4D printing combines additive layer manufacturing processes with smart materials to create structures that are able to change shape or properties over time under the influence of environmental stimuli. The article presents 3D printed multi-material shape-variable structures imitating a hinge. Fused deposition modelling was used because it provides the ability to pre-program structures during the printing process by varying printing parameters. The structures are printed with PLA and TPU and remain flat after printing until they are exposed to a stimulus - heat. The main objective of this article is to present the possibilities of the aforementioned pre-programming step which can be adapted by varying the printing process and design parameters of the printed part. Experimental results are presented investigating the influence of printing speed, temperature of the build plate and number of active layers in the structure. Furthermore, the repeatability of deformations after a small number of cycles is investigated. The obtained results prove that the deformation of the structures can be controlled by printing parameters and a variety of bending degrees can be obtained by manipulating them. Hot water is used as a stimulus in the study to activate the structures but it is believed that other direct and indirect heating sources are also applicable. The research could help predict the behaviour of deformation of shape-morphing structures by selecting certain printing and design parameter values.

Smart Structures technology called Adaptronics in Germany covers the entire field of making the elastomechanical behaviour of structures adaptable. The main objectives are vibration control, noise reduction and shape control. The latter is directly related to morphing of airframes, thus being a focus of DLR's aeronautical research program. DLR initiated the national morphing wing activities in the mid-nineties with research projects on morphing wing trailing egdes. The lessons learned where exploited continuously in different national and international projects up today. The research covers the most relevant application scenarios of morphing like smart trailing egde, smart winglet, or smart leading egde including the smart slat. Presently the gapless droop nose for laminar wings of future civil transport aircrafts is in the focus of the investigations at DLR in collaboration with European partners. This paper gives a survey of the related national and international activities where DLR has been involved in since the mid-nineties and elaborates the challenge of designing morphing wing structures, i.e. to provide flexibility for the deformation and stiffness for carrying high loads at the same time