Showing papers on "Eddy-current testing published in 1994"

Reconstruction of crack shape by optimization using eddy current field measurement

Abstract: The eddy current field perturbation due to a thin crack may be described as the field generated by a current dipole layer located on the surface of the crack. In this paper the Fourier-transform of the eddy current field by a known dipole layer is evaluated analytically if the dipole layer density function is given as, for example, a Taylor's or Fourier series. This result is used for the calculation of the impedance change of the exciting coil due to a crack by solving an integral equation. In the case of an unknown crack the measured impedance is used for reconstruction. By zeroth order optimization the shape of the crack is varied to fit the calculated impedance data to the measured ones. Several local minima of the objective function are found and statistically processed to give reliable approximation of the crack shape even in the case of sparse and noisy data. >

Structural analysis of CFRP using eddy current methods

Abstract: Nondestructive inspection of carbon fibre-reinforced plastics (CFRP) using eddy current methods is not only based on the conductivity of the fibres. High frequencies (up to 10 MHz) enable the exploitation of capacitive effects reflecting the properties of the matrix. The paper presents a method using rotating eddy current probes to measure the anisotropic electrical properties. Potential applications of the method are structural identification of laminates and estimation of their degradation. For example, the strength properties of constructions based on unidirectional CFRP are very sensitive to small differences in fibre direction angle. Best detection of these angles could be obtained above 7.5 MHz. In addition, a signal perpendicular to the fibre orientation was found and termed the plateau effect. It is a result of capacitive connections between the fibres and presents new opportunities for the characterization of matrix properties.

Self-Nulling Eddy Current Probe for Surface and Subsurface Flaw Detection

Abstract: An eddy current probe which provides a null-signal in the presence of unflawed material without the need for any balancing circuitry has been developed at NASA Langley Research Center. Such a unique capability of the probe reduces set-up time, eliminates tester configuration errors, and decreases instrumentation requirements. The probe is highly sensitive to surface breaking fatigue cracks, and shows excellent resolution for the measurement of material thickness, including material loss due to corrosion damage. The presence of flaws in the material under test causes an increase in the extremely stable and reproducible output voltage of the probe. The design of the probe and some examples illustrating its flaw detection capabilities are presented.

NDE handbook : non-destructive examination methods for condition monitoring

Abstract: Acoustic cross correlation acoustic emission coating thickness dye penetrant examination eddy current testing emission spectroscopy endoscopy ER probe ferrography hardness testing hydrogen cell isotope techniques laser distance measurements leak testing LPR probe magnetic plugs magnetic particle examination mechanical calibration NDE method combination NDE methods under development noise measurement optical pattern recognition P scan pinhole detection pressure testing radiography replical technique spectrometric oil analysis program (SOAP) strain gauge technology stroboscopy test coupons thermography ultrasonic leak detection ultrasonics vibration monitoring visual inspection X-ray crawlers X-ray diffraction.

Motion induced remote field eddy current effect in a magnetostatic non-destructive testing tool: a finite element prediction

Abstract: A hitherto unobserved phenomenon motion induced remote field eddy current effect, is presented in this paper. A numerical study of the non-destructive inspection of tubing with conducting walls, using a DC electromagnetic probe led to the detection of this interesting effect. This paper describes a bi-directional transmission of the electromagnetic field energy through the tube walls, similar to the phenomenon responsible for the Remote Field Eddy Current (RFEC) effect in eddy current (an AC electromagnetic nondestructive testing tool) inspection of tubing. Thus far it was considered that the RFEC effect by the nature of its physics was possible only in the presence of AC excitation in tubular geometries. However, it is shown in this paper that currents induced by magnetic flux moving over conducting material produce a RFEC effect even when a DC probe is used. This phenomenon may enable extraction of valuable information regarding the entire thickness of the tube wall from measurements made on the same side as the excitation source. >

Pulsed eddy-current characterization of corrosion in aircraft lap splices: quantitative modeling

Abstract: Pulsed eddy-current detection and characterization of wall- thinning in aircraft lap-splices due to corrosion is studied theoretically. The relevant lap-splices consist of two one mm thick sheets of aluminum bonded together by bolts and separated a small distance by a insulating sealant. Corrosion changes the thickness of both plates and the size of the gap between them. The problem is to determine nondestructively the thickness of both the 'top' and 'bottom' plates as well as the intervening gap. We calculate the time-domain current-voltage response function of a small cylindrically symmetric coil of wire that is placed next to a lap-joint and excited by a step-function current. The result for air-core coils is obtained as a simple quadrature, while coils that contain ferrite-cores are modeled with a finite element code. The characteristic features of the transient response are shown to depend sensitively on the thickness of the top plate, the gap and the bottom plate.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Eddy current testing system with scanning probe head having parallel and normal sensing coils

Abstract: An eddy current testing system consists of a multi-sensor probe, a computer and a special expansion card and software for data collection and analysis. The probe incorporates an excitation coil, and sensor coils; at least one sensor coil is a lateral current-normal coil and at least one is a current perturbation coil.

