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, Belgium3Faculty of Sciences, Ghent University, 9000 Gent, Belgium

Abstract: In this contribution, we report on ultrasonically stimulated mechanoluminescence. We em-ploy 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 ob-tained 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 vi-sualizing 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 emis-sion is proportional to the applied ultrasonic power [3, 4]. By coating the surface of an ultra-sonic 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 re -constructing the 3D radiation field of a piston-like ultrasonic transducer.

2. ResultsThe 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 authorE-mail address: mathias.kersemans@ugent.be (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 distri-bution 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 in-sonification 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 ex-pected. Our experimental results however reveal small variations in angular direction, indi-cating the poor performance of the used ultrasonic transducer. In order to assist the AL obser-vations, 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. ConclusionsWe 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.

References1. 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.