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The evaluation of the viscoelastic properties is a key topic for the analysis of the dynamic mechanical behaviour of polymers. In vehicle dynamics field, the knowledge of the viscoelasticity of tread compound is fundamental for tire-road contact mechanics modelling and friction coefficient prediction for the improvement of vehicle performance and safety, i.e. motorsport field. These properties are usually characterised by means of Dynamic Mechanical Analysis, which implies testing a compound sample obtained by destroying the tire of interest or a slab manufactured in different conditions respect to the final product provided by tiremakers. In this scenario, the non-destructive procedures are an advantageous solution for the analysis of the tread viscoelasticity, without affecting the tire integrity, allowing a great number of tests in the shortest possible time. For this reason, the authors propose an innovative instrument, called VESevo, for viscoelasticity evaluation by means of non-destructive and user-friendly technique. The purpose of the following work is the preliminary analysis of the dynamic response of the tires tested employing the VESevo in order to determine viscoelastic behaviour indexes for mechanical properties evaluation.
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Development Of An Innovative Instrument For Non-
Destructive Viscoelasticity Characterization: VESevo
Flavio Farroni1, Andrea Genovese1, Antonio Maiorano1, Aleksandr Sakhnevych1 and
Francesco Timpone1
1 University of Naples Federico II
flavio.farroni@unina.it
andrea.genovese2@unina.it
antonio.maiorano@unina.it
ale.sak@unina.it
francesco.timpone@unina.it
Abstract. The evaluation of the viscoelastic properties is a key topic for the anal-
ysis of the dynamic mechanical behaviour of polymers. In vehicle dynamics field,
the knowledge of the viscoelasticity of tread compound is fundamental for tire-
road contact mechanics modelling and friction coefficient prediction for the im-
provement of vehicle performance and safety, i.e. motorsport field. These prop-
erties are usually characterised by means of Dynamic Mechanical Analysis,
which implies testing a compound sample obtained by destroing the tire of inter-
est or a slab manufactured in different conditions respect to the final product pro-
vided by tiremakers. In this scenario, the non-destructive procedures are an ad-
vantageous solution for the analysis of the tread viscoelasticity, whitout affecting
the tire integrity, allowing a great number of tests in the shortest possible time.
For this reason, the authors propose an innovative instrument, called VESevo, for
viscoelasticity evaluation by means of non-destructive and user-friendly tech-
nique. The purpose of the following work is the preliminary analysis of the dy-
namic response of the tires tested employing the VESevo in order to determine
viscoelastic behaviour indexes for mechanical properties evaluation.
Keywords: Non-Destructive Testing, Tire Viscoelasticity, Vehicle Dynamics
1 Introduction
The evaluation of tire tread viscoelasticity is a fundamental topic in a wide range of
activities concerning the development of polymers for innovative compounds, the par-
ametrization of physical contact models and the optimization of vehicle performance
and road safety.
In these applications, the viscoelastic properties determination of tire block, which
depends on rubber temperature and frequency solicitation of bitumen asperities, is es-
sential for contact mechanics modelling and the prediction of the limit value of the local
friction coefficient [1][4].
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The Dynamic Mechanical Analysis (D.M.A.) is widely employed into the character-
ization of viscoelasticity in order define the hysteretic behaviour of the compound fol-
lowing the Time-Temperature Superposition principle [5][6]. On one hand, this testing
approach perfectly fits with polymer specimens manufactured with specific dimension
for the D.M.A. and it cannot be always applied for tread characterization because of the
need to destroy the tire; on the other hand, these common testing procedures involve
complex and very expensive machines for the analysis of a generic compound sample.
Regarding the evaluation of the parameters of contact and friction models, the avail-
ability of thermal and structural properties of the effective tread compound provides an
increase in in reliability of the prediction of magnitudes of interest by means of the
proposed models. The chance to determine the viscoelasticity by means of non-invasive
testing would allow to preserve the tire integrity and to exclude the boundary effects
due to testing specimen of specific dimensions.
