Tibor Goda

Finite element analysis of a polymer composite subjected to a sliding steel asperity Part I Normal fibre orientation

FE micro-models have been developed in order to determine contact, stress and strain conditions produced by a steel asperity sliding on the surface of a normally oriented fibre-reinforced polymer composite. A displacement coupling technique was introduced to model a “micro-environment” as part of a “macro-environment” and to get more realistic simulation results about the failure conditions in the composite structure, in comparison to the so far widely applied anisotropic analytical or numerical macro-models. On the basis of the results, conclusions may be drawn for the possible wear mechanisms of the fibre-reinforced polymer composite. Stress results in the vicinity of the fibers in the contact area show high shear loading of the matrix leading to the formation of stretched-out matrix wear debris. In addition, high repeated compression-tension stresses at the fibre/matrix interface near the surface can lead to fibre debonding phenomena. Considering the fibre ends in the contact region, high compression stresses at their rear edges can produce fibre cracking features. To study the wear mechanisms experimentally, a “single asperity” scratch test was also performed showing shear failure events of the polymer matrix, fibre/matrix debonding and fibre cracking effects, as expected from the modelling studies.

Finite element analysis of a polymer composite subjected to a sliding steel asperity Part II: Parallel and anti-parallel fibre orientations

Finite element (FE) micro-models have been developed in order to determine contact, stress and strain conditions produced by a steel asperity sliding on the surface of a fibre-reinforced polymer composite. Two cases were studied, i.e. a parallel and an anti-parallel fibre orientation relative to the sliding direction. In order to get more realistic simulation results relating to the failure conditions in the composite structure, FE contact macro/micro-models were used, contrary to the so far widely applied anisotropic analytical or numerical macro-models. To model a “micro-environment” as part of a “macro-environment”, the displacement coupling technique was introduced. The contact analysis operates on both the macro- and the micro-level, applying node-to-node contact elements. The contact results, especially the contact pressure distribution, can characterize the real fibre/matrix micro-system. Displacement and strain results lead to explanations of fibre related phenomena, matrix shear effects, and fibre/matrix debonding events. On the basis of the stress results, conclusions were drawn on the possible wear mechanisms of the fibre-reinforced polymer composite. For parallel fibre orientation, fibre/matrix debonding as a result of shear stresses at the interface, matrix shear type failure and fibre thinning are the dominant sliding wear mechanisms. If an anti-parallel fibre orientation is considered, matrix shear, tension/compression type fibre/matrix debonding and fibre thinning, associated with fibre cracking events, are the most dominant wear mechanisms. To study the wear mechanisms experimentally, diamond tip scratch tests were carried out, showing that the predicted failure events occur also in reality.

Friction Force Measurement at Windscreen Wiper/Glass Contact

In the present study, a novel windscreen wiper-on-cylinder machine has been used to investigate the influence of sliding speed and normal force on the coefficient of friction. Using this machine it is possible to measure the friction force not only on specimen level, as in former studies to be available in the literature, but also on structural level by considering the whole windscreen wiper. As measurement results are strongly influenced by both the real, non-circular cross-section, and the eccentricity of the rotating glass cylinder an analytical model has been developed to explain the measurement results. The good agreement to be found between theory and experiment confirms the validity of the model. Majority of the results belongs to partial contact where the wiper blade is not in contact with the glass countersurface along its total length. After the discussion of experimental results, as a last step, authors made an attempt to compare quantitatively the predictive capability of two different contact models widely used in mixed friction model of sliding rubber components. The results show that the difference in film thickness due to solid–solid contact can be larger than three orders of magnitude in case of a typical windscreen wiper.

Finite Element Simulation of the Fiber–Matrix Debonding in Polymer Composites Produced by a Sliding Indentor: Part I – Normally Oriented Fibers:

To study the contact and debonding behaviors between a CF–PEEK fiber-reinforced polymer composite specimen and a sliding diamond indentor, finite element macro- and micro-models have been developed. Around each fiber, interface elements were introduced in order to detect the tension-type and also the shear-type debondings for different cases. If initial or final debonding has occurred, a “control algorithm” checked the limit strain conditions for the interface elements and changed the material properties according to the actual debonding condition. As a final result, it can be concluded, that the dominant debonding under compression is due to the shear loading conditions.

