Page 15 - CEGE Magazine - Fall 2016

“WE CAN THINK ABOUT WAVE DISTURBANCES AT VARIOUS SCALES. THE VERY LARGE SCALE IS AN EARTHQUAKE, AND WE OFTEN SEE ITS DESTRUCTIVE EFFECTS. IN MY LAB, HOWEVER, WE STUDY VERY SMALL DISTURBANCES—MICROCRACKS—THAT ARE MILLIMETERS IN SIZE AND LAST ONLY MILLISECONDS IN TIME.”
The goal of Labuz’s research is to use waves called acoustic emission (AE) to study how brittle solids (rock, concrete, etc.) fail or break. This understanding
is useful in designing more economical ways to break rock and in protecting workers in underground mines.
Labuz and his students focus their research on three main areas: Developing and testing theoretical models related to quantitative analysis of AE data, in partic- ular calibrating transducers and modeling crack sources; Conducting experiments
Recent years have seen a breadth of new ideas and concepts pertaining to wave control and manipulation due in part to the advent of a new class of
to verify AE results through complemen- tary observations of fracture initiation and propagation; and Applying AE to monitor the failure process of structures composed of brittle solids under various loading conditions, from tensile fracture to shear localization.
Rock and concrete may appear solid, but microscopic observations have shown that small cracks are naturally present in these brittle materials. When mechanical or thermal loads of suf cient magnitude are applied, the small cracks can grow,
releasing some energy as elastic waves. The emitted waves, AE, can be detected by piezoelectric transducers attached to the surface of the material. “Listening”
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“MY STUDENTS AND I INVESTIGATE STRATEGIES TO CONTROL WAVES IN MEDIA CHARACTERIZED BY COMPLEX INTERNAL ARCHITECTURES. THE TECHNOLOGICAL GOAL IS TO DESIGN MATERIALS WITH DESIRED AND TAILORED DYNAMIC FUNCTIONALITIES. OUR ULTIMATE OBJECTIVE IS TO BE ABLE TO MOLD INTANGIBLE AND INVISIBLE ELASTIC WAVEFIELDS WITH THE SAME LEVEL OF FREEDOM AND FLEXIBILITY THAT WE NORMALLY OBSERVE IN THE MANIPULATION OF PHYSICAL MATTER.”
materials called mechanical or elas-
tic metamaterials. Metamaterials are structural materials that owe their unique behavior to their complex and unconven- tional internal structures rather than to their (chemical) compositions. Because of the unconventional way in which their internal structural components (beams, rods, shells, etc.) are arranged and connected geometrically, metamaterials behave in ways that are vastly superior to
what could be achieved by their individ- ual components taken as stand-alone elements.
The most exotic properties of metama- terials are observed in the realm of wave propagation. “We can control the way elastic waves propagate in metamateri- als,” Gonella says. “For example, we can design materials that block certain wave frequencies while allowing others, thus behaving as mechanical lters or wave-
CONTINUE ON PAGE 17 University of Minnesota College of Science and Engineering | DEPARTMENT OF CIVIL, ENVIRONMENTAL, AND GEO- ENGINEERING 15
JOSEPH LABUZ: LISTENING TO ROCK
STEFANO GONELLA: WAVE CONTROL WITH SMART MATERIALS