For these reasons, the development of innovative methodologies, as well as the non-
destructives, are an attractive solution replacing the standard test methods involving
complex and expensive benches for the investigation of a compound specimen manu-
factured in different conditions respect to the final product provided by tiremakers [7].
Further, Motorsport racing teams use to face with the restrictions due to the employ-
ment of confidential tires and not available to invasive testing. Therefore, an innovative
procedure for the acquisition of the data for tire viscoelasticity characterization within
the working thermal range could be very useful for vehicle setup optimization and def-
inition of vehicle simulation tools [8][9].
Hence, the Vehicle Dynamics research group of the Department of Industrial Engi-
neering of the University of Naples Federico II has designed and developed an innova-
tive and portable device, defined as Viscoelasticity Evaluation System Evolved
(VESevo), which allows users to characterise the tire tread viscoelasticity and its vari-
ations due to cooling or heating, due to wear phenomena [10][11], aging or different
compounding [12] depending on vehicle applications. Thus, engineers, especially Mo-
torsport ones, will be capable of analysing more useful information about confidential
tires and improving the vehicle performances and safety.
In the current work, the authors describe the development of the prototypal device
VESevo and the preliminary experimental activity aimed to analyse the viscoelastic
tires tread response taking into account the signals of interest. Therefore, the main task
of the paper is the employment of this innovative device for the extrapolation of the
parameters which mostly match with the viscoelasticity behaviour .
2 Viscoelasticity Properties and Characterization Methods
The Viscoelasticity Evaluation System Evo, also called VESevo, is a prototypal de-
vice developed by Vehicle Dynamics Research Group of the Department of Industrial
Engineering of the university of Naples Federico II. The principal aim of this device is
the evaluation of viscoelastic response of materials by means of a non-destructive
method.
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A viscoelastic material exhibits a mechanical behaviour depending on time and tem-
perature. Particularly, it halfway behaves between a purely elastic (Hookean Solid) and
a purely viscous one (Newtonian Liquid) [13]. Therefore, a phase angle between 0° and
90° occurs comparing the stress and the corresponding strain (Fig. 1).
Fig. 1. Stress-strain phase delay for a viscoelastic material
This means that the stress-strain relationship is defined by a complex dynamic mod-
ulus as amount of the overall resistance to deformation of the compound:
      (1)
The complex modulus is characterised by a real and an imaginary party. The first
one is defined Storage Modulus E’ and is a of the elasticity of the material linked to the
ability to story energy, the second one is the Loss Modulus E” is associated with the
aptitude of the compound to dissipate energy as heat. The ratio of the Loss Modulus to
the Storage Modulus defines the Loss Factor which is an index of the material overall
damping.
These viscoelastic properties of polymers, such as the tire tread compound, are usu-
ally determined by means of Dynamic Mechanical Analysis (D.M.A.). This method
requires testing a polymeric specimen of suitable dimension within a frequency range
from 0.1 Hz to 100 Hz [5]. Furthermore, the D.M.A. is carried out by means of expen-
sive rheometers distinguished in three point bending and dual cantilever for dynamic
modulus E* evaluation or torsional plates for shear modulus G*.
Fig. 2. Dual Cantilever Clamp (a), 3 Point Bending Clamp (b), Shear Sandwich Clamp (c)
The Dual Cantilever Clamp (Fig. 2, a) is suitable for testing highly damped materials
and it is the best mode for evaluating the cure of supported ones; the 3 Point Bending
Clamp (Fig. 2, b) is the best way for measuring medium to high modulus materials and
it guarantees the purest deformation mode since clamping effects are defected rather
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than Dual Cantilever; the Shear Sandwich Clamp (Fig. 2, c) is good for evaluating
highly damped soft compounds.
Tipical trend of the Storage and Loss Moduli depending on solicitation frequency is
shown in Fig. 3. At very low frequency range, the compound behaves as a pure viscous
solid (rubbery plateau region), whereas the behaviour is similar to a glassy solid (glassy
region plateau) at high stress frequency values. Considering the frequency range match-
ing with piece of curve between these regions, the compound behaviour is viscoelastic
and the maximum energy dissipation occurs.
