C. Ronchei et alii, Frattura ed Integrità Strutturale, 34 (2015) 74-79; DOI: 10.3221/IGF-ESIS.34.07 74 Focussed on Crack Paths Life estimation by varying the critical plane orientation in the modified Carpinteri-Spagnoli criterion Camilla Ronchei, Andrea Carpinteri, Giovanni Fortese, Andrea Spagnoli, Sabrina Vantadori Department of Civil-Environmental Engineering and Architecture, University of Parma - Parma, Italy Marta Kurek, Tadeusz Łagoda Department of Mechanics and Machine Design, Opole University of Technology - Opole, Poland A BSTRACT . The modified Carpinteri-Spagnoli (C-S) criterion is a multiaxial high-cycle fatigue criterion based on the critical plane approach. According to such a criterion, the orientation of the critical plane is linked to both the averaged directions of the principal stress axes and the fatigue properties of the material. The latter dependence is taken into account through a rotational angle,  . Then, the multiaxial fatigue strength estimation is performed by computing an equivalent stress amplitude on the critical plane. In the present paper, some modifications of the original  expression are implemented in the modified C-S criterion. More precisely, such modified expressions of  depend on the ratio between the fatigue limit under fully reversed shear stress and that under fully reversed normal stress (in accordance with the original expression), and can be employed for metals ranging from mild to very hard fatigue behaviour. Some experimental data available in the literature are compared with the theoretical results in order to verify if the modified  expressions are able to improve the fatigue strength estimation capability of the modified C-S criterion. K EYWORDS . Constant amplitude loading; Fatigue lifetime prediction; Modified C-S criterion; Multiaxial high- cycle fatigue; Critical plane. I NTRODUCTION n the high-cycle fatigue related to a linear-elastic material, several criteria available in the literature to assess fatigue strength are based on the so-called critical plane approach. This approach takes into account the crack nucleation and early growth mechanisms experimentally observed during cyclic loading. According to such criteria, fatigue failure assessment is performed on a specific plane (the critical plane) within the test specimen or component. The above criteria are characterized by different rules suitable to define the orientation of the critical plane but, for all these criteria, the fatigue life assessment is carried out by employing a combination of stresses acting on the critical plane itself [1] (see also the review on the critical plane and other approaches to multiaxial fatigue, published in Ref. [2]). For instance, several researchers define the critical plane as the plane where amplitude or some stress component or a combination of them exhibits a maximum value [3-6]. Alternatively, the position of the critical plane may be correlated with that of the principal stress directions by using appropriate weight functions [7]. I