Energy Dissipation and the High-Strain Rate Dynamic Response of Vertically Aligned Carbon Nanotube Ensembles Grown on Silicon Wafer Substrate

P Raju Mantena,L David Stephenson,Brahmananda Pramanik,Ashok Kumar,Veera M Boddu,Tezeswi Tadepalli,Matthew W Brenner

doi:10.1155/2013/259458

Abstract

The dynamic mechanical behavior and high-strain rate response characteristics of a functionally graded material (FGM) system consisting of vertically aligned carbon nanotube ensembles grown on silicon wafer substrate (VACNT-Si) are presented. Flexural rigidity (storage modulus) and loss factor (damping) were measured with a dynamic mechanical analyzer in an oscillatory three-point bending mode. It was found that the functionally graded VACNT-Si exhibited significantly higher damping without sacrificing flexural rigidity. A Split-Hopkinson pressure bar (SHPB) was used for determining the system response under high-strain rate compressive loading. Combination of a soft and flexible VACNT forest layer over the hard silicon substrate presented novel challenges for SHPB testing. It was observed that VACNT-Si specimens showed a large increase in the specific energy absorption over a pure Si wafer.

Highlights

Graded materials (FGMs) are a new generation of engineered materials wherein the microstructural details are spatially varied through nonuniform distribution of the reinforcement phase(s)
Three specimens each of pure Si wafer and vertically aligned carbon nanotubes (VACNTs)-Si were tested in the DMA under three-point oscillatory flexural load over a temperature range from ambient (30∘C) to 120∘C
The flexural rigidity and damping of these specimens were investigated in terms of the required force (F, line force at midspan to achieve 10 μm amplitude, Figures 3(a) and 3(b)), “apparent” storage modulus (E󸀠, Figures 3(c) and 3(d)), and the damping loss factor (tanδ, ratio of dissipated energy to stored energy, Figures 3(e) and 3(f)). These values remain constant over the 120∘C test temperature range, without a demonstrated peak of loss factor along with drop in storage modulus, which is more typical for viscoelastic materials around their glass transition temperature [5, 6]. Both the required force and “apparent” storage modulus show a negligible drop for the VACNT-Si specimens, but still within experimental scatter

Summary

Introduction

Graded materials (FGMs) are a new generation of engineered materials wherein the microstructural details are spatially varied through nonuniform distribution of the reinforcement phase(s) These FGMs are being investigated for applications in blast, ballistic protection of building structures and other armor applications. A fundamental knowledge and understanding of the dynamic behavior of vertically aligned carbon nanotubes (VACNTs) with functionally graded stiffness modulus is needed to understand the energy transfer mechanisms and their dependence on the microstructures of carbon nanotube (CNT) ensembles. This information is expected to lead to novel, strong, lightweight, and robust composites for enhanced protection. This concept of engineering the material’s microstructure allows designers to fully integrate the material and structural considerations into the final design of structural components

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Results

Conclusion