Analysis of hydrodynamic journal bearings lubricated with non-Newtonian fluids

A procedure for solving the Navier-Stokes equations for the steady, three-dimensional flow of a non-Newtonian fluid within a finite-breadth hydrodynamic journal bearing is described. The method uses a finite-difference approach, together with a technique known as SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) which has now become established in the field of computational fluid dynamics. The concept of ‘effective viscosity’ to describe the non-linear dependence of shear stress on shear rate is used to predict the performance of bearings having a single full-width axial inlet groove situated at the position of maximum film thickness. To illustrate the capabilities of the procedure, results are obtained for a range of non-linearity factors, and lead to the conclusion that the pressure distribution, attitude angle, end-leakage rate, shear force and load capacity can all be predicted for a variety of non-Newtonian lubricants using the SIMPLE numerical integration technique.     

Stress analysis of metallic rocket motor cases reinforced with a viscoelastic fibre overwind

Analytical stress solutions are presented for a metallic rocket motor case reinforced with a prestrained fibre overwind, with viscoelastic properties, under both constant and varying temperature histories. The treatment is in terms of the ‘reduced time’ proposed by Schwarzl and Staverman, with the Williams et al. form of the ‘shift function’.The analysis is reduced to the solution of a Volterra integral equation. For a constant temperature history and using the ‘standard linear solid’ representation of the material properties of the overwind, this equation can be solved in closed-form. For varying temperature histories the equation is solved using a finite-difference technique proposed by Lee and Rogers and improved by Margetson. The effects of fibre relaxation on the internal pressure required for the initiation of yield in the motor case are evaluated and discussed for a number of temperature histories and end conditions.     

On non-symmetrical plane contacts

Plane elastic contact problems are considered, with particular emphasis on asymmetrical punch profiles, in the case of ‘complete’, ‘partially complete’ and ‘incomplete’ contact. An explicit, analytical solution is presented for the case of a single area of contact where the overlap is described by a generic spline function, and examples presented. The interior stress field and strength of the contact, under full or partial slip conditions, are also discussed, and some example shown for representative cases. It is found also that the direction of sliding has a significant effect for the strength of non-symmetrical contacts.     

Electro-mechanical interactions during erosion–corrosion

This study aims to understand the physical processes invoked when solid particle erosion occurs in a corrosive media. The literature on wear-corrosion shows some effort has been placed in qualifying the interactions between erosion and corrosion leading to ‘synergistic’, ‘additive’ and ‘antagonistic’ terms. These terms are difficult to quantify experimentally with multiple experiments being required and generate considerable errors often suggesting these interactive terms are negligible. Hence the current work seeks to investigate these interactions, the errors associated with their measurement and gain understanding of the processes involved by careful examination of microstructural and mechanical property changes of surfaces subjected to erosion–corrosion. Cast nickel–aluminium bronze/NaCl solution has been chosen to study as this system has been studied at Southampton for several years. In situ electrochemical, gravimetric and topographical analyses have been evaluated and discussed using microstructural and hardness measurements.     

Contact and thermal analysis of transfer film covered real composite-steel surfaces in sliding contact

For composite-steel surfaces in sliding contact an anisotropic numerical contact algorithm has been developed to study the ‘layer type’ problems. An FE contact analysis was applied to evaluate the contact parameters (real contact area, contact pressure distribution and normal approach). The contact temperature rise was determined by using both a numerical thermal algorithm for stationary and a FE transient thermal technique for ‘fast sliding’ problems.The effect of a continuous transfer film layer (TFL), that had built up during wear of the PEEK matrix material on the steel counterpart, was considered. Its thickness was assumed to be t=1 μm, and its material properties were that of PEEK at room temperature or, in the case of frictional heating, at a temperature of 150°C (i.e. above the glass transition temperature of the polymer matrix).Results are presented for a spherical steel asperity, with/without TFL, sliding over composite surfaces of different fibre orientation, and in addition, for real composite-steel surfaces (based on measured surface roughness data) in sliding contact. The TFL has an effect on the contact parameters especially at higher operating temperatures (i.e. 150°C); it results in the production of a larger contact area and a lower contact pressure distribution. The contact temperature rise is clearly higher if a TFL is present. Due to the low thermal conductivity of PEEK, the TFL is close to the melting state or it even gets molten within a small vicinity of the contact area.     

