Nowadays, fringe projection profilometry is widely used in optical three-dimensional (3D) measurement. However, due to distortion, epipolar constraints are inaccurate for use in correspondence matching of a projector. This results in low accuracy of final 3D reconstruction. To address the issue, two simple and effective methods for undistorted correspondence matching are proposed in this paper. The first removes distortion from projector correspondences using double epipolar constraints; the second pre-distorts the fringe images for projection, thus the projected fringes become naturally undistorted. Comparative experiments are implemented, in a measurement volume of (50 × 30 × 8) mm. The results show that typical mean reconstruction errors of the two methods drop to 32.9% and 27.6% from that of an existing method based on single epipolar-constraint undistortion.

This paper investigates the Lamb wave imaging method combining time reversal for health monitoring of a metallic plate structure. The temporal focusing effect of the time reversal Lamb waves is investigated theoretically. It demonstrates that the focusing effect is related to the frequency dependency of the time reversal operation. Numerical simulations are conducted to study the time reversal behaviour of Lamb wave modes under broadband and narrowband excitations. The results show that the reconstructed time reversed wave exhibits close similarity to the reversed narrowband tone burst signal validating the theoretical model. To enhance the similarity, the cycle number of the excited signal should be increased. Experiments combining finite element model are then conducted to study the imaging method in the presence of damage like hole in the plate structure. In this work, the time reversal technique is used for the recompression of Lamb wave signals. Damage imaging results with time reversal using broadband and narrowband excitations are compared to those without time reversal. It suggests that the narrowband excitation combined time reversal can locate and determine the size of structural damage more precisely, but the cycle number of the excited signal should be chosen reasonably.

The focusing of time reversal acoustic fields for dispersive and multimodal Lamb waves is theoretically investigated and experimentally verified. It is demonstrated that the time reversal Lamb wave signal will reach the maximum amplitude when the observation point is located at the damage location. Based on the time reversal focusing theory, a damage imaging method is proposed for structural health monitoring using Lamb waves. The experiments employ a transducer network consisting of four piezoelectric transducers as actuators and as well sensors for excitation and measurement of Lamb waves. The results show that this method is able to accurately predict damage location and provides an estimation of the possible area even for damage close to one of the transducers or even slightly outside the transducer network.

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Considering the interaction between the piezoelectric transducer (PZT) and the plate, a frequency adjusting method of generating single mode Lamb waves using single piezoelectric transducer is presented in theory. The application of mode selection in Lamb wave structural health monitoring is experimentally given. The theory has the ability to predict the amplitude of each Lamb wave mode as a function of frequency for given plate material and thickness, and specific PZT size. Optimal actuating frequency can be identified at which the wave amplitude for a particular mode is maximized while the wave amplitudes for other modes are relatively minimized. Numerical results are presented to validate the theory and show the capability of single mode Lamb wave selection. Different frequencies that correspond to a preferential A0 mode, a preferential S0 mode, and both the A0 and the S0 modes are excited for damage imaging, respectively. The results show that the single Lamb wave mode detection can locate the damage more accurately, demonstrating the importance of the mode selection in Lamb wave structural health monitoring.