PSI - Issue 26

Jesús Toribio et al. / Procedia Structural Integrity 26 (2020) 368–375 Toribio / Structural Integrity Procedia 00 (2019) 000 – 000

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6. Fractographic analysis Fig. 4 shows the microscopic fracture modes in the hot rolled steel before cold drawing. After the fatigue pre crack (Fig. 4a), there is a hydrogen damage topography (HDT) in the form of a non-conventional fractographic mode associated with hydrogen effects in pearlitic microstructures: the so-called tearing topography surface (TTS), as identified by Thompson and Chesnutt (1979) and Costa and Thompson (1982), consisting mainly of micro tearing at the finest microscopic level, with a certain degree of plasticity, cf. Fig. 4b, and final fracture by cleavage (Fig. 4c). Therefore the HAMD is a sort of TTS and the HACP develops in mode I with no macroscopic deflections or deviations from the initial fatigue crack propagation in mode I.

Fig. 4 (left). Microscopic modes of fracture in steel 0: (a) fatigue precrack; (b) hydrogen damage topography; (c) cleavage . Fig. 5 (right). Microscopic modes of fracture in steel 0: (a) fatigue precrack; (b) hydrogen damage topography; (c) MVC + cleavage. Fig. 5 shows the microscopic fracture modes in the commercial prestressing steel (heavily cold drawn). After the fatigue pre-crack (Fig. 5a), there is a marked crack deflection ( deviation angle close to 90º or propagation step ) and a hydrogen damage topography (HDT) in the form of a sort of enlarged and oriented TTS (EOTTS) following the deflected crack path associated with mixed mode propagation , cf. Fig. 5b, and final fracture again in the original mode I propagation by a mixture of micro-void coalescence (MVC) and cleavage (Fig. 5c). Therefore the HAMD is a sort of EOTTS and the HACP develops in mixed mode in the form of a markedly deflected crack with a strong component of mode II and evidence of shear, as shown in Fig. 5b.