Studying the physical properties and auxetic behavior of 3D-printed fasteners

Abstract

Fasteners are used in every industry, and in virtually every component. The ideal fastener would be relatively easy to insert or push into a hole, but take much more force to remove or pull out of the hole. Such fasteners would not require nuts screwed onto bolts, or other retaining rings. The current work attempts to develop a simple fastener with a low Insertion Force / Removal Force ratio by exploiting auxetic behavior. Auxetic materials or negative Poisson's Ratio materials have properties that are counter-intuitive. A rubber band, for example, becomes thinner in width when pulled lengthwise Auxetic materials, however, will expand in width, when pulled along their length, or when compressed along their length, will also contract in width. Hence, normal materials have positive Poisson's Ratios while auxetic materials are revealed to have negative Poisson's Ratio. By exploiting the theory behind negative Poisson's Ratios, a suitably designed fastener can exhibit auxetic behavior. Specifically, a properly designed fastener will decrease in diameter when pushed/compressed through a hole and increase in diameter when pulled out of the hole. Using previously developed theory, several cylindrical fasteners were fabricated that exhibited auxetic behavior. Aluminum sleeves were fabricated with holes of slightly different diameters, such that when the fasteners were inserted they must contract slightly. Insertion/Removal force ratios as low as 18% were observed. The project is considered a success and could lead to patents and be a boon to the fastener industry. Based on the research experience a learning module to be implemented in an Algebra I class has been designed. The paper describes the learning module that is based on an active learning methodology known as Legacy Cycle. It will be implemented in the spring semester in Algebra I classes in a high school with majority of Hispanic students. © 2012 American Society for Engineering Education.