Speaker: Connor McMahan, Caltech

Thursday Sept 9th 2021, 4:00pm
Title: Modeling and programming shape-morphing structured media

Shape-morphing is an example of a mechanical behavior that can be “programmed” in structured media by designing geometric features at micro- and mesostructural length scales. This programmability is possible because the small-scale geometry often imposes local kinematic modes that are strongly favored over other deformations. In turn, global behaviors are influenced by local kinematic preferences over the extent of the structured medium and by the kinematic compatibility (or incompatibility) between neighboring regions of the domain. This considerably expands the design space for effective mechanical properties, since objects made of the same bulk material but with different internal geometry will generally display very different behaviors. This motivates pursuing a mechanistic understanding of the connection between small-scale geometry and global kinematic behaviors. This talk will address challenges pertaining to the modeling and design of structured media that undergo large deformations. The first part of the talk will focus on the relation between mesoscale patterning and energetically favored modes of deformation. This is discussed within the context of sheets with non-uniform cut patterns that buckle out-of-plane due to kinematic frustration. Motivated by computational challenges associated with the presence of geometric features at disparate length scales, we construct an effective continuum model for these non-periodic systems, allowing us to simulate their post-buckling behavior efficiently and with good accuracy. This method could be adapted to applications such as the prediction of how soft robotic frames will deform when squished. The second part of the talk will discuss a way to leverage the connection between mesoscale geometry and energetically favorable local kinematics to create “programmable media” that undergo prescribed shape changes when exposed to an environmental stimulus. We demonstrate the capabilities of an algorithm that inversely designs the small-scale geometric features that will enforce desired macroscopic behaviors. This method automates the design of structured plates that morph into target 3D geometries over time-dependent actuation paths. In the same way that folding an origami swan requires creases to be folded in sequence rather than simultaneously, this method of programming both shape changes and deformation rates enables initially planar structures to reach target geometries that would otherwise be impossible to achieve.

Connor McMahan is a recent graduate of Caltech’s Mechanical Engineering PhD program. His thesis work with Prof. Chiara Daraio focused on developing systematic approaches to the design of shape-changing structures. This entails studying how a structure’s global mechanical behaviors can be “programmed” by tuning geometric features at the micro/mesoscale, then using these concepts to create deployable devices and soft robots that change shape and locomote in response to environmental stimuli. In addition to his work at Caltech, Connor was a NASA Space Technology Research Fellow and spent two summer collaborating with researchers at the NASA Jet Propulsion Lab. Before moving to California for graduate school, he earned a B.Sc. in Mechanical Engineering at MIT.