Author:

Abstract:

Nonlinear ultrasonic techniques have proven to be extremely sensitive to early damage evaluation in materials. In short, the field of nonlinear acoustics in solids deals with the investigation of the amplitude dependence of material parameters, which can for instance be evidenced by thorough analysis of modifications in the spectral content and lack of scalability with amplitude. The degree to which these material properties depend on the applied dynamic amplitude can be quantified by nonlinear parameters. Several studies have shown that an instantaneous measurement of the nonlinearity parameters can efficiently diagnose the presence and monitor the degree of internal damage in a material. One drawback of the nonlinear NDT techniques is that high-power ultrasonic excitation is needed to activate defects and provoke nonlinear response. To overcome this problem, a novel and efficient NDT technique, called Local Defect Resonance (LDR) Spectroscopy, was recently suggested. Using this method, one searches for resonance modes related to local defects and takes advantage of these characteristic frequencies to enhance the defect’s nonlinear acoustic response. It has been shown experimentally that, using the concept of LDR, the ultrasonic excitation of defects can be optimized, and excitation amplitudes can be significantly reduced even to mWatt levels without compromising the quality of detection. In this study, the existence of LDR is first illustrated for Flat Bottom Holes (FBH), both experimentally and with numerical simulations. This allows us to define appropriate excitation methods and analysis tools that reveal the local defect resonance frequencies. The proposed method is then used to study the concept of nonlinear LDR spectroscopy in more detail using our customized 3D simulation code for wave propagation in materials containing closed but dynamically active delaminations. LDR frequencies of delaminations will be determined and corresponding defect vibration patterns will be identified. Using the model, we will also demonstrate that a strong increase in defect nonlinearity can be observed when the excitation frequency is tuned to the LDR frequency. The discussed simulation results will help us to obtain a better understanding of the concept of LDR and to assist in the further development of LDR spectroscopy techniques for the detection, localization and characterization of kissing bonds.