Mechanics of Metal-Nanocomposites at Multiple Length Scales: Case of Al-BNNT

Abstract

Metal-nanocomposites are drawing attention of the composites community due to improvements in stiffness, strength, crack-bridging ability, and resistance to creep and fracture. The analysis of nanocomposites involves studies at multiple length scales due to the small length of the reinforcement. This paper conducts a detailed study on the mechanical behavior of a metal nanocomposite (Al-BNNT)—made of an aluminum (Al) matrix reinforced with boron nitride nanotubes (BNNTs)—under compressive and shear loadings. First a representative volume element (RVE) is modeled and analyzed using molecular dynamics (MD) simulation. Then the elastic properties are derived for a specially orthotropic lamina using a hierarchical multiscale scheme in conjunction. This result is further extended to derive elastic and shear moduli of bulk nanocomposites with aligned and randomly oriented reinforcement. The result shows excellent agreement with previous experimental observations. The bounds of elastic moduli using Voigt and Reuss formulations diverge with an increase in volume fraction of reinforcement—unlike typical composites, in which these two bounds first diverge and then eventually converge. This anomaly is attributed to the weakness of nanotubes in the radial direction. However, most elastic properties are found to be improved by the reinforcement, especially by double-walled nanotubes. Depending on the type of loading, nanocomposite exhibits failure at the matrix, interface, or nanotubes. This reveals the importance of considering all three loading cases when modeling a nanocomposite.