PSI - Issue 79

A. Bacco et al. / Procedia Structural Integrity 79 (2026) 342–347

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3. Results Visual analysis of the samples after galvanising revealed non-uniform coating distribution around the notched area, altering also its geometry. In addition, the interface between the Zn–5%Al coating and the steel substrate showed microstructural discontinuities and pre-existing microcracks, potentially attributable to hydrogen embrittlement or residual stresses generated during the galvanising process, Fig. 2.

b

a

Fig. 2. Galvanised sample (a) with detail of the cut-out area (b).

Fatigue tests conducted on STRENX700E samples galvanised with Zn–5%Al alloy generated a Wöhler curve (Fig. 3) characterised by an inflection point at approximately 4·10 5 cycles. This transition marks the shift between the finite life regime, dominated by crack nucleation and rapid propagation, and the infinite life regime, in which the material shows greater stability under cyclic loading. In the finite life section, the slope of the curve was k=3.847, consistent with typical values for high-strength steels subjected to cyclic loads in the presence of notches. Above the knee, in the infinite life regime, the slope increased to k=45, indicating a sharp reduction in sensitivity to changes in the number of cycles, consistent with stable material behaviour in the absence of significant initial cracks.

Fig. 3. Wöhler’s curve.