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

Abstract

Structural Health Monitoring (SHM) processes for aeronautic composite structures are generally based on the comparison between healthy and unknown databases. The need for prior baseline signals is one of the barriers to an industrial deployment and can be avoided with “baseline-free” SHM (BF-SHM) methods based on the attenuations and reflections of symmetric and antisymmetric Lamb waves modes attributable to a damage. A promising mode decomposition method is based on the use of dual PZTs (concentric disc and ring electrodes lying on a single PZT). However, performances of such methods highly depend on the Lamb wave modes properties (propagation speed and attenuation that 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 optimal distance and orientation between neighbor PZTs elements. Finally, a network optimization solver applies these parameters to place dual-PZTs on a fan cowl of an aircraft nacelle and provides a candidate network covering the whole structure.