Ultrasonic sensor platforms for non-destructive evaluation

Robotic vehicles are receiving increasing attention for use in Non-Destructive Evaluation (NDE), due to their attractiveness in terms of cost, safety and their accessibility to areas where manual inspection is not practical. A reconfigurable Lamb wave scanner, using autonomous robotic platforms is presented. The scanner is built from a fleet of wireless miniature robotic vehicles, each with a non-contact ultrasonic payload capable of generating the A0 Lamb wave mode in plate specimens. An embedded Kalman filter gives the robots a positional accuracy of 10mm. A computer simulator, to facilitate the design and assessment of the reconfigurable scanner, is also presented. Transducer behaviour has been simulated using a Linear Systems approximation (LS), with wave propagation in the structure modelled using the Local Interaction Simulation Approach (LISA). Integration of the LS and LISA approaches were validated for use in Lamb wave scanning by comparison with both analytical techniques and more computationally intensive commercial finite element/diference codes. Starting with fundamental dispersion data, the work goes on to describe the simulation of wave propagation and the subsequent interaction with artificial defects and plate boundaries. The computer simulator was used to evaluate several imaging techniques, including local inspection of the area under the robot and an extended method that emits an ultrasonic wave and listens for echos (B-Scan). These algorithms were implemented in the robotic platform and experimental results are presented. The Synthetic Aperture Focusing Technique (SAFT) was evaluated as a means of improving the fidelity of B-Scan data. It was found that a SAFT is only effective for transducers with reasonably wide beam divergence, necessitating small transducers with a width of approximately 5mm. Finally, an algorithm for robot localisation relative to plate sections was proposed and experimentally validated

Electromagnetic Induction Imaging with Atomic Magnetometers

Electromagnetic induction imaging (EMI) is a technique for non-invasively mapping the passive electromagnetic properties of materials. It involves the active probing of samples with a radio-frequency magnetic field and recording the details of the magnetic field produced by the induced eddy current response. The performance of an EMI system is ultimately determined by the choice of magnetic field sensor used in the measurement. The sensor’s sensitivity, range of operation frequency, and sensing volume are all crucial characteristics when considering the imaging platform’s capabilities. Atomic magnetometers (AMs) – based on the coherent precession of a polarised alkali atomic vapour – currently rate amongst the most sensitive devices for magnetic field measurements. Radio-frequency atomic magnetometers (RF-AMs) are ultra-sensitive detectors of oscillating magnetic fields across a broad range of frequencies. As such, they are ideally suited to EMI applications. This work presents the development of EMI systems based on RF-AMs. The imaging performance and a wide range of applications are experimentally demonstrated. The continuous development of a single-channel rubidium RF-AM is described. The final device operates in unshielded environments and near room temperature with a measured sensitivity of 55 fT/√Hz, a photon-shot noise limit of 10 fT/√Hz, and a linewidth of 36 Hz. Tunability of the device is proven by consistent, narrow-linewidth operation across the kHz – MHz band – operating in magnetic fields significantly greater than previous AM designs. The sensor was developed with a small effective sensor volume, which increases the spatial resolution of the imaging. High-resolution EMI is performed across a broad range of materials. This spans the first EMI images with an RF-AM at 6x107 S/m to low-conductivity, non-metallic samples at 500 S/m. Typically, sample volumes are of a few cm3 and with an imaging resolution around 1 mm. These numbers make EMI with AMs (EMI-AM) suitable for numerous applications. Techniques – including multi-frequency image analysis – are employed to discriminate sample properties. Further work developed novel image reconstruction approaches – based on machine learning – to maximise the amount of information that can be extracted from EMI images. Finally, the potential of biomedical imaging is discussed and its feasibility verified by simulating the application of EMI-AM to imaging the conductivity of the heart

Proceedings of the 11th international conference on NDE in relation to structural integrity for nuclear and pressurized components

