Hussain J.M. Al-Alkawi et al./ Elixir Mech. Engg. 75 (2014) 27388-27396 27388 Introduction The ability of rubber to withstand very large strains with out permanent deformation makes it ideal for many applications such as tires, vibration isolators, seals, hoses and belts. As these applications impose large static and time-varying strains, durability and therefore mechanical fatigue is often the primary consideration. Such tools have been developed for other materials. However, these are commonly based upon theories of material behaviour that cannot be applied to rubber, due to rubber’s highly deformable and nonlinear nature. Typically, the fatigue failure process involves a period during which cracks nucleate in critical regions that were initially free of observed cracks, followed by a period of crack growth to the point of failure [1-3]. Mars and Fatemi [4] reported that many factors influence the fatigue behaviour of rubber. They include various aspects of the mechanical loading history, environmental effects, effects of rubber formulation and effects due to the dissipative aspects of the constitutive response of rubber. A primary consideration relating to the mechanical load history is rubber’s extreme sensitivity to not only the load range, but also the R ratio. Depending on the polymer type and the presence of fillers, the effect of increasing the minimum or mean loading may be either beneficial or harmful[1-3]. It was also reported that the fatigue life initiation and propagation in natural and synthetic rubbers are dependent upon the chemical composition, environment and the mechanical stresses applied to the sample [5]. Strain-crystallizing rubbers such as natural rubber were shown to be very sensitive to changes in the R-ratio as the fatigue crack growth rates drastically dropped for natural rubber as the minimum stress level was gradually increased. This sensitivity to R-ratio was not present during tests performed on SBR, which does not strain-crystallize. A small minimum stress level can yield very beneficial effects to the fatigue crack growth behavior [6]. Fatigue Crack Growth (FCG) Behavior: The most commonly used crack growth parameter for rubber is the energy release rate or tearing energy. This concept is based on the idea that crack growth is due to the conversion of stored potential energy to surface energy related to new crack surfaces. Measurements of the crack growth behavior under repeated stressing using various test pieces may be expressed as the crack growth per cycle (dc/dN) as a function of G or K. Results for a natural rubber (NR) compound indicate that the crack growth under repeated stressing is independent of the test piece geometry, hence, it is a true strength property of the material, and a logarithmic plot holds [7-11]: da/dN = BG α …………………………………..(1) where a is the crack length, N is the number of load applications (cycles) , and (B and ) are material constants. Crack growth is predicted by the calculation of the tearing energy for given specimen under prescribed loading conditions. It is well-adapted to tires problem in which the parts should be able to sustain crack propagation before failure [12,13]. The scientific basis for the optimization of fatigue life is to determine the rate of fatigue crack growth over a broad range of tearing energies. In practice, rubber products usually meet with progressive weakening of mechanical properties, and finally reach failure due to mechanical fatigue, i.e. continual crack growth under sinusoidal excitation with an extended period of time. The crack growth characteristics of rubbery materials must be, therefore, an important factor determining their strength and durability [14]. The fatigue crack growth is affected by not only material variables such as rubber type, but also test conditions such as test frequency, temperature and strain amplitude [15,16]. The Flexing Fatigue Properties of Filled Rubbery Compounds under Constant and Variable Amplitude Loading Hussain J.M. Al-Alkawi 1 , Dhafir S. Al-Fattal 2 and Nabel K. Abd-Ali 3 1 Electromechanical Engineering Department, University of Technology, Baghdad- Iraq. 2 Mechanical Engineering Department, University of Technology, Baghdad- Iraq. 3 College of Engineering, Al-Qadisiya University, Al-Dewaniya City- Iraq. ABSTRACT Crack growth characteristics of rubbery materials are an important factor in determining the strength and durability of the materials. The present work studied three stocks composed of Natural Rubber (NR), Styrene Butadiene Rubber (SBR) and Polybutadiene Rubber (BR cis ) filled with carbon black N330. Three percentages of (NR/SBR/BR cis ) were studied, namely (40/60/0), (40/50/10) and (40/40/20). The intent was to develop the Standard Italian Perilli Recipe (SPR) for Truck Tires Sidewall manufactured in Al-Dewaniya Tires Factory that have (30NR/70SBR) filled with carbon black N550. The results of constant amplitude loading have verified the applicability of the Paris law to the flexing fatigue behaviour of truck tire sidewall components. The accumulative fatigue damage was studied by the application of Miner’s rule to variable amplitude loading and gave unacceptable safe results at both sequence (L-H) and (H-L). The recipe that has (NR/SBR/BR cis ) blending with percentages of (40/50/10) was generally the best in combined properties. © 2014 Elixir All rights reserved ARTICLE INFO Article history: Received: 15 January 2013; Received in revised form: 20 September 2014; Accepted: 29 September 2014; Keywords Crack growth, Rubber, Sidewall, Flexing, Paris law, Accumulative fatigue, Miner Rule. Elixir Mech. Engg. 75 (2014) 27388-27396 Mechanical Engineering Available online at www.elixirpublishers.com (Elixir International Journal) Tele: E-mail addresses: nabelkadum@yahoo.com © 2014 Elixir All rights reserved