Y Wang

The influence of ring crack location on the rolling contact fatigue failure of lubricated silicon nitride: experimental studies

This paper presents an experimental study of the influences of ring crack location within the contact path on the rolling contact fatigue failure. The rolling contact tests are performed on silicon nitride/steel elements. Silicon nitride ball surfaces are examined before testing using a dye-penetrant technique and light microscopy. The surfaces during testing, post-test and after failure are examined using light microscopy. Research shows that fatigue failure under rolling contact loading is markedly sensitive to the location of ring crack on the contact track. Only a few locations on the contact track can lead to fatigue failure at the maximum Hertz contact pressure of 5.58 GPa.

Rolling contact fatigue failure modes of lubricated silicon nitride in relation to ring crack defects

The use of silicon nitride applied to rolling element bearings shows some practical advantages over traditional bearing steels. The contribution of this present study is to provide understanding of surface crack defect characteristics and the subsequent rolling fatigue modes of failure. Surface defects such as pressing faults and ring cracks are characterised using light microscopy. Experimental bench testing using an accelerated rolling contact fatigue rig produces results which identify the relationship between fatigue failure modes and surface defects. The rolling contact tests are performed on silicon nitride/steel elements using a variety of lubricants. Ball surfaces are examined before testing using a dye-penetrant technique and light microscopy. Post-test surfaces and failures are examined using light and scanning electron microscopy. A model of surface crack propagation in lubricated contact is described. The boundary element model is used to investigate the growth mechanism of ring crack defects during rolling contact.

The influence of ring crack location on the rolling contact fatigue failure of lubricated silicon nitride: fracture mechanics analysis

The influence of ring crack location within the contact path on rolling contact fatigue failure has been studied using numerical fracture analysis. The numerical calculations are based on a three-dimensional model for the ring crack propagation. The ring crack is considered as a conic shape with a curved line as the crack front. The rolling contact loading is simulated by repeated Hertzian point contact load with normal pressure and tangential traction. Fracture mechanics analysis is utilised to determine the stress intensity factors (SIFs) along the crack front and the SIFs are analysed using a three-dimensional boundary element model. The analytical results are verified by experimental studies, which show that present predictions of ring crack location influence are consistent with the experimental observations.

Ring crack propagation in silicon nitride under rolling contact

Abstract Silicon nitride has been found to have the optimum combination of properties which are suitable for rolling element bearing applications to withstand high loads, severe environments, and high speeds. Surface ring cracks are difficult to detect but are found on the surface of silicon nitride balls. These ring crack defects decrease the rolling contact fatigue life considerably. This paper presents an experimental study and numerical analysis of ring crack propagation in rolling contact. The contribution of this study is to provide understanding of ring crack propagation behaviour and life prediction in rolling contact. Rolling contact tests are performed on the silicon nitride/steel elements. Silicon nitride ball surfaces are examined before testing using a dye-penetrant technique and optical microscopy. The surfaces are examined using optical microscopy and scanning electron microscopy during testing and after failure. The numerical calculations are based on a 3D model of ring crack growth. The rolling contact loading is simulated by a repeated Hertzian surface load with normal pressure and tangential traction. Fracture mechanics analysis is utilised to determine the stress intensity along the crack front and the stress intensity factors are analysed using a 3D boundary element model. Life predictions from the present calculations are in line with the experimental observations.

Failure modes of ceramic rolling elements with surface crack defects

The properties of ceramics, specifically low density and high stiffness are of most interest to gas turbine and machine tool manufacturers. High hardness, low coefficient of thermal expansion and high temperature capability are properties also suited to rolling element materials. Much research over the past two decades on its structure, quality control and manufacturing techniques has produced ceramic materials which can seriously be considered for rolling contact bearing design. However, the difficulties of both sintering and machining the material may result in surface cracks. It is difficult to detect such cracks during high volume production processes and hence there is an important need to understand their influence and fundamental mechanism of the failures they cause. In the present study, the mechanisms of fatigue failure from surface cracks subjected to rolling contact have been studied experimentally and numerically. A three-dimensional boundary element model has been developed to study the failure mechanisms in rolling contact. The calculated results show that surface crack initiated fatigue failure involves fatigue crack propagation from original surface cracks and secondary surface crack formation when the crack reaches a critical condition. The secondary surface cracks play a dominant role in the formation of spalling fatigue failure. A comprehensive experimental study has been carried out to verify the numerical prediction. A modified four-ball machine is employed to perform rolling contact fatigue tests. Results from the experimental test are in good agreement with the results from the numerical analysis.

