INVESTIGATIONS OF SURFACE AND SUBSURFACE INITIATED FAILURES OF ROLLING CONTACT FATIGUE
thesisposted on 17.02.2020 by Zamzam Golmohammadi
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Rolling contact fatigue (RCF) is a common cause of failure in tribological machine components such as rolling-element bearings (REBs). The two most dominant modes of RCF failure are surface originated pitting and subsurface initiated spalling, both of which have been studied in this investigation. Surface originated pitting is characterized by the initiation of a crack at a surface irregularity i.e. dents or scratches. In this study, a coupled multibody elastic-plastic Voronoi finite element (FE) model was developed based on the continuum damage mechanics (CDM) to investigate the effects of surface defects, such as dents on RCF. The contact pressure, internal stresses and strains were determined using the coupled FE model. The spall patterns and Weibull slopes obtained from the numerical model were found to be in good agreement with the experimental observations from the open literature. The results demonstrated that the sharper the edges and higher the pile-up of the dent profile caused higher pressure spikes and consequently reduced the RCF life.
Subsurface initiated spalling mechanism dominates when bearings are operated under clean lubricated conditions and smooth surface finishes. In this failure mode, micro-cracks originate below the surface at material inhomogeneities such as an inclusion and propagate towards the surface. The fatigue life due to the surface defects is shorter compared to subsurface initiated spalling. In order to allow bearings to reach their critical damage mode (subsurface initiated spalling) and useful estimated life, surface-originated damage in components of REBs can be repaired. In this study, the effects of refurbishing on RCF failure for case carburized bearing steel are experimentally and analytically investigated. A thrust bearing test apparatus (TBTA) was designed and developed to characterize the rolling contact fatigue behavior of different materials. The experimental results for the case carburized AISI 8620 steel demonstrated that the refurbishing process extended the fatigue life of the test specimens. It was also observed that the remaining useful life of the refurbished specimens was improved by increasing the depth of the regrinding. For the analytical investigation, a 2D elastic-plastic FE model was developed to predict the RCF life for pristine and refurbished specimens of case carburized steel using CDM. The effect of the carburizing process (e.g., variations in hardness and residual stresses) were incorporated in the FE model. A sectioning approach was used to simulate the point contact using the 2D model. The
analytical results for the pristine and refurbished case carburized steel corroborated well with the experimental results obtained from TBTA for different grinding depths.
The 2D RCF model was then extended to a 3D efficient elastic-plastic FE model to predict the RCF lives of through hardened AISI 52100 steel at high loading conditions. In order to simulate the RCF for high loads, variations of contact configuration observed in the experimental work were implemented in the 3D model. The experimental results using TBTA demonstrated that the track width increased during RCF cycles, which caused a reduction in the contact pressure due to plastic strains. The compressive residual stresses due to the plastic deformation were also incorporated in the damage evolution law. The estimated RCF lives of through hardened domains corroborated well with the experimental results at high load conditions.
The developed 3D efficient FE model was then extended to investigate the effects of refurbishing on RCF behavior of through-hardened bearing steel. In order to simulate the bearing refurbishment, a layer of the 3D material microstructure was removed from the top surface while the damage state of each element at a specific loading cycle prior to refurbishing was preserved as an initial damage value in the domain. The refurbished domains were then subjected to RCF cycles until a crack reached the top surface designated final fatigue life. In order to investigate the effects of the regrinding depth, fatigue cycles prior to refurbishing and applied loads on the fatigue failure of the through-hardened bearing steel in a circular contact, a parametric study was performed. The numerical results revealed that deeper refurbishing depth and greater fatigue cycles before refurbishment increased the total fatigue life.