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Opis: A two-dimensional computational model for simulation of contact fatigue of gear teeth flanks is presented. In the model, it is assumed that the initial crack of length 0.015 mm is initiated at the surface due to previous mechanical or heat treatment of the material as well as a consequence of the running in process. The discretized model with the initial crack is then subjected to normal contact pressure, which takes into account the elasto-hydro-dynamic (EHD) lubrication conditions, and tangential loading due to friction between contacting surfaces. The model also considers the moving contact of gear flanks, fluid trapped in the crack and residual stresses due to heat treatment of the material on crack propagation. The virtual crack extension (VCE) method, implemented in the finite element method, is then usedfor simulating the fatigue crack growth from the initial crack up to the formation of the surface pit. The computational results show that the initial surface crack of length 15 m and the considered boundary conditions lead to the appearance of very small surface pits, which can be termed as micro-pitting on gear teeth flanks. The numerical results correspond well withavailable experimental data.
Ključne besede: machine elements, gearing, fracture mechanics, numerical analysis, micro pitting, pitting simulation, matematical model, fatigue, contact mechanics, crack propagation
Objavljeno v DKUM: 01.06.2012; Ogledov: 2660; Prenosov: 0

Opis: Contact fatigue is a phenomenon of important practical significance for engineering applications involving localized contacts, such as gears, rail wheel system and rolling bearings. The service lifetime of such components is related to damage, which results from the contact fatigue. The process in the material structure that causes this kind of failure is quite complicated. The aim of the present paper is to describe a contact fatigue initiation criterion, based on the critical plane approach for the general contact problem. On the basis of contact stress analysis with modified Hertzian boundary conditions, the loading cycle of characteristic material points in the contact area is determined. The Dang Van damage initiation criterion is based on the critical plane approach, which combines the largest allowable shearing and hydrostatic stresses (tensile and compressive), with an assumed elastic shakedown behaviour and it is used in this work. The material point of initial fatigue damage is then determined at the transition of the loading cycle stresses over the critical plane. The model assumed a homogeneous and elastic material model, without any imperfections or residual stresses, and elastic shakedown is considered. A proper determination of loading cycles and their characteristic values is of significance for contact fatigue initiation analysis. Finally, determination of the most critical material point on or under the contact surface and related number of loading cycles required for fatigue damage initiation is calculated with the strain-life (▫$epsilon$▫-N) method.
Ključne besede: machine elements, contact fatigue, crack initiation, strain life method, critical plane approach, numerical modeling
Objavljeno v DKUM: 01.06.2012; Ogledov: 2046; Prenosov: 117
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Opis: A computational model for simulation of surface and subsurface initiated fatigue crack growth due to contact loading is presented. The model is based on fracture mechanics theory where the required materials properties are obtained from common fatigue tests. For computational simulations an equivalent model of two contacting cylinders is used instead of simulating theactual contact of mechanical elements. The discretised model with the initial crack on or under the surface is then subjected to normal contact pressure, which takes into account the EHD-lubrication conditions, and tangential loading due to friction between contacting surfaces. The model considers also the moving contact of mechanical elements and for the surface initiated crack also the fluid trapped in the crack. The virtual crack extension method, implemented in the finite element method is then used for simulating the fatigue crack growth from the initial crack up to the formationof the surface pit. The numerical results correspond well with available experimental data. The described model can be used for simulation ofpitting phenomenon of contacting mechanical elements like gears, bearings, wheels, etc.
Ključne besede: fracture mechnaics, machine elements, gears, contact fatigue, pitting, subsurface crack initiation, computational analysis
Objavljeno v DKUM: 31.05.2012; Ogledov: 2353; Prenosov: 116
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