Emergent Mechanics of Auxetic Layered Anisotropic Composite Structures

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Auxetics are a special class of materials, which when stretched in one direction, expand in the other one or two directions, unlike conventional materials which contract. Auxetic materials and structures may be produced by laminating fiber reinforced composite layers in certain specific orientations and sequences. Producing auxetic composite structures significantly enhances their energy absorbing capability and delamination resistance, in comparison to equivalent non-auxetic structures. Although some experimental evidence exists, fundamental understanding of the underlying deformation mechanisms is severely lacking to use auxeticity for achieving optimum performance enhancements without compromising other properties. This award supports fundamental research to understand the mechanical behavior of auxetic layered composite structures with the help of an integrated experimental-modeling approach. Knowledge gained will enable composite structures with substantially improved toughness and damage tolerance for aircraft structures, automotives, and marine applications, while simultaneously providing weight, cost, and energy savings. New usages of auxetics facilitated by this research may extend to improved artificial bone implants, prosthetics, and dent-resistant laminated flooring. Furthermore, this award will facilitate recruitment and education of middle and high school students and foster their excitement about STEM fields. At the same time, undergraduate and graduate students will perform original research and attain new skills and talents in this critical area. In the conduct of these activities, the recruitment of women and minorities will be prioritized by leveraging institutional programs.The performance enhancements of auxetic layered anisotropic composite structures are fundamentally attributed to the underlying triaxial states of stress and the unique mechanics that lead to failure. To understand these issues, the constitutive and failure behaviors of auxetic composites (carbon fiber and carbon nanotube reinforced polymer matrix laminates) at varying levels of auxeticity will be examined through targeted experiments using state-of-the-art characterization and imaging. This will provide insights into the deformation and failure mechanisms at the microscopic level. Complementary micromechanical approaches will be used to model the observed stiffness degradation and to develop cohesive laws that reflect the physically observed behaviors. These experimentally validated micromechanical models will be integrated into an impact model developed using an energy-based phase field approach. The physical insights from this model, along with the ability to predict the response of auxetic structures under various loadings, will enable implementation of a science-based method of using auxeticity as a configuration constraint for achieving controllable performance enhancements of layered composite structures in various applications.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.