Vannina Linck

Finite Element Simulation of Dynamic Instabilities in Frictional Sliding Contact

This paper describes tools for numerical modelisation which enable the understanding of the appearance of the vibration of structure generated by the frictional contact between two bodies (the excitation source being friction). The dynamic finite element code PLASTD is used to reproduce transitory phenomena generated at the contact interface. This code includes contact and friction algorithms based upon the formulation of Lagrange multipliers. A numerical study of the dynamic response of a 2D mechanical model composed of a deformable plane in relative translation and unilateral contact with Coulomb friction with a rigid surface is presented. The steady sliding solution is generically unstable and leads to a dynamic response which leads to the generation of instabilities characterized by the appearance of sliding-sticking or sliding-sticking-separation waves. It is important to notice that those instabilities appear even with a constant friction coefficient. These simulations have permitted to obtain the local contact conditions (kinematics, tribological state, contact stresses, etc). The kinematics shows the existence of local impacts and sliding at high frequencies. Furthermore, local normal pressure is much higher than that expected for a smooth surface. Finally, a 3D simulation of braking is carried out. We focused on the vibration of the disc and the brake pad which caused noises due to the generation of interface instabilities.© 2003 ASME

Modeling the consequences of local kinematics of the first body on friction and on third body sources in wear

Abstract Wear can be considered as a third body flow that is definitely ejected from the contact. This implies a source flow (particle detachment from first bodies) and an internal flow (displacement of these particles in the contact surface). The formation and fragmentation involved in surface tribological transformation (STT) is the main origin of this source flow. Therefore, modeling wear, i.e. writing the equation of flow equilibrium, requires understanding the STT formation. In order to explain the source flow, the first step is to determine the phenomena responsible for particle detachment. To help this determination, a finite element simulation has been undertaken to model the friction contact between two perfectly smooth first bodies. These simulations have enabled the local contact conditions (kinematics, tribological state, contact stresses, etc.) to be obtained. The kinematics shows the existence of local impacts and local slidings at high frequencies moving through the contact surface. These local phenomena are responsible for much higher local normal and tangential stresses than those expected for a smooth surface. The repeated local impact (high normal stress) and local sliding (high tangential stress) on the contact interface can explain the particle detachment from the contact surface. An investigation on the influence of different parameters has shown the relative role of friction, speed and applied load on local contact conditions.

Finite element simulation of dynamic instabilities in frictional sliding contact.

This paper describes tools for numerical modelisation which enable the understanding of the appearance of the vibration of structure generated by the frictional contact between two bodies (the excitation source being friction). The dynamic finite element code PLASTD is used to reproduce transitory phenomena generated at the contact interface. This code includes contact and friction algorithms based upon the formulation of Lagrange multipliers. A numerical study of the dynamic response of a 2D mechanical model composed of a deformable plane in relative translation and unilateral contact with Coulomb friction with a rigid surface is presented. The steady sliding solution is generically unstable and leads to a dynamic response which leads to the generation of instabilities characterized by the appearance of sliding-sticking or sliding-sticking-separation waves. It is important to notice that those instabilities appear even with a constant friction coefficient. These simulations have permitted to obtain the local contact conditions (kinematics, tribological state, contact stresses, etc). The kinematics shows the existence of local impacts and sliding at high frequencies. Furthermore, local normal pressure is much higher than that expected for a smooth surface. Finally, a 3D simulation of braking is carried out. We focused on the vibration of the disc and the brake pad which caused noises due to the generation of interface instabilities.© 2003 ASME

Dry friction: influence of local dynamic aspect on contact pressure, kinematics and friction

Abstract When studying the contact with friction between two bodies with a non-negligible relative velocity, the steady state conditions do not enable a correct understanding of the transient effects to be obtained. In these problems it is important to take into account the dynamic aspect in order to have a better understanding of the phenomena involved during sliding with frication. The dynamic aspect of contact with friction has been the subject of several analytical and numerical studies. These studies have shown that stationary sliding between bodies can locally lead to an unstable response. These instabilities usually lead to a dynamic response characterized by the propagation on the contacting surface of sliding, sticking and separation areas (stick-slip-separation waves). The purpose of the present investigation is to model the contact with friction between a brake shoe and a wheel. The influence of the dynamic aspect on the tribological state of the instantaneous contact surfaces (sliding, sticking, separation) as well as the real contact pressure and the local kinematics of the contact surface has been studied. In fact, the knowledge of the local dynamic enable a better understanding of the mechanism responsible for wear and can explain the heat generation in the contact. Since it is really difficult experimentally to understand the influence of the material properties such as Youngs modulus and the Poissons coefficient on the phenomena occurring during sliding with friction, this paper also studies the influence of Youngs modulus and Poissons coefficient on the local dynamic. Simulations have revealed that there is a Youngs modulus limit and a limit Poissons coefficient limit from which the contact surface reaches a stable and rather large displacement cycle.

Influence of Contact Geometry and Third Body on Squeal Initiation: Experimental and Numerical Studies

Brake squeal is an everyday problem, so it has been widely studied, mostly in vibration parlance studying modes, noise energy and frequencies. We propose to study the phenomenon from its origin — i.e. the contact — in a tribological approach that combines contact geometry, third body aspect and squealing initiation: thus, a temporal analysis was necessary. Experimental and numerical approaches have been used to identify influential parameters and they give similar results: a really strong correlation exists between the disk geometry and the squeal initiation.Copyright

Finite Element Analysis of a Contact With Friction Between an Elastic Body and a Thin Soft Layer

When studying a mechanical system involving contact between two bodies such as a disc and brake pad system, finite element simulations are often used to predict the phenomena involved. However, due to model size and calculation time problems, when modeling this type of mechanical system on a scale of about 100 mm, it is difficult to model as well a layer (for example a third body layer) on a scale of approximately 10 pm. In quasi-static problems it is possible to simulate the contact between an elastic body and a thin elastic layer bonded to a rigid surface, by considering the contact between this elastic body and a rigid surface with a specific contact law. This paper shows that it is possible to implement this specific contact law in a dynamic finite element code to simulate thin layers undergoing quasi-static and dynamic problems without physical contact instabilities. This specific contact law saves a large amount of calculation time. Once the specific contact law has been validated, the influence of the layer thickness is studied.