An investigation into novel techniques for nonlinear NDT measurements and their applications

Abstract

Non-Destructive Testing (NDT) encompasses a diverse field of techniques that can be used to detect the presence of damage in mechanical components and parts. Among these techniques, ultrasonic pulse echo NDT is the most common method used to detect damage. Conventional ultrasonic NDT utilizes piezoelectric transducers to send and receive pulses of acoustic energy into a part. The interaction of these elastic waves with material defects can be measured as small amounts of reflected energy. However, current methods have significant limitations when it comes to defects such as micro-cracks, closed cracks or weak bonds.It is recognized that nonlinear acoustics methods can be used for the detection of early stage damage and material defects that are generally undetectable with traditional methods. One of these methods is nonlinear wave modulation also referred to as nonlinear vibro-acoustics. In this technique a sample is excited at two distinct frequencies, known as pumping and probing waves. The pumping wave is normally between 10-1000 Hz and the probing wave is in the ultrasonic range. Interactions with defects, such as cracks or weak bonds, causes intermodulation of these two frequencies due to nonlinear effects. These effects include nonlinear contact dynamics, hysteresis and contact acoustic nonlinearity (CAN). This intermodulation results in additional frequencies being present in the frequency domain that were not part of the original excitation. These new frequencies are known as sidebands or harmonics and can be directly observed by taking an FFT of the system response.This work attempts to address the disadvantages of conventional nonlinear NDT methods while also expanding the understanding of nonlinear phenomena through novel processing methods and apparatuses. In this research three novel methods have been developed. The first method utilizes nonlinear frequency mixing without requiring the use of a low frequency pumping wave. The sample is excited at several distinct but similar frequencies in the ultrasonic range using only one piezoelectric transducer. This eliminates the need of a shaker for low frequency excitation, greatly simplifying experimental design.The second technique is a novel data processing method. It is based on the down-conversion of the probing wave to recover a low frequency distorted wave, this provides a more direct and meaningful analysis of nonlinear wave modulation compared to conventional methods. This technique uses all of the sideband spectra around the probing frequency. This is in contrast to existing methods that typically only consider the first or second pair of sidebands in their analysis.Lastly, an entirely non-contact, air-coupled nonlinear vibro-acoustic measurement technique and apparatus has been developed and demonstrated. This apparatus uses non-contact ultrasonic transducers for probing wave excitation and a loudspeaker inside a resonant cavity to provide a pumping excitation. The technique is demonstrated for the detection of near surface defects for thin plates. A scanning platform has been developed which allows the generation of nonlinear c-scan images using the non-contact method.