Passive low frequency RFID for non-destructive evaluation and monitoring

Ph. D ThesisDespite of immense research over the years, defect monitoring in harsh environmental conditions still presents notable challenges for Non-Destructive Testing and Evaluation (NDT&E) and Structural Health Monitoring (SHM). One of the substantial challenges is the inaccessibility to the metal surface due to the large stand-off distance caused by the insulation layer. The hidden nature of corrosion and defect under thick insulation in harsh environmental conditions may result in it being not noticed and ultimately leading to failures. Generally electromagnetic NDT&E techniques which are used in pipeline industries require the removal of the insulation layer or high powered expensive equipment. Along with these, other limitations in the existing techniques create opportunities for novel systems to solve the challenges caused by Corrosion under Insulation (CUI). Extending from Pulsed Eddy Current (PEC), this research proposes the development and use of passive Low Frequency (LF) RFID hardware system for the detection and monitoring of corrosion and cracks on both ferrous and non-ferrous materials at varying high temperature conditions. The passive, low cost essence of RFID makes it an enchanting technique for long term condition monitoring. The contribution of the research work can be summarised as follows: (1) implementation of novel LF RFID sensor systems and the rig platform, experimental studies validating the detection capabilities of corrosion progression samples using transient feature analysis with respect to permeability and electrical conductivity changes along with enhanced sensitivity demonstration using ferrite sheet attached to the tag; (2) defect detection using swept frequency method to study the multiple frequency behaviour and further temperature suppression using feature fusion technique; (3) inhomogeneity study on ferrous materials at varying temperature and demonstration of the potential of the RFID system; (4) use of RFID tag with ceramic filled Poly-tetra-fluoro-ethyulene (PTFE) substrate for larger applicability of the sensing system in the industry; (5) lift-off independent defect monitoring using passive sweep frequency RFID sensors and feature extraction and fusion for robustness improvement. This research concludes that passive LF RFID system can be used to detect corrosion and crack on both ferrous and non-ferrous materials and then the system can be used to compensate for temperature variation making it useful for a wider range of applications. However, significant challenges such as permanent deployment of the tags for long term monitoring at higher temperatures and much higher standoff distance, still require improvement for real-world applicability.Engineering and Physical Sciences Research Council (EPSRC) CASE, National Nuclear Laboratory (NNL)

Characteristics of Eddy Current Attenuation and Thickness Measurement of Metallic Plate

Abstract(#br)In eddy current testing, the law of attenuation of eddy current (EC) is of great concern. In conductive half space under the excitation of uniform magnetic field, the EC density decreases exponentially in the depth direction. However, in conductor with finite thickness tested by coil, the distribution of EC in the depth direction is more complicated. This paper studies the characteristics of EC attenuation in metallic plate of finite thickness. Simulation results show that there is an EC reflection at the bottom of plate, which changes the law of EC attenuation. A new concept, namely the equivalent attenuation coefficient, is proposed to quantify the speed of EC attenuation. The characteristics of EC attenuation are utilized to explain the nonmonotonic relation between coil..

High-sensitivity eddy-current testing technology for defect detection in aerospace superalloys

Industrial eddy-current testing (ECT) inspections of aerospace superalloys, such as Titanium 6Al-4V, must reliably detect sub-millimetre surface breaking defects. The sensitivity of such measurements is hindered by the materials' low conductivity and high coherent background material noise, making the high sensitivity standards required by industry harder to achieve. It is demonstrated herein that using eddy-current array (ECA) technology also introduces inspection "blind-spots", whereby small defects could be missed. This supports the motivation to develop techniques for enhancing the sensitivity of typical ECT and ECA measurements. The early stage research and development of a novel ECT measurement method is presented, and shown to improve the standard measurement sensitivity of industrial ECT inspections. A defect signal enhancement phenomenon within a band of frequencies close to the electrical resonance of an ECT probe, termed near electrical resonance signal enhancement (NERSE), was observed and characterised. This phenomenon was demonstrated to be a direct result of the shifting resonant frequency of the probe in the presence of material discontinuities. Frequency sweeping chirp signals were used to generate electrical resonance traversing frequency spectra measurements of the inspection probe in the presence of material discontinuities. Critical feature analysis of the results demonstrated a correlation between defect dimensions and peak NERSE amplitude, but failed to draw any conclusive trends between discontinuity dimensions and the resonant frequency shift. This was attributed to the relatively small defect sample set used and the size of many of the machined defects being smaller than the diameter of the inspection coil. An ECT probe was excited at a single frequency carefully selected to correspond to the NERSE peak frequency. A study was performed to statistically analysis the sensitivity of this NERSE measurement compared to a standard excitation frequency measurement used in industry. The results demonstrated that a NERSE frequency inspection was able to reliably detect a defect size of 0.82mm, compared to 1.09mm achieved by a standard operating frequency

Diffusion and perfusion MRI and applications in cerebral ischaemia

Two MRI techniques, namely diffusion and perfusion imaging, are becoming increasingly used for evaluation of the pathophysiology of stroke. This work describes the use of these techniques, together with more conventional MRI modalities (such as T1, and T2 imaging) in the investigation of cerebral ischaemia. The work was performed both in a paediatric population in a whole-body clinical MR system (1.5 T) and in an animal model of focal ischaemia at high magnetic field strength (8.5 T). For the paediatric studies, a single shot echo planar imaging (EPI) sequence was developed to enable the on-line calculation of maps of the trace of the diffusion tensor. In the process of this development, it was necessary to address two different imaging artefacts in these maps: eddy current induced image shifts, and residual Nyquist ghost artefacts. Perfusion imaging was implemented using an EPI sequence to follow the passage through the brain of a bolus of a paramagnetic contrast agent. Computer simulations were performed to evaluate the limitations of this technique in the quantification of cerebral blood flow when delay in the arrival and dispersion of the bolus of contrast agent are not accounted for. These MRI techniques were applied to paediatric patients to identify acute ischaemic events, as well as to differentiate between multiple acute events, or between acute and chronic events. Furthermore, the diffusion and perfusion findings were shown to contribute significantly to the management of patients with high risk of stroke, and in the evaluation of treatment outcome. In the animal experiments, permanent middle cerebral artery occlusion was performed in rats to investigate longitudinally the acute MRI changes (first 4-6 hours) following an ischaemic event. This longitudinal analysis contributed to the understanding of the evolution of the ischaemic lesion. Furthermore, the findings allowed the acute identification of tissue 'at risk' of infarction

Aeronautical Engineering. A continuing bibliography with indexes, supplement 136, June 1981

This bibliography lists 424 reports, articles, and other documents introduced into the NASA scientific and technical information system in May 1981