Study of the new eddy current non-destructive testing sensor on ferromagnetic materials

Abstract: A new eddy current non-destructive testing (NDT) sensor developed at NASA Langley Research Center can accurately detect fatigue cracks and measure material and non-conducting coating thicknesses. This paper presents experimental and finite element modeling data on the characteristics of the sensor on ferromagnetic materials. Fatigue crack detection and lift-off characteristics in steel and aluminum are compared and discussed. >

An Eddy Current Technique for the Measurement of Constituent Volume Fractions in a Three-Phase Metal-Matrix Composite

Abstract: An eddy current technique is developed for the measurement of reinforcement and intermetallics concentrations in extruded particulate-reinforced metal-matrix composites. The technique employs a the...

Polyphase eddy current testing

Abstract: Polyphase eddy current testing is proposed. The operating principle of this system is as follows. The polyphase eddy current testing system may be regarded as one of the polyphase transformers. Primary balanced polyphase coils are star connected and secondary polyphase circuits are composed of the target. When balanced polyphase currents are fed into the primary coils, zero phase voltage is detected if the target has defects. No zero phase voltage is detected if the target has no defects. It is found that this method has a higher sensitivity compared with that of conventional eddy current testing.

A thin sheet finite element crack model in eddy current NDE

Abstract: A thin sheet finite element crack model has been developed to analyze the electromagnetic interaction between probe coils and crack-type defects in eddy current NDE (Nondestructive Evaluation). The crack type irregularities are treated here as nonconducting thin sheets with negligible thickness to avoid the discretization along their width. On the crack surfaces magnetic vector potential with zero normal component and jumping electric scalar potential ensure the proper behavior of the electromagnetic field. The model can be incorporated into the finite element formulation without disrupting the primary finite element scheme. A benchmark problem has been solved, and numerical results of the crack model have been compared to experimental data. >

An inverse analysis for crack identification in eddy current NDT of tubes

Abstract: This paper presents an inverse analysis for crack identification in eddy current nondestructive testing of tubes. The finite element modeling of axisymmetric eddy current phenomena is based on the formulation of the energy functional with the magnetic vector potential. An optimization algorithm in the inverse process employs the objective function expressed in terms of the impedance change of a differential probe which consists of two coils. The proposed method is applied to a simple axisymmetric model with a crack made arbitrarily. >

Combining eddy current and magnetic flux leakage for tank floor inspection

Abstract: Magnetic flux leakage (MFL) test technology has been applied to the inspection of aboveground storage tank (AST) bottoms since the late 1980s, when it was demonstrated that corrosion in flat carbon steel tank floor plates up to 10 mm (0.4 in.) thick could be detected using MFL techniques. This article will discuss a method that combines eddy current and MFL methods into one test probe design. This design not only detects discontinuities on the top and bottom sides of the tank bottom, but also discriminates between them.

Application of a perturbation method to the solution of eddy current testing problems

Abstract: This paper presents an analytical solution for the change of the impedance of a double conductor line due to eddy currents induced in presence of an infinitely long horizontal cylindrical flaw with vertically symmetric cross-section. The solution, found by a perturbation method, is applicable in the case the conductivities of the flaw and of the surrounding material do not differ by much. Numerical results for flaws of circular and elliptic cross-sections are presented. >

Virtually zero cross-talk dual-frequency eddy current analyzer based on personal computer

Abstract: The dual-frequency eddy current technique is a powerful tool for nondestructive testing of conducting materials, well established both for quality control in the manufacturing industry and for inservice inspection of plant components and structures. The common approach aiming at the extraction of two frequency components from a single signal is based on band filtering. A different approach, which contemporaneously carries out component separation and signal rectification, is proposed in this paper; it enables fast measurement response, virtually zero cross-talk, and hardware simplification. The described methodology has been implemented in the form of a card which can be plugged into a personal computer. Together with the managing software, the eddy current card gives the PC all the capabilities of a powerful and versatile eddy current inspection system. >

Finite element calculations for eddy current interactions with collinear slots

Abstract: The results of finite element calculations detailing the interactions of eddy currents with fine collinear slots in nonferromagnetic and ferromagnetic conductors are presented. These are applicable to both remote field eddy current inspection tools and conventional reflected impedance eddy current probes. The calculations show that, while fine slots have little interaction with collinear induced currents in nonferromagnetic conductors, there are much larger effects in ferromagnetic conductors. This is due to magnetic field interactions. The term eddy current inspection' is therefore somewhat restrictive and the much broader term electromagnetic inspection' is proposed.

