Stress-wave-based Damage Detection For Aerospace Structures Without Baseline Information

Most of the traditional Lamb-wave-based Structural Health Monitoring techniques rely on thebaseline information of structures in intact condition. These techniques realize damage identificationby comparing the difference between monitor signal and reference signal. However, environmentalfactors such as noise and temperature will affect Lamb wave propagation characteristics. This willmake signal changes with no relation to the damage, which is easy to cause false diagnosis ofstructure health status. In the purpose of solving this technical problem, damage identification inaircraft structures without baseline information was studied in this thesis.Firstly, by simulation, this thesis researches Lamb wave mode conversion caused by a suddenchange in structural thickness. Influences of temperature, damage symmetry and structural complexityon conversed modes are analyzed. Taking advantage of polarization characteristics and symmetricalarrangement of PZT sensors, conversed modes are extracted without any baseline data.Secondly, this thesis researches the reference-free crack detection in metal plate based on Lambwave mode conversion. Crack detection methods based on transfer impedance in flat plate and timeshift of conversed modes in stiffened plate are developed. Simulation and experiments wereconducted to verify the effectiveness and applicability of the proposed method. Crack locationidentification method based on pure mode Lamb waves is further studied. And the crack sizeidentification based on Lamb wave mode conversion is preliminary discussed.Finally, this thesis researches the reference-free damage detection in composite laminated platebased on Lamb waves. Lamb wave mode conversion leaded by matrix crack in laminated plate isstudied through numerical simulation, and the applicability of reference-free crack detection formatrix crack is verified. In order to achieve delamination damage detection in laminated plate withoutbaseline information, multipath delamination damage identification method based on Lamb wave timedelay is developed, and the feasibility of this method is verified by numerical simulation.