Phased electromagnetic acoustic transducer array for Rayleigh wave surface defect detection

A phased electromagnetic acoustic transducer (EMAT) array system has been developed for detection and characterisation of surface breaking defects. An array of four linear coils which are individually controlled are used to generate a Rayleigh wave. The high current electronics combined with the coil designs enables the array to generate either narrowband or broadband signals, and controlling the phase delay between the channels makes it possible to change the ultrasound wavelength without requiring the physical separation of the coils to be changed. Experimental results show that the four-coil phased array is able to generate a wavelength range from 3.0 mm to 11.7 mm. Surface breaking defects were characterised using a transmit-receive set-up with a broadband EMAT detector being used to detect the Rayleigh wave. Machined surface slots with different depths were used for technique validation. The results show that the array is sensitive to surface defects and that a wide depth sensitivity range for defect sizing can be easily achieved by applying phasing to tune the wavelength of operation. A large increase in detection flexibility is immediately shown

Surface defect characterisation using non-contact ultrasound

Electromagnetic acoustic transducers (EMATs) have been used as a non-contact ultrasound approach for detecting and characterising surface defects in aluminium bars and billet. The characterisation was made from understanding the interaction of broadband Rayleigh surface waves with surface crack growing normal or inclined to the sample surface, based on rolling contact fatigue (RCF) cracks in rail tracks. The interaction with normal cracks have been previously reported. For inclined cracks, mode conversion of Rayleigh waves to Lamb-like waves occur in the wedge section formed by the crack, resulting in strong and prominent enhancement in the signal detected. This is confirmed by finite element analysis (FEA) models and Lamb waves arrival times calculation. Signal enhancement from the interaction creates features in B-scan images, and they have been used for initial crack classifications. Then, a number of analyses were performed to estimate the crack inclination, and accurately determine the crack vertical depth. A feature extraction and image classification program based on genetic programming have been developed (through a collaboration work) to perform automated classification on the B-scans. The program produces more than 90% accuracy using the experimental data set. The viability of EMATs to detect and fully characterise narrow cracks have been investigated through experiments using laser interferometer and comparison with EMATs measurements. The results confirmed that narrow cracks can be detected with EMATs, with initial classification (in B-scans) to normal/inclined. However, the depth sizing may not be accurate, and suggestion for better designs of EMATs have been made. FEA models have been used to study the interaction of the Rayleigh waves with branched cracks. Interesting results are observed in terms of Rayleigh waves reflections, which helps to determine the presence of a branch on RCF-like cracks. A method has been proposed for calculating the length of the branch, following a number of analyses

Miniaturised SH EMATs for fast robotic screening of wall thinning in steel plates

Electromagnetic acoustic transducers (EMATs) are well suited to generating and detecting a variety of different ultrasonic wavemodes, without the need for couplant, and they can be operated through some coatings. EMATs can be used to generate shear horizontal (SH) waves, which show promise for fast screening of wall thinning and other defects. However, commercial SH-wave EMATs are not suitable for robotic implementation on ferritic steel due to the large magnetic drag force from the magnets. This article describes the design and characterisation of miniaturised SH guided wave EMATs, which significantly reduce the magnetic drag and enable mounting onto a small crawler robot for sample scanning. The performance of the miniaturised EMATs is characterised and compared to a commercial EMAT. It is shown that signal to noise ratio is reduced, but remains within an acceptable range to use on steel. The bandwidth and directivity are increased, depending on the exact design used. Their ability to detect flat bottomed holes mimicking wall thinning is also tested

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

Time of flight diffraction and imaging (TOFDI)

Time of flight diffraction and imaging (TOFDI) is based on time of flight diffraction (TOFD), adding cross-sectional imaging of the sample bulk by exploiting the scattering of ultrasonic waves from bulk defects in metals. Multiple wave modes are emitted by a pulsed laser ultrasound ablative source, and received by a sparse array of receiving electromagnetic acoustic transducers (EMATs), for non-contact (linear) scanning, with mode-conversions whenever waves are scattered. Standard signal processing techniques, such as band-pass filters, reduce noise. A B-scan is formed from multiple data captures (A-scans), with time and scan position axes, and colour representing amplitude or magnitude. B-scans may contain horizontal lines from surface waves propagating directly from emitter to receiver, or via a back-wall, and angled lines after reflection off a surface edge. A Hough transform (HT), modified to deal with the constraints of a B-scan, can remove such lines. A parabola matched filter has been developed that identifies the features in the B-scan caused by scattering from point-like defects, reducing them to peaks and minimising noise. Multiple B-scans are combined to reduce noise further. The B-scan is also processed to form a cross-sectional image, enabling detection and positioning of multiple defects. The standard phase correlation technique applied to camera images, has been used to track the relative position between transducer and sample. Movement has been determined to sub-pixel precision, with a median accuracy of 0.01mm of linear movement (0.06 of a pixel), despite uneven illumination and the use of a basic low resolution camera. The prototype application is testing rough steel products formed by continuous casting, but the techniques created to facilitate operation of TOFDI are applicable elsewhere

Miniaturised SH EMATs for fast robotic screening of wall thinning in steel plates

Electromagnetic acoustic transducers (EMATs) are well suited to generating and detecting a variety of different ultrasonic wavemodes, without the need for couplant, and they can be operated through some coatings. EMATs can be used to generate shear horizontal (SH) waves, which show promise for fast screening of wall thinning and other defects. However, commercial SH-wave EMATs are not suitable for robotic implementation on ferritic steel due to the large magnetic drag force from the magnets. This article describes the design and characterisation of miniaturised SH guided wave EMATs, which significantly reduce the magnetic drag and enable mounting onto a small crawler robot for sample scanning. The performance of the miniaturised EMATs is characterised and compared to a commercial EMAT. It is shown that signal to noise ratio is reduced, but remains within an acceptable range to use on steel. The bandwidth and directivity are increased, depending on the exact design used. Their ability to detect flat bottomed holes mimicking wall thinning is also tested

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

Applications on Ultrasonic Wave

This book presents applications on the ultrasonic wave for material characterization and nondestructive evaluations. It could be of interest to the researchers and students who are studying on the fields of ultrasonic waves