This study presents an effective solution for the optimization of piezoelectric (PZT) wafer placement in a network of convex and non-convex structures, toward the application in the field of structural health monitoring. The proposed objective function is to maximize the coverage of the monitored area, discretized by a set of control points, while minimizing the number of PZT wafers. In the optimum solution, each control point should be covered by a user-defined number of sensing paths, defined as the coverage level. The PZT locations were treated as continuous variables. Thus, during the optimization process, any location on the plate is considered as a potential position for a PZT wafer. The algorithm provides the flexibility of changing a wide range of parameters including the number of PZT wafers, the distance covered around the sensing path, the required coverage level, and the number of control points, in addition to identifying the most sensitive PZT wafer within the network. The tractability of the model proposed was improved by feeding the solver an initial solution. The model calculates the importance of each PZT wafer within the network, which allows for further reduction in the number of active PZT elements. The suggested model was solved using a genetic algorithm. Multiple sensor network configurations on composite and metallic structures were selected, including a large cargo door of an A330 airplane, and validated experimentally. The experimental validation was to evaluate the accuracy in damage localization within the optimized sensor networks. The results demonstrated the proficiency of the model developed in distributing the PZT wafers on non-convex structures and large metallic structures.

The interaction of guided waves with a material discontinuity is not well understood. This study investigates the propagation behavior of the fundamental Lamb-wave modes, the symmetric mode ( S₀) and the anti-symmetric mode ( A₀), upon interaction with welded joints of dissimilar materials. A plate with an intact AA6061-T6/AZ31B dissimilar joint was employed, and the interaction of the propagating wave with the material interface was scrutinized numerically and validated experimentally. Plane-wave approximation was also adopted to investigate the behavior of the symmetric modes, and its performance was compared to the numerical and experimental results. The effect of the angle of incidence on the reflection, transmission, and mode conversion of the incident modes was analyzed. The study was conducted as the excited Lamb wave propagated from AA6061-T6 to AZ31B (forward), and when the propagation direction was reversed (backward). Different techniques were developed to identify the in-plane and out-of-plane modes from the three-dimensional measurements and to separate wave reflections and transmissions of the joint. The fundamental shear-horizontal guided-wave mode ( SH₀ mode) has evolved upon the interaction of the obliquely-incident Lamb-wave S₀ mode with the interface. While the reflection of the SH₀ mode from the joint was found to be well-pronounced, its transmission to the other material is extremely weak. The analytical solution, using plane-wave approximation, was accurate for predicting the behavior of the in-plane modes ( S₀ and S₀– SH₀ modes). Despite the peaks appearing at the critical angle, the absolute values of the reflection coefficients of the studied modes have shown similar trends between the forward and the backward propagation directions. The total reflection of the excited wave, from the material interface, was not observed in any condition. The transmission coefficients of the S₀ and A₀ modes are almost constant until reaching very steep incidence angles [ ]. The results were experimentally validated on an intact AA6061-T6/AZ31B friction-stir-welded joint using an excitation frequency of 200 kHz. Measurements along the transmission and reflection directions were conducted using a three-dimensional scanning laser vibrometer. Experimental results showed very good agreement with both the analytical and the numerical ones.