Multi-helical Lamb Wave Imaging for Pipe-like Structures Based on a Probabilistic Reconstruction Approach

The special form of pipe-like structure provides the helical route for ultrasonic guided wave. Considering the pipe as a flattened plate but with periodical replications, the helical wave becomes intuitional and a corresponding imaging algorithm can be constructed. This work proposes the multihelical Lamb wave imaging method by utilizing the multiple arrival wavepackets which are denoted as different orders. The helical wave signal model is presented and the constant group velocity point is illustrated. The probabilistic reconstruction algorithm is combined with the separation and fusion of different helical routes. To verify the proposed scheme, finite element simulations and corresponding experiments are conducted. The cases of single-defect simulation and two-defect simulation indicate the successful and robust implementation of the imaging algorithm. The test on actual pipe damage is also investigated to show its capability in imaging an irregular defect. The comparison with imaging results from only first arrival demonstrates the advantage of multihelical wave imaging, including the better imaging resolution and higher localization accuracy

Sensor Placement for Damage Localization in Sensor Networks

The objective of this thesis is to formulate and solve the sensor placement problem for damage localization in a sensor network. A Bayesian estimation problem is formulated with the time-of-flight (ToF) measurements. In this model, ToF of lamb waves, which are generated and received by piezoelectric sensors, is the total time for each wave to be transmitted, reflected by the target, and received by the sensor. The ToF of the scattered lamb wave has characteristic information about the target location. By using the measurement model and prior information, the target location is estimated in a centralized sensor network with a Monte Carlo approach. Then we derive the Bayesian Fisher information matrix (B-FIM) and based on that posterior Cramer-Rao lower bound (PCRLB), which sets a limit on the mean squared error (MSE) of any Bayesian estimator. In addition, we develop an optimal sensor placement approach to achieve more accurate damage localization, which is based on minimizing the PCRLB. Simulation results show that the optimal sensor placement solutions lead to much lower estimation errors than some sub-optimal sensor placement solutions

Lamb wave based structural health monitoring using fibre bragg grating-based intrinsic fabry-perot interferometer sensor

The demand for an increase in structural safety and a reduction in structural maintenance costs can be achieved by applying a reliable monitoring system. Such systems are called structural health monitoring (SHM) systems. There are several SHM techniques suitable for engineering structures such as acoustic emission (AE), electromechanical impedance (EMI) or Lamb waves testing. The applications of Lamb waves for SHM systems are highly desirable owing to their advantages compared to other approaches. Lamb waves are elastic waves having dispersive behaviours that propagate in plates and shell-like structures. The detection of damage comes from the interaction of Lamb waves with defects in the structures. These defects triggered reflections and conversions of the Lamb modes. The changes in the Lamb wave signals are extracted to detect the damage in the monitored structure. These interactions are complex. Thus, a higher sensitivity sensor is required to capture the interaction of the Lamb wave for applications in SHM systems. The present work proposed an optical fibre sensor, fibre Bragg grating-based intrinsic Fabry-Perot interferometer (FBG-IFPI) sensor for damage detection using Lamb wave technique. It is a hybrid of the fibre Bragg grating sensor and the Fabry-Perot interferometer sensor. The effect of the variation of optical cavity length, the directional sensitivity, and the response of the sensor towards the strain were evaluated. A positive result was observed from that evaluation. It was found that the sensitivity was higher for the longer optical cavity length. The FBG-IFPI with a 10 mm optical cavity length has an FSR of 83 pm, while FBG-IFPI with a 20 mm optical cavity length has an FSR of 34 pm. A smaller FSR indicates a steeper edge and yields a higher strain on optical power amplification, which presents higher sensitivity and resolution in the sensing. The result shows that the sensitivity enhancement in the longer optical cavity is 2.46 times higher. The FBG-IFPI sensor was at its maximum performance when it was oriented in the radial direction (0°) of the source Lamb wave, which shows an increment of 75% for amplitude detection compared to when it is oriented at the right angle with the radial direction (± 90°). In addition, the sensor performance in strain sensing was improved by identification of the optimum wavelength. This wavelength would maximise the sensitivity of the sensor to small changes in strain. It shows an increment of about 7.4% compared to the strain sensitivity of the single FBG. Other than that, the improvement of the FBG-IFPI sensor can also be observed using the absolute gradient curve, which shows the sensitivity is 7.5 times higher than the FBG sensor. The FBG-IFPI sensor was surface mounted on the intact and damaged plate. The sensor was placed at two different positions, that is, 8 and 23 cm from the actuator, which represent the pulse-echo and pitch-catch methods. For the pulse-echo method, additional wave packets are observed due to the reflection from the defect. Meanwhile, for the pitch-catch method, the observation of the amplitude decrements was 66% compared to the intact plate signal. The characteristics of the Lamb waves during interaction with defects for the two methods were successfully detected by the FBG-IFPI sensor and they were validated by Finite Element Analysis (FEA). For the pulse-echo method using FEA, the finding shows that there is also one additional wave packet that appears after the wave interacts with the defect. The analysis of the pitch-catch method shows about 76% amplitude decrement. These characteristics are the damage features that can be used as an indicator for the structure health status. Thus, the main results of the work presented here are the extracted damage indicator from the measured signal using the proposed sensor. This finding shows the applicability of the proposed sensor in damage detection, which can be utilised as the alternative sensor for SHM applications

Lamb-Wave-Based Multistage Damage Detection Method Using an Active PZT Sensor Network for Large Structures

A multistage damage detection method is introduced in this work that uses piezoelectric lead zirconate titanate (PZT) transducers to excite/sense the Lamb wave signals. A continuous wavelet transformation (CWT), based on the Gabor wavelet, is applied to accurately process the complicated wave signals caused by the damage. For a network of transducers, the damage can be detected in one detection cell based on the signals scattered by the damage, and then it can be quantitatively estimated by three detection stages using the outer tangent circle and least-squares methods. First, a single-stage damage detection method is carried out by exciting a transducer at the center of the detection cell to locate the damaged subcell. Then, the corner transducers are excited in the second and third stages of detection to improve the damage detection, especially the size estimation. The method does not require any baseline signal, and it only utilizes the same arrangement of transducers and the same data processing technique in all stages. The results from previous detection stages contribute to the improvement of damage detection in the subsequent stages. Both numerical simulation and experimental evaluation were used to verify that the method can accurately quantify the damage location and size. It was also found that the size of the detection cell plays a vital role in the accuracy of the results in this Lamb-wave-based multistage damage detection method