A nonlinear ultrasonic modulation approach for the detection and localisation of contact defects

Critical metallic and composite structures are periodically inspected for contact defects such as kissing bonds and delamination, using phased array techniques based on linear ultrasound. The detection of contact flaws at multiple depths in the material can be challenging due to high signal attenuation and noise level. In this study an alternative ultrasonic phased array approach relying on the nonlinear modulation of dual-frequency excitation was introduced to improve the sensitivity and accuracy in the detection of contact defects. A phased array probe was used for the generation of single-frequency and dual-frequency waves, and the capturing of echoes. The flaws were detected using a new nonlinear modulated parameter characterising the response of the material arising only from the modulation sidebands at the sum and difference frequencies f2+f1 and f2-f1. Ultrasonic tests were conducted on materials with multiple contact interfaces. The novel parameter was plotted against the linear response, and the peaks indicating the contact interfaces were compared based on their signal-to-noise ratio (SNR), their width at half-height (6 dB drop) and their positioning error. The peaks of the nonlinear modulated parameter offered up to 103 times higher SNR, up to 10 times smaller width at half-height and around 45% smaller localisation error than the peaks in the classical linear ultrasonic response. The results showed that the proposed approach could lead to more effective detection and more accurate localisation of contact defects in structural materials such as kissing bonds and closed delamination.</p

Modeling Of Dynamic Behavior In Closed Crack And Nonlinear Ultrasonic Array Imaging

Ultrasonic testing (UT) utilizes the traveling time and amplitude of a scattered wave from cracks in a material. A distinct scattered wave can be obtained from a crack with opening faces. It is difficult, by contrast, to detect signals from closed cracks such as stress corrosion and fatigue cracks using the conventional UT. Since the crack faces are in contact due to a residual stress, most of the incident wave penetrates the crack faces and a little scattered wave will be generated. A nonlinear ultrasonic method based on contact acoustic nonlinearity (CAN) which utilizes the dynamic behaviors of the contact and separation states of the crack faces is a promising method. The clapping motion of the crack faces generates harmonics in the frequency spectrum. However, the generation of the harmonics from the crack faces is so sensitive that the voltage, angle, cycle, and frequency of the incident wave should be set in a well-chosen method.In this thesis, a modeling of the generation of the harmonics wave from the closed crack was performed to enhance the reliability of the nonlinear ultrasonic method. Here, an elastodynamic finite integration technique (EFIT) was introduced to simulate a transient motion of the scattered wave from the closed crack. The EFIT adopted a set of split computational nodes at the interface of the closed crack to show the contact and separation depending on the stress and opening displacement of the interface. The numerical results for one-dimensional wave field showed good agreement with the analytical solutions. The simulation results revealed that a closing velocity of the interface was determined by the compressive pressure of the material and was validated by the experimental measurement with polymethylmethacrylate (PMMA) specimens. The appropriate conditions to obtain the nonlinear ultrasonic wave in the case of ultrasonic array testing were determined by performing two-dimensional simulations.An imaging method of the closed crack using an array transducer was investigated using the EFIT simulation. The full waveforms sampling and processing (FSAP) was applied as the imaging technique. For the generation of the nonlinear ultrasonic wave from the closed crack, the FSAP was modified to an algorithm which can transmit a strong beam from the array transducer by setting the delay for all elements electronically. The second harmonic component which extracted from the scattered wave using a band-pass filter was used as the input to the FSAP imaging technique. From the results, it was found that the shape and the location of the closed crack can be reconstructed when the amplitude, frequency, cycle, and angle of the incident wave are set at appropriate values

Nonlinear imaging (NIM) of flaws in a complex composite stiffened panel using a constructive nonlinear array (CNA) technique

Recently, there has been high interest in the capabilities of nonlinear ultrasound techniques for damage/defect detection as these techniques have been shown to be quite accurate in imaging some particular type of damage. This paper presents a Constructive Nonlinear Array (CNA) method, for the detection and imaging of material defects/damage in a complex composite stiffened panel. CNA requires the construction of an ultrasound array in a similar manner to standard phased arrays systems, which require multiple transmitting and receiving elements. The method constructively phase-match multiple captured signals at a particular position given multiple transmit positions, similar to the total focusing method (TFM) method. Unlike most of the ultrasonic linear techniques, a longer excitation signal was used to achieve a steady-state excitation at each capturing position, so that compressive and tensile stress at defect/crack locations increases the likelihood of the generation of nonlinear elastic waves. Moreover, the technique allows the reduction of instrumentation nonlinear wave generation by relying on signal attenuation to naturally filter these errors. Experimental tests were carried out on a stiffened panel with manufacturing defects. Standard industrial linear ultrasonic test were carried out for comparison. The proposed new method allows to image damages/defects in a reliable and reproducible manner and overcomes some of the main limitations of nonlinear ultrasound techniques. In particular, the effectiveness and robustness of CNA and the advantages over linear ultrasonic were clearly demonstrated allowing a better resolution and imaging of complex and realistic flaws.</p