Stress Tensor and Gradient of Hydrostatic Pressure in the Half-Space Beneath Axisymmetric Bodies in Normal and Tangential Contact

The stress state in the volume of contacting bodies may essentially influence the material behavior. For evaluating various modes of inelastic behavior and/or failure, such as plastic deformation, crack initiation, and propagation or fatigue, the complete stress tensor beneath the contact interface may be of importance. For many geotechnical and biomechanical applications, the hydrostatic pressure gradient beneath the contact is of interest as well. However, most theories for normal and tangential contact provide only few stress components in the contact surface. In the present paper, we show that the full stress state in the half-space can be easily found for axisymmetric bodies. We provide expressions in form of one-dimensional integrals for all components of the stress tensor and the hydrostatic pressure gradient inside the half-space. In terms of numerical complexity, the proposed method can be advantageous to other elaborate methods.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität Berli

A RIGOROUS MODEL FOR FREQUENCY-DEPENDENT FINGERPAD FRICTION UNDER ELECTROADHESION

In the electroadhesive frictional contact of a sliding fingerpad on a touchscreen, friction is enhanced by an induced electroadhesive force. This force is dominated by the frequency-dependent impedance behavior of the relevant electrical layers. However, many existing models are only valid at frequency extremes and use very simplified contact mechanical approaches. In the present paper, a RC impedance model is adopted to characterize the behavior in the relevant range of frequencies of the AC excitation voltage. It serves as an extension to the macroscopic model for electrovibration recently developed by the authors, which is based on several well-founded approaches from contact mechanics. The predictions of the extended model are compared to recent experimental results and the most influential electrical and mechanical parameters are identified and discussed. Finally, the time responses to different wave forms of the excitation voltage are presented

Epiphanie des Augenblicks. Zeit im Werk Thomas Glavinics

Les trois romans Die Arbeit der Nacht, Das Leben der Wünsche et Das größere Wunder de l’écrivain autrichien Thomas Glavinic traitent de la vie et de l’amour du narrateur Jonas. Ils ne sont pas liés par une continuité de l’action, mais par des sujets communs. Dans les textes – c’est la thèse de ce travail – la perception du temps et le temps lui-même jouent un rôle important. Le narrateur se trouve dans des situations extrêmes et il réfléchit entre autres sur la possibilité de saisir le temps de manière durable par des signes propre à la médialité et par la perception humaine. Il s’agit d’une épiphanie de la perception de l’instant. Dans ces romans, les instants liés au bonheur et à l’amour ne peuvent être ressentis que dans l’instant même. Ainsi les textes de Glavinic peuvent être lus comme un plaidoyer pour la perception consciente et attentive des instants.In drei Romanen Die Arbeit der Nacht, Das Leben der Wünsche und Das größere Wunder des österreichischen Romanciers Thomas Glavinic (*1972), wird das Leben und die Liebe des Erzählers Jonas beschrieben. Die Verbindung zwischen den Romanen ist vor allem in ihren gemeinsamen Sujets zu sehen und nicht in einer fortlaufenden Handlung. In den Texten – so die These meines Beitrags – spielt die Wahrnehmung von Zeit und Zeit an sich eine wichtige Rolle. Der Erzähler ist Extremsituationen ausgesetzt und reflektiert darin unter anderem die Möglichkeiten Zeit auf medialen Zeichenträgern und mittels der menschlichen Wahrnehmung dauerhaft rezipierbar zu machen. Es wird eine Epiphanie des Augenblicks und der Augenblickswahrnehmung betrieben. Denn Augenblicke sind in den drei Romanen mit Glück und Liebe verknüpft und diese können nur in den Augenblicken selbst empfunden werden. So sind Glavinics Texte als ein Plädoyer der bewussten Wahrnehmung von Augenblicken im Leben lesbar.The novels entitled Die Arbeit der Nacht, Das Leben der Wünsche and Das größere Wunder by Austrian writer Thomas Glavinic (*1972) deal with the life and love story of the narrator, Jonas. They tackle the same questions but their plots loosely connected. This article argues that time and the perception of time are very important. The narrator is going through ordeals and he ponders the possibility of controlling the passing of time either through mediality or human perception. Capturing the instant becomes an epiphany. In these novels, moments in time are connected with the metathemes of happiness and love, which can only be experienced at a given moment. So Glavinic’s novels can be read as a plea for paying attention to the value of instants of time

An Analytical Model for Almost Conformal Spherical Contact Problems: Application to Total Hip Arthroplasty with UHMWPE Liner