This Conference, the eleventh in a series on NDE in relation to structural integrity for nuclear and pressurized components, was held in Jeju Island, Korea, from 19th to 21st of May 2015. The scientific programme was co-produced by the European Commission’s Joint Research Centre, Institute for Energy and Transport (EC-JRC/IET). Previous conferences were held in Amsterdam in October 1998, New Orleans in May 2000, Seville in November 2001, London in December 2004, San Diego in May 2006, Budapest in October 2007, Yokohama in May 2009, Berlin in September 2010, Seattle in May 2012, and Cannes in October 2013. All were highly successful in the quality and scope of the technical programs and the number of attendees from all countries with an interest in the structural integrity of nuclear and pressurized components. The overall objectives of the Conference were to provide an up-to-date assessment of the development and application of NDE and to allow technical interchange between experts on an international basis. The Conference covered all aspects of this extremely important subject, with special regard to the links between structural integrity requirements and NDE performance. The development of improved NDE systems and methods was highlighted. Determination of NDE performance by development of qualification systems or performance demonstration, and experience of their use in practice was prominently featured.JRC.F.5-Nuclear Reactor Safety Assessmen

Proceedings of the 11th international conference on NDE in relation to structural integrity for nuclear and pressurized components

This Conference, the eleventh in a series on NDE in relation to structural integrity for nuclear and pressurized components, was held in Jeju Island, Korea, from 19th to 21st of May 2015. The scientific programme was co-produced by the European Commission’s Joint Research Centre, Institute for Energy and Transport (EC-JRC/IET). Previous conferences were held in Amsterdam in October 1998, New Orleans in May 2000, Seville in November 2001, London in December 2004, San Diego in May 2006, Budapest in October 2007, Yokohama in May 2009, Berlin in September 2010, Seattle in May 2012, and Cannes in October 2013. All were highly successful in the quality and scope of the technical programs and the number of attendees from all countries with an interest in the structural integrity of nuclear and pressurized components. The overall objectives of the Conference were to provide an up-to-date assessment of the development and application of NDE and to allow technical interchange between experts on an international basis. The Conference covered all aspects of this extremely important subject, with special regard to the links between structural integrity requirements and NDE performance. The development of improved NDE systems and methods was highlighted. Determination of NDE performance by development of qualification systems or performance demonstration, and experience of their use in practice was prominently featured.JRC.F.5-Nuclear Reactor Safety Assessmen

Characterization of vertical cracks using lock-in vibrothermography

214 p.Esta tesis se centra en la aplicación de la vibrotermografía lock-in para la detección y caracterización dedefectos verticales sumergidos. En esta técnica, la pieza se excita mediante ultrasonidos, que generancalor en los defectos por fricción o deformación plástica. Este calor se difunde por el material y susefectos se pueden detectar midiendo la temperatura superficial mediante una cámara infrarroja. Con el finde caracterizar defectos es necesario resolver el problema inverso, que consiste en recuperar la geometríade las fuentes de calor a partir de la distribución de temperatura superficial medida. Éste es un problemamal puesto, ya que su solución es fuertemente dependiente de pequeños errores en los datos y la inversiónes inestable. Se ha implementado un algoritmo de inversión robusto, basado en minimización pormínimos cuadrados estabilizados mediante términos de penalización basados en los funcionales deTikhonov, Total Variation y L1, capaz de reconstruir distribuciones de fuentes de calor partiendo de datosde vibrotermografía. El algoritmo se ha analizado con datos sintéticos y se ha optimizado con el fin deextender su aplicación a la caracterización del mayor rango de geometrías de fuentes de calor posible.Los resultados obtenidos se han verificado con datos experimentales obtenidos en ensayos devibrotermografía lock-in, utilizando muestras con fuentes de calor verticales calibradas. Finalmente, se hahecho uso del algoritmo de inversión para caracterizar grietas reales en una muestra soldada de Inconel718 y los resultados están en buena correlación cualitativa con los resultados del ensayo de líquidospenetrantes realizado posteriormente

Inspection of composite aerospace structures using capacitive imaging and guided waves

