Tailoring of Hierarchical and Gradient Lengthscales for Extreme Stiffness and Damping
Abstract
Superior stiffness and damping are critical for numerous structural applications that require extreme vibration damping and impact mitigation. However, simultaneously improving both stiffness and damping in a material is often challenging because of the mutually exclusive nature of these two mechanical properties. Recent advancements in hierarchical materials offer new opportunities to develop lightweight stiff materials that also have superior damping capacity because of the presence of structural features across multiple lengthscales that can be tailored during synthesis. Developing a fundamental understanding of the relationships between the hierarchical structure and their effective mechanical properties such as specific stiffness and specific damping is crucial for creating superior lightweight stiff damping materials. This research aims to develop such key structure-property descriptions of hierarchical materials in quasistatic and dynamic regimes using vertically aligned carbon nanotube (VACNT) foams as model systems. A characteristic buckle wavelength emerges in VACNT foams under compression, because of an inherent competition between the tortuosity of CNT fiber bundles that tends to reduce the critical buckling load and the neighbor interactions of the CNTs that tend to increase the critical buckling load. This lengthscale predominantly governs the bulk stress-strain response of the VACNT foams and their bulk properties such as modulus and damping. This emerging lengthscale primarily depends on the hierarchical structure of the VACNT foams that span a broad lengthscaleÑa multi-walled CNT structure in the nanoscale, their entangled forest-like system in the microscale, and their organization into vertically aligned bundles in the mesoscale. VACNT foams also present controllable gradient functional properties such as mass density and stiffness gradients along the height of the sample. Correlating the hierarchical and gradient structures of the VACNT foams to their effective mechanical properties will enable us to develop scaling laws that govern their behavior. The specific objectives of this research are to: (i) develop a fundamental understanding of the emergence of the characteristic lengthscale of collective progressive buckling in VACNT foams and their mesoscale architectures, (ii) identify and describe the underlying key relations among the structure, intrinsic structural gradients, and the stiffness-damping properties of the VACNT foams in quasistatic and dynamic regimes, and (iii) investigate the ability to achieve synergistic enhancement of mutually exclusive properties such as stiffness and damping through mesoscale architecturing of VACNT foams. The structure of the CVD-grown VACNT foams and their mesoscale architectures across different lengthscales will be characterized using synchrotron X-ray scattering, scanning and transmission electron microscopy, and Raman spectroscopy. Their mechanical response will be measured using a custom-built broadband (1-2000 Hz) large-amplitude (up to 90 microns) dynamic mechanical analyzer with high-speed imaging in dynamic regime and in situ SEM micro-compression in quasistatic regime (strain rates: 0.01-1/s). Based on the observed scaling laws in VACNT foams in both linear and nonlinear dynamic regimes, approaches to synergistically improve both stiffness and damping through mesoscale architecturing will be investigated. Developing a comprehensive fundamental understanding of the emergence of the collective buckle wavelength and its role in quasistatic and linear and nonlinear dynamic response of VACNT foams and their mesoscale architectures will provide new pathways to develop lightweight hierarchical materials with extreme tunable damping and stiffness. Moreover, the mechanistic insights developed in this study can potentially help in understanding the mechanical behavior of other similar hierarchical materials systems such as composites and biological materials.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 09, 2020
- Source ID
- W911NF2010160
Entities
People
- Ramathasan Thevamaran
Organizations
- Army Contracting Command
- United States Army
- University of Wisconsin–Madison