Due to its high relevance for designing ball joints in mechanical engineering and (artificial) hip joints in biomechanics, the almost conformal elastic contact between a sphere and a spherical cup represents an important contact problem of current research. As no closed-form analytical solution to the problem has been found to date, full computational methods such as the finite element method are needed for analysis. However, they often require incredibly long, unacceptable calculation times, making parameter studies hardly practicable. For this reason, approximate analytical and semi-analytical models are applied, capable of predicting quantities of interest with sufficient accuracy. In the present work, a very simple model based on a radially directed Winkler foundation is presented, which provides (approximate) closed-form analytical solutions for both the pressure distribution and the dependencies between macroscopic contact quantities such as normal force and indentation depth. To ensure an optimal mapping of a specific contact problem, only the foundation modulus must be defined in a suitable way. As an example, the proposed model has been successfully adapted to adequately simulate the frictionless normal contact for hard-on-soft hip implants. For this purpose, the foundation modulus was approximated with the aid of a finite element analysis instead of adopting it from already well-established models, as the latter produce clearly erroneous results for large liner thicknesses and large Poisson’s ratios. By a comparison with extensive parameter studies of finite element simulations, it is demonstrated that the proposed model provides acceptable results for all commonly used hard-on-soft hip implants. On this basis, the influence of geometrical changes of the femoral head and the acetabular cup on the maximum pressure as well as the half-contact angle is discussed, and consequences on the wear behavior are deduced.DFG, 414044773, Open Access Publizieren 2021 - 2022 / Technische Universität Berli

Macroscopic Modeling of Fingerpad Friction Under Electroadhesion: Possibilities and Limitations

Electrovibration is one of the key technologies in surface haptics. By inducing controlled electrostatic forces, the friction within a sliding contact between the human finger and a capacitive screen is modulated, which in turn gives effective tactile feedback to the user. Such powerful haptic displays can be built into mobile phones, tablets, navigation devices, games consoles and many other devices of consumer electronics. However, due to the layered structure and complex material of human skin, the underlying contact mechanical processes have not yet been fully understood. This work provides new continuum-based approaches to macroscopic modeling of the electro-adhesive frictional contact. A solution of pure normal contact between a human finger and a rigid, smooth plane under electroadhesion is derived by applying Shull's compliance method in the extended regime of large deformations. Based on these results and assuming pressure-controlled friction, a model for the sliding electro-adhesive contact is developed, which adequately predicts the friction force and coefficient of friction over the whole range of relevant voltages and applied normal forces. The experimentally observed area reduction caused by the tangential force is incorporated in a more empirical than profound contact mechanical way. This effect is studied with the help of a two-dimensional finite element model of the fingertip, assuming non-linear elastic material for the skin tissue. Although the simulations are restricted to non-adhesive tangential contacts, they show a significant reduction of the contact area, which is caused by large deformations of the non-linear elastic material around the distal phalanx. This result indicates that adhesion is only of secondary importance for the area reduction.TU Berlin, Open-Access-Mittel – 202

Wear Analysis of a Heterogeneous Annular Cylinder

Wear of a cylindrical punch composed by two different materials alternatively distributed in annular forms is studied with the method of dimensionality reduction (MDR). The changes in surface topography and pressure distribution during the wear process is obtained and validated by the boundary element method (BEM). The pressure in each annular ring approaches a constant in a stationary state where the surface topography does not change any more. Furthermore, in an easier manner, using direct integration, the limiting profile in a steady wear state is theoretically calculated, as well as the root mean square (RMS) of its surface gradient, which is closely related to the coefficient of friction between this kind of surface and an elastomer. The dependence on the wear coefficients and the width of the annular areas of two phases is obtained

Hannah Montana como modelo aspiración de valores y consumo para niñas mexicanas: un análisis cualitativo en el noreste de México

Este trabajo reporta los resultados de una investigación de corte cualitativo que tuvo como objetivo identificar como se genera el proceso de recepción televisiva Hannan Montana y la asimilación de costumbres, actitudes y estilos de vida del personaje principal por parte de niñas de 6 a 12 años

NUMERICAL IMPLEMENTATION OF FRETTING WEAR IN THE FRAMEWORK OF THE MDR

Two numerical methods are proposed to improve accuracy of the numerical calculation of fretting wear in the framework of the Method of Dimensionality Reduction (MDR). Due to the singularity of the transformation equations, instabilities appear at the border between the stick and slip regions after many transformations from the one-dimensional to the three-dimensional contact and back. In these two methods, the transformation equations are reformulated to weaken the singularity of the integrals and a stable simulation of fretting wear is realized even with the wear models which go beyond the classical Archard law. With an example of dual-oscillation, we show the change in the worn profile of a parabolic indenter as well as the stress distribution on the contacting surface during the oscillating cycles under the Archard’s law of wear and Coulomb’s law of friction