Fig. 3. Viscoelastic properties with the respect the solicitation frequency
The viscoelastic curves shown in Fig. 3 match with a reference temperature at which
the frequency sweep characterization is carried out. The same trend can be analysed
taking into account the temperature dependence, as shown in Fig. 4. On one hand, the
compound behaviour at lower temperature values is the same at high frequency ones
(glassy region); on the other hand, the same rubbery plateau region of low frequencies
occurs at high temperature ranges.
Fig. 4. Viscoelastic properties with the respect the compound temperature
The above diagrams exhibit the equivalence between temperature and time effects
on viscoelastic properties of viscoelastic material. This means that the Time-Tempera-
ture superposition principle is satisfied and the viscoelastic curves can be generated by
means of William-Landel-Ferry equation starting from the master curve [14][15]:


(2)
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In equation 2, C1 and C2 are fitting coefficients of the William-Landel-Ferry rela-
tionship, T0 is the reference temperature of the master curve and aT is the shift factor
that satisfies the following relationship:
    (3)
All the viscoelastic materials whose behaviour is in accordance with equations 2
and the Time-Temperature superposition principle are defined simple thermoreologi-
cally materials.
3 Prototypal device description
The Dynamic Mechanical Analysis described in the previous paragraph requires
very expensive and complex machines to determine the viscoelastic properties of a
compound of interest. Furthermore, this procedure only involves test on specimen of
standard dimension depending on the clamp system (Fig. 1). Thus, the viscoelasticity
characterization cannot be carried out on systems that do not allow the realization of
the samples required, such as a piece of tread compound of a Motorsport confidential
tire.
In this scenario, a prototypal device, called VESevo, has been developed by UniNa
Vehicle Dynamics research group with the aim to overcome the limits of traditional
D.M.A. testing and extrapolate the viscoelastic properties of the system of interest by
means of a non-destructive procedure.
The device VESevo has been designed taking into account a gun-shape handle. Thus,
the ergonomics of the instrument allows a high number of tests with a satisfying repeat-
ability (see Fig. 5).
Fig. 5. 3D representation of the prototypal device VESevo
The inner structure of the device is characterised by a steel rod with a semi-spherical
indenter. This rod is free to bounce on the surface of the compound of interest sliding
inside a suitable guide so that the damping phenomenon during the rod motion inside
the case can be neglected. A spring is arranged in the system in order to guarantee a
minimum preload.
The motion of the rod always starts from the same initial position thanks to an in-
novative system based on an magnet: this magnet is mounted on a suitable slider and
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it is capable of holding the upper plate of the rod and lifting it up to the maximum ascent
point. The described system has been patented by the Vehicle Dynamics research group
of the University of Naples Federico II [16].
During each test, the motion of the rod starting from initial position against the
compound surface needs to be analysed. To acquire properly the displacement signal,
an optical sensor has been chosen for its compact dimensions and very high frequency
response rather than others commonly available in similar devices, such as LVDT.
The temperature of the compound during a single test has to be acquired together
with the displacement data. Therefore, a compact IR pirometer has been chosen and set
up in the suitable sensor housing in order to analyse the signals at different viscoelastic
behaviour of the specimen of interest.
4 Signal acquisition and processing
The signal acquisition of the VESevo needs a suitable case containing the data ac-
quisition devices, i.e. a National Instrument data logger and the conditioning devices
for the optical sensor and IR pyrometer provided by the corresponding manufacturers.
Furthermore, a self-made customized software for raw data acquisition has been de-
veloped in LabVIEW environment to find out any acquisition anomalies and check the
goodness of the whole test session.
A typical raw signal of the displacement curve during a single test is shown in Fig.
6. The signal is acquired within a time range of 0.1s. The compound temperature is
determind as the average of the values acquired before the first impact of the road on
the sample of interest.
