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In the paper, we combined air-coupled Lamb wave scan method and virtual time reversal (VTR) algorithm and proposed a composite baseline-free delamination inspection technique of composite plates. According to VTR algorithm, time reversal process is virtually performed through signal operations and the hardware manipulation for time reversal is not required. Baseline-free damage inspection can be achieved by comparing the first input actuation signal with the reconstructed final signal obtained by VTR algorithm. An air-coupled Lamb wave scan method combined with VTR-based probabilistic imaging algorithm is developed for delamination inspection of composite plates. Carbon fiber-reinforced composite plates with the delaminations of different shapes and sizes were experimentally tested. The testing results are well in accordance with the actual delamination locations and sizes as well as the results obtained with the commercial point-to-point immersion C-scan system.

Magnetic flux leakage (MFL) sensors, with their compact configuration and high sensitivity to small defects, have attracted much attention in recent years for the non-destructive testing of ferromagnetic structures. Tunnel magneto-resistive (TMR) devices have superior performances in sensitivity and linear operation range over conventional magneto-resistive devices. In this paper, a commercial TMR device is employed for developing an electromagnet-based MFL sensor. The electromagnet magnetizer includes Helmholtz-like coils together with a custommade magnetic shield. The orthogonal test method is applied to aid the structural parameter optimization to the magnetizer based on the finite element analysis results of magnetic field distribution. In this study a prototype of a TMR-based MFL sensor is developed, and its performances on detecting various types of defects are tested on a scanning apparatus. The experimental results show that the MFL signal induced by a blind hole with dimensions of 0.3 mm in both depth and diameter is detectable. In addition, two adjacent notches located more than 2.0 mm from each other can be clearly distinguished from the received MFL signal. The detectable angular detection range for a single TMR device is estimated as 52°in the tested linear shaft rod. The consistency between the simulated and received MFL signal induced by a row of notches inspires confidence in the proposed sensor design method, which in the future can be transplanted for TMR-based sensor array design. Finally, the TMR-based MFL sensor is used for detecting a flaw of a single broken wire with a diameter of 0.5 mm, and the induced MFL signal can be clearly recognized from the oscillation signal that is generated by the twisted rope surface. Therefore, the presented TMR-based MFL sensor has great potential for steel wire rope inspection with enhanced sensitivity to small defects, and it is capable of being integrated into production lines due to its compact configuration.

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Combined with the modified damage imaging algorithm based on Reconstruction Algorithm for the Probabilistic Inspection of Damage (RAPID), a baseline-free detection method of the Lamb waves based on time reversal method is developed to detect the delamination in the composite plate. Through the analysis of the focus position of time reversal signal, a new calculation method of Damage Index (DI) values is proposed. The modified damage imaging algorithm based on RAPID is integrated with two image fusion methods: full summation method and full multiplication method. According to the analysis of limitations of the modified damage imaging algorithm, the transducer array is adopted, and the problem of the uneven probability distribution caused by the uneven density of sensing network is discussed. Modified methods are proposed to eliminate the influence of the uneven probability distribution and improve the accuracy and reliability of the detection results. The parameter β significantly affects the effective detection range of each sensing path in the modified damage imaging algorithm. The value of parameter β was experimentally determined and reasonably applied to detect the delamination. Based on the calculation of the DI of individual sensing path and the imaging results of delamination, the identification and localization of delamination were experimentally achieved. The imaging results demonstrated the problem of the uneven probability distribution and the validity of modified methods.

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