Development of Programmable Mechanical Metamaterials

As requirements for materials behavior become increasingly complex, traditional concepts of material selection and design are reaching their limits. In the Fraunhofer Cluster of Excellence Programmable Materials CPM, we are collaborating with other Fraunhofer institutes to conduct research on so-called “programmable materials” that adapt to environmental conditions, replace traditional system approaches, or can be switched between different properties.

Development of Programmable Materials Behavior at the Meso- and Microscales

Mechanical metamaterials consist of homogeneous materials whose macroscopic properties are specifically tuned through a mesoscale (µm–cm) structure composed of so-called unit cells. As a result, the properties (e.g., stiffness, Poisson’s ratio, damping, thermal expansion, wettability) can differ significantly from those of the corresponding bulk materials and can even take on values that do not occur in nature (negative Poisson’s ratio, i.e., auxetic behavior).

In programmable materials, the mesostructure is no longer static but changes reversibly under certain boundary conditions or in response to external stimuli. The deformation causes the unit cell to transition to a different state with different properties. For example, it is possible to program the material so that, starting at a certain strain, the stiffness increases abruptly by a multiple, or so that bistable behavior arises through a targeted design of the energy landscape. In addition, geometric parameters within the material can be varied gradually to optimize a component’s functionality at the macroscopic level. This enables combinations of component properties for your products and components that cannot be achieved with conventional solid materials.

The development of these novel materials thrives on interdisciplinary collaboration. In the Fraunhofer cluster of excellence Programmable Materials CPM — a consortium of various Fraunhofer institutes — we develop programmable materials to solve industrial challenges.

Our services:

• Design of specific mechanical materials behavior (e.g., locally tailored shape and stiffness for shoe soles, protective gear, etc.)
• Integration of switchable shape change and stiffness into technical applications (e.g., prosthetics, grippers)
• Design, prototyping, and characterization of lattice structures
• Development of fluid-filled metamaterials with controllable and self-regulating damping
• Customized characterization methods for hierarchically structured materials and multiphysical effects

Do you think programmable materials could add value to your product or application? Feel free to contact us. Our collaboration can range from joint research projects to custom research services.

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Design of Specific Mechanical Materials Behavior

Programmable materials allow for the specification and local adjustment of specific force-displacement curves. For the application example of a shoe sole, for instance, a monomaterial structure was developed whose properties result solely from the local configuration of the unit cells.

For systematic development, we use analytical and numerical methods (e.g., the finite element method, FEM) to predict behavior and accelerate the design of the geometry and topology of the unit cells. Tools developed at CPM enable us to digitally model the materials. Multiscale simulations based on homogenization allow for the optimization of mesostructures. This makes it possible to adapt materials to application-specific boundary conditions and to numerically evaluate the functionality of different parameter distributions or unit cells even before manufacturing.

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Integration of Switchable Shape Change and Stiffness into Engineering Applications 

To replace existing engineering solutions with programmable materials, we first abstract the models (analytically and/or numerically) to understand how they work. From this, we derive the necessary dependencies and conditions (if/then/else) that materials must satisfy to meet the desired requirements — thereby expanding the existing solution space.

A key design principle for switching between states is bistability: structures have two states that are stable without an energy input. This property has been integrated, for example, into a passive finger prosthesis that can be switched back and forth between different shapes. Implementing it as a metamaterial enables direct 3D printing from a single material without additional assembly steps (non-assembly). In addition to mechanical force, other triggers such as temperature or magnetic fields can also be used to switch materials.

In addition to shape, stiffness can also be switched. This allows components to be specifically adapted to changing operating conditions and enables functions that would be only partially possible — or not at all — with conventional, static materials — such as gripper systems that require different levels of stiffness depending on the object.

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Design, Prototyping, and Characterization of Lattice Structures

By combining nonlinear materials behavior with suitable lattice structures, programmable behavior can be achieved. For example, nonlinear characteristic curves of lattice structures can be specifically designed. On the one hand, this enables a high degree of reversible deformability in metals, with strains of several percent. On the other hand, changes in stiffness can be achieved through the engagement of additional beam elements once a defined strain or compression is reached.

When using polymers, materials characteristics such as viscoelasticity, glass transition, or shape memory effects can be leveraged to tailor the grids’ responsiveness to strain rate, temperature, or other stimuli.

Depending on the application, we manufacture grids on various scales. Our additive manufacturing includes other manufacturing processes that we use and continue to develop with our partners such as deep drawing of origami and folded structures, 4D printing, and Laser Powder Bed Fusion (LPBF).

• Micrometer scale: 2-photon polymerization
• Millimeter scale: Stereolithography
• Macro scale: Filament printing

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Development of Fluid-Filled Metamaterials with Controllable and Self-Regulating Damping

When combined with fluids, metamaterials enable adjustable damping. Through targeted shape changes, flow channels in fluid-filled metamaterials can be varied in such a way that fluid friction — and thus energy dissipation — can be precisely controlled. The damping is both actively controllable and passively self-regulating.

Programming is achieved through triggered shape changes, such as those caused by temperature, impact velocity, or impact mass. As a result, the damping adaptively adjusts to the load conditions. The mechanism is pressure-independent, since the fluid does not flow through a single constriction with a defined pressure difference; rather, the damping is primarily determined by variable flow paths. In arrays of such metamaterial cells, locally varying damping can also be achieved by grading the structure.

