Development of programmable material behavior on the meso and microscale

Increasingly complex material behavior requirements are pushing the classic concepts of material selection and design to their limits. In the Fraunhofer Cluster of Excellence Programmable Materials CPM, together with other Fraunhofer institutes, we are researching so-called programmable materials that adapt to environmental conditions, replace classic system approaches and allow for the development of new materials and material properties.

Development of programmable material behavior on the meso and microscale 

Mechanic metamaterials consist of homogeneous materials whose macroscopic properties are specifically parameterized by a mesoscale (µm-cm) structuring of so-called unit cells. The properties (e.g. stiffness, transverse strain coefficient, damping, thermal expansion, wettability) can therefore differ greatly from the properties of the corresponding solid material and even take on values that do not occur in nature (negative Poisson's number, i.e. auxetic behavior).

In programmable materials, the mesostructure is no longer static, but changes reversibly under certain boundary conditions or as a result of an external stimulus. This deformation brings the unit cell into a different state with different properties. For example, it can be programmed into the material that the stiffness abruptly increases many times over from a certain strain or that the material exhibits bi-stable behavior through a targeted parameter of the energy landscape. Furthermore, a distribution of geometric parameters (gradient) can be carried out in the material in order to optimize the functionality of a macroscopic component. In this way, combinations of component properties can be achieved for your products and components that cannot be achieved with conventional solid materials.

Our services:

Design and development of unit cells, metamaterials and surfaces that perform system functions (e.g. targeted shape change or controllable stiffness) tailored to your systemic requirements

Prototypical production of material samples (3D printing)

Digital imaging, modeling of material behavior and topology optimization of metamaterials

Characterization of metamaterials and their components (e.g. individual beams) and evaluation of reliability under mechanical and thermal loads 

Design and development of metamaterials and unit cells

The material behavior that is made possible via this method is: targeted shape change under external load (e.g. for adaptable surfaces), control of fluid damping (especially depending on temperature and speed), switchable stiffness (e.g. for soft robotics, orthoses) and adjustable wettability (self-cleaning surfaces).

In Materials Design, local and global effects can be incorporated through a hierarchical structure of unit cells. For this purpose, we develop unit cells and arrays in which mechanical mechanisms (contact, rotation, bumps, fluid friction...) are incorporated. These can consist of a mono-material (e.g. PLA or TPU) or several base materials. The former is particularly advantageous in terms of sustainability, the latter allows us to incorporate functional materials and control responsive behavior with triggers such as temperature (see video 1), speed or magnetic field or to use effects of smart materials such as shape memory polymers. Furthermore, symmetries and the targeted incorporation of anisotropies in beam or shell structures can lead to unusual properties. Here, instabilities (targeted buckling or kinking) can be exploited, for example, to generate history-dependent behavior (storage of deformations). The finished unit cells can perform non-linear or linear functions (e.g. transverse strain as a function of the strain rate) and fulfill conditional conditions (if-then). Ideally, these can be combined like functions in an algorithm.

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Modeling and digital imaging of metamaterials

For the systematic development of materials, we use analytical and numerical methods (e.g. finite element method, FE) to predict behavior and accelerate the design of the geometry or topology of the unit cell. Tools developed at CPM enable us to digitally map the materials. The multiscale simulation used, based on homogenization, allows the mesostructures to be optimized. Materials can be adapted to application-specific boundary conditions and the functionality of different parameter distributions or unit cells can be numerically checked before production.

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Prototype production (3D printing)

We can produce the designed unit cells and materials as prototypes by 3D printing (see Figure 2) on various size scales. Our additive manufacturing options range from printing on the micrometer scale (2-photon polymerization) to the millimeter scale (stereolithography) and the macroscale (filament printing). The use of different polymers allows us to adjust the properties of the structures not only through the geometry, but through a combination of different characteristics of the base materials (e.g. stiffness, glass point, etc.).

Other manufacturing options that we are developing and using with our partners include the deep-drawing of origami and folded structures, 4D printing and laser powder bed fusion (LPBF).

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Characterization and evaluation of reliability

The service life of the materials described above no longer depends solely on the base material, but also on the failure of the individual mechanisms. When examining programmable materials, we therefore distinguish between the loss of switchability, followed by the loss of functionality as well as component failure. In some applications, programmed materials do not need to be toggled at all or only very rarely, in others very frequently. The number of load cycles can therefore differ greatly from the number of switching cycles. By loss of functionality, we mean the irreversible loss of the programmed function due to the failure of the mechanisms. This includes, for example, wear on contact surfaces, loss of shape due to stress relaxation in polymers or the failure of individual bars/beams. The breakdown of a single unit cell does not necessarily lead to failure of the overall function in a structure consisting of many unit cells. The use of a large number of unit cells can therefore lead to resilient material behavior. In addition to the design-dependent factors, there are also effects from the manufacturing process and the base materials used. In particular, we consider the influence of manufacturing errors (pores, material accumulations) and tolerances in additively manufactured components.

To this end, we develop micromechanical characterization methods to ensure long-term functionality. The macroscopic geometries of the samples or demonstrators of the programmable materials often do not correspond to the standard test specimens. We therefore develop adapted test setups for mechanical characterization. This generally concerns the adaptation for carrying out classic tensile-compression tests under quasi-static, dynamic and cyclic loading. The development of a test rig for measuring the fluid resistance of immersed and dynamically loaded structures at different temperatures (20°-100°) also enables us to characterize metamaterials for fluid damping. In addition, we realize customized test methods for thermomechanically stressed programmable materials using our furnaces in the micromechanics laboratory.

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The development of these novel materials is being driven forward in particular by interdisciplinary cooperation. In the Fraunhofer cluster of excellence Programmable Materials CPM, a cluster of various Fraunhofer institutes, we are working on solving industrial problems with programmable materials. This can involve the improvement of everyday products such as shoes, mattresses or seats in which comfort (shape, cushioning) is increased through customization. On the other hand, programmable materials can also bring added value to complex systems by reducing the number of components or increasing the range of applications. For example, shape-adaptive connection components for stress reduction or the integration of programmable materials in orthoses are research topics in this area. The combination of programmable responsiveness to external stimuli and specifically designable actuators through a direct change in shape or properties also opens up applications in the area of sensors and activators. In this context, it is envisaged that complex signal processing systems can be replaced.

Another research field is bio-inspired programmable materials. Here we are working on a BMBF project together with the University of Freiburg (LivMats Cluster, IMTEK) and the companies Phoenix and Festo to research new possibilities for shock absorption with the help of programmable materials.

Do you think programmable materials can offer added value for your product or application? Please feel free to contact us. Collaboration can range from joint research projects to the provision of R&D services.

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Publikationen

• 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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