USING FUZZY LINGUISTICS TO SELECT OPTIMUM MAINTENANCE AND CONDITION MONITORING STRATEGIES

Continued pressure on companies to reduce costs and improve customer satisfaction has resulted in increasingly detailed examinations of maintenance practices and strategies. The justification of any given maintenance strategy or practice within an organisation must consider multiple criteria. It should also be based on the overall objectives of the organisation, many of which are ‘intangible’ or ‘non-monetary’. A fuzzy linguistic approach to achieve the inclusion of somewhat subjective assessments of maintenance strategies and practices in an objective manner is outlined in this paper. This approach is also demonstrated with two examples. Implementation of this approach will assist decision makers in the evaluation and selection of maintenance strategies and particular condition-monitoring techniques.     

A ‘3-body’ abrasion wear study of bioceramics for total hip joint replacements

The current study focuses on the effect of the material type and the lubricant on the abrasive wear behaviour of two important commercially available ceramic on ceramic prosthetic systems, namely, Biolox^® forte and Biolox^® delta (CeramTec AG, Germany). A standard microabrasion wear apparatus was used to produce ‘3-body’ abrasive wear scars with three different lubricants: ultrapure water, 25 vol% new-born calf serum solution and 1 wt% carboxymethyl cellulose sodium salt (CMC-Na) solution. 1 μm alumina particles were used as the abrasive. The morphology of the wear scar was examined in detail using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Subsurface damage accumulation was investigated by Focused Ion Beam (FIB) cross-sectional milling and Transmission Electron Microscopy (TEM). The effect of the lubricant on the ‘3-body’ abrasive wear mechanisms is discussed and the effect of material properties compared.     

Effect of friction cycles on wear particle generation of carbon nitride coating against a spherical diamond

The in-situ observations of wear particle generation of carbon nitride coating on silicon repeatedly sliding against a spherical diamond have been studied in terms of the critical friction cycles and normal loads. An environmental scanning electron microscope (E-SEM), in which a pin-on-disk tribotester was installed, has in-situ provided direct evidence of when and how the wear particle generation do occur during the repeated sliding of carbon nitride coating against a spherical diamond. The in-situ observations of non-conductive carbon nitride coating are therefore available free from surface charging with controllable relative humidity. The repeated sliding tests at a sliding speed of 50 μm/s have been carried out with the purpose of observing the ‘No wear particle generation’ region when varying normal load from 10 to 250 mN. It appears that until 20 friction cycles, the maximum Hertzian contact pressure P_max for ‘No wear particle generation’ can be improved from 1.39 Y to 1.53 Y if silicon is coated by carbon nitride with a thickness of 10 nm, where Y is defined as the yield strength of silicon. The applicable enlargement of the ‘No wear particle generation’ region of carbon nitride coating has therefore been comparatively discussed with the silicon substrate from the view points of the friction coefficient and the specific wear rate. The mode transition maps have also been summarized for the repeated sliding of carbon nitride coating in terms of ‘No wear particle generation’, ‘Wear particle generation by microcutting’ and ‘Wear particle generation by microcutting and microfracturing’ three typical modes.     

An incremental formulation of hereditary constitutive laws applied to uniaxial problems

The paper describes an incremental formulation of hereditary constitutive laws for uniaxial stress states which admits convenient ‘exact’ treatment of this type of constitutive relation for linear and nonlinear spring/dashpot models. The formulation and solution technique introduced here are applied to the detailed analysis of imperfect viscoelastic columns made of ice or concrete.The difference between such an ‘exact’ treatment and solutions based on approximate constitutive laws, in which hereditary effects are neglected, is analyzed. It is shown that the apparent increased material rigidity associated with such a constitutive approximation, referred to as Shanley's hypothesis, can significantly affect the behaviour of columns made of this type of material and may even lead to totally erroneous conclusions.     

