Structural Health Monitoring of Adhesively Bonded Joints With Piezoelectric Wafer Active Sensors

The present research proposes a new approach to structural health monitoring of adhesively bonded joints using small, unobtrusive piezoelectric wafer active sensors (PWAS) which are permanently affixed on the surface of the structure. PWAS can be placed in restrictive spaces, like in built-up aerospace structures. The surface-bonded PWAS can produce guided waves traveling parallel to the surface and could detect damage that would escape some ultrasonic methods. The major focus of this research was directed towards the electromechanical impedance method for disbond detection. The electromechanical impedance (EMI) method was modeled extensively with analytical and finite element method; the theoretical predictions were compared with carefully conducted experiments on geometrically tractable specimens. Other damage detection methods, i.e. picth-catch and pulse-echo, were also introduced but used only experimentally. The transfer matrix method was used to develop the analytical model for a uniform beam using the Euller-Bernoulli beam theory (shear deformation and rotary inertia were not considered). Next, the method was expanded to address the more complex case of a multi-layer adhesively bonded beam. The length of the beam was divided in smaller segments, and for each segment the state vectors (displacements and internal forces) were calculated using the field transfer matrices and the point transfer matrices. If damage (disbond) is present in the structure the model becomes more complicated and the beam is now divided in branches. For each branch, the mathematical formulation for the equivalent material properties, field and point transfer matrices were developed. Finally, the state vectors, hence the frequency response function at any location on the beam was calculated. Using the frequency response function the electromechanical impedance was calculated. A finite element analysis (FEA) was also performed for comparison with the analytical results. A conventional FEA analysis in which a harmonic force applied at the nodes to simulate the PWAS two ends was performed first, using the commercial code ANSYS. Then, a coupled-field FEA was performed using the couple-field option of the PLANE13 element in ANSYS. Both analytical and FEA results were compared against experimental results from two simple beam specimens: one pristine and one damaged with artificially simulated disbond. A set of damage indices were developed to detect the presence of disbond damage in the structure. From the results, it was shown that by using only one damage index it may be difficult to have positive damage detection. Different damage indices respond in different ways to changes in the impedance spectrum. The last part of this research addressed the possibility of using PWAS transducers for damage detection on real world specimens. Experiments were conducted on (a) a fabricated large scale adhesively bonded lap-joint; (b) a spacecraft simulated panel specimen, and (c) on a full scale helicopter blade. The experiments successfully proved the possibility to excite and receive guided Lamb waves in real world specimens despite the attenuation and dispersion encountered while travelling in the structures. Detection of damage was achieved with several methods (pitch-catch, pulse-echo, electromechanical impedance), which were critically compared. The dissertation ends with conclusions and suggestions for future work