High spatial resolution imaging for structural health monitoring based on virtual time reversal

Abstract

Lamb waves are widely used in structural health monitoring (SHM) of plate-like structures. Due to the dispersion effect, Lamb wavepackets will be elongated and the resolution for damage identification will be strongly affected. This effect can be automatically compensated by the time reversal process (TRP). However, the time information of the compensated waves is also removed at the same time. To improve the spatial resolution of Lamb wave detection, virtual time reversal (VTR) is presented in this paper. In VTR, a changing-element excitation and reception mechanism (CERM) rather than the traditional fixed excitation and reception mechanism (FERM) is adopted for time information conservation. Furthermore, the complicated TRP procedure is replaced by simple signal operations which can make savings in the hardware cost for recording and generating the time-reversed Lamb waves. After the effects of VTR for dispersive damage scattered signals are theoretically analyzed, the realization of VTR involving the acquisition of the transfer functions of damage detecting paths under step pulse excitation is discussed. Then, a VTR-based imaging method is developed to improve the spatial resolution of the delay-and-sum imaging with a sparse piezoelectric (PZT) wafer array. Experimental validation indicates that the damage scattered wavepackets of A 0 mode in an aluminum plate are partly recompressed and focalized with their time information preserved by VTR. Both the single damage and the dual adjacent damages in the plate can be clearly displayed with high spatial resolution by the proposed VTR-based imaging method.

Structural Health Monitoring for Composite Materials

“…In delay-and-sum imaging method [29][30], every point of the tested structure is considered as a potential flaw. As shown in figure 7, the traveling time (,) ij tx y of Lamb waves from actuator i at (, ) ij xy to an imaging point O at (,) xy and then to sensor j at (,) jj xy is computed assuming that only one Lamb wave mode exists 22 22 ( , ) ( …”

Signal Construction-Based Dispersion Compensation of Lamb Waves Considering Signal Waveform and Amplitude Spectrum Preservation

“…With piezoelectric (PZT) wafers applied as actuators and sensors, a Lamb wave signal, supposed of single wavepacket for simplicity, can be represented in the frequency domain as [19,20] $V_{0}false(\omega false)=V_{a}false(\omega false)Hfalse(\omega false)$ where $\omega$, $r$, $V_{a}false(\omega false)$ and $V_{0}false(\omega false)$ are the angular frequency, propagation distance, frequency-domain excitation signal and sensor signal, respectively. $Hfalse(\omega false)$, regarded as the transfer function of the whole procedure including Lamb wave exciting, propagating and sensing, can be expressed by $Hfalse(\omega false)=Afalse(r,\omega false)e^{-i{K}_{0}false(\omega false)r}$ where $Afalse(r,\omega false)$ is actually the amplitude spectrum of $Hfalse(\omega false)$, $K_{0}false(\omega false)$ is the wavenumber that determines the dispersion relation of the Lamb wave mode, and $c_p(\omega )=\omega /K_0(\omega ),c_g(\omega )=d\omega /d\left[K_0(\omega )\right]$ where $c_{p}false(\omega false)$ and $c_{g}false(\omega false)$ are the phase and group velocities of the mode, respectively.…”

Applications of diffraction tomography on quantitative imaging using ultrasonic guided plate waves

“…In the rst step, the damage is localised by using the current reconstructed image as shown in Figure 7.11d or other localisation method, e.g. beamforming [82,86] and time-reversal mirror [111,165]. In the second step, the defect is interrogated by a highly directional beam excited by a phase array [79] according to the detected damage location in the rst step .…”

“…The results also show that the reconstructions for small and shallow damages are prone to suer from residual errors introduced by the baseline subtraction process because of the poor SNR of the scattered waves. One possible approach to improve the SNR hence the reconstruction is to use a two-step procedure that uses phased array technique [79] to enhance the strength of the interrogating wave based on the damage location estimated by localisation methods such as beamforming [82,86] and time-reversal [111,165]. However, the improvement of this two-step procedure is still not clear, and hence further investigations are required.…”

“…Cai et al [111] have developed the virtual time reversal (VTR) method and have applied it to generate images combined with the DAS imaging algorithm. An array of transducers acting as both transmitters and sensors were employed to emit and record wave signals.…”