PSI- Issue 9

Jesús Toribio / Procedia Structural Integrity 9 (2018) 317–322 Author name / Structural Integrity Procedia 00 (2018) 000–000

319

3

3. Macroscopic fatigue crack paths in global mode I

Toribio and Toledano (2000), as well as Toribio et al. (2007a, 2007b, 2009, 2010, 2014, 2015, 2016, 2017) have thoroughly studied the fatigue performance of pearlitic steel wires with different cold drawing degree, from a hot rolled pearlitic steel bar with randomly oriented microstructure regarding colonies and lamellae (cf. Fig. 1; left) to a fully cold drawn pearlitic steel wire with a markedly oriented microstructure regarding colonies and lamellae (cf. Fig. 1; right) aligned along the wire axis or cold drawing direction. The first conclusion of the research is the experimental fact that in all cases the macroscopic fatigue crack paths develop globally in mode I following the original macro-crack propagation direction, i.e., they 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.

4. Microscopic fatigue crack paths in 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. 3. 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. 3. 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. 4.

Fig. 4. Specific profiles of the fatigue crack paths for different Δ K levels in the hot-rolled bar (left) and the cold drawn wire (right).