Page 16 - CEGE Magazine - Fall 2016

BOJAN GUZINA CONT...
develops theories addressing two funda- mental questions: how does microstruc- ture affect the global wave motion? and what can we learn about the microstruc- ture of an anomaly using tomography with long-wavelength illumination? Guzina directs his research toward analysis of media with microstructures that are random, periodic, or fractal (Figure 2); in this way he develops next-generation tools to characterize and diagnose earth’s subsurface, engi- neered materials, and human tissue in ways that would have been unthinkable only a few years ago.
Human tissue, unlike rock or concrete, are easily deformed and so propagate nonlinear ultrasound waves, even at small amplitudes. This nonlinearity endows ultrasound waves with an ability to help diagnose human tissues in ways other
JOSEPH LABUZ CONT...
to these waves can help researchers
understand what is going on beneath the surface. AE has proven to be a powerful engineering technique to evaluate the integrity of a structure, such as an under- ground mine or a concrete bridge, by determining the locations of weak planes or unseen fractures within the structure.
AE has been widely used as a monitor- ing technique to determine cracking and assess degradation of materials to assure safety of various systems. One of the rst observations of acoustic emission, made by the US Bureau of Mines in 1938, was associated with an underground mine. Current applications typically focus on structural health monitoring in industries
Figure 2. Sub-wavelength sensing: shear wave (wavelength λ~3mm, middle panel) propagating in a gelatin sample containing polystyrene microspheres (diameter 10μm~ λ/300, left panel). The wave senses the microstructure via dispersion—the dependence of wave speed on frequency (right panel). Joint work with Ralph Sinkus, King’s College, London, and Sverre Holm, University of Oslo.
imaging techniques cannot. For instance MRI scans are capable of revealing very small lesions in a human body; however, a biopsy is often required to diagnose a dark spot in an MRI image. Guzina is develop- ing models and theories by which nonlin- ear, focused ultrasound waves can help
a radiologist make a diagnosis without a
from aerospace to construction, nuclear to petrochemical, mining to offshore drill- ing. AE has even been used to evaluate crunchiness of breakfast cereal!
Commonly used detection systems consist of a number of transducers con- nected to a recording device (Figure 4). Based on the time it takes the elastic wave to reach each transducer, the positions of the transducers, and the speed with which the wave can move through the material, the location of each crack or source event can be calculated. The location of small (unseen) cracks is probably the most valuable information that can be obtained from AE data.
In general, the theoretical models of AE use principles and techniques similar
to those developed by seismologists to study earthquake mechanisms. Obvi- ously, the scales from the lab to the earth’s crust are vastly different. A more important difference, however, is that an earthquake is associated only with a sliding crack. When studying cracks in subsurface and infrastructure applica-
biopsy by remotely pushing on a lesion via the so-called Acoustic Radiation Force.
Guzina’s research group tests their wave models and imaging theories by prop- agating ultrasonic waves though rock
specimens, engineered materials, and tissue-mimicking phantoms using the 3D Scanning Laser Doppler Vibrome- ter (SLDV) located in CEGE’s Waves & Imaging Laboratory (Figure 3).
Figure 3. 3D ultrasonic scan of a solid block of nuclear graphite, hiding a cylindrical hole, using surface vibrations captured by SLDV. The sectional images on the right expose the cylindrical anomaly in 3D.
tions, the mechanism of crack opening must also be considered.
In addition to using arrival times to deter- mine source locations, AE waveforms (Figure 5) can be analyzed to charac- terize a crack: the extent of sliding or opening and orientation.
microcrack
elastic wave
transducers
Figure 5. AE waveform
Part of the research has concentrated on modeling an AE event as a small crack that slides and opens (or closes). This theoretical model relates a crack at a known location within the solid to the wave displacements at each transducer, and the transducer output (voltage) must be calibrated to actual displacements. The model can then be used to charac-
Figure 4. AE detection: material containing a microcrack with transducers attached.
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