This thesis describes a possible new approach for the future of the NDT of aerospace materials by using both ultrasonic guided wave and capacitive imaging (CI) techniques. The two techniques complement each other and are selected depending upon the area inspected and the resolution required. Guided waves are used for long range defect detection, while capacitive imaging is used for localised characterisation. The guided waves are generated by means of electromagnetic acoustic transducers (EMATs). These devices employ a coil, for inducing eddy currents, and a magnet (or an array of them), for generating a static magnetic field. The interaction of these two quantities produces ultrasonic guided waves based on the Lorentz force mechanism, but needs an electrically conductive surface to operate. In this thesis the conductive surface is provided by using thin, self- adhesive, removable metallic patches for both insulating and conducting samples. Conversely, the CI technique employs a pair of electrodes to establish a quasi-static electric field within the sample, and requires the sample to have a low (basically zero) electrical conductivity for allowing the field to probe it. Both techniques are non-contact and non-invasive nature. Guided waves have been studied using periodic permanent magnet (PPM) EMATs, which here have been designed to generate shear horizontal waves, and predominantly the SH0 mode is used in the thesis. In the aerospace field, the materials used are composites, whose electrical conductivity is often too low for efficient EMAT use, even when they contain carbon fibres. There is a notable exception, where a copper mesh for lightning strike protection is integrated into composite, as direct use of an EMAT on these samples is possible. For the cases where removable metallic patches are used, analytical models were designed to predict the forces and the generated wave within the sample. The predictions show good agreement with experimental measurements for the propagation of SH guided waves within different samples such a carbon fibre and glass fibre composites. Consequently, the methodology has been used for the detection of several types of defect, such as impact damage, delamination and lightning strikes. Furthermore, the production of images via a SAFT algorithm allows preliminary evaluations of the severity of the defects detected. The CI technique has been investigated for various designs of CI probes using 2D and 3D finite element (FE) models in COMSOL. It is shown how conductivity affects performance, and the results from simulations of different probe designs has been compared to experiments in insulating materials, with good agreement. These results indicate that CI is a suitable NDT technique for samples such as glass fibre composites. Conventionally, the images from CI measurements are based on the amplitude of the received signal rather than phase, due to the higher signal-to-noise ratios that can be obtained with the amplitude measurement. In this work, an improved image processing method has been introduced. The method combines amplitude and phase information to form clearer images, and thus improving the evaluation of both sizing and location of defects. The use of both techniques has been illustrated for the case of damage within pultruded glass fibre composites. It is shown that guided waves using EMATs and a removable copper patch can be used to detect defects at extended ranges, and that these can be characterised further at higher resolution using a localised inspection, the CI technique

Advances in Potential Drop Techniques for Non-Destructive Testing

In the field of Non-Destructive Testing, Potential Drop (PD) techniques have been used for decades, especially in the petrochemical and power generation industries, for monitoring crack growth and wall thickness variations due to corrosion and/or erosion in pipes, pressure vessels and other structures. Inspection is carried out by injecting currents in the specimen to be tested and measuring the arising electrical potential di erence between two or more electrodes placed on its surface. The presence of a defect generally increases the resistance and hence the measured voltage drop; inversion of these data can give information on the size and shape of the defect. However, while the principle underlying these techniques is relatively simple, some di culties have been encountered in their practical applications. Many commercial systems based on PD methods, for instance, require the injection of very large currents in order to obtain su ciently large signals; doubts have been raised on the stability of these methods to variations in the contact resistance between the electrodes and the inspected material. The present work aims to show that some of these problems can be easily overcome, and to evaluate the capabilities of PD techniques for crack sizing and corrosion mapping. After a brief review of the advantages, disadvantages and applications of the main electromagnetic methods for Non-Destructive Testing, an experimental setup for Potential Drop measurements which was developed for this work and which uses small alternating currents (AC) is described. The setup is benchmarked against existing PD systems and then used to validate a model that allows AC PD simulations to be run with a commercial Finite Element code. The results of both numerical simulations and experimental measurements are used to investigate the possibility of sizing defects of complex geometry by repeating the analysis at several di erent frequencies over a broad range, and of reconstructing the depth pro le of surfacebreaking defects without the need for assumptions on their shape. Subsequently, the accuracy to which it is possible to obtain maps of corrosion/erosion on the far surface of an inspected structure is discussed, and results obtained with an array probe that employs a novel arrangement of electrodes are presented. Finally, conclusions are drawn and suggestions for further research are made