Multi-functional lattice structures utilising metamaterials have the potential to radically change the future of products that we use in our daily lives and the way in which industries like aerospace and the medical field operate. There are many benefits to 3D printed metamaterials that go way beyond the lightweighting offered with topology optimisation or lattice structures and other common design for AM techniques. Let’s look in more detail at the benefits of this innovative geometric design phenomenon.
Going further with metamaterials
One of the key benefits of additive manufacturing (AM) is its ability to reduce the mass of parts, e.g. lightweighting. For many applications, the extra cost of AM compared to traditional methods are offset during the lifetime of the product. For other applications, lightweighting such as utilising lattice structures has little or no effect on the lifetime cost. In these cases, it might be a good idea to consider structures that can provide multiple benefits to the part by utilising metamaterials. Benefits of 3D printed metamaterials include structural, thermal, acoustic and more.
How mechanical metamaterials differ from conventional materials
All materials are made of constituent elements known as atoms and the way these atoms connect with each other define the material properties of the part. Atoms are an inherent feature of a natural material. A metamaterial, from the Greek, “go beyond” is a class of structures that have been rationally designed and exhibit properties that go beyond those of the constituent bulk materials, e.g. it is engineered to have a property that is not found in naturally occurring materials.
Within a metamaterial, the constituent building blocks are designed and optimised. The final structure is then comprised of these specific building blocks.
The idea of artificial materials and structures that outperform those that currently exist has, until recently, been solely in the realms of fiction. However, in the last twenty years, advances in CAD, simulation and manufacturing have allowed engineers to make these structures a reality.
From a structure perspective, metamaterials can be used to create extreme material properties whilst maintaining lightweight structures. Structural elements like closed-cell plate lattices have been shown to be close to the theoretical limit for the stiffness of any material.
This process uses 3D printing on the microscale level called Two-Photon Polymerization Direct Laser Writing (TPP-DLW). Find out more about this in this open-access research paper written by Crook et al.
Another type of structural metamaterial is an auxetic lattice. These structures demonstrate a negative Poisson’s ratio. In materials science, Poisson’s ratio is a measure of the expansion or contraction of a material in the direction perpendicular to the direction of loading. Most naturally occurring solid materials have a positive Poisson’s ratio in the range of 0.2-0.3.
Auxetic metamaterials are interesting to designs as they can be used for a number of applications including energy absorption in body armour or protective sports equipment, drug delivery, stents, or filters.
Lattices have been used in thermal applications regularly. These applications can include traditional thermal applications such as heat exchangers and heat sinks. The reason for this is that lattice structures have a high surface area to volume ratio.
Looking to the future, additive manufacturing can be used for thermal metamaterials that exhibit unusual thermal properties such as constant or negative thermal expansion. Most materials expand when they are heated, however, with these metamaterials, they will either stay the exact same size or even shrink when heated. These structures can be used for many applications including high precision devices or for space antennas, optical systems or thermal actuators.
Another structure produced by additive manufacturing is the acoustic metamaterial. Acoustic metamaterials are used to manipulate and control soundwaves within a structure. They are of interest as they have many applications in cloaking, imaging, noise reduction or enhancement.
Advantages of lattice structures
The principal advantage of multi-functional metamaterials is their ability to gain functions beyond what is possible in the natural world. If we can unlock the full advantages that metamaterials can bring, we will be able to design components and products that have properties that far exceed what we have been capable of making up until now.
Additionally, The ability to combine these structures with 3D printing means that you can get the advantages of freeform structures. As with all the metamaterial structures, this means that you can get multi-functionality within a single component. This is particularly interesting for applications where space is restricted. These include aerospace where you may wish to incorporate sound reduction into a standard panel or in motorsport where space is at a minimum.
However, many applications of metamaterials have only been demonstrated in the research world. There are many challenges that have to be overcome before we are seeing multi-functional lattice structures applied across a wide range of commercially available products.
Challenges with 3D printed metamaterials
There is a lack of design tools that allow designers to create large scale metamaterial structures. One reason for this is the need to design large parts composed of many small scale elements. Most typical CAD systems utilise a geometry representation called boundary representation or BRep. The speed of processing this type of geometry is very slow when you reach thousands of faces in the model. However, many companies are now including implicit modelling capabilities within their design tools, these allow geometry to scale to larger amounts of complexity making the design of components using metamaterial possible.
At Gen3D, we have simplified the creation of the unit cells for metamaterial structures, by providing users with the option to create their own custom unit cells. By specifying the positions of the strut connections and the connectivity, users can create any unit cells and insert that unit cell into their designs. To learn more about this feature, check out this tutorial video.
Simulation of metamaterials is a challenge. Usually, multiphysics simulation tools are expensive and require expertise to use. Furthermore, using small scale unit cells inside a large structure can lead to a data-intensive simulation.
The size of the constituent elements of a material determines the type of frequency that the metamaterial can interact with. The acoustic spectrum lends itself to additively manufactured metamaterials as the wavelength of sound waves from ultrasonic which can be as small as microns to the audible frequency on the scale of metres. These lend themselves well to the current additive manufacturing technologies available.
In contrast, the visible light wavelength exists between 400 and 700 nanometres. This is generally smaller than the resolution of the majority of additive manufacturing processes, which more specialist AM techniques like TPP-DLW used at these scales.
In the future, multi-functional lattice structures could radically change the products that we use in our daily lives. Although they’re not without their challenges, lattice structures have been demonstrated in both academic and industrial applications. At Gen3D, we look forward to supporting designers and engineers create custom lattice structures that can be used to advance and improve the types of components we interact with in the future.
If you’re interested in learning more about design for additive manufacturing and the advantages that it can bring, please check out our free course. It contains over three hours of video, 50+ pages of lesson notes and interactive tutorials within Sulis software.