Ultrasound waves are best known for being used in medical applications, where high end devices can
produce impressive images of submerged structures. In the broader context of non-destructive
testing, ultrasound waves are routinely used to inspect engineering parts, e.g. for early crack
detection in airplanes. In our project, we aim to use ultrasonic waves to investigate the mechanical
properties and the thickness of plate-like samples, together with their thickness. A straight forward
approach to measure the thickness of a plate is to measure the time-of-flight of an ultrasound pulse
traveling through the plate. The thickness yields from the time-of-flight multiplied with the materials
sound velocity. We developed a concept where a priori knowledge on the sound velocity is not
required. This could find application for example in cases where a plate is glowing hot, and the
temperature is influencing its sound velocity.
In our study, we use lasers to excite and detect ultrasonic waves. A laser pulse is focused onto the
surface of a sample, producing heat in a small area, which leads to sudden heat expansion and
mechanical stresses, which travel through the sample as an ultrasound wave. This ultrasound wave
leads to surface displacements in the range of picometers, which we are able to detect with a laser
interferometer. Excitation and detection work contact-free, and have minimum influence on the
wave propagation we aim to study.
In plates, the propagating sound waves are reflected between the surfaces. This leads to the
formation of so called guided waves, which propagate along the plate. These plate modes are of
great interest for the testing of plates, ranging from cm thick concrete walls to m thick foils. They
feature interesting physical effects, which are the focus of our study: At certain frequencies,
resonances with large amplitudes appear. These can be detected extraordinary well, and
mathematical theories exist, which allow us to calculate their frequencies. By comparing calculations
and experimental results, the properties of the plate can be found. We will combine these
resonances with additional information we gain from shaping the excitation laser. This leads to
additional interferences in the plate modes, which appear at certain frequencies. Our goal is to
produce these different types of resonant and interfering modes with a single laser pulse. From the
detected response, we aim to find the plates properties: thickness and sound velocities.
Besides proofing and optimizing this basic concept we need to clarify its limits and its applicability to
a range of materials and geometries.