Optimal dual-PZT sizing and network design for baseline-free SHM of complex anisotropic composite structures

Structural health monitoring (SHM) processes for aeronautic composite structures are generally based on the comparison of healthy and unknown databases. The need for prior baseline signals is one of the barriers to industrial deployment and can be avoided with baseline free SHM (BF-SHM) methods, based on the attenuations and reflections of symmetric and antisymmetric Lamb wave modes attributable to damage. A promising mode decomposition method is based on the use of dual-PZTs (concentric disc and ring electrodes lying on a single piezoelectric transducer of lead zirconate titanate). However, the performance of such methods highly depends on the Lamb wave mode properties (propagation speed and attenuation , which vary with material orientation and inter-PZT distance), the number and the sensitivity of the dual-PZT to each mode (which depends on the frequency and element size). Considering these constraints, an original three-step process able to design a full dual-PZT network and the optimal range of excitation frequencies to consider on a highly anisotropic and arbitrarily complex aeronautic structure is presented. First, the dispersion curves of Lamb waves in the investigated material together with the minimal size of the damage to detect are used to estimate the size of the dual-PZT, as well as convenient excitation frequencies. A local finite element model representative of the full-scale structure is then used to estimate the optimal distance and orientation between neighboring PZT elements. Finally, a network optimization solver applies these parameters to place dual-PZTs on the fan cowl of an aircraft nacelle and provides a candidate network covering the whole structure.