PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 718–723 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000 – 000

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4. Macroscopic fatigue crack paths in global mode I: isotropic fatigue behavior As reported by Toribio (2018) and Toribio et al. (2020), the macroscopic fatigue crack paths develop globally in mode I following the original macro-crack propagation direction, i.e., they exhibit an isotropic fatigue behavior and do not show any evidence of strength anisotropy in the form of macro-crack deflection in spite of the afore-said oriented and aligned microstructure of the fully drawn wire (produced after manufacturing by cold drawing) that

apparently does not affect the macroscopic crack path developing in global mode I. 5. Microscopic fatigue crack paths in local mixed mode: anisotropic fatigue behavior

As analyzed by Toribio (2018) and Toribio et al. (2020), 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 heavily cold drawn pearlitic steel wire (commercial prestressing steel wire) as shown in Fig. 2. 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).

Fig. 2. Fatigue crack paths in the hot rolled bar (left) and the cold drawn wire (right).

Therefore, 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 orientation, affecting the angle of micro-deflections in the tortuous fatigue crack paths with zigzag shape, see Fig. 2. Fig. 3 shows the locally anisotropic fatigue crack paths in both the hot rolled bar and in the cold drawn wire. As the drawing degree (or strain hardening level) rises, the average micro-deflection angle increases and the average micro-deflection length decreases. This implies more frequent deflections (and with higher angle) in the heavily cold drawn wire that in the hot rolled bar, as indicated in the schematic crack paths of Fig. 3.

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Fig. 3. Locally anisotropic fatigue crack paths in the hot rolled bar (left) and in the cold drawn wire (right).