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Faraday discovered that when a magnetic field passes through a conductor a material in which electrons move easily -or when a conductor passes through a magnetic field-an electric current will flow through cjd conductor if there is a closed path through which the current can circulate. Eddy current NDT can examine large areas very quickly, and it does not require use of coupling liquids.
Thus, variations in the conductivity of the test material, its magnetic permeability, the frequency of the AC pulses driving the coil, and coil geometry will all have an effect on test sensitivity, resolution, and penetration.
Pencil probes – Smaller diameter probes housing coils built for high frequencies for high resolution of near surface flaws. The top curve represents a 0. This example was a 0. In some cases, eddy current and ultrasonic testing are used together as complementary techniques, with eddy current having an advantage for quick surface testing and ultrasonics having better depth penetration. Other companies soon followed. Bolt Hole Inspection – Cracking inside bolt holes can be detected using bolt hole probes, often with automated rotary scanners.
Both cracking and thickness variations can be detected. In flaw detection applications, this calibration process typically involves the use of reference standards of the same material, shape, and size as the test piece, containing artificial defects such as saw cuts, drilled holes, or milled walls to simulate flaws.
Some common probe types are listed below. This in turn affects the movement of electrons in the coil courahts varying the electrical impedance of the coil. Types of probes Eddy current instruments can perform a wide variety of tests depending on the type of probe being used, and careful probe selection will help optimize performance.
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Some inspections involve sweeping through multiple frequencies to optimize results, or inspection with multiple probes to obtain the best resolution and penetration required to detect all possible flaws.
Donut probes – Designed to inspect aircraft fastener holes with fasteners in place. Proper calibration with appropriate reference foucqult is an essential part of any eddy current test procedure.
The phenomenon of eddy currents was discovered ccnd French physicist Leon Foucault inand for this reason eddy currents are sometimes called Foucault foicault. Foucault built a device that used a copper disk moving in a strong magnetic field to show that eddy currents magnetic fields are generated when a material moves within an applied magnetic field. In addition to finding cracks, eddy current can also be used to check metal hardness and conductivity in applications where those properties are of interest, and to measure thin layers of nonconductive coatings like paint on metal parts.
The screen display below shows a nonmetallic coating over aluminum.
This measurement is most commonly used to determine d conductivity of a material but it will also provide the thickness of a coating which is considered the “Liftoff” from the material or how far the probe is above the surface of the conductive material. The operator observes the response from the reference standards and then compares the indications from test pieces to these reference patterns to categorize parts.
Many advances were made throughout the s and s, especially in the aircraft and nuclear industries. Menu More Pour nous joindre.
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If the probe and its magnetic field are brought close to a conductive material like a metal test piece, a circular flow of electrons known as an eddy current will begin to move through the metal like swirling water in a stream. Both field portable and fixed system instruments are available to meet a wide variety of test needs.
The horizontal line is the lift off in which the probe has been “nulled” balanced on the aluminum part and when it is lifted in the air, the signal moves directly to the left. ECA can permit use of simpler fixturing, and can also simplify inspection of complex shapes through custom probes built to fit the profile of the test piece. Another common test involves measurement of nonconductive coatings like paint over metals.
Eddy current testing began largely as a result of the English scientist Michael Faraday’s discovery of electromagnetic induction in In an eddy current probe, an alternating current flows through a wire coil and generates an oscillating magnetic field. This data is referenced to an encoded position and time and represented graphically as a C-scan image showing structures in a planar view.
Conductivity Testing – Eddy current testing’s ability to measure conductivity can be used to identify and sort ferrous and nonferrous alloys, and to verify heat treatment. That eddy current flowing through the metal will in turn generate its own magnetic field, which will interact with the coil and its field through mutual inductance.
The inspection measurements are dependent entirely on the comparison of the signal against the reference calibration. Some of the most common are listed below. In addition to providing visualization through C-scan imaging, ECA allows coverage of larger areas in a single pass while maintaining high resolution. Higher test frequencies increase near surface resolution but limit the depth of penetration, while lower test frequencies increase penetration.
Weld Inspection – Many weld inspections employ ultrasonic NDT for subsurface testing and a complimentary eddy current method to scan the surface for open surface cracks on weld caps and in heat affected zones. Larger coils inspect a greater volume of material from any given position, since the magnetic field flows deeper into the test piece, while smaller coils are more sensitive to small defects.
By Tom Nelligan and Cynthia Calderwood Magnetism, the underlying principle behind electric motors and generators, relays and stereo speakers, is also the force that enables an important category of NDT tools called eddy current instruments.
Eddy current arrays Eddy Current Array testing, or ECA, is a technology that provides the ability to simultaneously use multiple eddy current coils that are placed side by side in the same probe assembly. After this calibration is done, the inspector would measure on their parts and watch for the distance that the signal travels across the screen. Eddy current testing is now a widely used and well-understood inspection technique for flaw detection as well as for thickness and conductivity measurements.
Menu More Pour nous joindre. Variations in permeability of a material generate noise that can limit flaw resolution because of greater background variations.
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Tubing inspection – Both in-line inspection of tubing at the manufacturing stage and field inspection of tubing like heat exchangers are common eddy current applications. Each individual coil produces a signal relative to the phase and amplitude of the structure below it. Bolt hole probes – Designed to inspect the inside of a bolt hole. While conductivity and permeability are properties of the test material that are outside of the operator’s control, the test frequency, coil type, and coil size can be chosen based on test requirements.
OD probes – Used for inspection of metal tubing and bars from the outside, with the test piece passing through the coil Reference standards An eddy current system consisting of an instrument and a probe must always be calibrated with appropriate reference standards at the start of a test.
Impedance plane displays While some older eddy current instruments used simple analog meter displays, the standard format now is an impedance plane plot that graphs coil resistance on the x-axis versus inductive reactance on the y-axis. However, it was not until the Second World War that these developments in the transmitting and receiving of electromagnetic waves were put to practical use for materials testing.