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.

A method of ultrasonic imaging detection is presented for quick non-destructive testing (NDT) of concrete structures using synthesized aperture focusing technology (SAFT). A low cost ultrasonic sensor array consisting of 12 market available low frequency ultrasonic transducers is designed and manufactured. A channel compensation method is proposed to improve the consistency of different transducers. The controlling devices for array scan as well as the virtual instrument for SAFT imaging are designed. In the coarse scan mode with the scan step of 50 mm, the system can quickly give an image display of a cross section of 600 mm (L) × 300 mm (D) by one measurement. In the refined scan model, the system can reduce the scan step and give an image display of the same cross section by moving the sensor array several times. Experiments on staircase specimen, concrete slab with embedded target, and building floor with underground pipe line all verify the efficiency of the proposed method.

Concrete is one of the main construction materials in civil engineering. Among the existing Nondestructive Testing (NDT) methods, the imaging techniques that can give a graphic display of the inner structure of the concrete elements have received great attention. Because of several kinds of disturbing signals in the concrete structures imaging by low frequency ultrasound, low signal-noise ratio and poor resolution are engendered. Synthetic Aperture Focusing Technique (SAFT) is one of effect methods in the ultrasonic NDT area, and can focus effectively to reflected waves, and improve the reconstructed image's resolution through rough B-scan method. For lowfrequency excitation signal, however, wavepacket distortion is produced during the focusing process, which decreases test resolution. Therefore, Wavepacket Decomposition Technique (WDT), which can put the whole reflected wavepacket into focusing calculation, is introduced here to improve the reconstructed resolution. This technique solves the problem of wavepacket distortion, and better focusing effect and resolution are obtained. Furthermore, calculating velocity is increased because the points joined into SAFT calculation are cut down greatly. The presented technique has been verified by the simulation experiment. From the results, the method can improve more the ability of localizing object and give better imaging effect.

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