Discrete laser-ultrasonic spectroscopy using guided waves

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.

In the project, various innovative methods for investigating samples using ultrasound were explored. Ultrasound was generated and measured contactlessly using lasers. We further developed the concept of laser ultrasound for specific samples or demonstrated it under challenging conditions. For plates, we utilized a combination of resonant modes (where the cross-section of the plate vibrates like a string) with surface waves to determine both the thickness of the plate and its sound velocities simultaneously. Typically, one of these quantities must be known to determine the other from simple ultrasound experiments. For example, the thickness of the plate can be directly determined from the travel time of a sound pulse, provided the sound velocity is known. In our method, CoMLAS (combined mode local acoustic spectroscopy), we use a laser pattern of periodic lines to simultaneously determine plate resonance frequencies and surface wave velocity. From this, thickness, longitudinal, and transverse sound velocity can be calculated. A single laser pulse is sufficient for the measurement. In experiments, we determined thickness variations of plates. Additionally, we measured the change in sound velocity induced by heating a plate, while the thickness was correctly determined during this process. Resonant plate modes were also used without combination to observe steel plates at very high temperatures >700C during a phase transition. Knowing the plate thickness, longitudinal and transverse sound velocities could be determined. The same method was applied during the process of cold aging of aluminum plates, where small changes in the measured quantities correlate with the material process. We also used the laser excitation with periodic lines to measure the surface wave velocity of samples as a function of their wavelength. For samples made of a single material, this velocity is the same for every wavelength. For samples with a coating, it depends on the wavelength, and from this dependence, the thickness of the layer or its elastic properties can be inferred. We demonstrated that sufficient information can be measured with a single laser pulse for many cases using the periodic lines. This exploits the fact that waves that fit exactly between two adjacent lines interfere constructively. This applies to the wavelength that exactly matches the line spacing, but also to those that fit 2x, 3x, 4x, etc. Thus, we determined the surface velocity at 3-4 wavelengths and compared it with the lengthy method. Finally, a calculation method for waves in piezoelectric plates was developed and confirmed with experiments. Waves in such materials behave differently than in metals, for example, because in addition to elastic forces, electrical forces also act during small deformations. This is intended to be used in a follow-up project to characterize piezoelectric materials.