Pulsed Eddy Current Imaging of Inclined Surface Cracks

Inclined fatigue cracks can potentially cause severe damage to metallic structures as they affect larger region in the tested structure compared to crack perpendicular to the sample surface. The abilitiy to detect and characterize such cracks is paramount in non-destructive testing (NDT). Pulsed eddy current testing (PEC) is known to offer a broadband of excitation frequencies, which in conjuction with C-scan imaging, may offer discrimination of inclination angles of cracks. Finite element modelling (FEM) was carried out to study the effects of different crack inclination angles, while experimental results were used to verify the FEM results. Selection of both time and frequency domain features for C-scan image construction was also presented, where C-scan images of peak value and amplitude at 200 Hz were shown to be potentially capable in determining different inclination angles. Nevertheless, between these two signal teatures, the amplitude at 200 Hz was found to be more effective in the discriminataion of inclined cracks

Characterisation of surface and sub-surface discontinuities in metals using pulsed eddy current sensors

Due primarily to today's rigorous safety standards the focus of non-destructive testing (NDT) has shifted from flaw detection to quantitative NIDT, where characterisation of flaws is the objective. This means information such as the type of flaw and its size is desired. The Pulsed Eddy Current (PEC) technique has been acknowledged as one of the potential contenders for providing this additional functionality, due to the potential richness of the information that it provides. The parameters mainly used to obtain information about the detected flaws are the signal's peak height and arrival time. However, it has been recognised that these features are not sufficient for defect classification. In this research, based on a comprehensive literature survey, the design of PEC systems and the interpretation of PEC signals, mainly for flaw classification, are studied. A PEC system consisting of both hardware and software components has been designed and constructed to facilitate the research work on PEC signal interpretation. After a comparative study of several magnetic sensing devices, probes using Hall device magnetic sensors have also been constructed. Some aspects related to probe design, such as coil dimensions and the use of ferrite core and shielding have also been studied. A new interpretation technique that uses the whole part of PEC responses and is able to produce more features has been proposed. The technique uses Principal Component Analysis (PCA) and Wavelet Transforms, and attempts to find the best features for discrimination from extracted time and frequency domain data. The simultaneous use of both temporal and spectral data is a logically promising extension to the use of time domain only with the signal-peak-based technique. Experiments show that the new 1 technique is promising as it performs significantly better than the conventional technique using peak value and peak time of PEC signals in the classification of flaws. A hierarchical structure for defect classification and quantification has been presented. Experiments in the project have also shown that the signal-peak-based technique cannot be used for flaw detection and characterisation in steels, both with and without magnetisation. The new proposed technique has shown to have potential for this purpose when magnetisation is used. The new technique proposed in the report has been successfully used for ferromagnetic and non-ferromagnetic materials. It has also been demonstrated that the new proposed technique performs better in dynamic behaviour tests, which shows its better potential for on-line dynamic NDT inspection which is required in many industrial applications. In addition to testing calibrated samples with different discontinuities, a study case using an aircraft lap joint sample from industry has further supported the statement regarding the potential of the new technique.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Pulsed magnetic flux leakage techniques for crack detection and characterisation

Magnetic flux leakage (MFL) techniques have been widely used for non-intrusively inspecting steel installations by applying magnetization. In the situations where defects may take place on the near and far surfaces of the structure under inspection, current {MFL} techniques are unable to determine their approximate size. Consequently, an extra transducer may have to be included to provide the extra information required. This paper presents a new approach termed as pulsed magnetic flux leakage (PMFL) for crack detection and characterisation. The probe design and method are introduced. The signal features in time�frequency domains are investigated through theoretical simulations and experiments. The results show that the technique can potentially provide additional information about the defects. Lastly, potential applications are suggested

Effect of temperature on ultrasonic signal propagation for extra virgin olive oil adulteration

Fraud cases involving adulteration of extra virgin olive oil has become significant nowadays due to increasing in cost of supply and highlight given the benefit of extra virgin olive oil for human consumption. This paper presents the effects of temperature variation on spectral formed utilising pulse-echo technique of ultrasound signal. Several methods had been introduced to characterize the adulteration of extra virgin olive oil with other fluid sample such as mass chromatography, standard method by ASTM (density test, distillation test and evaporation test) and mass spectrometer. Pulse-echo method of ultrasound being a non-destructive method to be used to analyse the sound wave signal captured by oscilloscope. In this paper, a non-destructive technique utilizing ultrasound to characterize extra virgin olive oil adulteration level will be presented. It can be observed that frequency spectrum of sample with different ratio and variation temperature shows significant percentages different from 30% up to 70% according to temperature variation thus possible to be used for sample characterization

