Auxetic behavior in some fiber network structures
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Auxetic materials are a rare class of materials that exhibit negative Poisson’s ratio. While most substances (like a rubber band) become thinner in lateral direction when stretched, auxetic materials grow thicker. The broad objective of this research is to study the origins of auxetic behavior in fibrous networks and to develop predictive processing-structure-property relations for these materials systems. We start by examining out-of-plane Poisson's ratio in paper by investigating a range of carefully chosen commercial paper samples. Laboratory handsheets were also produced and examined for their out-of-plane auxetic response. Geometrical and finite element models were built to help understand the origin of and underlying mechanism responsible for this auxetic response. Additionally, we were able to create a similar auxetic response in needle-punched nonwoven fiber networks by a heat-compression treatment. A series of microscopic and tomographic characterization was performed. From results on paper and nonwovens, it is evident that the type of network stabilization (hydrogen bonding in paper and needle-punching in nonwovens) and the choice of subsequent processing conditions have a significant influence on the out-of-plane Poisson’s ratio in these materials. Ultimately, a fundamental understanding of the origins of deformation behavior in these fiber networks should lead to the prospect of rational design of new auxetics and, in turn, to new product development opportunities for fiber-network materials.