Study on Hertzian cone crack in brittle materials and experimental investigation of fretting wear in bearings using bearing fretting test rig
This thesis investigates two major failure phenomena that bearing industry face, (i) Brittle materials failure phenomenon commonly referred to as Hertzian cone crack, which is formed due to impact or indentation, and (ii) the bearing assembly is prone to failure due to fretting wear at the bearing/shaft and the bearing/housing interface. Finite Element Method was utilized to study the nature of Hertzian cone crack formation and the fretting wear was investigated using an experimental setup. ^ Brittle materials such as silicon carbide SiC, Alumina Al2O 3, other ceramics and soda lime glass develop Hertzian cone cracks when indented or impacted by a hard sphere. Ceramics are now being used in various applications and the cone cracks formed during the manufacturing or due to other contact interactions, limit the fatigue life of the component. A Hertzian cone crack is thought to be the origin of catastrophic failure. In this thesis, ABAQUS explicit finite element software is utilized to investigate Hertzian cone crack occurring in various materials. This model employs axisymmetry and cohesive-law to initiate and propagate cracks in arbitrary directions along the grain boundaries. Voronoi tessellation process is used to simulate microstructure of these materials of Hertzian cone crack. The kinematic contact algorithm is used for defining the contact between a sphere and the substrate. A sphere made of the desired material is impacted on a semi-infinite domain at a variety of speeds and the effective impact load is calculated for different materials to compare with the experimental results. Generally, three types of cracks are found in brittle materials, i.e. conical cracks, lateral cracks and radial cracks. Conical crack formations and behavior are studied here. The crack initiation and propagation nature compares well with the experimental results where multiple cracks initiate and gets arrested as the contact radius increases and finally a crack is formed at or just outside the contact radius which propagates as a dominant cone crack. In this study, various parameters such as cone crack initiation radius, crack length, coefficient of restitution are investigated for various materials. The crack length results match closely with the experimental results. From the crack length results for different materials and for different velocities, a correlation between the crack length, material properties and the load is established. ^ Fretting wear occurs due to small oscillatory motion between two bodies in contact. Fretting phenomenon is divided into fretting wear and fretting fatigue, where fretting wear is characterized by debris formation which results in damage and fretting fatigue is a failure caused by crack formation. This study aims to understand the fretting wear occurring in the bearing-housing assembly by varying different parameters such as fits & tolerances, surface roughness, hardness and type of housing material for different load conditions. A test rig is developed to observe the effect of different parameters of housing in the assembly. Reduced factorial design in design of experiments (DOE) is followed to obtain conclusive results with optimum number of experiments. This study provides a guideline of the parameters that should be controlled in a bearing assembly in order to prevent fretting wear and failure due to fretting. Wear volume in the bearing and the number of revolutions after which onset of wear track is observed are used as the governing output parameters in statistically deciding the optimum parameters.^
Farshid Sadeghi, Purdue University.
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