Issue 38

F. Berto et alii, Frattura ed Integrità Strutturale, 38 (2016) 215-223; DOI: 10.3221/IGF-ESIS.38.29 216 I NTRODUCTION ealing with torsional and multiaxial fatigue, an anomalous phenomenon of the notch-strengthening effect was observed in circumferentially notched specimens made of austenitic stainless steels [1–3]. The fatigue life of notched specimens resulted longer than that of smooth ones, and increased with increasing stress concentration factor under the same amplitude of the nominal shear stress. This notch-strengthening effect was also observed in NiCrMo steel [4], pure titanium [5], but it was not found in carbon steels [3,6,7]. In circumferentially notched bars subjected to torsion fatigue loadings, factory-roof type fracture surfaces are obtained under low stress amplitudes and the sliding contact of the fracture surfaces causes the retardation of crack propagation [8–12]. At high stress amplitudes, instead, flat fracture surfaces are observed and the crack retardation due to sliding contact is reduced. The presence of a superimposed static tensile stress also reduces the crack surfaces contact [11]. Recently, Tanaka [13] has deeply investigated this phenomenon dealing with the fatigue behaviour of notched bars made of austenitic stainless steel, SUS316L, and carbon steel, SGV410, subjected to torsion loadings and characterized by different notch tip radii. In the present work, the experimental fatigue results have been reanalysed by means of the averaged strain energy density (SED) approach, first proposed by Lazzarin and Zambardi [14]. The crack initiation life has been considered, in such a way to exclude all extrinsic effects acting during the fatigue crack propagation phase, such as sliding contact and/or friction between fracture surfaces. E XPERIMENTAL FATIGUE RESULTS he materials tested in [13] were an austenitic stainless steel (SUS316L) and a carbon steel (SGV410) for structural use in nuclear power plants. The yield strength and tensile strength of SUS316L were 260 and 591 MPa, and those of SGV410 were 275 and 470 MPa. Fig. 1 reports the geometry of the cylindrical specimens weakened by circumferential notches with three different root radii. The specimens with a notch radius  equal to 4.5, 1.07, and 0.22 mm are named NA, NB, and NC, respectively. The elastic stress net-section concentration factor for the shear stress under torsion for NA, NB, and NC specimens calculated by the finite element method (FEM) was 1.17, 1.55, and 2.54, respectively, while that for the tensile stress was 1.50, 2.50, and 5.07, respectively. Figure 1 : Geometry of the cylindrical notched specimens [13] (dimensions are in mm). D T