The scope of my research was to develop a better understanding of the engineering variables that influence the interaction of PWAS with structure during activation of the transducer. This is a key feature needed to develop more power/energy efficient structural health monitoring (SHM) systems. SHM is the field of engineering that determines the health of a structure while it is in service. Active SHM can be performed through piezoelectric transducers such as piezoelectric wafer active sensors (PWAS) that can be permanently attached to the structure through a bonding layer. PWAS transducers can actively interrogate the structure by exciting and receiving Lamb waves propagating in the structure or by passively listen to changes in the structure. PWAS-structure interaction modeling is fundamental in order to achieve single mode excitation, i.e., tuning, a requirement for most of the SHM algorithms (time reversal, phase-array, and imaging).
To achieve our research goal, we had to go beyond the current state of the art in modeling and understanding the load transfer from PWAS to the structure. The existing modeling methods rely on the low frequency assumption of axial/flexural waves only. This assumption is not true in the high frequency range of ultrasonic SHM applications. We derived, through the normal mode expansion methods (NME), the interfacial shear stress, hence, the load transfer from the PWAS to the structure through the bond layer, without limitations on the frequency and the number of modes. This allowed us to derive more accurate predictions of the tuning between PWAS and Lamb waves which compared very well with experimental measurements.
This dissertation is constructed in three major parts. In Part I, we developed a generic formulation for ultrasonic guided waves in thin wall structures. The formulation is generic because, unlike many authors, in many parts of our derivation (power flow, reciprocity theorem, orthogonality, etc.) we stayed away from specifying the actual mathematical expressions of the guided wave modes and maintained a generic formulation throughout.
In Part II, we addressed some unresolved issues of the PWAS SHM predictive modeling. We extended the NME theory to the case of PWAS bonded to or embedded in the structure. We developed the shear layer coupling between PWAS and structure using N generic guided wave modes and solving the resulting integro-differential equation for shear lag transfer. We applied these results to predicting the tuning between guided waves and PWAS and obtained excellent agreement with experimental results.
Another novel aspect covered in this dissertation is that of guided waves in composite materials. The increasing use of composites in aeronautical and space applications makes it important to extend SHM theory to such materials. For this reason, the NME theory is extended to the case of composites. We developed a generic formulation for the tuning curves that was not directly dependent on the composite layup and can be easily extended to various composite formulations. We conducted carefully-planned experiments on composites with different orientations. The comparison between our predictions and experiments was quite good.
In Part III, SHM applications and related issues are addressed. We discussed the reliability of SHM systems and the lack of specifications for quality SHM inspections with particular focus on the case of composites SHM. We determined experimentally the ability of PWAS to detect damage in various composite specimens. We tested the performance of the PWAS for damage detection on composite plates, on unidirectional composite strips, on quasi-isotropic plates, on lap-joints junctions, and composite tank sections. We also tested the ability of PWAS transducers to operate under extreme environments and high stress conditions, i.e. the survivability of PWAS-based SHM. We proved the durability of the entire PWAS-based SHM system under various different load conditions. We also tested the influence of bond degradation on PWAS electrical capacitance as installed on the structure, which gives a measure of the quality of the PWAS installation, a key feature in PWAS-based SHM. We developed theoretical models for shear horizontal waves scattering from a crack and Lamb waves scattering from change in material properties. We studied the acoustic emission (AE) in infinite plate and we used NME to model AE phenomena.