Neutrally stable shells with embedded actuation

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Abstract

Elastically neutrally stable structures are compelling within the field of compliant mechanisms, since no force is required to deform them. Several examples of this state of elastic neutral stability exist, which often use equal but opposing structures to balance internal forces. This thesis is on the design of an active neutrally stable structure by embedding responsive materials. Responsive materials can undergo a significant change in properties in response to stimuli, such as humidity, temperature, light, pH or electric or magnetic fields. These materials create many novel opportunities in a wide range of disciplines: from medical devices that open in the body to release drugs, to architecture which adapts to the environment. These materials do not add much, if any, mass or volume to a structure.
With the goal of creating an active neutrally stable structure, a novel actuation concept for shape mor­phing of neutrally stable compliant shells is presented. The concept is based on locally changing the stiffness in order to eliminate neutral stability and deform the structure. This concept allows for the actuation of complex deformation with a relatively simple and universally applicable actuation design. This is achieved practically by embedding Ni-­Ti wires, which undergo a significant change in stiffness upon being heated beyond their Austenite transition temperature. The wires are locally heated by ex­ternal forced convection. With that, the structure can change its shape and can exert force and work. In contrast to existing shape morphing structures, the presented structure is capable of fully reversible shape morphing while also preserving its shape after removing the stimulus. This allows for positioning without continuous actuation. The actuation concept shows potential to be widely applicable on zero stiffness compliant mechanisms.
To assess the effect of adding wires to a shell, which require prestress to be embedded in the shell, a modelling method is presented, in which the wires are projected on the deformed shell geometry. The simulated results not only show that a constant energetic path can be maintained in an already neutrally stable structure, but the model further predicts opportunities to create novel neutrally stable mechanisms by strategic design of the hybrid structure. The findings are supported by observations on hybrid shell prototypes. This model lays the foundation for the design of neutrally stable hybrid shells by internal static balancing with preshaped wires.