- Project title: Discrete Laser-Ultrasonic Spectroscopy using Guided Waves
- Acronym: DISCUSS
- Duration: 36 months (start 01.01.2021)
- Funding: FWF Stand-Alone Project
Projects
Discrete Laser-Ultrasonic Spectroscopy using Guided Waves
Project information:
Wider research context:
The simultaneous measurement of the thickness of a structure and its sound velocities using ultrasound poses a scientific problem. We propose a solution of this problem for plate-like structures which guide elastic waves. This would benefit applications, from the monitoring of sheet metal production to the characterization of microscopic layers. We suggest to excite discrete frequencies using laser-ultrasound (LUS), exploiting a combination of zero-group velocity (ZGV) resonances and surface acoustic waves (SAW). In this context, we will also investigate the influence of microstructure on ZGV modes in thin, real world samples.
Objectives:
⇒ shape a thermo-elastic LUS source to couple into one point on the SAW mode and simultaneously into ZGV resonances
⇒ assign the peaks in the response spectrum to the according modes with high precision
⇒ find the local thickness and bulk sound velocities of simple plates and more complex waveguides from the frequency positions of the peaks
⇒ implement this characterization with a single shot, pulsed laser excitation and a static phase- or amplitude mask
⇒ spatially resolve the properties of a plate with varying thickness and elastic properties with an according setup
⇒ quantify the range of wavelength to grainsize ratio which allows us to consider a microscopically polycrystalline material to behave isotropic (in this concept) and
⇒ research a statistical procedure to retrieve the averaged properties beyond this range
Methods:
We aim to...
⇒ Finite elements simulations of thermo-elastic excitation of elastic waves and their propagation in homogeneous and polycrystalline media
⇒ Semi-analytic finite elements calculations of dispersion curves
⇒ LUS experiments, including the shaping a laser beam by a spatial light modulator and static masks
Figure 1: The combination of ZGV frequencies and the frequency of a SAW generated at a fixed wavenumber allows for the determination of thickness and bulk sound velocities of a plate.
Innovation:
We combine two essentially different ways to access specific points within the dispersion curves of a sample: Resonant zero-group velocity points and a narrow band SAW source (see Figure 1). The resulting frequency peaks allow for a local and fast characterization of plates and more complex waveguides. We think that the influence of micro-structure of common polycrystalline materials on SAW and ZGV modes poses a limitation to the method, which we take as an impulse to quantify it, using simulations and experiments.
