Showing papers by "Gui Yun Tian published in 2010"

Quantitative evaluation of angular defects by pulsed eddy current thermography

Abstract: Pulsed eddy current (PEC) thermography employs a combination of PEC and thermographic non-destructive testing (NDT) techniques. This study considers the capabilities of PEC thermography for obtaining quantitative information about cracks set at an angle to the surface. The investigation is implemented by simulating the transient thermal distribution for angular slots, via time-stepping 3D finite element analysis (FEA), with the experimental work undertaken for verification. A slope inclination feature of the transient temperature distribution has been extracted and presented for estimating the angle of slots that is independent of slot depth and length inside the sample. With the identification of the slot angle, quantification of the length/depth of the slot inside the sample can then be made through a maximum temperature amplitude feature. Experimental studies have been undertaken for evaluation of the numerical simulation and transient feature extraction methods.

Modelling and evaluation of eddy current stimulated thermography

Abstract: Thermographic inspection with eddy current (EC) excitation is an emerging integrative NDT&E method with the ability to inspect for defects over large areas. The resultant surface heat distribution from direct EC heating and diffused heat can be obtained easily with a thermal camera, but techniques for the determination of heating mechanisms around a particular defect for quantitative defect characterisation are required. In this paper, numerical modelling and experimental studies are applied to understand EC stimulated thermography on simple discontinuity defects, including transient EC distribution and heating propagation for slots and notches. This fundamental understanding of transient EC distribution and heating propagation will aid in the development of feature extraction and pattern recognition techniques for the quantitative analysis of EC thermography images and defect characterisation.

Pulsed eddy current thermography: System development and evaluation

Abstract: There is a need for fast and efficient techniques to inspect engineering structures and complex components such as aircraft turbine blades to identify potential sites of failure. Pulsed eddy current (PEC) thermography is a new inspection technique which allows the user to capture the eddy current distribution in a component or structure using infrared imaging and detect defects over a relatively wide area. The technique is applicable to materials with a reasonable level of electrical conductivity and has the ability to detect defects under coatings. However, PEC thermography has received relatively little attention compared to other thermographic inspection techniques. In this paper, the design, development and optimisation of a PEC thermography inspection system is detailed, including coil design for global and local heating of samples, optimisation of excitation parameters (frequency, power, pulse duration etc) and camera selection. The system is used to inspect several real-world samples, using different coil designs, and the results are assessed using newly developed feature extraction techniques. The work shows that with judicious coil design and selection of excitation parameters, PEC thermography can be used to obtain quantitative information for defect characterisation through analysis of the surface heating pattern and the transient temperature change.

The pulsed eddy current response to applied loading of various aluminium alloys

Abstract: In this paper the pulsed eddy current (PEC) response of a range of heat treatable and non-heat treatable aluminium alloys (AA-1050, 2024, 5083 and 7075) to applied stress well below the elastic limit is studied. The effect of prior heat treatment and prior plastic deformation on the stress dependency of the pulsed eddy current response is quantified using the peak value of the PEC difference signal. In certain cases non-linear stress responses are measured. Because it is well known that the effect of stress on conductivity is anisotropic both isotropic (circular) probes and directional (rectangular) probes are compared. It is found that both prior plastic work and heat treatment condition can have a strong effect on the stress coefficients, even changing the sign of the stress dependence. The implications of these effects for the future characterisation of residual stress state by PEC are discussed.

Noncontact evaluation of the dependency of electrical conductivity on stress for various Al alloys as a function of plastic deformation and annealing

Abstract: This paper presents a method of noncontact evaluation of the anisotropic dependency of electrical conductivity of metallic specimens exposed to tensile stress. The method combines eddy current (EC) measurements using a directional probe with numerical modeling to infer the electrical conductivity longitudinal and transverse to an essentially static uniaxial applied tensile stress. The inversion method has been applied to various aluminum alloys (AA1050, AA2024, AA5083, and AA7075). The recovered changes in electrical conductivity lie within the theoretically expected range of a few percent at the yield stress. The effect of prior plastic strain and heat treatment on electrical conductivity has also been evaluated. The electrical conductivity tensor for hardenable alloys exhibit a distinct dependence on cold work compared to the nonhardenable ones. The outcomes are discussed in terms of their implications for the use of EC measurements for diagnosing the state of residual stress for aluminum alloys.

