Effect Of The Different Loads Onto The Friction And Stick-Slip Of Polyamide Composites

Our present work is connected to a research project, which is based on PA6, POM, PET and PEEK polymer matrix and aimed at making a map of the features of tribology. Our work analyses the friction and connection with this, and the stick-slip behaviour of polyamide composites with different load relations. The friction tests are prepared in alternating (Reciprocating) cylinder/plane model system, developed in both the dynamic and the static friction force which is continuously measured. This special tribology test system is extremely suitable to investigate among other things the stick-slip phenomenon also. You can find detailed presentation in our earlier articles. Expected results from our research may serve in a context the effect of the different loads have on a static and dynamic friction. Our research shows the effects of friction on different levels and susceptibility of the tested materials in this direction in the stick-slip PA6 and PA66 composites

Shear localization in a model glass

Using molecular dynamics simulations, we show that a simple model of a glassy material exhibits the shear localization phenomenon observed in many complex fluids. At low shear rates, the system separates into a fluidized shear-band and an unsheared part. The two bands are characterized by a very different dynamics probed by a local intermediate scattering function. Furthermore, a stick-slip motion is observed at very small shear rates. Our results, which open the possibility of exploring complex rheological behavior using simulations, are compared to recent experiments on various soft glasses.Comment: 4 pages, 4 figures (5 figure files

First-passage time of Brownian motion with dry friction

We provide an analytic solution to the first-passage time (FPT) problem of a piecewise-smooth stochastic model, namely Brownian motion with dry friction, using two different but closely related approaches which are based on eigenfunction decompositions on the one hand and on the backward Kolmogorov equation on the other. For the simple case containing only dry friction, a phase transition phenomenon in the spectrum is found which relates to the position of the exit point, and which affects the tail of the FPT distribution. For the model containing as well a driving force and viscous friction the impact of the corresponding stick-slip transition and of the transition to ballistic exit is evaluated quantitatively. The proposed model is one of the very few cases where FPT properties are accessible by analytical means

On the effect of the wall slip boundary conditon

This work describes the implementation and assessment of the wall slip boundary condition in a 3D numerical modelling code, based on the finite volume method, that is being developed by the research team. Several phenomenological models relating the velocity and the shear stress at the wall were implemented. The capabilities of the new numerical code are illustrated with three case studies where the wall slip boundary conditions play an important role, namely the flow in a smooth contraction, the stick-slip phenomenon an the flow in a profile extrusion die. The results obtained are qualitatively in accordance with the theoretical expectations and evidence the importance of wall slip

Nonlinear vibration in powertrain systems induced by friction couplings

University of Technology, Sydney. Faculty of Engineering and Information Technology.The friction related noise and vibrations are concerned by all researchers and engineers in the automotive field. These vibrations widely exist within brakes, clutch discs and bearings affecting the vehicles’ dynamic performance. These friction caused vibrations can even undermine the vehicle safeness. Vehicle powertrain system is treated as a multi-body system combining the transmission characteristics (transmission gear ratio, gear backlash and shifting process). The model and methodology has been developed to perform both linear and transient analysis to identify its dynamics. The focus of this thesis is on the friction induced vibration within vehicle powertrain systems. These vibrations are commonly non-linear and less researched before. The stick-slip behaviour within brake and rotor sub-system is studied through a piecewise linear function. The system dynamics and its impact is studied. Various stick-slip motions are found based on carefully designed criteria. Simulation and analytical results both are used to verify each other. One-Way clutch is another friction mechanism widely used in mechanical systems. A thorough study is carried out in this thesis to investigate the dynamic responses of systems with OWC. Numerical and analytical results prove that, as OWC is not always engaged, and its transition introduces new transients into system dynamics. The experimental works are carried out on UTS powertrain test rig. The stick-slip phenomenon between driving tires and flywheels are visually verified from the observed noise and wear on the driving tires

Numerical and experimental studies of stick-slip oscillations in drill-strings.

