Fractal Shape Analysis of Tire Debris Particles: Preliminary Results and Applications
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Fractal Shape Analysis of Tire Debris Particles: Preliminary Results and Applications MARINA CAMATINI¹, GIOVANNI F CROSTA¹, GAIA M CORBETTA¹, MARA OTTIN-BOCAT¹, STEFANO AMBROSIO¹, GIANPAOLO GIULIANI², SIMONE CENCETTI ³ and CLAUDIA REGAZZONI³ ¹ Università degli Studi Milano - Bicocca, Dept. of Environmental Sciences, 1, Piazza della Scienza, I 20126 MILAN, Italy; contact: [email protected] ² Consorzio Milano Ricerche del CNR, MILAN, Italy ³ Pirelli Pneumatici S.p.A., MILAN, Italy. ABSTRACT Particles of tread rubber debris produced by laboratory wear tests are imaged by light microscopy. The fractal dimension, DF , of the image perimeter is estimated. The sensitivity of DF with respect to the image acquisition and processing parameters is determined. Particles belonging to two types of undifferentiated debris, low severity (LS) and rasping (R), respectively, are analyzed. The values of DF have distributions, which, if suitably interpreted, allow the identification of either debris type with the following conditional probabilities: p LS|LS = 0.71 and p R|R = 0.81. The procedure can be eventually applied to environmental impact studies. INTRODUCTION The tread of a tire is subject to wear during normal use. In fact, tires and other products e.g., brakes, have to wear in order to accomplish the task they have been designed for. The material, which results from tread wear is known as tire debris or tire dust. It consists of particles, which have irregular shapes and are distributed in size over a wide range (hundreds of micrometers down to hundreds of nanometers). The properties, by which tire debris can be characterized and later on identified among other particles are of three classes: morphological, physical and chemical. The morphology of tire debris has been investigated by many Authors, among which [1] and [2]. Interest in tire debris aims at evaluating its impact on the environment and on human health. Some related results are presented in [3], [4]. Irregular and complicated shapes call for a method of analysis, which attaches one or a few numerical descriptors (indices) to a particle. One such method is fractal analysis, by which the fractal dimension, DF , of the particle boundary is estimated. This paper gives a preliminary account on fractal analysis carried out on the perimeters of particle images. Tire debris particles come from laboratory wear tests. Fractal analysis is routinely applied to material particles and to worn surfaces. STUPAK et al. [5], [6] examined debris and tread surfaces and related DF to rubber composition and wear conditions. KINDRATENKO et al. [7] analyzed microcrystal aggregates and particles of various origin and determined the DF of their perimeters. KINDRATENKO’ s computer code has been used in the present study. MATERIALS, SPECIMEN PREPARATION The approximate composition of the tread rubber under test is: masterbatch = { 75% SBR + 10% polybutadiene + 15% natural rubber + 60 phr (parts per hundred rubber) fillers } e.g., 30 phr carbon black + 30 phr precipitated SiO2 ; 3 to 4 ph
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