The longer the rubber band is stretched, the narrower it becomes? This is the Poisson's ratio at work!

Everyone must have had this experience: when we play with elastic rubber bands in our daily lives, as we continue to stretch them, the length of the rubber band will become longer, while the width will become narrower! This interesting phenomenon reflects a wonderful material property, so what is going on?

In material mechanics, we use a special term, Poisson's ratio , to describe the way a material deforms when subjected to external forces. Poisson's ratio simply refers to the ratio of the transverse normal strain to the axial normal strain when the material is subjected to unidirectional tension or compression. This ratio is actually an elastic constant that reflects the lateral deformation of the material, which helps us understand how the material deforms when subjected to external forces.

For most materials, their Poisson's ratio is positive, usually between 0.2 and 0.5, which means that when the material is stretched in one direction, it will shrink in the radial direction, and when it is compressed in one direction, it will expand in the radial direction.

However, there are some special materials whose Poisson's ratio is negative. This means that when such materials are stretched in one direction, their radial direction will become wider; and when the materials are compressed in one direction, their radial direction will become narrower. Such materials are called negative Poisson's ratio materials, also commonly known as "auxetic materials".

Negative Poisson's ratio materials have attracted much attention because they have unique properties different from ordinary materials and have advantages that other materials cannot match. They have good performance in fatigue resistance, energy absorption, porosity, shear resistance and fracture resistance, and are widely used in automobiles, ships, aerospace and medical fields. Typical negative Poisson's ratio materials include pyrite, cat skin, pyrolytic graphite, human arterial endothelium, sunflower foam core and rocks with microcracks. The characteristics of these materials make them play an important role in scientific research and engineering applications.

Source

This work was completed under the guidance of Professor Fan Yubo and Professor Wang Lizhen from the Biomechanics Science Communication Expert Team of the China Association for Science and Technology and the Biomechanics Science Communication Team of the Chinese Society of Biomaterials.

Authors: Yao Yan, Shi Yanzhu, Yu Jiayu, Yang Xinyi, Huang Huiwen

Supporting units: Beijing Biomedical Engineering Advanced Innovation Center Science Education Base, Materials Biomechanics Branch of the Chinese Society of Biomaterials.

This work is funded by the China Biomaterials Society's Excellent Science Popularization Project