Contact Mechanics

The current study investigates the characteristics of particles generated from the wear of braking materials, and provides an applicable index for measuring and comparing wear particle emissions. A pin-on-disc tribometer equipped with particle measurement instruments was used. The number concentration, size, morphology, and mass concentration of generated particles were investigated and reported for particles 10 nm-32 m in diameter. The particles were also collected on filters and investigated using EDS and SEM. The effects of wear mechanisms on particle morphology and changes in particle concentration are discussed. A new index, the airborne wear particle emission rate (AWPER), is suggested that could be used in legislation to control non-exhaust emissions from transport modes, particularly rail transport.

Wire electro discharge machining (WEDM) has become one of the most popular processes for producing precise geometries in hard materials, such as those used in the tooling industry. The so-called tapercutting involves the generation of inclined ruled surfaces, and it is especially important in the manufacturing of tooling requiring draft angles. In this paper a new approach to the prediction of angular error in wire-EDM taper-cutting is presented. A systematic analysis of the influence of process parameters on angular error is carried out using Design of Experiments (DoE) techniques. A quadratic equation for the prediction of angular error that takes into account electrical parameters and part geometry is derived. Validation results reveal a dominant influence of the mechanical behaviour of the wire, rather than that of EDM regime. Following this assertion an original finite element model (FEM) to describe the mechanical behaviour of soft wires, typically used in taper-cutting operations, has been developed taking into account non-linear phenomena such as contact mechanics, plastic behaviour, stress-stiffening and large displacements. Both the results of DoE techniques and FEM simulation have been validated through experimental tests in industrial conditions. Deviations of accuracy of WEDMtapered parts can be reduced to values below 4 0 using these models.

The aim of this article is to give a detailed account of the plane harmonic generalized elasto-thermodiffusive (ETNP) waves in semiconductive materials. The shear (purely transverse) waves get decoupled from the rest of the motion and remain independent of the influence of other fields. These waves propagate without dispersion and attenuation in semiconductors. The coupled system of partial differential equations, governing the rest of the interacting fields, has been solved to obtain a complex secular equation. According to the frequency equation, four coupled longitudinal waves, namely, the quasithermoelastic (QTE), quasielastodiffusive (QEN/QEP), quasithermodiffusive (QTN/QTP), and quasithermal (T-mode), can exist and propagate in an infinite semiconductor. The complex secular equation of plane harmonic waves in semiconductors is solved by using Descartes' algorithm and the irreducible case of Cardan's method in order to obtain phase velocities and attenuation coefficients of all possible coupled waves. The thermoelastic (ET), elastodiffusive (EN/EP) and thermodiffusive (TN/TP) waves have also been investigated as special cases. The derived theoretical results have been illustrated and verified numerically for germanium (Ge) and silicon (Si) semiconductors. The computed phase velocity and attenuation profiles have been presented graphically.

Contact mechanics is a fundamental field of tribology and generally refers to the interaction of solid surfaces. This interaction or contact can occur on many different scales, ranging from nanoscale asperities up to tires on roads and even contact between tectonic plates. This chapter reviews the basic technical information available in predicting the contact area, pressure, stresses, and forces that occur when surfaces interact. The chapter considers different geometries such as spheres and wavy surfaces and also outlines how to consider elastic and plastic deformation. The phenomena of creep and adhesion that are important for many tribological applications, and especially biological contacts, are also discussed. Finally, the chapter concludes by covering methods used to model the complicated situation of contact between rough surfaces that contain many different geometrical features.

This paper introduces a new computer program called WEAR. It contains a new and more accurate method for calculating wheel–rail contact stresses to predict degradation due to wear, deformation and fatigue. WEAR accounts for conformal contact by using influence numbers that are based on quasi-quarter spaces instead of the traditional half-spaces and considers the varying geometrical spin due to the varying contact angle through a contact patch. This new contact method is applied to two cases: wheel–rail contact in a turnout and conformal contact in a curve with a heavily worn wheel profile. In both cases, many of the assumptions commonly made in excising solution methods for wheel–rail contact problems, such as a small contact patch and a constant spin creepage, may be violated. The case of conformal contact demonstrates the effect of the influence numbers and the varying spin creepage on the resulting stresses and creep forces and provides a comparison of this new method with the well-established contact method CONTACT. For the turnout case, the first task is to obtain realistic input for all timesteps of a vehicle coasting through a turnout (i.e., diverging route) in a simulation. This step is necessary because the contact forces, which cause wear, deformation and fatigue of the wheels and rails, and the dynamics of the vehicle-turnout interaction strongly influence each other. WEAR converged for all timesteps, including many cases with multiple-point contacts at the switch blade and with extremely high creepages. This robustness demonstrates suitability of the new method for online contact force evaluation in vehicle dynamics simulations.