Modelling one-dimensional unsteady flows in ducts: Symmetric finite difference schemes versus galerkin discontinuous finite element methods

The Euler equations for one-dimensional unsteady flows in ducts have been solved resorting to classical symmetric shock-capturing methods with second-order accuracy and to the recent discontinuous Galerkin finite-element method, with second- and third-order accuracy. In particular, the finite difference techniques adopted are the two-step Lax—Wendroff method and the MacCormack predictor—corrector method, with the addition of the flux corrected transport (FCT) or of the Davis nonupwind TVD scheme to suppress the spurious oscillations in the vicinity of discontinuous solutions. The finite-element method adopted is based on the weak formulation of the Euler equations, which are solved by introducing a discontinuous finite-element space discretization. A dissipative mechanism has been considered to supplement the FEM with a “discontinuity capturing” operator, adding a “viscous like” term to damp minor numerical overshoots arising in proximity of steep gradients of the solution. The numerical tests chosen to carry out a comparison between these schemes are the shock-tube problem and the shock—turbulence interaction problem. Both the test cases considered show the superiority of third-order FEM calculations, whereas the comparison between the computer run times points out the greater computational effort required.     

Classical, local buckling of tubes having rectangular cross-sections

The stress level for local buckling of a general rectangular elastic tube, loaded in axial compression, has been worked out by Bulson [The Stability of Flat Plates. Chatto and Windus, London (1970)] according to classical bifurcation theory, for a range of values of and . Bulson's results are re-examined with a view to understanding them in simple, physical terms. Computer graphics are used to replot these results in various ways according to different hypothetical schemes of explanation. It is found that most of them can be reproduced to a satisfactory first approximation by considering one set of plates as ‘active’ and the other as ‘passive’; only in a relatively narrow ‘transition region’ does this scheme break down. Finally some simple, rational approximate formulae are presented. The motivation for this study is the thought that non-linear interactive buckling of tubular and other thin-walled members—which is an important practical problem—may be understood more readily if the criterion for classical buckling can be expressed economically.     

Numerical validation of the new track equations for static problems

This paper presents a numerical validation of the new equations for the rail-tie structure, derived recently by Kerr and Zarembski [Acta Mech. 40, 253–276 (1981)], for the track response in the lateral plane. Equations of this type are needed for the analysis of track buckling and lateral track dynamics. This is achieved by comparing the results based on the new equations with those of a finite-element analysis, for a 24 m (80 ft) long rail-tie structure. The comparison revealed very close agreement for the lateral deflections and bending moments, for a wide range of rail and tie spacings currently in use, which includes narrow and wide gauge tracks. The effect of the boundaries, where the rail-tie structure experiences a discontinuity, was found to be negligibly small. The findings show that the new equations are suitable for the analysis of cross-tie tracks in the lateral plane. They also confirm that, for tracks with stiff fasteners, the use of the standard beam bending equation with a ‘substitute’ track bending stiffness is of questionable validity.     

Physical nature and properties of dynamic surface layers in friction

Simulations of the dynamic processes in micro contacts with the Method of Movable Cellular Automata (MCA) show that their common feature is formation of a boundary layer where intensive plastic deformation and mixing processes occur. The boundary layer is well localized and does not spread to deeper layers. We call this layer a ‘quasi-fluid layer’. The thickness of the boundary layer is roughly proportional to the viscosity of solid. This parameter thus should play an important role in determining the wear rate of materials in friction.To better understand the physical nature of the dynamic surface layers, we consider a simplified model of a solid consisting of many thin sheets, interacting with each other according to a ‘friction law’ of Coulomb type. A quasi-fluid layer is always developing if the ‘friction law’ does allow a bi-stability in some range of stresses with one static and one dynamic state at the same stress.The existence of the boundary layer motivates us to change the existing approach to calculating wear in frictional contacts. The wear should be understood not as ‘fracture’ but as ‘mass transport out of friction zone’. The process of stochastic transport of wear particles in the closed friction zone is at the same time the main mechanism of development of surface topography.A very important fact is that the conditions for appearance of a quasi-fluid layer depend on the minimal size of structural elements of the medium, which means that this effect cannot be principally described in the frame of a continuum model.     