Eddy current testing method for ferromagnetic roller bodies - magnetically saturating test zones and superimposing alternating magnetic field and measuring eddy current reaction for detecting and measuring depths of cracks

Abstract: Test zones in the bodies, esp the running surfaces (16), are at least approximately magnetically saturated An alternating magnetic field is superimposed on the test zones using a stimulation coil (32), and the reaction of the induced eddy currents on the alternating field detected using a measurement coil (36) outside the stimulation coil at a distance from the surface of the test object Two measurement coils are connected in a difference arrangement The two measurement coils are arranged at the same distance from the surface of the test object and in opposition on either side of the stimulation coil USE/ADVANTAGE - Testing ferromagnetic roller bodies for cracks, esp for roller bearings, crane and high speed train wheels Detects concealed cracks, accurate crack depth measurement

Eddy current flaw detector

Abstract: (57) [Abstract] [Purpose] The imbalance of the characteristics of a pair of sensor coils can be eliminated, and highly accurate absolute value measurement can be performed without using an external reference, and absolute value measurement and comparative measurement can be performed simultaneously. To do so. [Structure] An eddy current generated by applying an AC test signal to a pair of sensor coils 26a, 26b forming a bridge circuit 25 and causing an AC magnetic field to act on an object to be inspected is detected by the pair of sensor coils. Then, in accordance with the state of the inspection object, the differential output generated in the bridge circuit is obtained as a detection output, and the eddy current inspection device for measuring the inspection object by using this as the inspection information signal of the inspection object, An opening / closing means SW for connecting and disconnecting at least one of the pair of sensor coils in a circuit manner and for disconnecting the sensor coil, Adjustment means 21 for adjusting the phase of the test signal given to the sensor coil connected to the opening / closing means is provided.

An automatic system measuring electromagnetic parameters for oil field pipes

Abstract: The fault detection and diagnosis of oil field pipes are very important to the petroleum companies, as if known, the fault of steel products can be detected based on the eddy current technique. The depth of a defect on an oil field pipe will determine the change in amplitude and phase of the detected signal. However, this signal is also affected by electromagnetic parameters in the vicinity of the fault. It is impossible, therefore, to accurately interpret the severity of the fault from a detected signal, without the knowledge of electromagnetic parameters. A system which can automatically measure electromagnetic parameters of oil field pipes has been developed, so that the correct interpretation of a pipe defect can be provided. An electronic emulator was built for differentiating the permeability of pipes accurately. The electronic emulator takes the signal from the transmission circuit, and converts it to the compensation signal with a certain amplitude and phase shift according to presettings. The combined received signal and compensation signal are conditioned and compared with the transmitted signal to give the expanded phase shift signal at the output. To stand the rigorous working condition in the underground pipes, the system is made capable of working at high temperature. Also taken into account is the variation of the internal diameter and the wall thickness of casing pipes, which affect the measurement of electromagnetic parameters. Therefore, the system can provide high sensitivity electromagnetic parameters for interpreting various types of defects in oil field pipes. >

Simulation of a remote field eddy current detector in eccentric thin-walled non ferrous tubes using a 3D finite-element program

Abstract: Use of a 3D eddy current program to simulate an eddy current probe system is described. Whilst there is great difficulty in accurate representation of fine cracks, the finite-element method works reasonably well for the investigation of thin tubes, and it can model the effects of eccentricities and variations in wall thickness, so long as attention is paid to the meshing. >

New methods of signal excitation and processing in magneto-inductive testing of structures and alloys

Abstract: This paper describes some results of applying noise excitation to eddy current testing Pseudo random binary sequences (PRBS) were used as a noise equivalent The advantage of this technique compared with conventional methods is mainly the simplicity of the equipment used It is possible to produce a device using noise excitation on the basis of a microcalculator Also, this technique gives a direct comparison between the reference object and the object under test as in the well known CS (controlled signals) method The performance of the PRBS method is compared with a multifrequency eddy current method by solving the problem of separating various grades of steel Both methods show nearly the same results

Conversion of NDE Inspection Signals into Aural Signals for Enhanced Material Characterization

Abstract: Nondestructive evaluation (NDE) technologies are often used to characterize materials for quality. Usually they rely on visual interpretation of electronic signals generated from conventional NDE instrumentation [e.g., ultrasonic testing (UT) A-scan trace on an oscilloscope or a Lissajous figure on an eddy current testing (ET) instrument]. In some inspections, such as ultrasonic wall thickness measurement, the signals can be easily interpreted. In many other inspections, however, the signals observed on the oscilloscope have low signal-to-noise and are difficult to visually interpret.

Signal simulator for the calibration of eddy current probes

Abstract: This paper describes a signal simulator which is designed to calibrate eddy current probes. By using simultaneously an excitation coil and an electrically conducting plane, the simulator can emulate signals which correspond to spot welds with different flaws. The presence of a conducting plane allows a wide variation of the amplitude and phase of the excitation coil voltage during the calibration of a probe. A mathematical model for the simulator is derived and used to study the variation of the impedance change upon the parameters of the probe. Numerical computation shows that the impedance change depends in an important way on the frequency of the excitation coil current. >