17th International Conference on Experimental Mechanics (ICEM 17)
July 3-7, 2016, Rhodes, Greece
ACOUSTO-LUMINESCENCE: SOUND INTO LIGHT
M. Kersemans1,2, P.F. Smet3, N. Lammens2, J. Degrieck2 & W. Van Paepegem2
2Faculty
of Engineering and Architecture, Ghent University, 9000 Gent, Belgium
3Faculty
of Sciences, Ghent University, 9000 Gent, Belgium
Abstract: In this contribution, we report on ultrasonically stimulated mechanoluminescence. We employ
this acousto-luminescent (AL) effect for the reconstruction of the 3D radiation field of an
ultrasonic piston transducer (f = 3.3 MHz).We found that the AL activated region has a specific geometrical shape, representing the local spatial distribution of the incident pressure
field. By varying the insonification distance, multiple 2D slices of the transducer's radiation
field have been obtained by which the full 3D radiation field may be reconstructed. The obtained AL results are confronted with classical scanning hydrophone experiments as well as
with ultrasonic holography, showing good correspondence.
1. Introduction
A material is called mechanoluminescent (ML) when it emits light when mechanically stressed.
Such materials are already used for non-destructive pressure sensing [1] and for visualizing
quasi-dynamic crack propagation [2]. Recently, the ML effect has also been used in combination with ultrasound waves. As such, it was found that the intensity of the light emission is
proportional to the applied ultrasonic power [3, 4]. By coating the surface of an ultrasonic
transducer with a ML film and measuring the light emission, it was demonstrated to provide a
rough image of the power distribution of the transducer [3].
In this study, we take this acousto-luminescent (AL) effect one step further and use it for reconstructing the 3D radiation field of a piston-like ultrasonic transducer.
2. Results
The recently designed BaSi2O2N2:Eu powder [5], with an emission peak at 498 nm resulting
from a 4f65d-4f7 transition within divalent europium, is embedded as an active component in
an epoxy matrix using a 1 to 10 weight mixture rule. After optically charging the circular AL
sample by means of a 15 Watt UV black light, a sufficient time is respected in order to reduce
the persistent luminescence. The charged AL sample is then placed under water and insonified
at different distances z by an ultrasonic transducer operating at 3.3 MHz, using an electrical
power input of 2 W/cm2. The circular area of the transducer equals 4.7 cm2. A photograph of
the experimental setup is displayed in Figure 1.
Figure 1. Photograph of experimental setup
1
Corresponding author
E-mail address: [email protected] (M. Kersemans)
During the insonification, a photograph is taken with a digital camera in order to capture
the AL light emission, which should be representative for the cross-sectional pressure distribution produced by the transducer. The experiments have been performed in a darkened room to
enhance the contrast. Figure 2 (top row) displays the obtained AL results at different insonification distances z, showing a clear variation of the cross-sectional pressure field. The pressure
distribution evolves from an annular ring with a disc inside towards a more or less Gaussian
distribution. This evolution is easily understood considering the diffractive nature of a spatially
bounded beam, resulting in the formation of near-field and far-field features. For a perfect circular radiator no angular variations of the radiated sound pressure are expected. Our experimental results however reveal small variations in angular direction, indicating the poor performance of the used ultrasonic transducer. In order to assist the AL observations, the radiation
field was measured alternatively by scanning hydrophone experiments (see Figure 2, bottom
row) as well as reconstructed by applying ultrasonic holography on the basis of a Fourier transform beam propagation method.
Figure 2. Recorded cross-sectional pressure field at several insonification distances z: AL (top row)
and scanning hydrophone (bottom row)
3. Conclusions
We demonstrated ultrasonically stimulated mechanoluminescence, and applied it for the 3D
mapping of the radiation field of an ultrasonic transducer. The AL results have been validated
with classical hydrophone scanning experiments and ultrasonic holography. Currently, we are
investigating the AL effect as a tool for sensing and inspecting (composite) components.
References
1.
Xu, C.N., et al., Dynamic visualization of stress distribution by mechanoluminescence
image. Applied Physics Letters, 2000. 76(2): p. 179-181.
2.
Kim, J.S., et al., Mechanoluminescent SrAl2O4 : Eu, Dy phosphor for use in
visualization of quasidynamic crack propagation. Applied Physics Letters, 2007.
90(24).
3.
Zhan, T.Z., et al., Direct visualization of ultrasonic power distribution using
mechanoluminescent film. Ultrasonics Sonochemistry, 2011. 18(1): p. 436-439.
4.
Terasaki, N., H. Yamada, and C.N. Xu, Ultrasonic wave induced
mechanoluminescence and its application for photocatalysis as ubiquitous light
source. Catalysis Today, 2013. 201: p. 203-208.
5.
Botterman, J., et al., Mechanoluminescence in BaSi2O2N2:Eu. Acta Materialia, 2012.
60(15): p. 5494-5500.