Over the past decade, RNA sequencing (RNA-seq) has become an indispensable tool for transcriptome-wide analysis of differential gene expression and differential splicing of mRNAs. However, as next-generation sequencing technologies have developed, so too has RNA-seq. Now, RNA-seq methods are available for studying many different aspects of RNA biology, including single-cell gene expression, translation (the translatome) and RNA structure (the structurome). Exciting new applications are being explored, such as spatial transcriptomics (spatialomics). Together with new long-read and direct RNA-seq technologies and better computational tools for data analysis, innovations in RNA-seq are contributing to a fuller understanding of RNA biology, from questions such as when and where transcription occurs to the folding and intermolecular interactions that govern RNA function.

RNA sequencing: the teenage years

Time-reassigned synchrosqueezing transform: The algorithm and its applications in mechanical signal processing

Foreword Chapter 1 Introduction to SHM (Daniel L Balageas) Chapter 2 Vibration-based techniques for SHM (Claus-Peter Fritzen) Chapter 3 Fiber-optics sensors (Alfredo Guemes and Jose M Menendez) Chapter 4 SHM with piezoelectric sensors (Philippe Guy and Thomas Monnier) Chapter 5 SHM using electrical resistance (Michelle Salvia and Jean-Christophe Abry) Chapter 6 Low frequency electromagnetic techniques (Michel B Lemistre) Chapter 7 Capacitive methods for SHM in civil engineering (Xavier Derobert and Jean Iaquinta) Short Bibliographies of the Contributors Index

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Structural health monitoring

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.

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

This review introduces several areas of importance in acoustic emission (AE) technology, starting from signal attenuation. Signal loss is a critical issue in any large-scale AE monitoring, but few systematic studies have appeared. Information on damping and attenuation has been gathered from metal, polymer, and composite fields to provide a useful method for AE monitoring. This is followed by discussion on source location, bridge monitoring, sensing and signal processing, and pressure vessels and tanks, then special applications are briefly covered. Here, useful information and valuable sources are identified with short comments indicating their significance. It is hoped that readers note developments in areas outside of their own specialty for possible cross-fertilization.

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Review on Structural Health Evaluation with Acoustic Emission

In recent years, many researchers have explored the use of guided ultrasonic waves for nondestructive testing (NDT). When guided waves are transmitted into a structure, any geometric and material discontinuities in the waves’ path modify these waves. Using appropriate signal processing methods for the waves received at a sensor, information about these features can be extracted. However, little research has been conducted to locate the features and automatically generate maps without using a priori knowledge. For NDT of large-scale structures, such as the wings of an airplane, many (automated) measurements need to be conducted, and localization of identified features on a map is crucial for successful damage detection. Hence, in this work, methods to detect edges are investigated in an effort to generate a map of the structure using Lamb waves. Measurements are conducted with contact and air-coupled ultrasound transducers in laboratory experiments. While the used contact transducers do not exhibit any directional sensitivity, air-coupled transducers are only sensitive to incoming waves from one direction. Therefore, different data processing methods have to be applied, depending on the applied actuator or sensor technology. Even though the experiments are conducted for a pristine aluminum plate, an outlook for composite plates is given as well. In addition, it is explored whether guided-wave based methods also allow for the detection of other structural features, such as stiffeners. The accuracy of the applied identification methods is validated against the structures’ true dimensions. Even though substantial assumptions have to be made, the investigated methods show promise for successful application in real scenarios.

Experimental investigation of Lamb wave-based edge detection methods

Advanced fuzzy arithmetic for material characterization of composites using guided ultrasonic waves

For the purpose of nondestructive testing (NDT), guided waves can be transmitted into a structure, and any defects or anomalies in the waves’ path modify the measured waves. Signal processing methods can be used to extract information about these features. In this work, an NDT method is demonstrated based on laboratory experiments for the case of a flat, rectangular, aluminum plate, which has a stiffener mounted underneath along the middle axis, such that the stiffener cannot be seen from the upper “outside” surface. Piezoelectric transducers are set up in a pitch-catch arrangement on this surface with the assumption that the location of the stiffener is unknown. When guided waves are induced in the plate by one of the transducers, the waves that are received by the other carry the information of the stiffener, as well as any defects in or boundaries of the structure. By transmitting from different points on a grid on the plate, the location and size of any geometry or material discontinuities can be identified. Hence, the developed algorithm reverse engineers the plate by mapping its edges and identifying the region of the stiffener.

Reverse engineering stiffened plates using guided wave-based nondestructive testing methods

Lamb waves propagating in thin plates and shells are being widely studied for their potential applications in nondestructive inspection of large-scale structures. These structures are generally characterized by the presence of geometrical discontinuities such as stiffeners, mechanical joints or variable thicknesses that affect the propagation characteristics of Lamb waves that can be very similar to those from defects occurring in service (delamination, disbond, etc.). Therefore, the knowledge of the effects of such discontinuities on the propagation of guided waves is essential to avoid their false identification as defects. In this work Lamb waves propagating in a metal plate with a downward step are studied through laboratory experiments. A single 10 mm piezoceramic disk (PZT) bonded to the host structure using cyanoacrylate gage adhesive is utilized for Lamb waves generation and the responses are measured at multiple locations, along a line crossing the step, using a scanning laser Doppler vibrometer (LDV). The interaction of the fundamental Lamb mode A0 with the geometrical discontinuity in the isotropic plate is investigated and discussed.

Laser Doppler velocimetry and PZT sensing for the study of guided waves in a stepped aluminum plate

Stiffeners are important structural components in modern composite and honey-comb structures. The safe operation of such composite structures, which are com-monly used in aeronautical applications, requires careful monitoring as hidden de-fects may compromise the structural safety. In order to improve the reliability of ultrasonic damage detection in stiffened plate structures, experimental and numeri-cal studies are conducted, in this paper, in an effort to understand the interaction of guided ultrasonic waves with simple models of the stiffener. Through a series of measurements in different positions, the amplitudes of scattered waves for various configurations of the stiffener are determined. Moreover, the group velocity of the waves in the stiffened structure is analyzed. The experimental findings are compared with results from numerical simulations. doi: 10.12783/SHM2015/248

Christoph Schaal

Papers

Experimental investigation of Lamb wave-based edge detection methods

Advanced fuzzy arithmetic for material characterization of composites using guided ultrasonic waves

Reverse engineering stiffened plates using guided wave-based nondestructive testing methods

Laser Doppler velocimetry and PZT sensing for the study of guided waves in a stepped aluminum plate

The Influence of Stiffeners on the Propagation of Guided Ultrasonic Waves