Research On Dispersion Compensation Methods In Lamb Wave Damage Imaging

Lamb waves, a kind of guided ultrasonic waves in plate-like structures, are widely used instructural health monitoring (SHM), because of the ability of long-distance transmission and highsensitivity to both the surface and the internal defects. However, Lamb waves have multi-mode anddispersion characteristics. Even for the single-mode signals, the wavepackets can spread out in timeand space with their envelopes deformed and amplitudes decreased, due to the dispersion effect. Theeffect will become more serious with longer traveling distance, resulting in extremely poorerresolution and signal-to-noise-rate (SNR) for sensor signals. In the dissertation, the single modegeneration and dispersion compensation methods are studied and adopted to improve the damageimaging resolution. The main works are as follows:(1) After mechanical modeling for the procedure of Lamb wave exciting, propagating andreceiving with PZT wafers, the analytical expressions of the sensor signals under different excitationmanners are derived. The multi-mode characteristic of Lamb waves is then analyzed and validated byexperiments in different structures. Moreover, the sensing modal of Lamb waves is also established.The effects of different dispersion relations on sensor signals are theoretically and numericallyinvestigated.(2) With the mechanical modeling results, two single mode generation methods, frequencytuning and double-side excitation, are compared. To prove the ability of double-side excitation ofimproving Lamb wave monitoring resolution, the one-dimension localization for adjacent multipledamages is carried out in an aluminum plate. Hereafter, the delay-and-sum imaging theory is studiedtogether with signal envelope extraction method using complex wavelet transform. Double-sideexcitation is then used to enhance the delay-and-sum imaging resolution and its efficiency isexperimentally verified.(3) Two dispersion removal methods based on theoretical wavenumber curves, i.e.,non-dispersive signal construction (ND-SC) and time-to-distance domain mapping (TDDM) areresearched. The implement procedures of these two approaches are also discussed consideringbroadband or narrowband excitation. The method to estimate the structural impulse responses withstep pulse excitation is experimentally studied. The high-resolution imaging methods based onND-SC and TDDM are developed and validated.(4) To solve the problems of seriously distorbing the signal spectra and requiring absolute wavenumber curves in ND-SC and TDDM, linear-dispersive signal construction (LD-SC) andimproved TDDM (ITDDM) methods are studied, respectively. The method of measuring the relativewavenumber curve is introduced and the feasibility of using LD-SC and ITDDM with relativewavenumber curves is testified. The methods of LD-SC and ITDDM are then applied to improve theimaging resolution in both the aluminum plate and the glass-fiber composite plate with unknownmaterial parameters.(5) Automatically compensating dispersion effect in Lamb wave signals through time reversal(TR) is analyzed. To overcome the problems that traditional TR method eliminates propagation timeof Lamb waves and its procedure is complicated, a virtual time reversal (VTR) method is developed.In VTR, changing-element exciting and receiving mechanism (CERM) rather than fixed exciting andreceiving mechanism (FERM) is adopted for time information reservation. Furthermore, the physicalmanipulations of CERM-based TR are replaced by signal operations. The VTR realization under steppulse excitation is discussed. The high-resolution imaging method based on VTR is introduced. Toverify the proposed imaging method, the experiments are arranged in an aluminum plate, a glass-fibercomposite plate and a carbon-fiber composite plate, respectively.