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V. Di Cocco et alii, Frattura ed Integrità Strutturale, 34 (2015) 415-421; DOI: 10.3221/IGF-ESIS.34.46 420 a) b) c) Figure 7 : Fracture surface SEM analyses in martensitic phase: a) R=0.10, b) R=0.50 and c) R=0.75 . According to the authors, different morphological and mechanical parameters should be taken into account in order to define the damaging micromechanisms. The main parameters are: - Alloy microstructure: large austenitic grains (diameters mean value equal to 600  m) and needles-like substructure; - Shape memory micromechanisms with the presence of a hysteresis during the loading-unloading process. Considering the peculiar behavior of the investigated alloy (Fig. 1), classic relationships [12] can’t be applied to quantify the reversed plastic zone (rpz) radius. Anyway, this radius increases with the increase of the applied  K and, considering that the microstructure is characterized by large grains and that these grains transform from austenite to martensite due to the applied local stress, damaging micromechanisms change with the applied  K, becoming more and more fragile (intergranular secondary cracks + cleavage) when more critical loading conditions are applied (high R and  K values). These more critical conditions imply a large martensitic transformation with a reduced importance of the M ↔ A hysteresis. Further investigations are still necessary. C ONCLUSIONS n this work, the fatigue crack propagation in a Cu-Zn-Al SMA was investigated. Fatigue crack propagation tests were performed considering three different stress ratios (R=0.10, 0.50 and 0.75, respectively). According to the observed stress induced microstructural transformations and considering that the investigated alloy I