65 research outputs found
Relaxation Tribometry: A Generic Method to Identify the Nature of Contact Forces
Recent years have witnessed the development of so-called relaxation
tribometers, the free oscillation of which is altered by the presence of
frictional stresses within the contact. So far, analysis of such oscillations
has been restricted to the shape of their decaying envelope, to identify in
particular solid or viscous friction components. Here, we present a more
general expression of the forces possibly acting within the contact , and
retain six possible, physically relevant terms. Two of them, which had never
been proposed in the context of relaxation tribometry, only affect the
oscillation frequency, not the amplitude of the signal. We demonstrate that
each of those six terms has a unique signature in the time-evolution of the
oscillation, which allows efficient identification of their respective weights
in any experimental signal. We illustrate our methodology on a PDMS
sphere/glass plate torsional contact
Tuning of frictional properties in torsional contact by means of disk grading
The contact of two surfaces in relative rotating motion occurs in many practical applications, from
mechanical devices to human joints, displaying an intriguing interplay of effects at the onset of sliding due to
the axisymmetric stress distribution. Theoretical and numerical models have been developed for some typical
configurations, but work remains to be done to understand how to modify the emergent friction properties in
this configuration. In this paper, we extend the two-dimensional (2D) spring-block model to investigate friction
between surfaces in torsional contact. We investigate how the model describes the behavior of an elastic surface
slowly rotating over a rigid substrate, comparing results with analytical calculations based on energy conservation.
We show that an appropriate grading of the tribological properties of the surface can be used to avoid a
non-uniform transition to sliding due to the axisymmetric configuration
Plastic and tribological properties of polytetrafluoroethylene (PTFE) under conditions of high pressure and shear
We investigate experimentally the behavior of a thin sheet of polytetrafluoroethylene between a steel plate and a cylindrical steel indenter under combined action of high normal force and torsion. Under these actions, the polytetrafluoroethylene layer is partially squeezed out of the contact area. The thickness of the remaining layer is studied as function of the applied normal force, the torsion angle, and the radius of the indenter. We suggest a simple semi-empirical material model which describes both the process of squeezing out of the layer and the force of friction produced by the layer
2D Spring-block model to study the transition from static to kinetic friction of complex-micro-textured contact surfaces
The capability of complex micro-texturing technique for tuning the transition from static to kinetic friction is investigated based on a two-dimensional (2D) lattice spring block model. Results reveal that implementation of micro-texturing remarkably decreases the static friction coefficient even for a small amount of covering percentage, however this effect gets slight after covering percentage of about 10%. It is observed that elongation of micro-texturing cavities perpendicular to the sliding direction can improve its reducing effect on static friction coefficient. Furthermore, as simulations prove, using complex shapes of micro-texturing cavities with sharp vertexes slightly modifies the frictional response
Mechanics of the solid-state bonding under severe thermomechanical processes
Friction stir welding (FSW) has found increased applications in automotive and aerospace industries due to its advantages of solid-state bonding, no fusion and melting, and versatility in various working conditions and material combinations. The extent and quality of the solid-state bonding between workpieces in FSW is the ultimate outcome of their industrial applications. However, the relationship among processing parameters, material properties, and bonding extent and fidelity remains largely empirical, primarily because of the lack of the mechanistic understanding of (1) tool-workpiece frictional behavior, and (2) bonding formation and evolution.
In this dissertation, to study the underlying mechanism of tool-workpiece frictional behavior and bonding evolution at workpiece-workpiece interface during solid-state bonding process, firstly, a numerical model that take advantage of Coupled Eulerian Lagrangian (CEL) method is implemented to investigate the stick-slip behavior at tool-workpiece interface. An analytical model is also developed to correlate the stick-slip fraction to processing parameters such as the tool spin rate, and further to derive dimensionless functions for torque and heat generation rate predictions. These analyses provide the critical strain rate and temperature fields that are needed for the bonding analysis. Then, we note that the interfacial solid state bonding process under applied thermomechanical loading histories is a reverse process of the high temperature creep fracture of polycrystalline materials by grain boundary cavities, in this regard, a general modeling framework of bonding fraction evolution was derived, which directly depends on the stress, strain rate, and temperature fields near the interface. Finally, Based on the stick-slip contact analysis and the understanding of solid-state bonding mechanism, an approximate yet analytical solution has been developed to derive the bonding fraction field from the given processing, geometric, and material constitutive parameters, and the predicted ultimate bonding extent with respect to these parameters becomes a figure of merit for the study of processing window for industrial applications and design of the FSW process
Axisymmetry in Mechanical Engineering
The reprint is devoted to the phenomena associated with exact or approximate axial symmetry in different areas of technical physics and mechanical engineering science. How can the symmetry of the problem be used most efficiently for its analysis? Why is the symmetry broken or why is it still approximately retained? These and other questions are discussed based on systems from different fields of engineering
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