Tallinn University of Technology

Recently, a researcher at the TTÜ Department of Cybernetics, Martin Lints, defended his doctoral thesis "Optimised Signal Processing for Nonlinear Ultrasonic Nondestructive Testing of Complex Materials and Biological Tissues".

Carbon Fibre Reinforced Polymer (CFRP) is a complex layered material consisting of carbon fibre textile layers bound by polymer. It is lightweight and highly strong material. CFRP is therefore a material of the future seeing increasing usage in new industries. It is currently used in aerospace industry, sports goods manufacturing, automotive industry (in load-bearing parts of cars, where reduction in weight allows better dynamics and lower fuel consumption) and increasingly in civil engineering (for increasing the load-bearing capacity of structures, such as bridges and buildings).

"Although CFRP is lightweight and strong, it may be very susceptible to external mechanical impacts (stress and impacts may cause cracking and delaminations, which may become fatal for a product) and therefore load-bearing CFRP products need to be tested regularly. The thesis focused on the analysis of material properties of CFRP by ultrasonic wave spectroscopy. Ultrasonic wave spectroscopy is currently the most economical method for nondestructively testing material properties and finding defects, applied in manufacturing industry and also medical imaging. The disadvantage (compared to the more expensive and complex methods, such as X-ray spectroscopy and tomography) is limited wavelength and high attenuation of ultrasound in CFRP," Martin Lints explains.

Ultrasonic wave spectroscopy has been used for nondestructive testing of materials for over a century. Most common methods rely on linear wave propagation, but recently methods relying on nonlinear wave propagation have been under significant development. The advantage of the latter is that analysis of nonlinear effects allows detecting defects that are smaller than the wavelength of the ultrasound. Large wavelengths penetrate CFRP better, which allows us to "see" further and deeper.

Martin Lints says, "In the doctoral thesis we developed the delayed Time Reversal–Nonlinear Elastic Wave Spectroscopy (delayed TR-NEWS) method, which uses the internal reflections of the material to create focusing of the ultrasonic wave (i.e. analysis of the waves which have impacted the internal surfaces of the material). The future prospect of this method is that a product or device will test the “health” of its components by itself. So the device (aeroplane, car, etc.) no longer needs to be stopped for testing, it will give a signal of the occurrence of a possible defect itself."

The supervisor of the doctoral thesis was Professor Andrus Salupere (TTÜ) and the co-supervisor was Professor Serge Dos Santos (INSA Centre Val de Loire).
The opponents were Professor Claes Hedberg (Blekinge Institute of Technology, Sweden) and Professor Patrick Marquié (Université Bourgogne Franche Comté, France).

The doctoral thesis has been published in the digital collection of TTÜ library: https://digi.lib.ttu.ee/i/?8437

Kersti Vähi, TTÜ Research Administration Office