Pulsed eddy current imaging of inclined surface cracks

Inclined fatigue cracks can potentially cause severe damage to metallic structures as they affect larger region in the tested structure compared to crack perpendicular to the sample surface. The abilitiy to detect and characterize such cracks is paramount in non-destructive testing (NDT). Pulsed eddy current testing (PEC) is known to offer a broadband of excitation frequencies, which in conjuction with C-scan imaging, may offer discrimination of inclination angles of cracks. Finite element modelling (FEM) was carried out to study the effects of different crack inclination angles, while experimental results were used to verify the FEM results. Selection of both time and frequency domain features for C-scan image construction was also presented, where C-scan images of peak value and amplitude at 200 Hz were shown to be potentially capable in determining different inclination angles. Nevertheless, between these two signal teatures, the amplitude at 200 Hz was found to be more effective in the discriminataion of inclined cracks

Investigation of the effects of various defects in metallic specimens on the spatial distribution of pulsed eddy currents

attention from researchers around the globe. Thanks to its richness of spectral components, various applications of this technique have been proposed and reported in the literature covering both structural integrity inspection and material characterization in various industrial sectors. In this project it was hypotesized that the spatial and temporal magnetic field signals can offer more information of the defects compared to single point data, which have been proven in this project, through numerical modelling and analysis of experimental data gathered by using a PEC system developed in the project. In this project, it has been shown that there is some unique correlation between the spatial distribution of the eddy current and the characteristics of defetcs, including their sizes. In this project, particulary this was studied by using normal and inclined cracks that are located on both surface and sub-surface. This finding can be utilized to enrich even further the information that we can extract from PEC signals

Feature extraction and selection for defect classification of pulsed eddy current NDT

Pulsed eddy current (PEC) is a new emerging nondestructive testing (NDT) technique using a broadband pulse excitation with rich frequency information and has wide application potentials. This technique mainly uses feature points and response signal shapes for defect detection and characterization, including peak point, frequency analysis, and statistical methods such as principal component analysis (PCA). This paper introduces the application of Hilbert transform to extract a new descending feature point and use the point as a cutoff point of sampling data for detection and feature estimation. The response signal is then divided by the conventional rising, peak, and the new descending points. Some shape features of the rising part and descending part are extracted. The characters of shape features are also discussed and compared. Various feature selection and integrations are proposed for defect classification. Experimental studies, including blind tests, show the validation of the new features and combination of selected features in defect classification. The robustness of the features and further work are also discussed

Dual EMAT and PEC non-contact probe: applications to defect testing

For many non-destructive testing (NDT) applications, more information and greater reliability can be gained by using different techniques for defect detection, especially when the methods are particularly sensitive to different types of defects. However, this will often lead to a much longer and more expensive test and is not always practical due to time and cost constraints. We have previously discussed initial experiments using a new dual-probe combining electromagnetic acoustic transducers (EMATs) generating and detecting ultrasonic surface waves, and a pulsed eddy current (PEC) sensor 1. This enables more reliable detection and sizing of surface and near-surface defects, with a reduced testing time compared to using two \{NDT\} techniques separately. In this paper, we present experiments using the dual-probe on samples which are more representative of real defects, for example testing for surface defects in rails. Several aluminium calibration samples containing closely spaced and angled slots have been measured, in addition to rail samples containing manufactured and real defects. The benefits of using the dual-probe are discussed

Modelling of scanning pulsed eddy current testing of normal and slanted surface cracks

Thanks to its wide bandwidth, pulsed eddy current (PEC) has attracted researchers of various backgrounds in the attempt to exploit its benefits in Non-destructive Testing (NDT). The ability of modelling PEC problems would be a precious tool in this attempt as it would help improve the understanding of the interaction between the transient magnetic field and the specimen, among others. In this work, a Finite Element Modelling (FEM) has been developed and experimental test data have been gathered for its validation. The investigated cases were simulated surface cracks of different sizes and angles. The study involved looking at time-domain PEC signals at different spatial distances from the cracks’ faces, which would particularly be useful for modelling scanning PEC probes. The obtained results show a good agreement between the FEM and experiment, demonstrating that the modelling technique can be used with confidence for solving similar problems. In addition, the extracted features from signals were also studied to discover the influence of crack geometries to the PEC responses