The cyclic nature of the stick-slip phenomenon may cause catastrophic failures in drill-strings or at the very least could lead to the wear of expensive equipment. Therefore, it is important to study the drilling parameters which can lead to stick-slip, in order to develop appropriate control methods for suppression. This paper studies the stick-slip oscillations encountered in drill-strings from both numerical and experimental points of view. The numerical part is carried out based on path-following methods for non-smooth dynamical systems, with a special focus on the multistability in drill-strings. Our analysis shows that, under a certain parameter window, the multistability can be used to steer the response of the drill-strings from a sticking equilibrium or stick-slip oscillation to an equilibrium with constant drill-bit rotation. In addition, a small-scale downhole drilling rig was implemented to conduct a parametric study of the stick-slip phenomenon. The parametric study involves the use of two flexible shafts with varying mechanical properties to observe the effects that would have on stick-slip during operation. Our experimental results demonstrate that varying some of the mechanical properties of the drill-string could in fact control the nature of stick-slip oscillations

Imaging high-speed friction at the nanometer scale

Friction is a complicated phenomenon involving nonlinear dynamics at different length and time scales[1, 2]. The microscopic origin of friction is poorly understood, due in part to a lack of methods for measuring the force on a nanometer-scale asperity sliding at velocity of the order of cm/s.[3, 4] Despite enormous advance in experimental techniques[5], this combination of small length scale and high velocity remained illusive. Here we present a technique for rapidly measuring the frictional forces on a single asperity (an AFM tip) over a velocity range from zero to several cm/s. At each image pixel we obtain the velocity dependence of both conservative and dissipative forces, revealing the transition from stick-slip to a smooth sliding friction[1, 6]. We explain measurements on graphite using a modified Prandtl-Tomlinson model that takes into account the damped elastic deformation of the asperity. With its greatly improved force sensitivity and very small sliding amplitude, our method enables rapid and detailed surface mapping of the full velocity-dependence of frictional forces with less than 10~nm spatial resolution.Comment: 7 pages, 4 figure

Oblique Frictional Impact of a Bar: Analysis and Comparison of Different Impact Laws

In this paper a basic, easily to multi-contact problems extendable, non-smooth approach is applied to analyze a bar striking an inelastic half-space. Coulomb contact is assumed and modeled by using set-valued Newtonian impact laws in normal as well as in tangential direction. The resulting linear complementarity problem contains all possible impact states and provides an instantaneous collision operator that respects all inequality constraints. This operator depends on the orientation of the bar and determines uniquely the post-impact velocities as functions of the pre-impact state. Different types of solutions may occur, including "stick'' and "slip''. In this context, stick and slip have to be understood as the two cases characterized by the tangential impulsive force as an element of either the set-valued or of the single-valued domain of the friction law. Depending on the choice of parameters, sign reversal of the tangential contact velocity is possible. For certain inertia properties and initial conditions, the collision operator yields an impact, even for initially vanishing normal contact velocity. This phenomenon is well known as the Painlevé paradox. The results obtained by this fully non-smooth rigid body approach are compared with those of other impact models, such as a lumped mass model with compliance elements, and a collision operator used for particle interactions in flow

Fracture and Friction: Stick-Slip Motion

We discuss the stick-slip motion of an elastic block sliding along a rigid substrate. We argue that for a given external shear stress this system shows a discontinuous nonequilibrium transition from a uniform stick state to uniform sliding at some critical stress which is nothing but the Griffith threshold for crack propagation. An inhomogeneous mode of sliding occurs, when the driving velocity is prescribed instead of the external stress. A transition to homogeneous sliding occurs at a critical velocity, which is related to the critical stress. We solve the elastic problem for a steady-state motion of a periodic stick-slip pattern and derive equations of motion for the tip and resticking end of the slip pulses. In the slip regions we use the linear viscous friction law and do not assume any intrinsic instabilities even at small sliding velocities. We find that, as in many other pattern forming system, the steady-state analysis itself does not select uniquely all the internal parameters of the pattern, especially the primary wavelength. Using some plausible analogy to first order phase transitions we discuss a ``soft'' selection mechanism. This allows to estimate internal parameters such as crack velocities, primary wavelength and relative fraction of the slip phase as function of the driving velocity. The relevance of our results to recent experiments is discussed.Comment: 12 pages, 7 figure