Keywords: surface pitting, fracture mechanics, short cracks, contact parameters.

Mechanical components, subjected to repeated contact loading of high intensity, commonly suffer from surface deterioration due to material fatigue. This process originates from small, surface-breaking or subsurface initial cracks, which grow under repeated contact loading. Usually the cracks curve back towards the surface, which eventually causes the material surface layer to break away when the cracks reach the free surface. The resulting surface void is called a pit and associated process is called surface pitting. Initial pitting is characterised by pits, whose depth usually does not exceed 20 m, and is commonly termed micro-pitting. It is usually the first indication of inevitable progressive macro-pitting [1,2,3]. The latter can seriously hamper the operating conditions of the component, and can lead to complete component failure.

This contribution describes a new computational model, which attempts to account for different parameters influencing the pitting process in the lubricated contact of mechanical components. Computational simulation of the crack growth leading to pitting starts from the initial surface-breaking fatigue crack, which is a consequence of mechanical treatment of the component material. The proposed model is based on a 2-dimensional plain strain fatigue crack analysis, utilising the fracture mechanics theory, where the required materials properties are obtained from common fatigue tests [4,5,6]. The crack growth model does not consider friction on the developing crack faces. The model is intentionally kept as simple as possible with the aim to be used in practical engineering design applications.

The discretised equivalent contact model [4], with the assumed size and orientation of the initial crack, is subjected to contact loading conditions, accounting for the EHD-lubrication effects and tangential loading due to sliding. The influence of a lubricating fluid, driven into the crack by hydraulic mechanism, is also considered. The short crack growth is simulated with use of the virtual crack extension method and the strain energy density method in the framework of the finite element analysis. The aim of the study is to determine the nature of short crack growth under various influential contact loading parameters and the resulting size of the estimated pit shapes.

The model is applied to a real pitting problem on a gear and corresponding computational results in terms of evaluated pit sizes correlate well to experimentally observed micro-pitting of gear teeth flanks [7,8]. If combined with appropriate short fatigue crack growth theories, the evaluated functional relationship between the stress intensity factor and the crack length can be used to predict the time required for development of pits, i.e. the service-life of contacting components in regard to pitting [6,8].

The virtual crack extension method, i.e. the criterion, predicts the most conservative fracture load. This is a highly desirable quality to be found in a fracture criterion since it leads to more reliable engineering designs. With its theoretical superiority and practical conservatism, the criterion appears to be superior to alternate mixed-mode fracture criteria for short cracks close to loading boundary. However, the reliability of the SED criterion could be improved by introduction of relevant constraint parameters for more accurate determination of the needed stress intensity factors or direct evaluation of the crack path from the strain energy density distribution around the crack tip.

1

R.S. Zhou, H.S. Cheng, T. Mura, "Micropitting in Rolling and sliding contact under mixed lubrication", ASME Journal of Tribology, 111, 605-613, 1989.

2

M. Kaneta, H. Yatsuzuka, Y. Murakami, "Mechanism of crack growth in lubricated rolling/sliding contact", Tribology Transactions, 28, 407-414, 1985. doi:10.1080/05698198508981637

3

W. Cheng, H.S. Cheng, T. Mura, L.M. Keer, "Micromechanics modelling of crack initiation under contact fatigue", ASME Journal of Tribology, 116, 2-8, 1994. doi:10.1115/1.2927042

4

S. Glodez, Z. Ren, "Modelling of crack growth under cyclic contact loading", Theoretical and Applied Fracture Mechanics, 30, 159-173, 1998. doi:10.1016/S0167-8442(98)00053-6

5

S. Glodez, Z. Ren, J. Flasker, "Simulation of surface pitting due to contact loading", International Journal for Numerical Methods in Engineering, 43, 33-50, 1998. doi:10.1002/(SICI)1097-0207(19980915)43:1<33::AID-NME410>3.0.CO;2-Z

6

S. Glodez, J. Flasker, Z. Ren, "A new model for the numerical determination of pitting resistance of gear teeth flanks", Fatigue and Fracture of Engineering Materials and Structures, 20, 71-83, 1997. doi:10.1111/j.1460-2695.1997.tb00403.x

7

G. Fajdiga, J. Flasker, S. Glodez, Z. Ren, "Numerical simulation of the surface fatigue crack growth on gear teeth flanks", Journal of Mechanical Engineering, 46(6), 359-369, 2000.

8

G. Fajdiga, "Analysis of the surface fatigue crack growth on gear teeth flanks under EHD lubrication conditions", Ph.D. dissertation, University of Maribor, 2001.

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