A thermally sensitive energy-absorbing composite functionalized by nanoporous carbon
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Aijie Han Department of Chemistry, University of Texas—Pam America, Edinburg, Texas 78539
Taewan Kim Program of Materials Science & Engineering, University of California—San Diego, La Jolla, California 92093
Yu Qiaoa) Department of Structural Engineering, University of California—San Diego, La Jolla, California 92093-0085; and Program of Materials Science & Engineering, University of California—San Diego, La Jolla, California 92093 (Received 30 April 2009; accepted 21 July 2009)
A polypropylene-matrix composite material functionalized by nanoporous particulates was produced. When the temperature is relatively low, the matrix dominates the system behavior. When the temperature is relatively high, with a sufficiently large external pressure the polymer phase can be intruded into the nanopores, providing an energy absorption mechanism. I. INTRODUCTION
Composite materials, especially polymer matrix composites, have been intensively studied for energy absorption applications.1 When the temperature is close to or higher than the glass transition temperature (Tg), the polymer matrix is usually viscoelastic. With an external loading, either dynamic or quasi-static, the relaxation of the network polymer chains would cause a considerable energy dissipation effect, which works quite well under cyclic loadings.2 The reinforcements can be continuous fibers, short fibers, particulates, and/or platelets.3–5 They can significantly enhance the overall stiffness, strength, and toughness, as well as the anisotropic and heterogeneous properties.6 If the bonding between the matrix and the reinforcements is relatively weak, debonding can take place when the local stress exceeds the critical value, which is often promoted by the stress concentration and/ or wave redispersion.7 As a first-order estimation, debonding of reinforcements of an overall interfacial area of A would result in an energy dissipation of E ¼ gd A ;
ð1Þ
where gd is the effective surface free energy. It can be seen that if the interfacial area, A, increases, the theoretical upper limit of the energy absorption capacity is higher, and thus using nanometer (nm)-sized a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0408
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 11, Nov 2009 Downloaded: 17 Mar 2015
fillers of ultralarge surface to volume ratios becomes an attractive concept.8 For instance, if 1 g of carbon nanotubes (CNTs) are embedded in a polymer matrix and if they can fully debond, 102 to 103 Joules (J) of energy can be dissipated. The high stiffness and strength of CNTs assure that they have the ability to store sufficient strain energy to trigger the interface debonding. However, under a compressive loading, especially when the strain rate is high, debonding would inevitably cause local weakening, which makes the stress wave distribution highly nonuniform.9 As a result, shear localization can significantly limit the overall energy absorption efficiency. In a few narrow shear bands, local defor
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