Ultrasonic guided wave propagation across waveguide transitions applied to bonded joint inspection

Adhesively bonded joints are increasingly used in safety critical applications such as load bearing elements in aerospace structures. The quality of surface preparation is crucial to the strength of an adhesive joint and also the adhesives are susceptible to environmental conditions and undergo degradation. The possibility of the presence of defects and interfacial weakness make nondestructive inspection techniques a valuable tool to evaluate the structural reliability of the adhesive joints.;Two problems related to the inspection of aircraft adhesive joints---the adhesive repair patches and adhesive skin-stringer joints are nondestructively evaluated using ultrasonic guided waves in this thesis. The former is a life extending patch bonded at defect locations on aircraft while the latter is a structural stiffener found in skin and wings in aircraft. Ultrasonic guided waves are thickness resonances that propagate under stress-free boundary conditions in plate-like structures called as waveguides. The guided waves display frequency dispersion or velocity variation with frequencies that are depicted using dispersion curves. Based on the nature of the cross-sectional displacement distribution at each point on the dispersion curves, the guided waves are categorized into modes. Selection of modes for sensitivity to interfacial weakness in a waveguide, and understanding the mode conversion at geometric discontinuities or transitions in waveguides is a major challenge to the inspection using guided waves.;The objective of this thesis is to enhance the understanding of guided wave interaction with waveguide transitions and interfacial defects in bonded assemblies in order to develop a field implementable solution for nondestructive inspection.;At the beginning, the problem of inspecting aircraft adhesive repair patches applied to epoxy bonded aluminum---titanium joint is presented. Weakness at the aluminum-epoxy interface and bulk defects in epoxy, simulated on repair patch samples prepared in the lab, were successfully detected and sized by selecting modes with large in-plane displacement at the interface of interest.;The problem of inspecting adhesive skin-stringer joints requires an understanding of the guided wave mode conversion and scattering at a waveguide transition in addition to the need to detect interfacial weakness in the stringer joint region. The existence and unique displacement characteristics for modes in a bonded joint that envelope or appear near to modes in one of the adherends, observed in the study of skin-stringer joints, is reported in the thesis. These modes are coined here as 'mode pairs' and show a matching trend in the phase velocity vs. frequency curve with the adherend. A quantitative model combining the Semi-Analytical Finite Element (SAFE) and Normal Mode Expansion (NME) was developed to handle guided wave scattering at a waveguide transition. Further, a qualitative model using wavestructure matching coefficient was also developed to determine the mode conversion for a single mode incidence at the waveguide transition.;The models showed equal excitation of the mode pairs and up to 100% energy transmission for matching group velocity vs. frequency curve. Using a commercial Finite Element package numerical experiments were conducted that agreed with the hybrid model and the wavestructure matching model. Fourier transform based signal processing algorithms for orthogonal decomposition of guided wave data into constituent normal modes, directional and mode matching filters, computationally efficient element-less receiver and wavestructure data processing were developed. The processing of numerical data revealed the instantaneous mode formation at a source and also at a transition thus enabling guided wave inspection of the edge of a bonded joint.;The mode conversion and scattering study were coupled with the interfacial inplane displacement parameter to form a combination parameter called Effectiveness index. Higher effectiveness index modes (>0.3) successfully detected simulated interfacial weakness in the skin-stringer joints prepared in the lab.