Mercedes García Rodríguez

Ph.D. Thesis title:
CONTRIBUCIÓN A LA MEJORA DEL MARGEN DINÁMICO EN SISTEMAS DE ENSAYOS NO DESTRUCTIVOS EN AIRE DE MATERIALES LAMINADOS

Author:

Director:

Dr. Juan Antonio Chávez Domínguez

Reading day:

5th May 2011

Abstract:

Lamb waves are guided waves which propagate in thin plates with free surfaces and with thickness on the order of the wavelength. Lamb waves offer the great advantage of long-rage testing in small time. An important characteristic of these waves is they are dispersive and multimodal. The propagation velocity of the wave depends on the plate thickness and the frequency, and there are multiple number of modes at a given frequency-thickness product. This makes difficult generate one mode and the dispersion can produce distortion when the signal travel inside the plate. The propagation of these waves is different in each material and depends on mechanical properties, elastic properties and thickness.

The main contribution of this Thesis is the development of the techniques and methods needed to develop air-coupled non-destructive systems that can be used to characterize highly attenuating plate materials with Lamb waves.

Air coupled concave array transducers have been used to generate and receive Lamb waves. The concave profile allows keeping the impact point constant independently of the steering angle. In this manner it is also constant the distance of wave flight and the losses.

The use of air as a coupling media, instead of liquids, provides more flexibility and speed in the measurements of large structures, and allows to inspect any material without damage its properties. Nevertheless, the major drawbacks of air-coupled ultrasound are attenuation of the ultrasound in air and energy losses at the transducer¿air interface. To overcome these problems and be able to inspect attenuating materials, the aim of the techniques and methods proposed is improve the signal-to-noise ratio (SNR). The work made to achieve it has been aimed to optimize the transducer excitation to increase the energy and propose signal processing techniques.

In the excitation system we propose an electronic subsystem where each piezoelectric transducer is driven by a power square bipolar burst circuit, with amplitude to ±250 V. The main advantage of this signal is it is easy control the signal bandwidth varying the number of pulses in the burst. This topology can be replicate easily in small area for the 32 transducer of the array. A matching network has been also proposed to match the output transmitter impedance to high impedance transducer, to maximize the transfer of power and improve the transmitting efficiency of the transducer, and so achieve better signal-to-noise ratio.

To transfer more energy to the transducer without loss resolution, it is proposed to excite with coded signals and employ pulse compression techniques that allow to increase signal to noise ratio. The use of frequency modulated square chirp signal has obtained an improvement of 12 dB in the SNR with respect the temporal signal without correlate. It has been also proposed the use of coded signal, employing Golay codes. The sum of the auto-correlation function of a pair of Golay codes has a main peak and zero side-lobes, which improve the SNR. In this work the use of this technique has provided us with a 21 dB gain in SNR.

To validate the contributions, a system based on Lamb wave has been designed and implemented. This system has allowed to test materials with attenuation of 19.59 dB/cm, like nylon. Other materials tested were: copper, brass, aluminium, polyester, polycarbonate and PETG.