PSI - Issue 28

Jesús Toribio et al. / Procedia Structural Integrity 28 (2020) 2378–2381 Jesús Toribio et al. / Procedia Structural Integrity 00 (2020) 000–000

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Fatigue cracking presents at a microscopic level a propagation path with frequent deflections, ramifications, bifurcations, changes in the crack opening and discontinuities (Fig. 2a). In the spheroidized steel the microcracking runs through the thin layer of proeutectoid cementite surrounding the PAG, without fracture the globular cementite particles (Fig. 2b).

(a)

(b) Fig. 2. Fatigue cracking: (a) intermediate zone; (b) crack tip.

3.1. Fracture cracking At the macroscopic level, the steel presents a fracture in mode I with the fracture surface contained in the wire cross section. The fracture advances in a fan shape from the fatigue crack to a narrow ductile ring, corresponding to the fracture end. The fractographic study showed that the fracture begins at the end of the fatigue crack in a fibrous zone (ductile) formed by nucleation of micro-voids (Fig. 3a), which is transformed into a fractography constituted by cleavage facets (propagation unstable) and zones of microvoids between them (Fig. 3b).

(a)

(b) Fig. 3. Fracture surface: (a) fibrous zone; (b) intermediate zone.

Fracture tests stopped before the catastrophic rupture (separation of the specimen in two parts) showing that the plasticity at the fatigue crack tip and the crack blunting are high, producing stable crack propagation (Fig. 4a). This phenomenon corresponds to the appearance of microvoids on the fracture surface forming the initial fibrous zone (Fig. 3a). In the region near the crack tip, it can be observed how the initiation of microvoids occurs in the places of maximum plastic incompatibility, the large particles of cementite being located in the proeutectoid layer, by decohesion with the matrix or by breakage (Fig. 4b).