Shear-induced migration and clustering mechanisms of kaolinite clay particles under the oscillating face seal by Jinchul Hong
The migration and clustering mechanisms of clay particles under the contact band of an oscillating face seal are investigated. There are two seperate stages in the wear, which are called the break-in period and the aggressive wear period. First small particles in the slurry penetrate through a short distance under the seal lip, where a thin oil film is present and there is no sign of significant wear. This period is called breakin period. However, after a number of cycles, the particles aggregate and start to form clusters. These clusters grow and the seal wears rapidly. Thus cluster formation plays a critical role in the whole wear process. As a first step, we have compared the possible forces which drive particles inwards. These include the effects of Van der Waals interactions, hydrodynamic shear, Brownian diffusion, etc. In the initial stages of the process, when the particle concentration is not high, prior experiments are consistent with shear-induced migration as the primary mechanism. Particles migrate inwards due to gradients in concentration, shear rate, viscosity and the effects of curved-streamline. Van der Waals force are likely to be dominant when the concentration is high enough so that the distance between the particles is small. In order to confirm these mechanisms, a new experiment using steel seals was carried out and corroborates earlier studies. Finally, some clustering and dispersion mechanisms such as the effects of dissolved ions (leading to aggregation) and thermal expansion of the seal lip (leading to dispersion) were also studied. Thesis Supervisor: L.Mahadevan Title: Associate Professor
Some rheological and electrokinetic properties of kaolinite suspensions
The rheological behaviour of kaolinite sols has been studied as a function of pH and in the presence of adsorbates. The Bingham yield value shows a pronounced maximum at about neutral pH due, it is suggested, to the presence of extended chains of particles attached together by electrostatic interactions between edges and faces, as has been suggested by Street. At high pH values the particles behave as individuals and the yield value can be very effectively correlated with the attractive and repulsive forces between them, using the basic notions of colloid stability theory. Adsorption of long-chain quaternary ammonium compounds on the clay surface at high pH enabled the particle interaction energy to be altered over a wide range and this adsorption itself allowed a study of the van der Waals interaction energy between the CH2 groups of the adsorbate; a value of 1.2kT was found for this latter energy, in excellent agreement with estimates based on micellization in solution.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME