Numerical/experimental investigation of the contact fatigue damage mechanisms under lubricated rolling/sliding conditions

Abstract

Many machine components, such as bearings, cams and gears, suffer Rolling Contact Fatigue (RCF) culminating in surface damage known as pitting. The lifetime of components can be extended by mitigating wear and eliminating other causes of premature failure, but pitting represents the upper boundary of the lifetime of the machine. Lubricant has been demonstrated to accelerate the initiation and propagation of rolling contact fatigue cracks, but its role is still the object of debate. Despite numerous investigations carried out via numerical/analytical modelling in the framework of LEFM, interpretation of the numerical results is still not clearly understood. Thus, this work is intended to shed light on the contribution of lubricant to RCF crack propagation by means of numerical analyses. 

Firstly, a 2D Finite Element model of an inclined edge crack subject to travelling contact load was devised. The contact load is approximated by analytical pressure distribution of Hertz. We investigated how fluid pressurization culminates in fluid entrapment. Pressurization was modelled by uniformly transmitting the external contact pressure acting on the crack mouth to the crack faces. As soon as the crack mouth is closed by hertzian load, despite hydraulic pressure, pressurization is superseded by entrapment. From that point on, the fluid pressure due to entrapment is found iteratively by imposing a constant entrapped fluid  volume. An extensive parametric analysis highlighted the contribution of entrapment to SIFs. Subsequently, the FE model was used to study the evolution of real cracks produced in disk-on-disk wear tests under lubricated rolling/sliding conditions. The numerical outcomes, along with experimental evidence, provide useful information on the effect of lubricant on the crack path.

As a second step, the same FE model, in which only entrapment of lubricant was implemented, has been improved. Entrapment has been assumed to be caused by closing action of the second body on the crack mouth or by crack lips closure. Instead of modelling an analytical contact pressure distribution, the actual mating bodies were modelled as well. Presence of lubricant between the mating bodies and in the crack cavity has been simulated via hydrostatic fluid elements. Comparison between the first and second model were carried out. Contribution of fluid entrapment to coplanar crack propagation is investigated. The results indicate that pressurization and entrapment tend to produce similar effects for short cracks. Furthermore, the branches are most likely to extend in tensile mode toward the contact surfaces and their growth could be influenced by other adjacent cracks.

The calculated SIFs history presents high non-proportionality and the magnitude of the peaks do not reach the fatigue threshold usually found for long cracks in steels and cast irons. Thus, a RCF crack is not expected to propagated. To shed light on this issue, the calculated SIF-cycles were reproduced experimentally on a macro crack in laboratory via an alternative non-standard procedure. Lubrication has been found to be necessary to give arise to crack propagation, which can be in form either of coplanar extension followed by branching or of branching only. A certain fatigue threshold activating coplanar extension in mode II, well below the typical fatigue threshold, has been measured. Furthermore, the scenario is similar to those of RCF cracks and coplanar extension seems to be pronounced in the interior of the sample where plane strain conditions prevails. The empirical results indicate that our previous models tend to underestimate the actual magnitude of SIFs cycle. But the experimental activity may provide some indications to improve the understanding and modelling of RCF.

The final part of this work concerns the elasto-hydrodynamics (EHD) lubrication related to asperity contact of layered contact surfaces. The micropitting, which is the precursor of pitting, is governed and promoted by asperity contact stresses under severe lubricated contact conditions. In one respect, the lubricant causes the pitting and micro-pitting. But, in another respect, it can alleviate severe contact conditions by reducing the friction between two mating surfaces. In addition, originated EHD oil film reduces the contact asperity, which may be detrimental.To improve tribological properties of bearings, the industry is aiming at protecting the load-bearing surface of bearings with deposition of coating layers. 

The presented modelling is focused on the study of asperity contact in elasto-hydrodynamics (EHD) lubrication affected by addition of protective coating layers.

The outcomes indicate that the use of coating layers along with a feasible thickness of EHD fluid film is capable to reduce asperity contact, extending thus the life of contact surface.