Issue 41

S.M.J. Razavi et alii, Frattura ed Integrità Strutturale, 41 (2017) 440-446; DOI: 10.3221/IGF-ESIS.41.55 441 notched components. This method was then enhanced by other scholars to consider different parameters which affect the fatigue crack growth behaviour in different materials [13-19]. Prediction of the branch crack threshold condition under mixed mode (I+III) was suggested by Pook and Sharples [20] through the analysis of the main crack tip stress field. Pook [21] revealed that there are different thresholds for the initiation of crack growth, crack arrest and specimen failure under multiaxial fatigue loading. Additionally, definition of a fatigue threshold ΔK th under poly-modal loading is difficult because under torsion loading an extensive plastic zone is developed at the vicinity of the crack [22]. The presence of plastic deformation around the crack tip in conjunction with the dissipative phenomena due to the possible contact of the crack surfaces result in a strong influence of the specimen geometry on the test data. Christopher et al. [23] developed a novel mathematical model of the stress fields around the crack tip under fatigue loading, taking into account the effects of plasticity through an analysis of their shielding effects on the applied elastic field. Lin et al. [24] studied the complex initiation of crack growth under a combination of opening and out of plane shearing mode loading showing that the cracks do not grow through a continuous evolution of the crack surface but rather by means of an abrupt fragmentation or segmentation of the crack front. In the last years the ASED approach which is based on the average value of strain energy density over a control volume has been extended for various brittle and quasi-brittle materials under tensile loading and a wide range of materials under fatigue loading. In particular, this method was used to assess the fatigue behaviour of axisymmetric specimens made of a medium carbon steel (C40) [25] and a 39NiCrMo3 [26] steel subjected to uniaxial and multiaxial loading. In addition, the dissipative phenomena and non-propagating cracks in the run-out specimens were studied by Berto et al. [27] and [28]. They reported that the control volume radius was found to be strongly influenced by extrinsic shielding mechanisms under mode III loading conditions. In the current paper, the multiaxial fatigue strength of circumferentially V-notched and semicircular notched specimens made of 40CrMoV13.9 were investigated under combined tension and torsion loading. The ASED criterion is used in order to predict the fatigue strength of notched components subjected to uniaxial and multiaxial loading. For 40CrMoV13.9 alloy which is a high strength steel the ASED approach is essentially able to overcome the complexity of non-proportional loading using a single control volume which was used independent of the loading mode. M ATERIAL AND GEOMETRY OF THE SPECIMENS ll the specimens were made of 40CrMoV13.9 steel. Static tensile tests were carried out to evaluate the elastic and the strength properties of the material; the relevant mean values are listed in Tab. 1, while the chemical composition of the material is reported in Tab. 2. Ultimate tensile strength (MPa) Yield stress (MPa) Elongation to fracture (%) Brinell Hardness 1355 1127 15.2 393-415 Table 1: Mechanical properties of 40CrMoV13.9. C Mn Si S P Cr Ni Mo V Al W 0.38 0.5 0.27 0.006 0.003 3.05 0.24 1.04 0.24 0.013 0.005 Table 2: Chemical composition wt.%, balance Fe. The geometries of the specimens tested in the present investigation are shown in Fig. 1. The V-notch depth d was equal to 4 mm for both cases. The axis-symmetric V-notched specimens were characterized by a constant notch tip radius (1 mm) and a V-notch opening angle equal to 90 o . The geometry of semicircular specimens instead was characterized by a constant notch tip radius equal to 4 mm. A

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