Finite Element Simulation of the Fiber– Matrix Debonding in Polymer Composites Produced by a Sliding Indentor: Part II – Parallel and Anti-Parallel Fiber Orientation:

A debonding simulation technique, based on a series of nonlinear FE evaluations, has been developed to study the contact behavior and the debonding process of unidirectional fiber-reinforced polymer composites subjected to a sliding diamond indentor. In Part I, the normal fiber orientation was studied, while in Part II, the parallel and anti-parallel fiber orientations relative to the sliding direction are analyzed. The technique developed can consider both the shear and the tension type debonding events, and also the effect of friction causing limited debonding. The results show a dominant limited shear type debonding under compression for both fiber orientations.

Fe micro-models to study contact states, stresses and failure mechanisms in a polymer composite subjected to a sliding steel asperity

Abstract FE micro-models have been developed in order to determine contact, stress and strain conditions produced by a steel asperity sliding on the surface of a fibre-reinforced polymer composite. Three cases were studied, i.e. a normal, a parallel and an anti-parallel fibre orientation relative to the sliding direction. In order to get more realistic simulation results about the failure conditions in the composite structure, FE contact macro/micro-models were used, contrary to the so far widely applied anisotropic analytical or numerical macro-models. To model a “micro-environment” as part of a “macro-environment”, the displacement coupling technique was introduced. On the basis of the stress results, conclusions were drawn on the possible wear mechanisms of the fibre-reinforced polymer composites. For each fibre orientation, surface failure of the matrix material occurs due to high shear stresses. The other characteristic source of failure is fibre/matrix debonding, eventually followed by fibre cracking events.

Numerical Analysis of Sliding Friction Behaviour of Rubber

A study has been made of asperity interaction of unlubricated steel/rubber sliding pair. The aim is to study the effect of the internal friction (hysteresis) of rubber on the friction force. In the two-dimensional finite element analysis, asperities are modeled by cylinders and both the interfacial adhesion and the friction at steel-rubber interface are neglected. Rate-dependent material behavior of rubber is described, as a first approximation, by a three-parameter Zener-model. It is found that the viscoelastic properties of rubber have a strong influence on the hysteresis component of friction. Distribution of energy loss generated over a cycle of contact in the rubber asperity is also studied. It is concluded that the energy dissipation is most intensive at a certain depth below the rubber surface.

Numerical Prediction of Friction, Wear, Heat Generation and Lubrication in Case of Sliding Rubber Components

Modeling strategies and algorithms have been presented for the numerical prediction of hysteretic friction, wear, frictional heat generation and lubrication state of rubber components subjected to sliding friction. All the algorithms presented base on different numerical techniques - such as finite element and finite difference method-, and mathematical methods (e.g. discrete Fourier transformation, numerical integration, etc.). This numerical approach allows the integration of the algorithms and, as a direct consequence, the development of design tools. As an example for the design tool, in the second part of this contribution, a very recently published numerical model has been adopted an applied to a widely used, standardized hydraulic O-ring. The model takes into consideration the effect of surface roughness, deformation of seal, pressure dependency of viscosity and cavitation, respectively. As an asperity type contact model is incorporated into the model it can be used not only for full film but also for mixed lubrication. By using the design tool developed both the pressure distribution within the lubricating film, and the amount of fluid flow transport during outstroke and instroke (their difference defines the amount of leakage) have been predicted.

On the Relation between Stress Relaxation and Constant Strain Rate Tensile Behavior for Linear Viscoelastic Materials; An Engineering Approach

The present paper, as a first step summarizes briefly the master curve construction methods applying the stress relaxation and DMTA based approach. Then, authors make recommendation to increase the covered time (frequency) domain of relaxation modulus master curve coming from standard tensile tests-performed at wide temperature range-by utilizing the time-temperature superposition principle. The proposed approach is used for natural rubber, whose tensile tests, for the sake of simplicity, are replaced by calculated engineering stress-strain curves. All in all, the proposed method gives fast and reliable way for engineers to identify the parameters of spring-dashpot models.

Parameter Identification Methods for Generalized Maxwell Models: Engineering Approach for Small-Strain Viscoelasticity

The present paper surveys briefly the parameter identification methods widely used in case of generalized Maxwell-model. Beside those basing on dynamical-mechanical thermal analysis (DMTA) and stress relaxation measurement authors have presented a technique using simple tensile tests. The latter can be effectively used by combining it with the genetic algorithm of Matlab for parameter identification in a limited frequency/time domain. The proposed method can be generalized easily for example for shear and compression tests too. After comparing it with other existing methods author make a proposal for the strain rate of the uniaxial tensile test.