Chapter 12 Effect of Micro-Cracks on the Thermal Conductivity of Particulate Nanocomposite Addis Tessema, Dan Zhao, Addis Kidane, and Sanat K. Kumar Abstract The effect of micro-cracks on the thermal conductivity of particle-reinforced nanocomposites is investigated. Two different particles (Carbon nanotube and Silicon dioxide) with different geometries are considered to account for the effect of particle aspect ratio. Three batches of specimens, two with and one without nano-fillers are fabricated. First, the thermal conductivity of the as-fabricated samples were measured using steady state linear heat transfer unit. Afterwards, the samples were subjected to cyclic loading and at the end of every 5000 cycles the samples were taken out and the thermal conductivity was measured. At the same time, the Modulus of Elasticity of the specimens were determined using uniaxial compression test. Based on these results, the effect of micro-cracks on the thermal conductivity of the nanocomposites is presented. In addition, the relation between micro-cracks, stiffness, and thermal conductivity are presented. Keywords Micro-cracks • Nanocomposite • Thermal conductivity • Fatigue loading • Stiffness 12.1 Introduction It has been well recognized that different types and amounts of damage could occur in composites during their service time, which negatively affects the physical properties of the composite materials. Thus it is critical to investigate the damage characteristics and its effect on the mechanical, thermal and electrical properties of the bulk materials. The presence of damage, such as micro to macro cracks, and discontinuity inside the material will hinder flow of load (mechanical, thermal and electrical) through the material network. Therefore, the extent and amount of damage has direct implication on the response of the materials. In fact, many studies [1, 2] have shown that it is possible to detect the damage size and sate based on the change in property of the material relative to the undamaged state. Most common types of damage in fiber reinforced composite are, Fiber breaking, interface failure and matrix microcracking [3]. However, for particle filled composites, interface delamination and matrix micro-cracking are the main dominant damages under quasi-static loading [4]. Under fatigue loading the quantity and size of these micro-cracks grows in progress as the number of cycles grows [1, 3, 5]. In parallel as the loading cycle marches, due to the increase in the quantity and size of the cracks developed, the stiffness of the material degrades proportionally [6]. Another material property which could be affected by the presence of damage is thermal conductivity. In general, heat is transported in two ways within the material, one through vibration of lattice (phonon) and other is through electrons wave (similar to electrical conductivity in metals) [6]. In polymers and their composites, phonon is the dominant way to conduct heat [7]; furthermore since polymers are amorphous or semi-crystalline in the solid state, vibration of molecular chains is the primary heat carrier instead of lattice vibration in crystalline materials. The presence of any damage such as, interface between the matrix and filler particles, voids and cracks results in the discontinuity of the polymer chain which induces phonon scattering. Indeed, micromechanics models have shown that presence of micro-cracks has direct influence on the thermal conductivity [2], and also the amount and shape of cracks affects the effective conductivity of the materials. Studying the effect of micro-cracks/damages within the material on the thermal conductivity, and interrelate the damage extent with variation in effective thermal conductivity using experimental techniques is the motive of this study. Former studies [1, 8] showed that, under fatigue loading the damage has been encountered and these damages has reduced A. Tessema ( ) • A. Kidane Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA e-mail: atessema@email.sc.edu D. Zhao • S.K. Kumar Department of Chemical Engineering, Columbia University, 500 West 120th Street, New York, NY 10027, USA © The Society for Experimental Mechanics, Inc. 2016 A.M. Beese et al. (eds.), Fracture, Fatigue, Failure and Damage Evolution, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-21611-9_12 89
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