Integrity 00 (2021) 000–000

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2. Microscopic fatigue crack paths in pearlitic steel: local mixed mode & anisotropic fatigue behavior

From the microscopic point of view, fatigue crack growth develops locally in mixed mode with micro-crack deflections and deviations in both the hot-rolled pearlitic steel bar (not cold drawn at all) and the cold drawn pearlitic steel wire (heavily drawn to produce prestressing steel) as shown in Fig. 1. The fatigue crack paths in both steels are transcolonial and translamellar with frequent micro-deflections, branches and bifurcations, especially in the cold drawn pearlitic steel that exhibits a tortuous crack path (TCP), as described by Toribio (2018a, 2018b, 2019, 2020).

Fig. 1. Fracto-metallographic analysis of the fatigue crack paths in the hot-rolled bar (left) and the cold drawn wire (right).

Therefore, at a finer microstructural level, the associated fatigue crack paths in both the hot rolled bar and the heavily cold drawn wire develop in local mixed mode in both materials ( anisotropic fatigue behavior; locally multiaxial fatigue crack growth ), i.e., a microscopic ( real ) anisotropic effect arise in the matter of fatigue crack propagation linked with the lamellae alignment and orientation, affecting the angle of micro-deflections in the tortuous fatigue crack paths with zigzag shape, see Fig. 2.

Fig. 2. Specific profiles of the fatigue crack paths for different Δ K levels in the hot-rolled bar (left) and the cold drawn wire (right).

3. Anisotropy of fatigue resistance and crack paths

Fig. 3 shows the locally anisotropic fatigue crack paths in the hot rolled bar and in the cold drawn wire. As the drawing degree rises, the average deviation angle increases and the average deflection length decreases. This implies more frequent deflections in the cold drawn wire that in the hot rolled bar (Toribio, 2018a, 2018b, 2019, 2020).