Guided wave is advantageous to inspect pipe-like structures because of its long-distance propagation. Due to the curved structure of pipeline, guided wave can spread spirally with a helix angle. However, the current transducers focus more on the narrow or omni-directional beam than the helical form. This work presents the designed transducer to generate helical Lamb wave with concentrated beam. The transducer is developed using permanent magnets and arc-coil based on the principle of electromagnetic-acoustic conversion. The numerical analysis applying finite element method is conducted to verify the theoretical model of the transducer. The angular profiles are obtained to study the propagation of helical Lamb wave. Further, experiments are designed to demonstrate the performance of proposed transducer. The results from the experiments show good agreement with the numerical simulations. Besides, the parameter of central width is investigated and the half-angle of divergence is calculated. The results are compared with that from traditional transducers. The comparison indicates the superiority of the proposed transducer in generating helical Lamb wave. Therefore, the designed transducer could be a potential alternation for pipeline inspection based on guided wave.

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Design of Electromagnetic Acoustic Transducer for Helical Lamb Wave With Concentrated Beam

The electromagnetic ultrasound detection technology provides an efficient method to inspect the structural health status. For the pipeline structure, the guided ultrasonic wave propagating along the axial direction has been the focus of the current research. However, the helical wave owns the advantage of obtaining the defect information in extra angles and has not been explored sufficiently. In this work, a transducer with a sector configuration is designed for generating helical shear horizontal waves in a pipeline. The magnetostrictive effect is analyzed, and the model is built in the finite element simulation software of COMSOL Multiphysics. The electromagnetic field and displacement distribution are studied. To evaluate the performance of the transducer, the half divergence angle is defined to measure the acoustic beam and evaluate the capacity in generating helical waves. Corresponding experiments are also implemented to analyze the influencing factors in the structural design of the transducer. The results indicate that the pulse cycle affects the half divergence angle slightly. The compromise should be considered when designing the central angle of the transducer because it influences the signal amplitude and further imaging resolution in a contradictory way.

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Characteristic analysis of electromagnetic acoustic transducers for helical shear horizontal wave based on magnetostrictive effect.

Aircraft pipelines are mainly used for the storage and transportation of fuel, hydraulic oil and water, which are mostly bent pipes of non-ferromagnetic materials. We used PPM (Periodic Permanent Magnet) EMAT (Electromagnetic Acoustic Transducer) to detect the defects at 90-degree bends. A simulation model was established by finite element software to study the propagation characteristics and defect detection capability of T (0, 1) mode-guided wave in aluminum pipe bend. In terms of propagation characteristics, the energy of the guided wave was focused in the extrados of the bend, and the guided waves in the intrados and extrados of the bend were separated due to the difference in propagation distance. Regarding defect detection capability, T (0, 1) mode-guided wave had the highest detection sensitivity for the defect in the extrados of the bend and the lowest detection sensitivity for the defect in the middle area of the bend. We designed a PPM EMAT for 320 kHz to verify the simulation results experimentally, and the experimental results are in good agreement with the simulation results.

/pdf/ppm-emat-for-defect-detection-in-90-degree-pipe-bend-dy11ioo7.pdf

PPM EMAT for Defect Detection in 90-Degree Pipe Bend

Guided waves (GW) are one of the most promising tools for structural health monitoring (SHM). They allow fast inspection of a large area. Thus GW based SHM is finding applications in several fields like aerospace, automotive, wind energy, etc. 

Recently, the use of optical fiber sensors and more specifically fiber Bragg grating (FBG) sensors are getting increasing attention from researchers for GW sensing. The FBG sensors are deployed in the edge-filtering configuration for the sensing. The FBG sensors offer several advantages such as light weight and small size. They also offer the chance to be embedded, and multiplexed. Unfortunately, they are passive sensors and show directional sensitivity. Also they are relatively expensive. Thus there is a need to optimize the usage of the sensors to obtain maximum information with minimum cost. 

Several different optimization strategies have been proposed in the literature including gradient based as well as evolutionary algorithms. For a non-linear, non-differentiable cost function evolutionary algorithms (EA) such as genetic algorithm, particle swarm optimization, etc. have shown a lot of promise. In this paper the Covariance-Matrix Adaptation Evolution Strategy (CMAES) has been employed for the sensor network optimization for GW based SHM. First, the ability of the algorithm to obtain the global optima is investigated and then the results are compared with the genetic algorithm (GA) which is the most commonly used EA. 3 different criteria for the optimization are used namely, the coverage with 3 actuator-sensor (AS) pairs (cov3), coverage with 1 AS pair (cov1), and the scalarized combination of cov3 and cov1. The results indicate that indeed CMAES is a powerful optimization tool for the sensor placement optimization problem.

Ultrasonic fiber Bragg grating sensor placement optimization in structural health monitoring usingcovariance matrix adaptation evolutionary strategy

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.

/pdf/multihelical-lamb-wave-imaging-for-pipe-like-structures-4c67h78x7k.pdf

Multihelical Lamb Wave Imaging for Pipe-Like Structures Based on a Probabilistic Reconstruction Approach

Pipeline structures are important structural components that cannot be replaced in actual engineering applications. Damage to a pipeline structure will create substantial safety hazards and economic losses in a project. Therefore, it is extremely important to study damaged pipeline structures. In this paper, L(0,2) mode guided waves are used to identify, locate, and image single and double defects in straight pipe structures. For the case where there is a single defect in the straight pipe section, the influence of different excitation frequencies on the reflection coefficient of L(0,2) modal guided wave is studied, and the optimal excitation frequency of L(0,2) guided wave is 70 kHz when single damage is determined. For the case of double defects in the straight pipe section, the double-defect size, the distance between the defects, and the relative defect positions are studied, and the influence of the defect recognition effect is analyzed. The propagation path of the ultrasonic guided wave in the double-defect pipe section is analyzed. Finally, the effectiveness of the three-point axial positioning method and damage imaging method is verified by the single-defect tube segment ultrasonic guided wave flaw detection experiment.

/pdf/research-on-pipeline-damage-imaging-technology-based-on-4rdwu1x8z6.pdf

Chen Zhou

Papers

Research on Pipeline Damage Imaging Technology Based on Ultrasonic Guided Waves