Auxetic materials

Auxetic materials have a negative Poisson’s ratio: they expand laterally when stretched and contract laterally when compressed. Their unusual deformation often arises from engineered structures rather than unusual constituent substances.

Poisson’s ratio: A dimensionless measure of how a material’s transverse strain relates to its longitudinal strain. Its negative value defines auxetic behavior.
Longitudinal strain: Relative deformation measured along the direction of an applied load. Auxetic response is defined by stretching or compression along this direction.
Auxetic foam: A porous polymer foam processed to exhibit a negative Poisson’s ratio. It is a lightweight auxetic material studied for padding and impact protection.
Positive Poisson’s ratio: A Poisson’s ratio above zero, for which a material contracts laterally when stretched. This is the ordinary response that auxetic materials reverse.
Arthur Lakes: A nineteenth-century American geologist and inventor who described an unusual re-entrant honeycomb geometry. His 1880s descriptions anticipated a structure later used to explain auxetic behavior.
Re-entrant honeycomb: A cellular structure whose inward-pointing ribs rotate and bend as the structure deforms. Its inward-pointing cells open sideways when pulled.
Transverse strain: Relative deformation measured perpendicular to an applied load. Auxetic materials reverse the usual transverse response to longitudinal loading.
Impact protection: The design of structures and materials that reduce damage from collisions or sudden loads. Lateral thickening can help auxetic structures resist indentation.
Zero Poisson’s ratio: A Poisson’s ratio of zero, for which longitudinal strain produces no transverse strain. It marks the boundary between ordinary and negative lateral response.
Roderic Lakes: An American materials scientist known for research on cellular solids and auxetic materials. His 1987 work on converted polymer foam helped establish modern auxetics research.

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