Issue34

F. Curà et alii, Frattura ed Integrità Strutturale, 34 (2015) 447-455; DOI: 10.3221/IGF-ESIS.34.50 448 means of three dimensional simulations. Flasker et al. [9] evaluated the residual life of the wheel with a crack along the tooth root for different loading conditions considering the effect of the contact area on the crack propagation direction. Only few works are available in literature about the effect of speed on thin rimmed gears: in particular Lewicki [6] investigated by experiment and 2D finite element models, the effect of the centrifugal load on crack propagation path; in this work a slotted spur gear is considered with notch machined along entire tooth face width. While Li, in its two works [10, 11] investigated effects of centrifugal load on bending strength, contact strength and deformations of a thin-rimmed spur gear used at high speed with the finite element method. The aim of this work is to investigate the influence of centrifugal load on crack propagation path in thin rimmed and webbed gears. According to literature, crack path is influenced by the initial crack position [12]. If it is assumed that initial crack is located at the maximum equivalent stress point, this point may shift its position according to both wheel rotation speed and external bending load. In this work two effects of centrifugal load have been considered: the motion of the crack nucleation point and the change of crack propagation direction. The first aspect has been investigated by means of finite element models where the centrifugal load has been varied and its influence on crack nucleation point has been observed. The second analysis has been performed by means of extended finite elements models (XFEM) [13], where a crack has been growth in case of only bending stress and in the case of both bending and centrifugal load showing different behaviours. Generally speaking, from the analysis of the obtained results it is possible to conclude that the wheel speed may cause the shifting of the point of maximum equivalent stress towards the tooth root, causing, in determinate cases, the failure mode changing (passing from safe failure, to catastrophic failure). On the other hand if a nucleation point is fixed, the centrifugal load tends to growth the crack in radial direction by turning down the crack propagation direction. N UMERICAL M ODELS irst of all a numerical investigation about the effect of centrifugal load on crack initiation point has been carried on. Assuming that the crack may nucleate at the point where the maximum equivalent stress is achieved, the effect of speed (with respect to the bending load) has been investigated by means of 2D FE models developed by MSC.Partan/Nastran® software. Figure 1 : 2D finite elements model and mesh refinement. A steel gear with the following geometrical parameters has been modelled: modulus 7.4mm, 28 teeth, 30° pressure angle, 8mm face width, as in [1, 2]. Concerning the 2D model, in order to reduce the computation time, the gear has been cut and only a sector of four teeth has been considered. The geometry of two teeth has been subdivided in small sectors in order to obtain little portions with regular shapes to allow a more regular and an easier meshing process (see Fig. 1). The average element edge in the root region where the maximum stresses are located is 0.1 mm. The mesh consists in “quad” elements with four nodes. Boundary conditions consist in displacement loads on the lateral edges of the sector to F