Conference Paper

AN EXPERIMENTAL PROCEDURE FOR THE CHARACTERIZATION OF TIRE THERMAL BEHAVIOR AND A MODEL BASED STUDY OF PARAMETERS VARIATION EFFECT

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Designers and technicians involved in vehicle dynamics face during their daily activities with the need of reliable data regarding tyres and their physical behaviour. The solution is often provided by bench characterizations, rarely able to test tyres in real working conditions as concerns road surface and the consequential thermal and frictional phenomena. The aim of the developed procedure is the determination of the tyre/road interaction curves basing on the data acquired during experimental sessions performed employing the whole vehicle as a sort of moving lab, taking into account effects commonly neglected.
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Viscoelastic contacts are present in countless industrial components including tires, dampers and rubber seals. The effective design of such components requires a full knowledge of viscoelastic contact mechanics in terms of stresses, strains and hysteric dissipation. To assess some of these issues, this paper describes a series of experiments on the contact area and penetration in a rolling contact between a nitrile rubber ball and a glass disk. The experimental results are compared with the theory proposed by Carbone and Putignano1 showing close agreement at low speeds. However, discrepancies arise at speeds above 100 mm/s because of the frictional heating. In order to evaluate this effect, the temperature of the sliding interface is measured for different rolling speeds using infrared microscopy. Thermal results showed that interfacial temperature remained constant at low rolling speeds before rising significantly when speeds above 100 mm/s were reached. These temperature effects are incorporated into the numerical simulations by means of an approximated approach, which corrects the viscoelastic modulus based on the mean measured temperature in the contact. The result of this approach is to extend the region of agreement between experimental and numerical outcomes to higher speeds.
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In the paper a new physical tyre thermal model is presented. The model, called Thermo Racing Tyre (TRT) was developed in collaboration between the Department of Industrial Engineering of the University of Naples Federico II and a top ranking motorsport team. The model is three-dimensional and takes into account all the heat flows and the generative terms occurring in a tyre. The cooling to the track and to external air and the heat flows inside the system are modelled. Regarding the generative terms, in addition to the friction energy developed in the contact patch, the strain energy loss is evaluated. The model inputs come out from telemetry data, while its thermodynamic parameters come either from literature or from dedicated experimental tests. The model gives in output the temperature circumferential distribution in the different tyre layers (surface, bulk, inner liner), as well as all the heat flows. These information have been used also in interaction models in order to estimate local grip value.
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A new technique for the determination of the thermal diffusivity of a tyre compound is proposed. The diffusivity is defined as the ratio between the thermal conductivity and the product of the specific heat and density. This technique is based on infrared measurement and successive analysis of the tyre cooling. Tyre samples were heated up by a laser at constant power rate and the heating and the next cooling of the tyres were registered versus time by mean of thermocouples and infrared cameras. Determination of the thermal diffusivity was thus estimated by mean of home-made model. The research activity was carried out in the laboratories of the department of Mechanics and Energetics of the University of Naples Federico II, in cooperation with the Combustion Institute of the CNR in Naples.
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We present a simple theory of crack propagation in viscoelastic solids. We calculate the energy per unit area, G(v), to propagate a crack, as a function of the crack tip velocity v. Our study includes the non-uniform temperature distribution (flash temperature) in the vicinity of the crack tip, which has a profound influence on G(v). At very low crack tip velocities, the heat produced at the crack tip can diffuse away, resulting in very small temperature increase: in this "low-speed" regime the flash temperature effect is unimportant. However, because of the low heat conductivity of rubber-like materials, already at moderate crack tip velocities a very large temperature increase (of order of 1000 K) can occur close to the crack tip. We show that this will drastically affect the viscoelastic energy dissipation close to the crack tip, resulting in a "hot-crack" propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt, which may result in unstable crack motion, e.g. stick-slip motion or catastrophic failure, as observed in some experiments. In addition, the high crack tip temperature may result in significant thermal decomposition within the heated region, resulting in a liquid-like region in the vicinity of the crack tip. This may explain the change in surface morphology (from rough to smooth surfaces) which is observed as the crack tip velocity is increased above the instability threshold.
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The thermomechanical behavior of pneumatic tires is a highly complex transient phenomenon that, in general, requires the solution of a dynamic nonlinear coupled thermoviscoelasticity problem with heat sources resulting from internal dissipation and contact and friction. This highly complex and nonlinear system requires in-depth knowledge of the geometry, material properties, friction coefficients, dissipation mechanisms, convective heat transfer coefficients, and many other aspects of tire design that are not fully understood at the present time. In this paper, a simplified approach to modeling this system that couples all of these phenomena in a straight forward manner is presented in order to predict temperature distributions in static and rolling tires. The model is based on a one-way coupling approach, wherein the solution of mechanical rolling contact problem (with friction and viscoelastic material properties) provides heat source terms for the solution of a thermal problem. The thermal solution is based on the thermodynamics of irreversible processes and is performed on the deformed tire configuration. Several numerical examples are provided to illustrate the performance of the method.
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A finite element-based method is demonstrated to predict tire rolling resistance and temperature distributions. Particular attention is given to the material properties and constitutive modeling as these have a significant effect on the predictions. A coupled thermomechanical method is described where both the stiffness and the loss properties are updated as a function of strain, temperature, and frequency. Results for rolling resistance and steady state temperature distribution are compared with experiments for passenger and radial medium truck tires. An extension of the method for transient temperature predictions is also demonstrated.
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A simplified three-dimensional model has been developed to predict the temperature distribution in a tyre during rolling or skidding, allowance being made for cooling. Calculated temperatures are shown to be in reasonable agreement with surface temperature measurements obtained during the running of a tyre against a rotating drum. The analysis is also used to investigate the effects of varying certain parameters and operating conditions.
An Evolved Version of Thermo Racing Tyre for Real Time Applications
  • F Farroni
  • A Sakhnevych
  • F Timpone
F. Farroni, A. Sakhnevych, and F. Timpone, "An Evolved Version of Thermo Racing Tyre for Real Time Applications", Lecture Notes in Engineering and Computer Science: Proceedings of The World Congress on Engineering 2015, 1-3 July, 2015, London, U.K., pp 1159-1164
The results of two tests for the same tire at different power values are shown. The simulated temperatures are with continuous lines while the acquired data are with dotted ones
  • Fig
Fig. 9. The results of two tests for the same tire at different power values are shown. The simulated temperatures are with continuous lines while the acquired data are with dotted ones. In order to handle properly the infrared measurements, the both tire surfaces emissivity has been set constant equal to 0.95.
Principles of Heat Transfer
  • F Kreith
  • R M Manglik
  • M S Bohn
F. Kreith, R. M. Manglik and M. S. Bohn, "Principles of Heat Transfer". 6 th ed. Brooks/Cole USA, 2010.