A Topologically Designed Metamaterial Filter for Nonlinear-guided-wave-based Structural Health Monitoring Application

Nonlinear guided waves (NGW) show great promise for structural health monitoring (SHM) due to their high sensitivity to the early detection of material micro-structural defects. In an SHM system, however, there exist inevitable non-damage-related nonlinear sources that may overwhelm the damage-induced nonlinear wave components, which in turn may jeopardize the practical implementation of the NGW-based SHM methodology. To eliminate these deceptive nonlinear interferences, this study introduces the concept of metamaterials to SHM. A wave filtering device, referred to as a meta-filter (MF), is developed to be surface-mounted on the structure under inspection under a second harmonic Lamb wave based SHM paradigm. Through a topological design, the MF enables ultra-wide stopbands to eliminate the secondary Lamb waves of the probing waves while preserving their strong fundamental wave components. The band structure, underlying wave filtering mechanism and the wave filtering function of the MF are investigated through finite element simulations. Upon tactically introducing deceptive nonlinear interferences at the actuation area, exemplified by adhesive bonding layers in a PZT-activated SHM system, the performance of the MF is examined from an SHM perspective, and finally validated experimentally using a metal specimen containing local plasticity-related incipient damage. Results demonstrate that the designed MF entails significant enhancement of the detection ability of NGW-based SHM system for incipient damages on one hand, and also allows for flexible selection of the excitation frequency on the other hand thanks to the customized band features enabled by the topological optimization.