Fig. 6. Displacement raw signal of the rod testing a compound at 30°C
In a single acquisition, the rod bounce exhibits three different phases which are es-
sential for the evaluation of the indexes connected with the viscoelastic behaviour :
The first one is characterised by the impact of the sensor rod on the tire surface
and it matches with the minimum point of the acquired curve;
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During the second phase, the rod bounces to the maximum point of the curve
because of its interaction with the external tire rubber layer; particularly, different
bounces could occur after the first one depending on surface temperature and
starting position;
The last phase is characterised by a stabilized rod displacement value established
within the contact at the end of transient reciprocal dynamics.
In order to characterise the tire tread compound within the temperature range of -
20°C to 100°C, a heating gun and a climatic cell (Fig. 7) were used. Thus, the glass
transition phenomenon could be identified for the compound of interest, with the po-
tential advantage to improve the evaluation of grip coefficient by means of specific
physical models [17][18].
During the test session, the tread surface is cooled and heated without degradation
until its temperature stabilises; then, three consecutives acquisitions at the same tem-
perature are stored in order to have a suitable amount of data for statistical processing.
The data are acquired by means of a self-made customized software developed in Lab-
VIEW environment.
Fig. 7. Instruments employed in the experimental characterization procedure
Taking into account the typical shape of the signals provided by the VESevo (Fig.
6), a set of indicators of the viscoelastic behaviour of the compound can be estimated
for the time-temperature characterization.
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Fig. 8. VESevo data acquisition on passenger tire A at different tread temperatures
In Fig. 8, the data acquired on a passenger tire compound (named A) at different
temperatures are shown. Particularly, increasing the temperature, the motion of the rod
changes due to different material responses. The signal is characterised by small am-
plitudes and reduced number of bounces within a low temperature range; on the con-
trast, more bounces with substantial amplitudes values occur at high temperatures. This
phenomenon is strictly depending on the viscoelasticity changes due to temperature
effect [13]: when the tread compound is solicited at lower temperatures, the strain en-
ergy loss peak occurs and the rod cannot reach the maximum amplitude during the
rising phase.
Taking into account the equation 2, the loss factor master curves with respect the
frequency at different test temperatures can be analysed. As observable in Fig. 9, the
maximum loss factor values occur in the frequency range of 500-5000 Hz, which
matches with the VESevo solicitation one, and between -15° and 15°C. This data are
in accordance with the phenomena described in Fig. 8.
Fig. 9. Loss factor viscoelastic master curves at different reference temperatures
A further analysis on the viscoelastic behvior of the tested tire compound can be
carried out considering the velocity signals of the rod motion (Fig. 10). Particularly, the
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slope of the displacement curve before and after the maximum indentation depth
changes due to the rebound on a viscoelastic surface. Thus, two different velocity val-
ues for each test next to the indentation area can be identified.
Fig. 10. VESevo velocity signals on passenger tire A at different tread temperatures
A preliminary experimental test session has been carried out on other tire com-
pounds, identified as tire B and tire C, whose viscoelastic behaviour is different in ac-
cordance with their vehicle dynamics application [12]. These compounds mostly differ
in percentage of fillers, silica, carbon filler and other chemicals added to the standard
stirene butadiene rubber.
Aspreviously described, the VESevo testing procedure requires warming and cool-
ing the tire by means of a professional heating gun (Fig. 7) or a climatic cell. The sur-
face tread temperature is measured through the IR pirometer set-up in the special hous-
ing built-in the device. Both temperature and displacement signals can be real-time an-
alysed through the customized acquisition software.
Fig. 11. Viscoelastic index at 1 Hz for the tested tire compounds
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Thus, a preliminary index of viscoelastic behaviour has been estimated as the varia-
tion of kinetic energy of the rod pre- and post- the first indentation, within the temper-
ature range of interest for each tested tire. The kinetic energy variation due to the
bounce on a viscoelastic sample is expressed as follows:
  

 
(4)
where  is the mass of the indenter,  and  the velocity pre- and post- the
first indentation. Such index has been chosen for this preliminary analysis because of
its physical coherence with the intrinsic concept of dissipation due to viscoelasticity
and for the good fitting with the available loss factor data, coming from D.M.A. analysis
carried out at 1Hz, once properly re-scaled. In Fig. 11, the viscoelastic index of the
tested tire compounds (A, B and C) as function of the temperature together their fit
curves are plotted. Each marker of a single curve corresponds to 1 Hz solicitiation fre-
quency for the compound. These iso-frequency curves have been generated shifting the
acquired temperature through eq. 2 and considering the fitting coefficients provided by
the tiremaker [15].
