Issue34

F. Curà et alii, Frattura ed Integrità Strutturale, 34 (2015) 447-455; DOI: 10.3221/IGF-ESIS.34.50 455 the position of the point on the tooth root where the maximum equivalent stress has been reached (this point is assumed as the crack initiation). Results show that the centrifugal load tends to shift the crack initiation point at the bottom of the tooth root fillet while, if bending load is predominant the nucleation point may shift to the top edge of tooth root fillet. An equation has been introduced to correlate the maximum equivalent stress position to the amount of bending and centrifugal loads. Then a propagation study has been carried on by means of extended finite elements models. The aim of these simulations has been to investigate how the centrifugal load may affect the crack path direction: cracks, initiated at the same point, have been propagated with and without centrifugal load, showing that, in presence of non negligible centrifugal loads, cracks tends to propagate in radial direction. From works available in the literature it is possible to state that in thin rim gears crack propagation direction mainly depends on gear geometry (rim and web thickness) and crack initiation point. From the analysis of the results presented in this work it is possible to conclude that the centrifugal load strongly influences both crack initiation point and crack propagation direction, by shifting the crack initiation point at the bottom of the tooth root fillet and by driving the crack propagation in the radial direction. Generally speaking centrifugal loads may be the key factor to promote failsafe or catastrophic failures in all those cases where the crack propagation path is not only defined by the gear geometry (uncertainty zone), causing the centrifugal load an important effect in both crack initiation position and crack propagation direction. R EFERENCES [1] Lewicki David G., Crack Propagation Studies to Determine Benign or Catastrophic Failure Modes for Aerospace Thin-Rim Gears, NASA Tecnical Memorandum 107170. [2] Lewicki David G., Three-Dimensional Gear Crack Propagation Studies, U.S. Army Research Laboratory, Lewis Research Center, Cleveland, Ohio, NASA/TM-1998-208827. [3] Lewicki David G., Ballarini R., Rim thickness effects on gear crack propagation life, International Journal of Fracture, 87 (1997) 59-86. [4] Lewicki David G., Gear Crack Propagation Path Studies, Guidelines for Ultra-Safe Design- NASA/TM-2001-211073. [5] Kramberger J., Flasker J. – Numerical Simulation of 3-D Crack Growth in Thin-Rim Gears – University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia. [6] Lewicki David G., Effect of Speed (Centrifugal Load) on Gear Crack Propagation Direction, U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio (2001). [7] Glodez S., Pehan S., Flasker J., Experimental results of the fatigue crack growth in a gear tooth root, Int. J. Fatigue, 20 (1998) 669-675. [8] Pehan S., Hellen Trevor K., Flasker J., Glodez S. – Numerical methods foe determing stress intensity factors vs crack depth in gear tooth roots, Int. J. fatigue, 19 (1997) 677-685. [9] Flasker, J., Glodez, S., Pehan., S., Influence of contact area on service life of gears with crack in tooth root, Communications in Numerical Methods in Engineering, 11 (1995) 49-58. [10] Li, S., Centrifugal load and its effects on bending strength and contact strength of a high speed thin-walled spur gear with offset web, Mechanism and Machine Theory, 43 (2008) 217–239. [11] Li, S., Effects of centrifugal load on tooth contact stresses and bending stresses of thin-rimmed spur gears with inclined webs, Mechanism and Machine Theory, 59 (2013) 34–47. [12] Curà, F., Mura, A., Rosso, C., Investigation about crack propagation paths in thin rim gears, Fracture and Structural Integrity, 30 (2014) 446-453. DOI: 10.3221/IGF-ESIS.30.54. [13] Sukumar, N., Moës, N., Moran, B., Belytschko, T., Extended finite element method for three-dimensional crack modelling, International Journal for Numerical Methods in Engineering, 48(11) (2000) 1549–1570. [14] Curà, F., Mura, A., Rosso, C., Effect of rim and web interaction on crack propagation paths in gears by means of XFEM technique, Fatigue Fract. Eng. Mater. Struct. (2015) (article in press).