Stepped Moduli in Layered Composites

Abstract

This paper describes adaptive composites that respond to mechanical stimuli by changing their Young's modulus. These composites are fabricated by combining a shorter layer of elastic material (e.g. latex) and a longer layer of stiffer material (e.g., polyethylene and Kevlar), and fixing them together at their ends. Tension along the layered composite increases its length, and as the strain increases, the composite changes the load-bearing layer from the elastic to the stiff material. The result is a step in the Young's modulus of the composite. The characteristics of the step (or steps) can be engineered by changing the number of layers (thereby changing the number of steps in the modulus), the difference in lengths of the layers (thereby changing the range of steps), or the type of materials (thereby changing the tensile modulus at each step). For composites with more than two steps in modulus, the materials within the composites can be layered in a hierarchical structure to fit within a smaller volume, without sacrificing performance.These composites can also be used to make structures with tunable, stepped compressive moduli.For example, when a compressive force is applied axially to an elastomeric cylinder that is wrapped with the composite around its circumference, it expands laterally and applies a tensile force on the composite. An adaptation of these principles can generate an electronic sensor that can monitor the applied compressive strain. Increasing or decreasing the strain closes or opens a circuit and reversibly activates a light-emitting diode.3

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