Stress measurement using magnetic Barkhausen noise and metal magnetic memory testing

Abstract: This paper introduces a comparison of the magnetic Barkhausen noise (BN) and metal magnetic memory (MMM) testing techniques for stress measurement. BN has become an important non-destructive technique due to its exceptional material and stress characterization capabilities. MMM is a recently developed technique with special ability for stress detection and stress history. In the applied tensile experiment, BN and MMM signals were acquired via a BN measurement system and EMS-2003 MMM instrument. Relationships between magnetic signals and applied tensile stresses were derived from experiment results. The difference and correlation of the two methods are investigated. Conclusions were derived based on the experiment results.

Optimal features combination for pulsed eddy current NDT

Abstract: This paper investigates current feature extraction techniques for pulsed eddy current NDT including peak value and peak time, spectral characteristics analysis, principal component analysis (PCA) projection coefficient and response shape curvature. Experimental study has been undertaken on samples with artificial surface and subsurface defects and metal thickness changes. Based on a comparative study of the feature extraction techniques, the most suitable features and their combination are discussed. The present study will provide a framework for future applications comprising the characterisation of real defects and stress profiles in engineering components.

Feature extraction and integration for the quantification of PMFL data

Abstract: If the vast networks of aging iron and steel, oil, gas and water pipelines are to be kept in operation, efficient and accurate pipeline inspection techniques are needed. Magnetic flux leakage (MFL) systems are widely used for ferromagnetic pipeline inspection and although MFL offers reasonable defect detection capabilities, characterisation of defects can be problematic and time consuming. The newly developed pulsed magnetic flux leakage (PMFL) system offers an inspection technique which equals the defect detection capabilities of traditional MFL, but also provides an opportunity to automatically extract defect characterisation information through analysis of the transient sections of the measured signals. In this paper internal and external defects in rolled steel water pipes are examined using PMFL, and feature extraction and integration techniques are explored to both provide defect depth information and to discriminate between internal and external defects. Feature combinations are recommended for def...

Feature extraction using normalized cross-correlation for pulsed eddy current thermographic images

Abstract: Pulsed eddy current (PEC) thermography is an emerging method in the field of non-destructive testing and evaluation (NDT&E). Defects can be characterized by tracking the diffusion of heat in a sample through the analysis of a sequence of PEC thermographic images. This study takes advantage of the capabilities of PEC thermography to gain quantitative information about complex geometrical defects, i.e. angular defect characterization through the analysis of the surface thermal distribution. To conduct the analysis, a new approach using a normalized cross-correlation technique is proposed. The strength of the proposed approach lies in its ability to track the heat diffusion through sequential PEC thermographic images in a metallic sample. The results of the analysis are used to determine the dimensions of defects in the sample under test. These results have shown the effectiveness of the proposed technique in providing features which have good agreement with defect detection and evaluation.

Simulation based on optimisation of pulsed eddy current probe design

Abstract: There is a need for a pulsed eddy current (PEC) to detect very small defects and deep buried subsurface flaws reliably in metallic structures such as aircraft wings. This paper reports an investigation into the sensitivity for a pancake PEC probe to variations in the material conductivity of specimens. Three experimental coil parameters are modelled: (a) coil inner radius, (b) coil width and (c) coil height. Based on the predicted signals, the sensitivity of the PEC probe as a function of coil parameters has been formulated. Through matrix numerical simulation experiments using orthogonal array, the weighting of each parameter to the sensitivity has been derived. Subsequently, a theoretical model for probe optimisation is established in order to realise the maximum sensitivity, based on which a probe is designed with improved sensitivity.

Investigation of directional eddy current complex measurements for defect mapping

Abstract: This paper reports an experimental study investigating the interaction of the real and imaginary components of eddy currents with defects and the resulting change in magnetic field under rectangular (directional) coil and circular (nondirectional) coil excitation, for the purposes of evaluating surface defects in conductive materials. Eddy currents are induced when an excitation coil is placed near the surface of a conductive material. Discontinuities such as cracks disturb the flow of the eddy currents and thus affect the induced current and magnetic field. These changes can be detected using a magnetic field sensor (Hall sensor). Measurements using an eddy current probe moved over a defect with an X-Y scanner were obtained experimentally. The results illustrate that the imaginary component of the magnetic field responses under directional magnetic field excitation using a rectangular-shaped excitation coil can provide further defect sizing information alongside the real component of the magnetic field, in comparison with a circular-shaped excitation coil.

