Development of Composite Leaf Springs Made by 4D Printing

4D printing is a manufacturing process which combines 3D printing with reconfiguration of the structure into a more complex form. The normal 4D printing would involve the deposition of polymeric materials with special properties to make flat layers. These flat layers are then subjected to some activation mechanism such as heat, light, water absorption etc. The materials in the flat layers then react to the activation mechanism to change the shape of the structure. 4D printing work was started in 2013 by Tibbits, and has received increasing attention. Most of the materials used in 4D printing have low mechanical properties. The modulus of these materials in only about 5 MPa, and these need to have special properties which can be expensive, and may not be widely available. 4D printing of composites is similar to the 4D printing mentioned above, except that the materials are regular composite materials that have been used to make structures such as airframes. These materials are light and stiff (modulus along fiber direction in order of 180 GPa), and strong (strength along fiber direction in the order of 1500 MPa). Flat layers of the composite are laid using either Hand Lay Up (HLU) or Automated Fiber Placement (AFP). The layers have different orientations to make unsymmetric laminates. Upon curing, the interaction of layer of different orientations will make the structure to be curved. This technique can be used to make structures of different curvatures, without the need to use complex molds. While the shape can be obtained, the question that remains is whether the structure is strong and stiff enough for engineering applications. This paper presents the formulation to determine the curvature, and the stiffness of the composite curved beams, intended for spring applications, that are made using 4D printing method.