PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2396–2403 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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3. Micro-approach to fatigue crack growth The fatigue tests consisted of applying a cyclic tensile load on cylindrical samples taken from the bar and the wire. A sinusoidal wave was used with 10 Hz and R -ratio = 0. As sketched in Fig. 4, the fatigue surfaces were examined by scanning electron microscopy (SEM) after the fatigue crack growth tests ( fractographic analysis ). In addition, the fatigue crack paths were observed after longitudinal cuts on the fatigued specimens, metallographic preparation and SEM observation (fracto-materialographic analysis ). With regard to the fractographies (Fig. 4a) the axes of the micrographs correspond to the radial and hoop coordinates. In the fracto-metallographic views (Fig. 4b) the axes of the micrographs follows the radial coordinate and the axial one (wire axis or cold drawing direction).

(a) (b) Fig. 4. Micro-approach to fatigue crack growth: (a) fractographic analysis ; (b) fracto-materialographic analysis .

The fatigue surface exhibits at the microscopic level ductile micro-tearing patterns (Figs. 5 and 6), consistent with previous research by Toribio and Toledano (2000) and associated with highly localized plastic strains, which can be classified as tearing topography surface or TTS, as identified by Thompson and Chesnutt (1979) and by Costa and Thompson (1982) in pioneering papers.

(a) (b) Fig. 5. Fractographic analysis for low regime of Δ K : (a) hot rolled bar; (b) cold drawn wire.

(a) (b) Fig. 6. Fractographic analysis for high regime of Δ K : (a) hot rolled bar; (b) cold drawn wire.