The research on the flaw classification and identification of the PEC NDT technology

Abstract: Pulsed Eddy Current (PEC) NDT has played a vital role in detection and classification of the surface and sub-surface defects in conductive structures. Normally, it uses peak values of the acquired transient field signals, and the combina tion of the feature values of the time of the peak to identify flaws wit h the help of Principal Component Analysis (PCA). However, it is found that the random noise undermines the classification results, because PCA works robustly only in the time domain. In the light of this drawback, the fundamental and the first-har monic components are investigated and taken as the new feature values in the frequency domain. Through the analysis of the feature values in both time and frequency domains, the influen ce of random noise is mitigated. Consequently, surface defects, subsurface defects and metal thickness changes are classified with much higher identification accuracy.

Simulation and Visualisation for Electromagnetic Nondestructive Evaluation

Abstract: This paper reviews the state-of-the art of modelling, simulation and visualisation and reviews the recent development of modelling, simulation and visualisation software for Nondestructive Evaluation (NDE) Simulation and visualisation can assist in the design and development of electromagnetic sensing and imaging techniques and systems for nondestructive testing, feature extraction and inverse problems for quantitative nondestructive evaluation After reviewing the state-of-the art of electromagnetic modelling and simulation, case studies from electromagnetic NDE research and development for eddy current distribution and thermography are discussed

Alternating winding magnetostrictive electromagnetic acoustic transducer for pipe torsional guided wave generation

Abstract: Ultrasonic guided waves propagating in bounded elastic media called waveguide are used extensively nowadays for rapid screening and long distance inspection of defect-prone structures. Various types of transducers are available for guided wave generation, including piezoelectric transducers and electromagnetic acoustic transducers (EMATs) etc.. EMAT is based on Lorentz force and/or magnetostrictive effect, and advantageous sometimes because of its non-contact nature. An alternating winding magnetostrictive type EMAT for torsional mode ultrasonic guided wave generation in a steel pipe is introduced in this paper. This new transducer design is composed of a pre-magnetized Nickel strip with circumferential static magnetic field and a coaxial coil in an alternating winding pattern. Theoretically this transducer design could be viewed as an extension of existing magnetostrictive type EMAT for SH guided wave generation in a steel plate. The alternating winding pattern enables accurate and easy control of guided wave mode. Results of experiments done on a steel pipe were shown to verify this transducer design.

The key technologies research on the large field-of-view and high resolution optical synthesis telescope

Abstract: It briefly introduces the international development status of the high resolution for air-to-ground remote sensing satellite. High resolution for the air-to-ground observation is also the civil and martial pursuing target. Because of the rising cost along with the large-diameter telescope, the weight, cubage will also become large. Nowadays, how to get high resolution with light weight, small cubage launch and large diameter is one of the important research directions in many countries. We raise a method of large field-of-view and high resolution optical synthesis telescope which can solve this problem. It is a co-phased segment mirrors which synthetic aperture diameter is about 1 m. Four 50cm diameter segment mirrors can fulfill the requirement. It is folded during its launch and is spread after it reaches to its working spot. In this way, it can reach the requirement of low launch weight, small launch cubage and can get high resolution observation. This method contains the key technologies of real-time UV coverage, optics design optimization, co-phase measurement and adjustment, micro-displacement sensor technology, the optics design and structure design. We explore the technology which can fulfill field-of-view of 1.86° and the resolution of 0.4m. We will discuss the UV-coverage method which includes the aperture arrangement, the relationship between the aperture number and the synthetic aperture diameter. There are much more detail calculation and analysis to it. Something is discussed about its structure design and optics design in the paper.

A study of inductance displacement sensor on optical aperture synthesistelescope

Abstract: The principle of the inductance displacement sensor on optical aperture synthesis telescope is given with a measure- ment of mutual inductance to deduce the axial distance. It illustrates an effective modelling and calculation method based on Partial Element Equivalent Circuit (PEEC). PEEC is the method to calculate mutual inductances between coils utilizing partial parameter concept. Simulation for the change of mutual inductances is done which is prepared for the next step work.