3D printing of morphing composite materials
Date of Issue2019
School of Mechanical and Aerospace Engineering
Technical University of Munich
In the recent years, composites have been widely studied thanks to their interesting properties and unique applications. One such innovative and novel approach is the development of bi-stable composites that exhibit a fast change between 2 stable shapes. Shape-adaptability in a material is highly demanded for autonomous sensing that could give life to static materials. Such materials have be developed in thin composite laminates with reinforcements aligned in perpendicular direction that build internal stresses. There are two ways those stresses are generated within the material using perpendicular directions of reinforcements: short fibers in hydrogel matrices that exhibit a high coefficient of thermal expansion, or swelling, and long fibers in thermoset matrices that exhibit low coefficient of thermal expansion. The first method leads to complex shapes and large deformation, whereas the second leads to geometrical basic shapes – rectangular shells- but with the possibility of a fast shape change through bi-stability. To create a composite material that has interesting shapes, fast morphing, and high stiffness and strength, the long carbon fibers typically used in prepregs could be replaced by short fibers with a local orientation controlled by a 3 dimensional (3D) printing technique. Since bi-stability occurs when sufficient stresses are built in between two layers with perpendicular orientations of reinforcement, the ideal material for each layer should exhibit high anisotropy in mechanical stiffness and in thermal shrinkage. In addition, if the material possesses other properties like electrical conductivity, this could be further beneficial in for an application as a sensor or an actuator in a more complex system. Epoxy composites comprising of aligned carbon nanotubes (CNTs) exhibit enhanced mechanical and electrical properties in the direction of alignment. However, due to the hydrophobicity of the CNTs as well as their large aspect ratio, only a small fraction can be added, limiting the extend of the anisotropic properties. This project aims at developing a composite ink that can be 3D printed to create bi-stable composites with fast change in shape. In this view, a material system comprising matrix and short reinforcements was selected and their combination optimized for extrusion-based 3D printing, mechanical strength, thermal properties and electrical conductivity were characterized as a function of the printing direction. A a first step toward the intended bistable composite, morphing bilayers were fabricated. Potential applications of these findings could range from morphing wings in aeronautics to automatic grippers in robotics.