Multifrequency Identification and Exploitation in Lamb Wave Inspection

Lamb wave inspection provides a promising method to assess the structural health status. However, Lamb wave modes exhibit different characteristics which vary with frequency significantly. The best excitation frequency usually cannot be determined in specific applications. This work proposes a multifrequency exploitation and identification method. Lamb waves of multiple frequencies are excitated simultaneously to utilize diverse attributes of Lamb waves in different frequency ranges. This paper firstly analyzes the detectability and sensitivity of Lamb wave. Then the multifrequency exploitation scheme and corresponding post-processing method are introduced. Relevant simulations by finite element method are conducted to verify its effectiveness. Experiments of single-frequency and multifrequency excitations are implemented. The investigations indicate that the proposed method can avoid the missing of defects compared with single-frequency excitation. In addition, a post-processing method is suggested and the results demonstrate that the multifrequency excitation also provides high accuracy in defect location

Experimental Study of the Guided Wave Directivity Patterns of Thin Removable Magnetostrictive Patches

© 2020 by the authors. The characteristics of removable magnetostrictive thin patches are investigated for the generation of guided waves in plates. The directivity patterns of SH, S0 and A0 modes have been measured in a thin metallic plate for different combinations of static and dynamic magnetic field directions. This used different coil geometries such as racetrack and spiral coils to generate the dynamic magnetic field, as well as separate biasing static magnetic fields from permanent magnets. This arrangement generated signals via both Lorentz and magnetostrictive forces, and the resultant emitted guided waves were studied for different dynamic and static magnetic field directions and magnitudes. It is demonstrated that different guided wave modes can be produced by controlling these parameters.Marie Skłodowska Curie Training Network in Non-Destructive Testing and Structural Health Monitoring of Aircraft structure

Structural Health Monitoring (SHM) and Determination of Surface Defects in Large Metallic Structures using Ultrasonic Guided Waves

Ultrasonic guided wave (UGW) is one of the most commonly used technologies for non-destructive evaluation (NDE) and structural health monitoring (SHM) of structural components. Because of its excellent long-range diagnostic capability, this method is effective in detecting cracks, material loss, and fatigue-based defects in isotropic and anisotropic structures. The shape and orientation of structural defects are critical parameters during the investigation of crack propagation, assessment of damage severity, and prediction of remaining useful life (RUL) of structures. These parameters become even more important in cases where the crack intensity is associated with the safety of men, environment, and material, such as ship’s hull, aero-structures, rail tracks and subsea pipelines. This paper reviews the research literature on UGWs and their application in defect diagnosis and health monitoring of metallic structures. It has been observed that no significant research work has been convened to identify the shape and orientation of defects in plate-like structures. We also propose an experimental research work assisted by numerical simulations to investigate the response of UGWs upon interaction with cracks in different shapes and orientations. A framework for an empirical model may be considered to determine these structural flaws

High temperature electromagnetic acoustic transducer for guided wave testing

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonThis research focuses on the theoretical analysis, development and experimental evaluation of a water cooled Electromagnetic Acoustic Transducer (EMAT) specifically designed for high-temperature Guided Wave Testing (GWT). Its novel design and detailed calculation of its optimum operating conditions resulted in its effectiveness at high temperatures for both short (500oC) and long-term inspection (250oC). All the steps followed for the theoretical and experimental investigation of the limitations of the existing technology and the development of a probe that can overcome these boundaries are presented. Finite Element Analysis (FEA) was performed for the optimization of the EMAT design and estimation of its ultrasonic and thermal properties at room and high temperatures over time. The wave mode purity profile of the Periodic Permanent Magnet (PPM) EMAT was theoretically studied as well as the effect of temperature rise on its ultrasonic performance. Thermal and Computational Fluid Dynamics (CFD) analysis was accomplished for the EMAT design optimization and calculation of its optimum operating conditions. The experimental validation of the theoretical study was also accomplished. The novel water cooled EMAT was developed and experimentally evaluated regarding its ultrasonic and thermal response at room and high temperatures. An empirical method for the enhancement of EMAT performance and its SNR was established. The wave mode purity characteristics of PPM EMAT were experimentally investigated via Laser vibrometry tests, which agreed with FEA results. The impedance analysis and ultrasonic evaluation of the EMAT at both room and high temperatures against various operating conditions were linked and compared to the results obtained from the ultrasonic, thermal and fluid FEA. In all cases, the experimental study is in good agreement with the theoretical results

Development of EMAT and piezoelectric transducers for high temperature ultrasonic thickness measurements

Improving reliability of components operating at high temperature, such as pipelines, boilers and reactors, within a range of industries is of importance in the asset management process. This thesis concerns the development and testing of ultrasound transducers for use at elevated temperatures, up to 500 _C, without the use of active cooling. Ultrasound thickness measurement applications employing these high temperature transducers includes both portable-type non-destructive testing (NDT) inspections and permanent condition monitoring, primarily towards detection of corrosion and erosion. The development and optimisation of an electromagnetic acoustic transducer (EMAT) design which generates and detects bulk radially polarised shear waves utilising a high temperature permanent magnet and a ceramic encapsulated spiral coil is discussed. This design was optimised for use on magnetite coated mild steel samples; it was shown that the magnetostriction mechanism tends to dominate, depending upon sample properties, producing large signals even at elevated temperatures. High temperature laboratory trials (up to 500 oC) demonstrated the non-linear change in signal amplitude with increasing temperature on magnetite coated mild steel samples, attributed to the complex non-linear relationship between magnetostrictive strains and applied external magnetic field. The EMAT provided good signal amplitude, even at relatively large sample-EMAT lift-off (up to 8.0 mm), demonstrating the applicability of this EMAT for high temperature scanning inspections. A longterm industrial field trial on a high temperature pipeline (≈ 350 oC) in a refinery exhibited the suitability of this design for high temperature continuous monitoring applications. A piezoelectric transducer with a novel compression-type design was optimised for application at high temperature, with the use of a waveguide, high temperature piezoelectric element and high temperature backing material; the optimisation of these components is discussed. This transducer design incorporates compression applied via a central bolt, to achieve acoustic coupling between the components, avoiding the use of adhesive layers, to generate bulk longitudinal waves. With the application of a bismuth titanate piezoelectric element, the transducer was able to generate signals on stainless steel whilst withstanding high temperatures (up to 500 oC) continuously without cooling