P. Gallo et alii, Frattura ed Integrità Strutturale, 34 (2015) 180-189; DOI: 10.3221/IGF-ESIS.34.19 183 Figure 1 : Microstructure of the specimens at different magnification values. Specimen geometry Considering the high temperature fatigue tests and the investigation of the cracks growth, three specimen geometries were considered: Hourglass shaped (smooth) specimens with a theoretical stress concentration factor close to 1.0 (see Fig. 2); Plates weakened by lateral symmetric V-notches, with a net cross section of 20 × 5 mm and a total length of 300 mm (Fig. 3). The notches were characterized by a depth, a , equal to 5 mm, an opening angle, 2α, equal to 90° and a notch tip radius ρ=1 mm. This geometry results in a theoretical stress concentration factor K t,n =3.84 (on the net transverse sectional area). Plates with a central hole (see Fig. 4), with a cross section of 5 x 30 mm and a total length of 300 mm; the hole radius is 5 mm, which results in a theoretical stress concentration factor K t,n =2.3 (on the net transverse area). The diameter of the hole matches the real dimension of the rolls cooling channels. Figure 2 : Hour-glass shaped specimen geometry, dimensions in mm. Figure 3 : Notched specimen geometry, dimensions in mm. Figure 4 : Plates with central hole, dimensions in mm.