As noticeable in these diagrams, the index trend looks similar to loss factor one,
exhibiting a peak matching with the glassy transition temperature at which occurs the
maximum energy loss. Moreover, a difference between the tested compounds can be
appreciated: on one hand, the tire C seems to be characterised by a higher loss factor
peak rather than the others; on the other hand, the tire B exhibits the lowest glassy
transition temperature. Consequently, this preliminary analysis, taking into account the
viscoelastic index, can provide suitable information concerning the viscoelastic behav-
iour among the tested tread compounds.
Fig. 12. Normalised loss factor 1 Hz D.M.A. master curves
The previous analysis results can be compared with the normalised values of the loss
factor determined by temperature sweep tests carried out in dual cantilever at 1 Hz. The
diagrams shown in Fig. 12 are mostly in accordance with the results described in Fig.
11. Actually, the tire C exhibits the maximum dissipation peak among the considered
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ones; whereas the compound B is marked by a low glassy transition temperature as
expected. Besides, the glassy transition temperature values, which are estimated by
means of VESevo acquisitions, are compared with D.M.A. ones in Table 1. As shown,
the results are very similar proving the goodness of the non-destructive method.
Table 1. Glassy transition temperature comparison
Tested Tire
Tg (D.M.A.)
Tg (VESevo)
Tire A
-26.8°C
-27.02°C
Tire B
-45.2°C
-46.42°C
Tire C
-25.5°C
-26.01°C
Conclusions
A prototypal device, called VESevo, has been proposed and described by the Vehi-
cle Dynamics research group of the University of Naples Federico II in the following
work. This device has been designed with the aim to perform a viscoelastic behaviour
analysis through an innovative and non-invasive testing approach.
Actually, the employment of the VESevo can provide a fast and accurate preliminary
evaluation of the viscoelasticity whitout using expensive rheometers and specific com-
pound samples, as traditional D.M.A. requires.
For these reasons, the VESevo could be very useful in Vehicle Dynamics field,
where the knowledge of viscoelastic properties of the tire tread compound could pro-
vide further information concering motorsport, truck or passenger vehicle applications.
Since the preliminary experimental activity carried on three different tire compounds
showed results in agreement with the expected D.M.A., the authors will focus on the
development of an algorithm capable of evaluating the viscoelastic properties, in terms
of Storage and Loss moduli, starting from the main index connected with the material
behaviour. Thus, the VESevo will be useful for monitoring the tread status by a non-
invasive approach, for analysing the dynamic behaviour of confidential motorsport tires
and for comprehending when changing tires configuration could be necessary.
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12 IFIT2020, 079, v1: ’Development Of An Innovative Instrument For Non-Destructive Visc. . .
... The second mechanism takes place in a microscale and is amplified by slippage. Therefore, in both tyre-road phenomena, the viscoelastic properties of the tread rubber and its hysteresis, which are characterized by destructive and non-destructive techniques Farroni et al., 2021;Genovese & Pastore, 2021), as well as ultrasound (Lionetto & Maffezzoli, 2009) and optical (Martorelli et al., 2020) methods, play an important role. In the road roughness effects analysis, two main features of the road geometry must be examined considering the tyre grip: the macro and micro roughness. ...
Chapter
This chapter deals with tyre mechanics and it has a particular focus on thermal effects on its dynamical behaviour. In the first part the typical tyre structure is introduced together with the tyre mechanical/dynamical behaviour according to a classical approach, so recalling the main kinematic and dynamic quantities involved in tyre pure and combined interactions. The core of this chapter is the description of a physical-analytical tyre thermal model able to determine the thermal status in each part of the tyre useful for vehicle dynamics modelling and driving simulations in order to take into account thermal effects on tyre interactions and consequently on vehicle dynamical behaviour. Successively also the tyre wear modelling is faced, after a brief introduction to the different models available in literature some considerations are reported concerning the thermal effects on wear.
