Development of Failure Limiting Diagram For Surface Modified Wheel-rail and Wind-turbine Bearing Materials Through Rolling Contact Fatigue Studies

Mechanical Engineering
Project type: 
Sponsored Projects
2019 - 2022
Principal Investigator: 
Dr. D. Kesavan
Project Number: 
Sponsoring Agency: 
SERB - Empowerment and Equity Opportunities for Excellence in Science (EMEQ)
Total Budget: 

The life of wheel-rail in railways and ball-raceway in wind turbine bearings are limited by rolling contact fatigue (RCF) and wear failures. Such failures are inevitable due to higher magnitude of normal and tangential stresses developed at the contact during the operation. RCF is caused by repetitive contact stress along with rolling and sliding appear simultaneously at the contact zone. Whereas, removal of material from the surface by wear is a function of the sliding and contact stress. The crack initiated by RCF process is the main source of the failure which implies physical separation of material from the surface of the component by the process called pitting or spalling or flaking. Wear is generally considered as an additional damage observed at higher load level with considerable amount of slip or sliding-to-rolling ratio (SRR) at the interface. Synergistic effect of RCF combining with wear leads to several other undesired phenomena like thermal cracks, undesired phase transformations, etc and leads to early catastrophic failure, and drive to the cost of component replacement and maintenance.

In rail-wheel, nature of the contact is dry and the amount of sliding between surfaces depends mostly on the contact geometry, normal force and friction coefficient. Amount of sliding and contact stress at the surface level vary its magnitude and orientation as wheel passes over the rail curves, crossings, etc. These quantities further depend on the system level dynamics that is affected by the change of wheel profile shape, also it is important to note that noise and vibration depend on the surface damage. The cracks that are generated by contact fatigue combined with wear lead to the damaging phenomenon called gauge corner which implies separation of chunks of material from the surface.

In ball-raceway wind turbine pitch bearings, damage mechanism is quite different from wheel-rail due to its lubricated contact. Extreme combination of moment load arises from blade rotation with wind load pushes the bearing balls beyond its designed position. As a result, elasto hydro-dynamic (EHL) film developed due to the lubricant between ball to raceway breaks and high local contact stress arises and causes initiation for the failure. White etching cracks (WECs) have been identified and widely reported cause of failure in wind turbine bearings. Such cracks propagate typically from surface and leads to spalling of raceway, such failure is commonly observed on field return bearings. The hypotheses put forward is hydrogen embrittlement due to hydrogen diffusion resulting from decomposition of lubricant, however its formation mechanism is different with wheel-rail crack formation, and it is still not fully understood.

Though these problems are well known in railway and bearing industries, failure detection is challenging because both RCF and wear interacts each other and play a complex role in real time operation. Therefore, finding an optimal combination of RCF and wear limit to prevent a failure is the key to run safe and cost-effective railways and wind turbines operations. To reduce failure, the interactions between RCF & wear mechanisms must be clarified under both dry and lubricated contact condition.