Laser-akustische Messtechnik in der Materialcharakterisierung: Numerische Schallfeldberechnung und praxisgerechte Auslegung für die kontaktlose Volumenprüfung
- Authors
- Publication Date
- Jan 01, 2016
- Source
- Fraunhofer-ePrints
- Keywords
- Language
- German
- License
- Unknown
- External links
Abstract
Testing equipment based on the propagation of elastic waves are commonly used for measuring specific material properties. As a prerequisite for accurate measurements a reliable acoustic coupling of probe and specimen is highly important. Therefore, high-resolution testing equipment is using fluids as couplant. In certain conditions, only non-contacting methods can be considered. This is the case for example, if particular high or low temperatures are present, if topographic features impede the use of ultrasonic probes, diffusion or solubility processes exist, measurements at vacuum are addressed and if high purity requirements need to be fulfilled. Hence, subject of this work is a method which offers to handle these constraints. With the emergence of modern laser systems the scientific basics for a non-contacting, laser-acoustic excitation of ultrasound were discovered. The tremendous development of commercially available laser systems during the last decade was taken as reason to investigate, to which extent former scientifically designed laboratory setups can now be merged into one single application oriented measuring system. All considerations are based on the thermoelastic excitation of ultrasound in combination with a likewise laser-based detection. By this, a self-contained measuring chain is built which combines the attributes non-destructive, non-contacting and application oriented within one ultrasonic measurement system for the first time. Thermal calculations lead to more precise equations which predict a laser-induced, local temperature rise of about 100 K. The examination of sound field simulations, as a prerequisite for the design of ultrasonic systems, identified an additional complex of problems. Although existing calculation approaches presuppose laser intensity profiles what can be described in analytical terms, real-world laser sources exhibit a complex shaped spatial distribution of laser energy. Based on a preceding CEFIT simulation, the developed CPSS method enables the calculation of the time resolved, 3D wave propagation of arbitrary shaped sources. A comparison to measured data successfully validated the results of simulation. By presenting selected scenario of measurements, the practical suitability of this non-contacting method is demonstrated. Using a transmission setup enables the characterization of open-pore ceramic coatings as well as the deduction of longitudinal and transversal speeds of sound. Equally, the imaging and estimation of the depth position of artificial defects with 0.7 mm in diameter is shown. Measurements based on a reflection setup provided evidence of a resolution limit of at least FBH = 1 mm in 4.5 mm depth. Additional examples demonstrate the ability to detect close-surface defects, the analysis of the challenging lamb waves zero-group-velocity S1 mode as well as the utilization of buried laser-acoustic sources.