... However, it cannot be always employed because of peculiar cases and circumstances where a compound specimen of a specific geometry should be extracted, and therefore requiring the development of novel innovative and nondestructive methodologies for the viscoelastic characterizations. Many of such procedures involve portable devices 24 in order to analyze in real-time the viscoelastic behavior and possible changes due to heating or cooling phenomena during the real working conditions. Further techniques can determine the viscoelastic behavior taking into account other properties, such as the permittivity 25,26 : the dielectric spectroscopy requires the dielectric constant measurement by placing the compound in contact with a pair of electrodes and applying a sinusoidal voltage. ...
Article
Background The ultrasound technique, usually based on the transmission mode, is capable of providing the viscoelastic properties of polymers. Further techniques involving pulse-echo methods were also described in literature, but they still exhibit inaccuracies in the evaluation of the acoustic properties. Objective The manuscript focuses on an innovative approach for the characterization of the viscoelastic behavior of polymers employing the ultrasound methodology. The proposed procedure is based on the pulse-echo method in order to overcome possible inaccuracies in acoustic properties evaluation and in issues related to transmitter mode applications. Methods Starting from the pulse-echo method adopted for the acquisition, a novel formulation for data processing has been developed and described, allowing to determine the wave attenuation coefficient, in comparison to the commonly employed procedures involving ultrasound in polymers characterization, based on transmitter mode inspections. To carry out the study, a specifically designed ultrasound bench has been set up and three different polymers have been tested in the temperature range of interest. Results According to the proposed methodology, the loss factor towards the temperature is determined starting from the data acquired considering the identified attenuation coefficient and the measured sound velocities. The trustworthiness of the novel procedure has been proved comparing the obtained viscoelastic loss factor quantities to the reference master curves obtained by the standard Dynamic Mechanical Analysis characterizations carried out on the same polymer specimens. Conclusions A novel methodology involving ultrasound technology aiming to evaluate the viscoelasticity of the polymers using non-destructive approach has been developed. The results obtained are agreement with the standard viscoelastic master curves determined through the DMA.
Chapter
Vehicle safety is of a fundamental importance in the automotive industry and, with an increasing level of automation, the impact of vehicles on the global environment has to be investigated from a totality of different perspectives starting from pollution impact objectives up to the vehicle risk-prevention capabilities. For this reason, the state estimation techniques and the advanced control logics have being developed in recent years with the aim of improving the safety of the semi-automated vehicles, able to assist the driver in emergency situations minimizing the connected risks. Furthermore, the vehicle stability topic has acquired more interest since it could allow to pre-determine the vehicle stability and maneuverability regions, optimizing both the real-time computational efficiency of the control-related logics and the correct identification of the optimum vehicle operating boundaries in completely different use scenarios. Since a vehicle is a strongly nonlinear system mainly because of tyres behaviour, the methodology able to adequately determine the stability region becomes crucial. Starting from a specific literature survey, this work aims to investigate control-oriented approaches, employing the local stability criteria method, able to determine stability regions within the system phase-plane potentially adoptable in a computationally-efficient vehicle onboard logic. The techniques presented and the sensitivity analyses conducted highlight which should be the research directions in this field to remove several not-negligible but yet present assumptions in the literature.