Rolling Contact Fatigue of Ceramics

High hardness, low coefficient of thermal expansion and high temperature capability are properties also suited to rolling element materials. Silicon nitride (Si{sub 3}N{sub 4}) has been found to have a good combination of properties suitable for these applications. However, much is still not known about rolling contact fatigue (RCF) behavior, which is fundamental information to assess the lifetime of the material. Additionally, there are several test techniques that are employed internationally whose measured RCF performances are often irreconcilable. Due to the lack of such information, some concern for the reliability of ceramic bearings still remains. This report surveys a variety of topics pertaining to RCF. Surface defects (cracks) in Si{sub 3}N{sub 4} and their propagation during RCF are discussed. Five methods to measure RCF are then briefly overviewed. Spalling, delamination, and rolling contact wear are discussed. Lastly, methods to destructively (e.g., C-sphere flexure strength testing) and non-destructively identify potential RCF-limiting flaws in Si{sub 3}N{sub 4} balls are described.

A study of line defect fatigue failure of ceramic rolling elements in rolling contact

Observations of failure mechanisms of silicon nitride rolling elements in relation to surface line defects under rolling contact has been carried out. The purpose of the present investigation is to study the way in which surface line defects fatigue fails and to interpret the failure processes. The rolling contact tests are performed on silicon nitride/steel elements. A modified four-ball machine is used to carry out fatigue tests. Silicon nitride ball surfaces are examined before experimental fatigue tests using a dye-penetrant technique and light microscopy. The surfaces of during testing, post-test and failure are examined using light microscopy and electronic microscopy. Secondary surface cracks play a dominant role in the formation of spalling failure. These cracks propagate conically away from the surface and meet the path from the fatigue crack propagation originated from the pre-existing line defect, and eventually result in the formation of an ellipse fatigue spall.

A mechanism for nucleating secondary fractures near a pre-existing flaw subjected to contact loading

The mechanism of surface failure from a pre-existing line defect under rolling contact has been studied using a boundary element method. A three-dimensional boundary element model has been developed in order to investigate how the pre-existing defect affects the surface fracture behaviour and to determine the geometry of acceptable line defects. Research shows that the pre-existing line defects significantly increases the magnitude of the surface tensile stress on the contact circle. This tensile stress leads to secondary fractures near the pre-existing flaw. Consequently, the secondary surface cracks are formed. These secondary surface cracks dominate the final pitting formation. The secondary fractures can be formed at either the trailing edge or the leading edge of the contact circle. Changes in the crack geometry have a significant effect on the magnitude and distribution of the surface stress. Increasing the gap between the crack faces increases the tensile stress at the edge of the contact circle. Also, increasing the crack length or crack depth increases the tensile stress at the edge of the contact circle. All the numerical analysis based on this model has been verified by a comprehensive experimental study and the predictions are consistent with the experimental observations.

Life prediction for surface crack initiated contact fatigue of silicon nitride bearing balls

Technical ceramics as materials for rolling contact bearing components show some practical advantages over traditional bearing steels. The properties of ceramics, specifically low density and high stiffness, are of most interest to gas turbine and machine tool manufactures. High hardness, low coefficient of thermal expansion and high temperature capability are properties also suited to rolling element materials. Silicon nitride has been found to have the optimum combination of properties suitable for this application. Much research over the past two decades on its structure, quality control and manufacturing techniques has produced material which can seriously be considered for rolling contact bearing design. However, the difficulties of both sintering and machining the material may result in surface crack defects. It is difficult to detect surface cracks during high volume production processes and, hence there is an important need to understand their influence and fundamental mechanism of the failures they cause. In the present study, a quantitative three-dimensional boundary element model has been developed which can be used to determine the geometry of acceptable surface defects and to predict rolling contact fatigue life. The concentrated contact analysis focuses on a hybrid ceramic/steel combination. A modified four-ball machine is employed to validate the predictions. Results from the experimental test are in good agreement with the results from the numerical analysis.