Another research project in the field of damping is the BMFTR project “ProBand.” Together with partners from Fraunhofer CPM, the University of Freiburg, and the company Festo, we are developing a novel, optimized shock absorber. It is modeled after the human intervertebral disc, whose mechanisms of action are abstracted into a programmable material and implemented as a technical shock absorber. Through appropriate programming, the shock absorber reacts to the speed of the approaching object and brakes it safely and quickly to its final position. Biological mechanisms are thus transferred to mechanical structures. By adjusting geometric parameters, stiffness and damping can be specifically tuned and tailored to different application scenarios.

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Customized Characterization Methods for Hierarchically Structured Materials and Multiphysical Effects 

In hierarchically structured materials, the service life depends not only on the base material but also on the failure of individual mechanisms. Therefore, they must be investigated at various hierarchical levels.

For programmable materials, we distinguish between loss of switchability, loss of functionality, and component failure. In some applications, programmable materials are not switched at all or only rarely; in others, they are switched very frequently — the number of load cycles and the number of switching cycles can vary greatly.

By “loss of functionality,” we mean the irreversible loss of the programmed function due to the failure of the mechanisms, such as wear on contact surfaces, loss of shape due to stress relaxation in polymers, or fracture of individual webs or beams. In a structure composed of many cells, the failure of a single unit cell does not necessarily lead to the failure of the overall function. The use of a large number of unit cells can thus contribute to resilient material behavior.

In addition to design factors, we take manufacturing influences and the properties of the base materials into account. Particularly for additively manufactured components, we analyze the influence of manufacturing defects (pores, material accumulations) and tolerances. Often, the macroscopic geometries of samples or demonstrators do not correspond to standard test specimens. Therefore, we develop customized test setups for mechanical characterization.

In addition to mechanical characterization, we investigate multiphysical effects of programmable metamaterials. Examples include:

• a test rig for measuring the fluid resistance of submerged, dynamically loaded structures at various temperatures (20–100 °C)
• customized test methods for thermomechanically stressed programmable materials, e.g., measuring the shrinkage force of FGP rings upon triggering the two-way effect (80 °C) over time or determining the coefficient of thermal expansion (CTE)

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Publications

• Chu, C.; Leichner, A.; Wenz, F.; Andrä, H., Exploring VAE-driven implicit parametric unit cells for multiscale topology optimization, Materials & Design 244 (2024) Art. 113087, 46 Seiten Link
• Wenz, F.; Schönfeld, D.; Fischer, S.C.L.; Pretsch, T.; Eberl, C., Controlling malleability of metamaterials through programmable memory, Advanced Engineering Materials,  3/2 (2023) Art. 2201022, 10 Seiten; 91/2022 Link
• Lichti, T.; Leichner, A.; Andrä, H.; Müller, R.; Wenz, F.; Eberl, C.; Schwarz, A.; Hübner, C., Optimal design of shape changing mechanical metamaterials at finite strains, International Journal of Solids and Structures,  252/ (2022) Art. 111769, 17 Seiten; 71/2022 Link
• Schwarz, A.; Lichti, T.; Wenz, F.; Scheuring, B.M.; Hübner, C.; Eberl, C.; Elsner, P., Development of a scalable fabrication concept for sustainable, programmable shape-morphing metamaterials, Advanced Engineering Materials,  24/11 (2022) Art. 2200386, 10 Seiten; 88/2022 Link
• Straub, T.; Fell, J.; Zabler, S.; Gustmann, T.; Korn, H.; Fischer, S.C.L., Characterization of filigree additively manufactured NiTi structures using micro tomography and micromechanical testing for metamaterial material models, Materials,  16/2 (2022) Art. 676, 14 Seiten; 143/2022 Link
• Lichti, T.; Andrä, H.; Leichner, A.; Müller, R.; Wenz, F., Optimal design of unit-cell based programmable materials, PAMM 20/1 Special Issue: 91th Annual Meeting of the International Association of Applied Mathematics and Mechanics (GAMM); Kuhl, D.; Mesiter, A.; Ricoeur, A.; Wünsch, O. (Eds.); John Wiley & Sons, Inc., Hoboken, NJ, USA (2021) e202000010, 2 Seiten Link
• Schönfeld, D.; Chalissery, D.; Wenz, F.; Specht, M.; Eberl, C.; Pretsch, T., Actuating shape memory polymer for thermoresponsive soft robotic gripper and programmable materials, Molecules 26/3 (2021) Art. 522, 20 S. Link
• Specht, M.; Berwind, M.; Eberl, C., Adaptive wettability of a programmable meta‐surface, Advanced Engineering Materials 23/2 (2021) Art. 2001037, 6 Seiten Link
• Wenz, F.; Schmidt, I.; Leichner, A.; Lichti, T.; Baumann, S.; Andrae, H.; Eberl, C., Designing shape morphing behavior through local programming of mechanical metamaterials, Advanced Materials 33/37 (2021) Art. 2008617, 8 Seiten Link
• Fischer, S.C.L.; Hillen, L.; Eberl, C., Mechanical metamaterials on the way from laboratory scale to industrial applications: Challenges for characterization and scalability, Materials 13/16 (2020) Art. 3605, 16 Seiten Link
• Berwind, M.F.; Kamas, A.; Eberl, C., A Hierarchical Programmable Mechanical Metamaterial Unit Cell Showing Metastable Shape Memory, Advanced Engineering Materials 20/11 (2018) 1800771 1-6 Link
• Nakanishi, K.; Aria, A. ; Berwind, M.; Weatherup, R. S.; Eberl, C.; Hofmann, S.; Fleck, N., Compressive behavior and failure mechanisms of freestanding and composite 3D graphitic foams, Acta Materialia 195 (2018) 187-196 Link

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