Defect detection and identification of point-focusing shear-horizontal EMAT for plate inspection

As a kind of nondestructive testing (NDT) method, shear-horizontal (SH)-guided wave detection technology is widely used on an electromagnetic acoustic transducer (EMAT). Although ultrasonic-guided waves perform well in defect location, it is difficult to obtain detailed information about defects, and the low efficiency of EMAT energy conversion still reduces the EMAT’s performance. Therefore, in this work, the defect detection method of different shapes and sizes by point-focusing shear-horizontal (PFSH)-guided wave EMAT with the use of periodic permanent magnet (PPM) is investigated through simulation and experiment. For the purpose of defect classification and quantification, the extraction principles of defect features are obtained through simulation based on the circumferential scatter diagrams, and the neural network (NN) is used to process the features extracted from the experimental data. The results show that by extracting effective defect features from the scatter diagram, high-accuracy classification and high-precision quantification of defects under the influence of the focusing transducer can be achieved

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

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

Acoustic and Elastic Waves: Recent Trends in Science and Engineering

The present Special Issue intends to explore new directions in the field of acoustics and ultrasonics. The interest includes, but is not limited to, the use of acoustic technology for condition monitoring of materials and structures. Topics of interest (among others): • Acoustic emission in materials and structures (without material limitation) • Innovative cases of ultrasonic inspection • Wave dispersion and waveguides • Monitoring of innovative materials • Seismic waves • Vibrations, damping and noise control • Combination of mechanical wave techniques with other types for structural health monitoring purposes. Experimental and numerical studies are welcome

Development of a guided wave EMAT online inspection system for Al/Al-Sn/Al/steel and CuSn/steel bimetal strip bond quality control used in the automotive industry

Cold roll bonded (CRB) Al/Al-Sn/Al/steel and sintered CuSnNi/steel bimetal strips are used in the automotive industry for the manufacture of engine bearings, bushes and thrust washers. Any defects such as delamination or porosity that occur in bimetal strips during manufacturing can cause problems at downstream production steps and if they remain undetected, could result in components failing in the field, which is a significant business risk.;One way to reduce this business risk is to install a final inspection system on a continuous production line as the strip passes a fixed inspection point. In process control this could alert the operators to reject defective material and correct process parameters when the defect occurs. As this system requires 100% volumetric inspection, installing it has its challenges due to the harsh manufacturing environment in which the strip moves at up to 20 m/min in the processing lines at room temperature.;A literature review and feasibility study on different non-destructive testing (NDT) techniques to inspect bond quality of CRBed Al/Al-Sn/Al/steel bimetal strips was conducted to assess technologies that could be developed for serial inspection. Guided waves generated using Electromagnetic Acoustic Transducers (EMATs) was identified as best suited for this application. Since this technology was not available off-the-shelf, significant research and experimental work was carried out to develop an automated prototype system.;The system was successfully installed at a strip processing line and demonstrated the online bond inspection capability for Al/Al-Sn/Al/steel and CuSnNi/steel bimetal strips, which is the main achievement of this EngD project. For CuSnNi/steel strips, causes of defects and preventative control measures were studied and examined. Industrialisation of the inspection system will significantly reduce the company business risk and improve bond quality of bimetal strips.Cold roll bonded (CRB) Al/Al-Sn/Al/steel and sintered CuSnNi/steel bimetal strips are used in the automotive industry for the manufacture of engine bearings, bushes and thrust washers. Any defects such as delamination or porosity that occur in bimetal strips during manufacturing can cause problems at downstream production steps and if they remain undetected, could result in components failing in the field, which is a significant business risk.;One way to reduce this business risk is to install a final inspection system on a continuous production line as the strip passes a fixed inspection point. In process control this could alert the operators to reject defective material and correct process parameters when the defect occurs. As this system requires 100% volumetric inspection, installing it has its challenges due to the harsh manufacturing environment in which the strip moves at up to 20 m/min in the processing lines at room temperature.;A literature review and feasibility study on different non-destructive testing (NDT) techniques to inspect bond quality of CRBed Al/Al-Sn/Al/steel bimetal strips was conducted to assess technologies that could be developed for serial inspection. Guided waves generated using Electromagnetic Acoustic Transducers (EMATs) was identified as best suited for this application. Since this technology was not available off-the-shelf, significant research and experimental work was carried out to develop an automated prototype system.;The system was successfully installed at a strip processing line and demonstrated the online bond inspection capability for Al/Al-Sn/Al/steel and CuSnNi/steel bimetal strips, which is the main achievement of this EngD project. For CuSnNi/steel strips, causes of defects and preventative control measures were studied and examined. Industrialisation of the inspection system will significantly reduce the company business risk and improve bond quality of bimetal strips

Novel Approaches for Structural Health Monitoring

The thirty-plus years of progress in the field of structural health monitoring (SHM) have left a paramount impact on our everyday lives. Be it for the monitoring of fixed- and rotary-wing aircrafts, for the preservation of the cultural and architectural heritage, or for the predictive maintenance of long-span bridges or wind farms, SHM has shaped the framework of many engineering fields. Given the current state of quantitative and principled methodologies, it is nowadays possible to rapidly and consistently evaluate the structural safety of industrial machines, modern concrete buildings, historical masonry complexes, etc., to test their capability and to serve their intended purpose. However, old unsolved problematics as well as new challenges exist. Furthermore, unprecedented conditions, such as stricter safety requirements and ageing civil infrastructure, pose new challenges for confrontation. Therefore, this Special Issue gathers the main contributions of academics and practitioners in civil, aerospace, and mechanical engineering to provide a common ground for structural health monitoring in dealing with old and new aspects of this ever-growing research field