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The wet grip performance of tire is one of the important performances affecting vehicle safety. The steering, acceleration, and braking of the vehicle are directly affected by the grounding characteristics between the radial tire and the ground. In order to study the influence of grounding characteristics of the tire on wet grip performance, ten 205/55R16 tires produced by different manufacturers were selected and tested. The grounding characteristics of the tires were tested using an optical test rig for tire grounding pressure distribution, considering inflation pressure distribution, load and wheel alignment. The tire-road contact area was subdivided into five parts, and 69 parameters were used to describe the grounding characteristics. A software was proposed to process the test results automatically, and 69 grounding characteristic parameters of each tire were obtained. Correlation analysis on tire wet grip performance and grounding characteristics was used for selecting the principal parameters. Finally, eight grounding characteristic parameters related to tire wet grip performance was obtained. Among them are five grounding characteristic parameters (central area rectangle ratio, central area width, internal shoulder length-to-width ratio, external and internal shoulder contact area ratio, external and internal shoulder impression area ratio) which have high correlation to tire wet grip performance, and three grounding characteristic parameters (external shoulder width, external shoulder length-to-width ratio, external and internal shoulder width ratio) which have low correlation to the wet grip performance of the tire. The principal component analysis method was used to analyze the highly correlated grounding characteristic parameters, and the regression equation for evaluating tire wet grip performance was fitted. The comparison of experimental and fitted values show that the errors are within 4%. The result demonstrates that, the method for evaluating wet grip performance of the radial tire through tire-road grounding characteristics was achieved.
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Full-text available
Since the early study by Grosch in 1963 it has been known that rubber friction shows generally two maxima with respect to speed-the first one attributed to adhesion, and another at higher velocities attributed to viscoelastic losses. The theory of Klüppel and Heinrich and that of Persson suggests that viscoelastic losses crucially depend on the "multiscale" aspect of roughness and in particular on truncation at fine scales. In this study, we comment a little on both theories, giving some examples using Persson's theory on the uncertainties involved in the truncation of the roughness spectrum. It is shown how different choices of Persson's model parameters, for example the high-frequency cutoff, equally fit experimental data on viscoelastic friction, hence it is unclear how to rigorously separate the adhesive and the viscoelastic contributions from experiments.
Conference Paper
The TRT model, developed to accurately reproduce the tire thermal dynamics in all the vehicle working conditions, has to be physically characterized [1][2]. An appropriate non-destructive procedure, that allows to obtain the thermal diffusivity of completely different tire layers, is described. The heat is directly supplied on the tire tread surface trough a specifically powered laser, while two thermal cameras acquire temperatures reached on both the outer and the inner layers. Using the above instrumentation layout to acquire the tire radial and circumferential temperature gradients and a specifically developed mathematical TRTLab model based on the use of Fourier's equation of diffusion applied to a three dimensional domain, allows to estimate the tire thermal diffusivity.
Conference Paper
The evaluation of the tire tread viscoelastic characteristics, especially by means of non-destructive procedures, is a particularly interesting topic for motorsport teams and companies, used to work with unknown and confidential compounds. The availability of such information would define new scenarios in vehicle analysis field, as the possibility to provide physical inputs to tire grip models or the study of the suspensions setup able to make tires work inside their optimal thermal working range. The employment of commercial devices allows to select by means of specific indices the optimal combination of tires to be installed on a vehicle, but it does not provide any information physically correlated with the tread polymers characteristics. The aim of the presented activity is the modelling of one of the cited devices, a dynamic dial indicator, interacting with a viscoelastic half-space. The obtained results allow, analyzing the signals acquired by the device, to identify the tread equivalent stiffness and damping as a function of tire working temperature, providing the basic guidelines for the development of an innovative procedure for a full non-destructive viscoelastic characterization of the tire compounds. Index Terms-Material non-destructive characterization, temperature effect, tire tread compound behavior, TSD, viscoelastic characteristics.
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Vehicle performances, especially in motorsport, are deeply affected by tire behavior and in particular by tire compound proper working conditions. In this research activity, a series of innovations have been introduced on the Thermo Racing Tire (a physical-analytical tire thermal model, based on Fourier’s law of heat transfer applied to a three-dimensional domain) in order to take into account all the main aspects actively involved in the thermal behavior of the tire, as the presence of exhausted gases eventually impacting at the rear axle and the inhomogeneous distribution of local variables (pressure, stress and sliding velocity) within the contact patch, caused in example by the tire camber angle. The new model developed considers the presence of the sidewalls, actively involved in the convective heat exchanges, respectively, with the external airflow and the inner gas fluid, located inside the inflation chamber. The aim of the new version of the tire thermal model is a better physical comprehension of all the phenomena concerning the contact with the asphalt and the prediction of the link between the thermal state and the frictional performance, crucial for the definition of an optimal wheel and vehicle setup.
