RESUMO
Air-coupled ultrasound sensors have advantages over contact ultrasound sensors when a sample should not become contaminated or influenced by the couplant or the measurement has to be a fast and automated inline process. Thereby, air-coupled transducers must emit high-energy pulses due to the low air-to-solid power transmission ratios (10-3 to 10-8). Currently used resonant transducers trade bandwidth-a prerequisite for material parameter analysis-against pulse energy. Here we show that a combination of a non-resonant ultrasound emitter and a non-resonant detector enables the generation and detection of pulses that are both high in amplitude (130 dB) and bandwidth (2 µs pulse width). We further show an initial application: the detection of reflections inside of a carbon fiber reinforced plastic plate with thicknesses between 1.7 mm and 10 mm. As the sensors work contact-free, the time of flight and the period of the in-plate reflections are independent parameters. Hence, a variation of ultrasound velocity is distinguishable from a variation of plate thickness and both properties are determined simultaneously. The sensor combination is likely to find numerous industrial applications necessitating high automation capacity and opens possibilities for air-coupled, single-side ultrasonic inspection.
RESUMO
Local ultrasonic resonance spectroscopy (LURS) is a new approach to material inspection, where the specimen is locally excited by a short mechanical impulse while its local mechanical response is recorded at a position nearby. The local material and geometrical properties can be extracted from the frequency spectrum of the response and visualized by performing a scan over the inspected area. In our experiment, the plate thickness and the reliefs of both plate surfaces (plate curvature) were obtained from thickness resonance and time of arrival analysis without physical contact to the specimen. Ultrasound was generated on the specimen surface by a laser pulse. Local mechanical response of a carbon fiber-reinforced polymer plate with a thickness ranging from 0.6 mm to 4.3 mm was recorded with a broadband optical microphone in through-transmission setup. The precision of this arrangement greatly exceeded the precision of conventional methods limited by the ultrasound wavelength. For thicknesses in the range around 1 mm, standard deviations of up to several µm were achieved. An influence of the through-plate ultrasound velocity on the measured relief of the plate surface nearest to the optical microphone was eliminated by a joint evaluation of thickness resonance and time of arrival. Furthermore, we demonstrated that internal delaminations have an influence on the spectrum of the local mechanical response and can therefore be detected by LURS.
