Thermosonic Testing with Phased Matched Guided Wave Excitation

Abstract. Vibrothermography, is an established NDT method praised for its full-field and defect-selective imaging. Especially for cracks and similar damage, defect signals are produced within seconds for a wide variety of materials. Usually high power sonotrodes that are applied with kN pressure are employed in order to maximize ultrasonic excitation of the specimens. An increase in excitation power is used for boosting the defect signal strength, i.e. the rise in temperature.
Improvements to the energy efficiency of the defect activation have been made by using local defect resonance (LDR). Via a frequency match of a defect’s resonance and the source’s driving frequency efficient pumping of the ultrasonic energy of guided waves to the defect is possible. As a consequence the use of simple, conventional excitation sources, e. g. piezo disc transducers, is viable while still achieving considerable enhancement in both vibration amplitude and temperature of the defect as well as signal-to-noise ratio of thermosonic images. However, due to the nature of most ultrasonic sources and their coupling methods to the specimen surface, the guided waves are usually radiated omnidirectionally from the source. Therefore, a significant fraction of the energy is radiated towards areas outside the infrared camera’s field of view.
We present an innovative, patented coupling method that significantly improves the distribution of ultrasonic energy of guided waves within the specimen. It is possible to channel the energy towards a specified direction by placing periodic linear structures between the transducer and the specimen phase matched to the wavelength of the guided wave excited. The distance and the wavelength are determined by the excitation frequency and the specimen properties. This antenna-like coupling device works exceptionally well in conjunction with the LDR technique, as both techniques require a frequency dependent setup.
A significant increase in the defect vibration amplitude was measured when using this ultrasonic line source. Finally, thermosonic testing results confirm that channeling ultrasonic energy towards a specified direction is advantageous for exciting defect vibrations while using low energy input sources.