Variable-Stiffness Protective Material

Abstract

The focus of this research is to develop a system for limiting the motion of a mechanical joint to prevent damage to the joint. One obvious application is enhancing the safety of human joints during athletic events. For maximum reliability, the designer should eschew electronic sensors or actuators. The system also must provide minimum mechanical inhibition or altered stiffness (even if elastic) inside the safe zone of motion, but significant strength as well as stiffness on the border of the joint’s safe zone. To this end, several domains were investigated, starting with an adaptation of Auburn’s patented “open structures” with a weave that gives some bending compliance, layup-varying approaches (just as aeroelastic tailoring uses special layup directions to produce deflection coupling for altered aerodynamic properties), and new chainmail weaves. These approaches were all unsuccessful, but later research found planar compliant mechanisms and fabric arrays of teeth stiffen sufficiently in simple bending through self-contact. The former were rejected because of their discrete, unconforming nature, but the latter proved successful in restricting a test fixture’s motion when wrapped into a cylindrical shape. Finally, the fabric arrays of teeth were successfully adapted into pre-curved arrays that can be worn around a human ankle and limit its motion; however, an attempt to generalize the compliant planar comb idea by revolving its shape about an offset axis produced insufficient stiffening up (even when special backbone geometries that stiffen in tension were used), indicating that fabric backbones are a key component of the material. Further research is needed into how to generate tooth geometries for the fabric-backed arrays that will conform to a complicated shape (like the lower leg) and lock up at a specific desired angle.