Collaborative Research: Investigations of Density-graded Auxetic Foams at Multiple Scales

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Polymeric foams have been used in many impact mitigation applications, affecting various aspects of our daily activities ranging from walking and running to protection gears in action sports. Nonetheless, shortcomings in their performance results in serious health hazards, e.g., concussions in football, despite attempts to increase efficacy through layers or reinforcement. This award seeks to exploit a novel derivative of foams, known as auxetic foams, to effectively mitigate the danger from impacts to humans by leveraging new deformation and energy dissipation mechanisms. To reach this goal, this award strives to uncover how different aspects of the material and geometry contribute to the efficiency of the auxetic padding in energy dissipation. The approach integrates computational and experimental methods to elucidate the structure-property-performance relations spanning multiple lengths and time scales. The successful implementation of the proposed collaborative activities will lead to novel impact-tolerant materials. The interdisciplinary nature of this research will facilitate the engagement and training of the next generation of scientists and engineers, with emphasis on students from underrepresented and underserved minorities. Students from different academic levels will be recruited and involved in different aspects of the research.Auxetic foams and their density-graded derivatives, e.g., auxetic polyurea foams, can simultaneously activate several energy dissipation mechanisms, making them ideal impact mitigating materials. The award aims to develop a comprehensive experimental-computational framework to elucidate the intrinsic energy dissipation mechanisms operative in these materials at multiple scales. The development of the integrated framework includes probing and modeling the material over a broad range of length (molecular to μm to mm) and time (μs to ms) spectra. For example, in-situ terahertz time-domain spectroscopy for changes in the intermolecular vibrational modes during mechanical loading, high-speed digital image correlation for dynamic deformation modes at multiple scales, scanning electron microscopy for characterization at the microscale, and direct numerical simulations using the finite element method. This expansive scope is expected to reveal the complex interplay between the material entrapped in the cellular microstructure and the change in their molecular arrangement, material nonlinearities, the geometrical attributes and discontinuities of the cells, elastic instabilities, and their fundamental role in the overall mechanical response of density-graded auxetic materials.This project is co-funded by the Mechanics of Materials and Structures (MOMS) and the Dynamics, Control, and Systems Diagnostics (DCSD) programs in the Civil, Mechanical and Manufacturing Innovation Division.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.