The paper describes a general computational model for the simulation of contact fatigue-damage initiation in the contact area of meshing gears. The model considers the continuum mechanics approach, where the use of homogenous and elastic material is assumed. The stress field in the contact area and the relationship between the cyclic contact loading conditions and observed contact points on the tooth flank are simulated with moving Hertzian contact pressure in the framework of the finite element method analysis. An equivalent model of Hertzian contact between two cylinders is used for evaluating contact conditions at the major point of contact of meshing gears. For the purpose of fatigue-damage analysis, the model, which is used for prediction of the number of loading cycles required for initial fatigue damage to appear, is based on the Coffin-Manson relationship between deformations and loading cycles. On the basis of computational results, and with consideration of some particular geometrical and material parameters, the initiation life of contacting spur gears in regard to contact fatigue damage can be estimated.
A computational model for simulation of surface pitting of mechanical elements subjected to rolling and sliding contact conditions is presented. The two-dimensional computational model is restricted to modelling of high-precision mechanical components with fine surface finishing and good lubrication, where the cracks leading to pitting are initiated in the area of largest contact stresses at certain depth under the contacting surface. Hertz contact conditions with addition of friction forces are assumed and the position and magnitude of the maximum equivalent stress is determined by the finite element method. When the maximum equivalent stress exceeds the local material strength, it is assumed that the initial crack develops along the slip line in a single-crystal grain. The Virtual Crack Extension method in the framework of finite element analysis is then used for two-dimensional simulation of the fatigue crack propagation under contact loading from the initial crack up to the formation of the surface pit. The pit shapes and relationships between the stress intensity factor and crack length are determined for various combinations of contacting surface curvatures and loadings. The model is applied to simulation of surface pitting of two meshing gear teeth. Numerically predicted pit shapes in the face of gear teeth show a good agreement with the experimental observations.
This article focuses on the development and experimental verification of a friction model to be implemented in a fully functional friction clutch. The resulting clutch model is intended to be employed in commercial software code AVL Excite, which imposes special requirements also for the underlying friction model. These requirements are related to model implementation, available input data, required output data, model complexity, numerical stability, and model parameters. Since for a fully functional clutch the ability to render true stick is crucial, the elasto-plastic friction model is chosen as a basis. This model is investigated in detail and modified adequately in order to meet all of the requirements and to deliver stable and satisfactory results. For validation purposes, a special test bed was built to measure the transmitted torques through the friction contact under various realistic load cases, including all operation phases of the friction clutch. Parallel to experimental measurements, multi-body simulations were done with the modified friction model within the target software. A very satisfactory agreement of simulation and measurement results was achieved.
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