ABSTRACT
In practice, measuring dynamic properties is limited by the accessible frequency range of the instrument. However, the modulus at high frequencies is available via the time-temperature superposition principle. According to this principle, the effect of changing the temperature is the same as applying a shift factor aT to the time scale. The time-temperature superposition for polymeric materials was proposed by Williams, Landel and Ferry (1955) and is generally referred to as the WLF principle. According to this principle, the horizontal shift factor aT is given by:
log( ) ( ) ( )
a C C
T T T C
=
−
< °
1 INTRODUCTION
A tire interacts with the hard road surface by deforming under load, thereby generating the forces needed for traction, cornering, acceleration and braking. It also provides increased cushioning for driving comfort. Traction, especially under wet conditions, is the most important tire property from a safety point of view. Traction is directly related to energy lost in each deformation cycle: each point in the tire passes through a stress-strain cycle once every rotation (Wang 2008, Grosch 2007). Due to the viscoelastic nature of the rubber compound, the deformation leads to energy loss in the form of heat in each cycle. Although it has been stated that the tread pattern and the roughness of the road surface are as important as the loss properties of the tire tread material in determining the skid properties, the current trend is basically to concentrate on the latter for improvements in performance.
