Page 17 - CEGE Magazine - Fall 2016

terize the crack mode (opening and/or sliding) and orientation.
New methods have been developed by Labuz and his team of researchers that provide a means to determine crack kine- matics even though the crack is not visible.
The capabilities of these methods can be demonstrated in the lab by loading a beam instrumented with AE trans- ducers and testing it to failure. AE clearly locates the initiation of a fracture
STEFANO GONELLA CONT...
guides (Figure 7). We can also engineer metamaterial architectures that can steer a wave, forcing it to follow a prescribed path within the material (Figure 8), instead of allowing it to propagate with equal speed in all directions, as is commonly observed in conventional media. This could be called an acoustic shield, capa- ble of de ecting harmful waves away from a sensitive target. With these capabilities, metamaterials could revolutionize how we manage vibration and sound problems.”
It is possible to establish a rigorous mechanistic relationship between the wave propagation phenomena observed at the macroscopic scale and the geo- metric architecture of these materials. “A big component of our research work is dedicated to developing mathematical models, predictive simulation tools, and laboratory experiments necessary to shed light on this complex relation. Once this link is fully understood, the internal archi- tecture of the material can be engineered at will and we can design materials with a wide range of desired functionalities.”
The applications for metamaterials stretch across a broad range of disciplines.
An example taken from the aerospace world is the design of smart airfoil skins. The skin of a wing airfoil is a thin layer of material that gives the airfoil its aerody- namic shape. It also protects the interior of the wing from excitations that might come from the outside--wind gusts or
the impact of a projectile. A conventional skin (typically just a curved sheet of metal) could be replaced with a metamaterial
through the clustering of events (Figure 6). The analyses indicate that the small cracks generating the AE were on the order of millimeters in length, similar to the average grain size of the studied material. Further, the analyses show a crack-opening tendency compatible with the failure mechanism of the beam. The AE method allows for non-invasive evaluation of fracture in the subsurface and built infrastructure.
Figure 6. Locations of AE up to about 95% of peak stress (blue) and around peak stress (red). Microcracking was more or less random prior to peak, but a localized region corresponding to the eventual fracture is clearly identi ed.
layer equipped with wave-controlling capabilities that could steer and reroute the energy, preventing it from reaching the interior of the airfoil and causing damage.
Over the past few years, concepts of mechanical metamaterials have been proposed as solutions to problems in infrastructural engineering and earth- quake control. Gonella’s group has embraced this challenge and is exploring concepts of seismic shields based on periodic arrays of tunable resonating units. “The application of metamaterials in seismic control is still fairly hypothetical, an idea that has only marginally been tested against realistic eld data. Many challenges (such as size constraints, affordability, variability in the frequency signature of seismic events) must be addressed before seismic shields could be implemented. However, all the elements are in place and the potential
is huge. We are not talking about a far- fetched idea; this is a powerful technol- ogy that is worth serious exploration.”
Gonella is very comfortable and even proud of the speculative nature of his research. “My main contribution is, rst and foremost, to generate new ideas and support them with innovative proofs of concept. The process requires conver- gence of long-term efforts by a large pool of researchers. My research group always keep applications in front of us as guiding stars. However, our signature contribution comes from the bottom up, pushing from the fundamentals.”
Figure 7: Wave manipulation in phononic metamaterials. Top: Experimental proof
of concept of waveguiding in a plate with resonating stubs realized using Lego® bricks. Bottom: Discretely telescopic stubs with variable resonant characteristics; the brick- enabled experimental platform allows switching con gurations and exploring designs with remarkable speed and ease.
Figure 8. Demonstration of wave control capabilities in lattice materials using laser vibrometry. Left: Lab setup for a scan of
a perforated aluminum slab experiencing exural waves. Right: Snapshots of measured wave elds revealing switching between x-shaped pattern at low frequencies and +-shaped pattern at high frequencies.
University of Minnesota College of Science and Engineering | DEPARTMENT OF CIVIL, ENVIRONMENTAL, AND GEO- ENGINEERING 17