Time accurate finite difference method for performance prediction of a silencer with mean flow and nonlinear incident wave

Transmission losses of various reactive silencers are predicted, using a time accurate finite difference method. The numerical scheme is the 3rd order upwind scheme for axisymmetric Euler equations. Main advantage of the present method is that it can simulate linear and nonlinear wave propagation phenomena in a flow field directly with minimum numerical oscillation errors. The special treatments of incident wave condition, i.e. multiple harmonics of the transparent acoustic condition are applied to the transmission loss prediction for calculation efficiency. For the validation of the present approach, circular expansion chamber silencers without mean flow and an exponential pipe with mean flow are simulated in case of linear incident wave. The computed transmission losses have quite good agreements with those of the others. The nonlinear incident wave case is also investigated to check the usefulness of this method. The periodicN wave is clearly captured without numerical oscillation errors, and the insertion losses of two different incident frequencies are compared.     

Derivation of dynamic couple-stress Reynold’s equation of sliding-squeezing surfaces and numerical solution of plane inclined slider bearings

Deriving a general dynamic Reynold’s equation of sliding-squeezing surfaces with non-Newtonian fluids is necessary for the assessment of dynamic characteristics of a lubricating system. Taking into account the transient squeezing-action effect and considering the effects of couple stresses resulting from the lubricant blended with various additives, the non-Newtonian dynamic Reynold’s equation applicable to the general film shape is derived by using the Stokes micro-continuum theory. As an application, the numerical analysis of a two-dimensional plane inclined slider bearing is illustrated. Based upon the small perturbation technique, two Reynold’s-type equations responsible for both the steady performance and the perturbed characteristics are obtained. The steady and perturbed pressures are then numerically calculated by using the conjugate gradient method. From the results obtained, the effects of couple stresses provide an improvment on both the steady-state performance and the dynamic stiffness and damping characteristics especially for the bearing with a higher value of aspect ratio.     

Wear particle analysis—utilization of quantitative computer image analysis: A review

This paper provides a general overview of developments and progress in quantitative computer image analysis as applied to wear particle identification/classification technology, over the last two decades. Since many technical disciplines are involved in this ‘infant-stage’ technical area, an attempt is made to put into perspective mechanical failure prediction/diagnosis and prevention through quantitative wear particle morphological analysis. The problems experienced with applying conventional wear particle analysis methods in machinery condition monitoring, notably the employment of wear debris morphological diagnostic systems, revealed that it is not prudent to rely solely on human interpretation in the analysis of ‘filtergram’ slides. This has highlighted the need for improving the provision of ‘intelligent’ objective methods for performing this type of analysis. In this paper, some of the developments reported in the literature relating to progress made with wear particle image analysis are reported and examined as a basis for establishing improved methods of diagnostic analysis.     

Stability characteristics of gas-lubricated bearings supported on rubber ‘O’ rings

A simplified design method is given against instability of an externally pressurized gas journal bearing supported on rubber ‘O’ rings. The stability characteristics (onset whirl speed and whirl ratio) are derived and expressed in closed form in terms of ‘O’ ring material properties and fluid film behaviour.     

Analysis of spread in ring rolling

Hill's general method of analysis (variational procedure), combining flexibility with rigorous principles, is applied to predict spread during plain ring rolling. The use of ‘dual stream functions’ in selecting a single approximating velocity field from a ‘sufficiently wide’ orthogonalizing family is demonstrated.