EXPERIMENTAL AND ANALYTICAL STUDY IN FATIGUE AND WEAR OF STEEL AND NICKEL-BASED ALLOYS
thesisposted on 29.10.2021, 12:53 authored by Arman AhmadiArman Ahmadi
Fretting wear occurs when two bodies in contact are subject to small oscillatory displacements. This wear phenomenon is common in many industrial applications, such as gears, couplings, bearings, screws, valves and joints where vibrations occur. It has been shown that many factors affect the fretting wear rate, e.g. the normal load, sliding distance, geometry of the bodies, surface roughness, material properties, lubrication status, temperature and presence of third bodies. Fatigue has also been the subject of much investigation over the past century. Fatigue damage is observed in the form of microcracks, debonding, etc. in the vicinity of stress risers (e.g. inclusions, voids etc.) within the materials. The stress component causing fatigue failure can be normal, shear, or a combination due to a compound state of loading. In order to investigate the shear mode of fatigue failure, torsional fatigue testing has been the subject of many studies. Shear mode of failure is of significant importance in triaxial state of stress present for ball and rolling element bearings and machine component which are subject to fretting fatigue. A number of different experimental and numerical techniques have developed to study the torsion fatigue and fretting wear of materials at different conditions. An in-situ fretting wear measurement technique was developed to investigate the effects of temperature on the coefficient of friction and wear rate of Inconel 617 in fretting wear in air and helium environments. Due to the importance of the shear mode of stress in fretting fatigue phenomenon, another set of experiments were designed to measure high cycle torsional fatigue properties of Inconel 617 at elevated temperatures. An MTS torsional fatigue test rig was modified with customized mechanical grips and cooling fins. In order to achieve the objectives of analytical aspects of this investigation, a 3D elastic-plastic finite element model was developed to examine the torsional fatigue damage in Inconel 617 material at high temperatures. Then, a 3D finite element model was developed to study fretting wear of similar materials in Hertzian line and circular contacts. The wear law incorporated in this model is based on the accumulated dissipated energy law. The FEM was used to investigate partial slip regime. Then, the model was verified by performing several experimental tests for the circular contact configuration. During fretting wear, the generated wear debris is of significant importance. A finite element model was created to study the third body effects on fretting wear of Hertzian contacts in the partial slip regime. Both first bodies and third bodies were modeled as elastic-plastic materials. The effect of the third body particles on contact stresses and stick-slip behavior was investigated. The influence of the number of third body particles and material properties including modulus of elasticity, hardening modulus, and yield strength were analyzed. Finally, A new thermally cured polymer-graphene-zinc oxide-based solid lubricant was developed that reduced friction and wear significantly during the sliding wear of bearing steel under extreme contact pressure and long duration. The dry solid coating composite was made from a mixture of graphene, zinc oxide, and a specific industrial binder and then laminated on the surface of 52100 steel disks using the spin-coating technique. After ∼3000 cycles, the 15 μm thick coating created a significant reduction in the steel's coefficient of friction (approximately 82%) and wear loss compared to the uncoated surfaces. Following the tribological examination, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy were conducted to determine the topography and morphology of the composite coating and resultant wear scars.