The Lawrence Livermore National Laboratory (LLNL) often uses 3D printed materials, including metamaterials, to do impressive work. Now, the lab has introduced a new class of metamaterials that respond and strengthen 3D printed structures almost instantly when exposed to a magnetic field. LLNL calls the material "field-response mechanical metamaterial" or FRMM. They involve viscous magnetically responsive fluids that are injected into hollow pillars and 3D printed lattice beams. Unlike other 4D printing materials, the overall structure of FRMM will not change. The ferromagnetic particles of the fluid located in the core of the beam form chains in response to the magnetic field, stiffening the fluid and the lattice structure. This happened in less than a second. The research is documented in a paper entitled "Responsive Mechanical Metamaterials in the Field".
Magnetically responsive metamaterial instantly hardens 3D printed structures
"In this article, we really want to focus on the new concept of metamaterials with adjustable properties, and even if it's just a hand-made process, it still highlights what can be done, and that's what I think is really exciting, Lead author Julie Jackson Mancini is an LLNL engineer and has been working on this project since 2014. "It has been proven that through materials, metamaterials can create mechanical properties that sometimes don't exist in nature or can be highly engineered, but once you build a structure you insist on using these attributes. The next evolution of these metamaterials is to adjust their mechanics Things that respond to external stimuli. Those that exist, but they respond by changing the shape or color, and the time it takes to respond may be minutes or hours. With our FRMM, the overall form does not change, the response is very fast, This sets it apart from other materials. "
Researchers inject magnetorheological fluids into hollow lattice structures based on the LLNL Large Area Projection Microlithography (LAPμSL) platform, which is capable of 3D objects with microscale features over a wide area using light and photopolymer resins print. According to Mancini, the LAPμSL machine has played an important role in the development of new metamaterials because complex tubular structures need to be made of thin walls and able to hold fluids contained, while withstanding the pressures generated during the filling process and the response to magnetic fields.
By changing the strength of the applied magnetic field, the strengthening of the fluid and the strengthening of the 3D printed structure are reversible and adjustable. "What really matters is that it is more than just a switch response. By adjusting the strength of the applied magnetic field, we can obtain a wide range of mechanical properties," Mancini said. "The idea of real-time remote adjustability opens the door for many applications."
These applications include shock absorption, such as car seats, where fluid-responsive metamaterials are integrated and sensors that can detect collisions. The seat will harden during an impact, which may reduce casualties. Other applications include helmets, neck supports, housings for optical components or soft robots.
To predict how the lattice structure responds to the applied magnetic field, former Messner, a former LLNL researcher working at the National Laboratory, has now developed a model from single-pillar testing. Starting from the model he developed to predict the mechanical properties of non-tunable static lattice structural materials, he added how magnetic field response fluids affect a single lattice element under a magnetic field, and incorporated a model of a single pillar into a designed cell Then, he calibrated the model to experiments, and Mancini experimented on fluid-filled tubes similar to the pillars in a grid. Researchers use this model to optimize the topology of the lattice and find structures that cause large changes in mechanical properties as the magnetic field changes.
Magnetically responsive metamaterial instantly hardens 3D printed structures
"We studied elastic stiffness, but models (or similar models) can be used to optimize different lattice structures for different types of targets," Messner said. "The design space for possible lattice structures is huge, so the modelling and optimization process helps us choose possible structures with favorable properties before printing, filling and testing actual samples."