Modeling forces applied to scissors during cutting of biological materials is useful for surgical simulation. Previous approaches to haptic display of scissor cutting are based on recording and replaying measured data. This paper presents an analytical model based on the concepts of contact mechanics and fracture mechanics to calculate forces applied to scissors during cutting of a slab of material. The model considers the process of cutting as a sequence of deformation and fracture phases. During deformation phases, forces applied to the scissors are calculated from a torque-angle response model synthesized from measurement data multiplied by a ratio that depends on the position of the cutting crack edge and the curve of the blades. Using the principle of conservation of energy, the forces of fracture are related to the fracture toughness of the material and the geometry of the blades of the scissors. The forces applied to scissors generally include high-frequency fluctuations. We show that the analytical model accurately predicts the average applied force. The cutting model is computationally efficient, so it can be used for real-time computations such as haptic rendering. Experimental results from cutting samples of paper, plastic, cloth, and chicken skin confirm the model, and the model is rendered in a haptic virtual environment.

This paper presents a study of the load-deformation behavior of geocell-stabilized subballast subjected to cyclic loading using a novel track process simulation apparatus. The tests were conducted at frequencies varying from 10 to 30 Hz. This frequency range is generally representative of Australian standard gauge trains operating up to 160 km=h. The discrete-element method (DEM) was also used to model geocell-reinforced subballast under plane strain conditions. The geocell was modeled by connecting a group of small circular balls together to form the desired geometry and aperture using contact and parallel bonds. Tensile and bending tests were carried out to calibrate the model parameters adopted for simulating the geocell. To model irregularly shaped particles of subballast, clusters of bonded circular balls were used. The simulated load-deformation curves of the geocell-reinforced subballast assembly at varying cyclic load cycles were in good agreement with the experimental observations. The results indicated that the geocell decreased the vertical and lateral deformation of subballast assemblies at any given frequency. Furthermore, the DEM can also provide insight into the distribution of contact force chains, and average contact normal and shear force distributions, which cannot be determined experimentally.

Durbi Takusheyi is a burial site composed of at least eight mounds located between the modern towns of Katsina and Daura in northern Nigeria. Parts of the mounds were first excavated in 1907 by Herbert Richmond Palmer in cooperation with the Emir of Katsina and later again in 1992 in the course of a German research project under the lead of Dierk Lange, Bayreuth. After the 1992 excavation, the retained blocks were stored in the Jos Museum, Nigeria, for further analyses. In 2007 the Römisch-Germanisches Zentralmuseum (RGZM) and the National Commission for Museums and Monuments in Nigeria (NCMM) started a project with the objective of completely restoring and analyzing the excavated artefacts.While the remains of the first excavation appear to be lost with only minimal information preserved, the three mounds excavated in 1992 each contained a single interment in the centre of the mound, all three with various burial goods produced from inorganic (metal, glass, stone, cowries) and organic material (cloth, wood, hides). Many artefacts are of regional provenance but some were also imported from distant regions of the Islamic world. Following the currently available radiocarbon measurements, one group of the burials would date to the earlier fourteenth century AD, and judging from typology and art history another burial dates to the later fifteenth/early sixteenth centuries. The site thus covers a crucial phase in history during which the Hausa city states emerged, indicating shifting contacts to the Mediterranean and to the Middle East.Durbi Takusheyi est un site funéraire constitué d'au moins huit buttes, situé entre les villes moderns de Daura et Katsina an nord du Nigeria. Une partie des buttes furent fouillées en 1907 par Herbert Richmond Palmer, en collaboration avec l’émir du Katsina, puis en 1992 au cours d'un projet de recherche allemand dirigé par Dier Lange, Bayreuth. À la suite des fouilles de 1992, des blocs de matériel furent déposés dans les remises du musée de Jos (Nigeria) dans l'attente d'analyses futures. En 2007 le Römisch-Germanisches Zentralmuseum (RGZM) et la National Commission for Museums and Monuments du Nigeria (NCMM) débutèrent un projet dont l'objectif est d'analyser et de restaurer totalement les artéfacts issus des fouilles.Les objets issus des fouilles de 1907 semblent avoir disparu, et seul le minimum d'informations subsiste. Pour ce qui est des fouilles de 1992, cependant, on sait que chacune des trois buttes fouillées contenait une sépulture en son centre, avec un mobilier divers constitué de matériaux inorganiques (métal, verre, pierre, cauris) et organiques (textiles, bois, peaux). Beaucoup des objets sont de provenance locale, mais d'autres furent importés de régions distantes du monde islamique. À la base des datations radiocarbon dont on dispose, il semble que l'un des groupes de sépultures date du début du XIVème siècle ap. J-C, et les données typologiques et d'histoire de l'art indiquent qu'une autre inhumation date de la fin du XVème/début XVIème. Le site date ainsi d'une période cruciale de l'histoire, durant laquelle émergèrent les cites-état hausa, indiquant des contacts changeants avec la Méditerranée et le Moyen-Orient.