Novel method of defect identification in bent structures through feature-guided wave detection using fiber Bragg grating sensors

Feature-guided wave-based health monitoring of bent plates using fiber Bragg gratings:

Abstract: Harnessing of ultrasonic guided waves confined in local features such as bends and welds, known as feature-guided waves, has emerged as a promising technique for non-destructive testing and structural health monitoring of industrial and aerospace structures. This article introduces a fiber Bragg grating based technique which uses feature-guided waves to detect anomalies or defects in plate structures with transverse bends. We are able to obtain good consistency between simulation and experimental results, both in the case of defect-free bent plates and those with transverse defects. Such results establish fiber Bragg gratings as a viable alternative to conventional techniques for structural health monitoring of bent plates.

Monitoring pipe wall integrity using fiber Bragg grating-based sensing of low-frequency guided ultrasonic waves.

Abstract: Recent literature shows that low-frequency ultrasonic guided waves experience mode confinement and loss of axi-symmetry in pipes with axially uniform features such as eccentricity. Considering extended wall loss as a case of uniform eccentricity, this paper proposes to monitor pipe integrity by measuring changes to the modal structure of low-frequency axisymmetric L(0,2) longitudinal guided waves. Fiber Bragg gratings are shown to be effective in detecting changes to L(0,2) modal characteristics, providing a novel route to health monitoring of pipe assets.

Comparative Study of Coupling Techniques in Lamb Wave Testing of Metallic and Cementitious Plates.

Abstract: Lamb waves have emerged as a valuable tool to examine long plate-like structures in a faster way compared to conventional bulk wave techniques, which make them attractive in non-destructive testing. However, they present a multimodal and dispersive nature, which hinders signal identification. Oblique incidence is one of the most known methods to generate and receive Lamb waves and it is applied in different experimental arrangements with different types of sensors. In this work, several setups were conducted and compared to determine the optimal ones to launch and detect ultrasonic Lamb waves, especially in non-homogeneous specimens. The chosen arrangements were contact with angle beam transducers, immersion in a water tank, localised water coupling using conical containers and air coupling. Plates of two different materials were used, stainless steel and Portland cement mortar. Theoretical and experimental dispersion curves were compared to verify the existence of Lamb modes and good correspondence was achieved.

Analysis of Energy Density Spectrum of Lowvelocity Impact in Honeycomb Sandwich Panel with FBG

Abstract: As a kind of structure commonly used in satellite, carbon fiber composite honeycomb sandwich panels are vulnerable to various kinds of damage, especially low-velocity impact damage in the process of using. In this paper, an impact monitoring system based on Fiber Bragg Grating (FBG) sensor is constructed. The frequency domain information of the signal can be obtained by Fourier transform. Monitoring results of different impact points show that the natural frequency of structure is 110Hz and 380Hz, which amplitude of different natural frequency is changed with position of impact point. The energy status of impact response signal can be obtained by wavelet packet analysis. The monitoring results of the same impact point shows that the change of the sensor signal is obvious in thirteenth order and energy size is related to the distance and angle between the impact point and sensor.

Ultrasonic Testing of Materials

Abstract: Nondestructive testing of solid material using ultrasonic waves, for defects such as cavities, nonbonding, and strength variations, is treated in this book from the physical fundamentals of ultrasonics and materials up to the most sophisticated methods. The book is written at a level which should make it accessible to readers with some knowledge of technical mathematics. Physical laws are explained in elementary terms, and more sophisticated treatments are also indicated. After the fundamentals, instrumentation and its application is extensively reported. Tricks and observations from thirty years of experience in the field are included. The third part of the book presents test problems related to special materials or ranges of modern heavy industry, including recent applications such as those in nuclear power plants. This fourth edition features improved presentation of certain fundamental physical facts, updated reports on electronic instrumentation, and new applications in the nuclear and space industries.

Rapid, Long Range Inspection of Chemical Plant Pipework Using Guided Waves

Abstract: Instrumentation for long range, guided wave inspection of pipework is now commercially available. This paper discusses the principles of the method and reviews the results of site experience. The technique was originally designed to work on pipes that were either un-coated or covered with, for example, epoxy paint. Recent tests have shown promising results with more attenuative coatings and these are discussed.

Highly sensitive dynamic strain measurements by locking lasers to fiber Bragg gratings

Abstract: A novel, sensitive, simple, and robust strain interrogation technique is analyzed and experimentally tested. By locking a laser wavelength to the midreflection wavelength of a standard fiber Bragg grating and measuring the error signal, we achieve high dynamic strain sensitivity of 45 picostrain/Hz rms at 3 kHz, where the dominant noise in the experiment is the laser frequency noise.

On the use of absorbing layers to simulate the propagation of elastic waves in unbounded isotropic media using commercially available Finite Element packages

Abstract: Finite Element models for simulating wave propagation and scattering from defects are vital for ultrasonic methods in NDE. This article addresses methods to dramatically enhance computational efficiency by only meshing a local region of the material surrounding the defect; this reduction requires some kind of boundary, or boundary condition, which absorbs, rather than reflects, any waves arriving at the exterior of the modelled domain. A variety of approaches exist and we take two approaches, Perfectly Matched Layers (PML) and Absorbing Regions, selected specifically as they are readily implemented in commercially available Finite Element packages without requiring the source code. We illustrate both bulk and guided waves, and analysis is used to guide the performance, and thus to plan the use, of each of them. Finally, application examples illustrate the gains yielded by absorbing layer methods in terms of reducing both model size and unwanted reflections.