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The results of an experimental activity, carried out using a prototype of pin on disk machine and aimed at investigating the frictional behavior of visco-elastic materials in sliding contact with rigid asperities, are presented. The pin is a rubber specimen coming from three different passenger automotive tires, while the disk is covered with glass, marble, or 3M anti-slip tape surfaces. Tests, performed both in dry and wet conditions, highlighted that the friction coefficient is strongly influenced by the effect that surface roughness plays on friction mechanisms of adhesion and hysteresis. The results confirmed the theoretical dependence of friction on vertical load, sliding velocity, rubber characteristics, and track conditions.
Chapter
On the contact interface between two bodies in frictional contact with one another, heat energy is released. Because the real contact area is, as a rule, only a fraction of the apparent contact area, the heat released in a tribological contact is very heterogeneous. The local temperature increases can be so high that they may influence the material properties and can even cause the material to melt. Furthermore, a local change in the temperature leads to a local heat expansion and, thus, a corresponding change in the contact conditions. This feedback reaction can, under certain conditions, lead to the development of thermo-mechanical instabilities in the contact. In this chapter, we investigate the various aspects of the frictionally caused heat release in tribological contacts.
Chapter
On the contact interface between two bodies in frictional contact with one another, heat energy is released. Because the real contact area is, as a rule, only a fraction of the apparent contact area, the heat released in a tribological contact is very heterogeneous. The local temperature increases can be so high that they may influence the material properties and can even cause the material to melt. Furthermore, a local change in the temperature leads to a local heat expansion and, thus, a corresponding change in the contact conditions. This feedback reaction can, under certain conditions, lead to the development of thermo-mechanical instabilities in the contact. In this chapter, we investigate the various aspects of the frictionally caused heat release in tribological contacts.
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The tire and vehicle setup definition, able to optimise grip performance and thermal working conditions, can make the real difference as for motorsport racing teams, used to deal with relevant wear and degradation phenomena, as for tire makers, requesting for design solutions aimed to obtain enduring and stable tread characteristics, as finally for the development of safety systems, conceived in order to maximise road friction, both for worn and unworn tires. The activity discussed in the paper deals with the analysis of the effects that tire wear induces in vehicle performance, in particular as concerns the consequences that tread removal has on thermal and frictional tire behaviour. The physical modelling of complex tire–road interaction phenomena and the employment of specific simulation tools developed by the Vehicle Dynamics UniNa research group allow to predict the tire temperature local distribution by means of TRT model and the adhesive and hysteretic components of friction, thanks to GrETA model. The cooperation between the cited instruments enables the user to study the modifications that a reduced tread thickness, and consequently a decreased SEL (Strain Energy Loss) and dissipative tread volume, cause on the overall vehicle dynamic performance.
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Understanding viscoelasticity is pertinent to design applications as diverse as earplugs, gaskets, computer disks, satellite stability, medical diagnosis, injury prevention, vibration abatement, tire performance, sports, spacecraft explosions, and music. This book fits a one-semester graduate course on the properties, analysis, and uses of viscoelastic materials. Those familiar with the author's precursor book, Viscoelastic Solids, will see that this book contains many updates and expanded coverage of the materials science, causes of viscoelastic behavior, properties of materials of biological origin, and applications of viscoelastic materials. The theoretical presentation includes both transient and dynamic aspects, with emphasis on linear viscoelasticity to develop physical insight. Methods for the solution of stress analysis problems are developed and illustrated. Experimental methods for characterization of viscoelastic materials are explored in detail. Viscoelastic phenomena are described for a wide variety of materials, including viscoelastic composite materials. Applications of viscoelasticity and viscoelastic materials are illustrated with case studies.