Engineering with Microlattices – Doing More By Using Less
Imagine the Eiffel Tower for one second - a solid web of steel girders and soccerball-sized rivets. Bridges and skyscrapers all rely on the same lattice design as well. If you took that entire mass of steel, all 7,300,000 tonnes of it and melted it down, you'd only have a sphere 12 meters across. For its size, the use of efficient use of material is a wonder - the steel structure is itself lighter than the air between the beams (proof). Replicating the same lattice structures on a smaller scale and the exact same principles apply.
Nickel-phosphorus metallic micro lattice sitting neatly on top of a dandelion (UC Irvine, Caltech and HRL Laboratories)
Small Things make a Big Difference
The wonder that is the Eiffel Tower couldn't have been possible without new developments in steel fabrication, construction, steam-power as well as in-depth knowledge of the engineering challenges behind it. A carpenter is only as good as his tools, as that worn-out statement goes. 130 years later and there are new tools at the hands of Engineers - micro-lattice structures. And with new tools comes braver concepts and ideas.
The metallic microlattice above was created by a complicated process involving 3D Printing and Electroless Nickel plating. Researchers at UC Irvine, Caltech and HRL Labs created a design in CAD and manufactured specifically via a unique Stereolithographic method. HRL manager Dr. William Carter explains.
The template is made by exploiting an optical trick for growing polymer fibers using light. The process passes UV light through a perforated mask into at a reservoir of UV-curable resin. Fiber-optic-like ³self-trapping² of the light occurs as the resin cures under each hole in the mask, forming a polymer fiber along the path of the light. By using multiple light beams, multiple fibers can then interconnect to form a lattice. We then wash out remaining uncured resin, coat the polymer template in a thin layer of nickel, and dissolve away the polymer, leaving a very thin hollow lattice that surrounds the original template.
The microlattice is extremely thin, up to a thickness of 100 nanometers, or 1/700th the thickness of a human hair. Composed of 99.99% air with a record-breaking density of about 0.9 mg/cm3, the structure is just like the Eiffel Tower, it weighs more than air within it.
For such a fine structure one would expect to have a material with the properties of aluminum foil. In fact, nothing could be farther from the truth.
'The bulk alloy is very brittle, but when the lattice structure is compressed, the hollow tubes do not snap but rather buckle like a drinking straw with a high degree of elasticity,' Schaedler says. The lattice can be compressed to half its volume but still springs back into its original shape." (Royal Society of Chemistry)
It's been suggested that these novel properties could be used to store energy, absorb shock or create microelectromechanical machines.
Not all microlattice structures are made equal. Engineers at the Lawerance Livermore National Laboratory have been researching types of microstructures that retain their low density yet are remarkably stiff. These sorts of material properties place it in a valuable place on Ashby material charts, giving Engineers and Designers a light yet strong material unheard of until now.
One area of the team is exploring is a lattice arranged in an Octet formation with two materials, allowing it to completely resist thermal expansion. Material Engineer Dr. Chris Spadaccini commented "By designing a structure with both high and low expansion materials, we can strategically place void spaces or small amounts of bending or twisting in a local structural member to accommodate growth or shrinkage from temperature changes,”
Optimize in CAD, Save Material Costs
Engineers at the University of Nottingham took these results to the next level, creating and evolving microlattice structures to suit particular circumstances. By constantly calculating and recalculating the relationship between the volume of the material used and the maximum load it could take, optimizing the thickness of the lattice. One model that was generatively evolved was a simple crane boom. The first generation was 100 times lighter than one from solid steel, yet a third generation structure was 10,000 times lighter. However, whatever imperfections existed in fabrication negated any significant gains.
"Even a small imperfection at a local scale could have a large impact as there is no extra material that could take the added stress and maybe that is why this kind of fabrication has not been practical to date," explains researcher Yong Mao to PhysicsWorld. Mao says that the team is also studying its models to better allow for such errors. But he is convinced that commercial techniques will improve over the coming year, providing the necessary precision tools. Mao also feels that the recently commercialized technique of 3D printing could really benefit the fabrication of these structures. "We could just upload our deferent designs to a program and people could download and print off the structures at home," (Sound familiar?)
The uses for all of these advances are endless and are really in the hands of the CAD user, heralding the primacy of CAD skills above all else. Regardless, 'micro-engineering' hasn't reached the point of mass adoption - the techniques are too new, too expensive and too unreliable to be so. For those that can afford it, interest is picking up speed - literally. F1 Teams have been bringing in 3D Printing companies like Within Labs to create super-light yet strong items to give them the tiniest of edges on weight.
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