Eddy current technique is an important electromagnetic non-destructive evaluation technique that is widely used in power, aerospace, petrochemical and other industries for detection of surface cracks and sub-surface damage in components made of metallic materials. Besides, it is also used traditionally for assessing the adequacy of heat treatment of alloys, as eddy currents are sensitive to changes in microstructure and stresses, which alter the electrical conductivity and magnetic permeability of the material. This paper gives a brief account of basic principle, features, applications, limitations of the eddy current technique. It also covers instruments and sensors to enable better appreciation of the technique and its capabilities.
EC technique is the most popular and widely used electromagnetic NDE technique. In industrial scenario, among other electromagnetic NDE techniques, this technique finds larger number of applications. This technique finds versatile applications in power, aerospace and petrochemical industries. It is not incorrect to say that worldwide almost all the heat exchangers and aircraft's are inspected using this technique. Two main aspects behind this widespread use are excellent sensitivity to surface as well as sub-surface defects and testing speed of as high as 10 m/s which no other NDE technique can match. This is especially profitable to industries as it enables rapid examination during manufacturing stages, while it drastically reduces the down time of operating plant components. Many developments are taking place in this existing NDE technique incorporating the rapid progress in the fields of microelectronics, instrumentation, sensors, computers, numerical modelling, digital signal & image processing.
BASIC PRINCIPLE 0F EDDY CURRENT INSPECTION
Eddy current testing works on the principle of electromagnetic induction to detect flaws in conductive materials.
Electromagnetic induction: When the magnetic flux through a conductor changes, induced currents are set up in closed paths or the surface of the conductor itself. So now, you can see that we are having that sample. So now we are having that induction coils and through that we are generating the magnetic field over there, and due to that magnetic field this reading colour the eddy currents is generating on to our work or maybe the onto our sample. These currents are in direction of perpendicular to the magnetic flux and are called Eddy current.
How it works?
First is that the generation of Eddy currents. In order to generate the Eddy currents for an inspection, a probe is used. Inside the probe is a length of electrical conductor which is formed into a coil. So you can see that coil. Alternating current is passed through the coil which generates an oscillating magnetic field. The probe and its magnetic field are brought close to a metal test piece. A circular flow of electrons form known as Eddy current will begin to move through the metal itself that red in color.
Secondary magnetic field is set up in opposite directions. Eddy currents flowing in the material will generate their own secondary magnetic field which will oppose the coil's primary magnetic field over there.
Impedance of the coil is changed. Changes in metal thickness or defects like near surface cracking will interrupt or alter the amplitude and pattern of the eddy current and thus resulting the magnetic field. This in turn affects movement of electrons in coil by varying the electrical impedance of the coil itself. So now you can see from this particular image, that how we are getting the signals, EC signal without crack, so it is good in nature. But, when we are having some cracks we are getting some erratic signals over there, so now change in impedance is analysed. Eddy current instrument plots changes in the impedance amplitude and phase angle, which can be used by a trained operator to identify changes in the taste piece.
APPLICATIONS OF EDDY CURRENT INSPECTION
A few specific practical applications of EC technique are given below for better appreciation of the technique.
• Quality assurance of austenitic stainless steel tubes, plates and welds
• Inspection of installed heat exchanger/steam generator/condenser tubes (single and multi-frequency)
• Detection of surface as well as sub-surface defects in multi-layer aircraft structures (single frequency, multi-frequency & pulsed techniques)
• On-line automated saturation based quality assurance of steel (ferromagnetic) tubes.
• Location of garter springs in PHWRs and measurement of gap in coolant channels
• Detection of intergranular corrosion (IGC) in stainless steels (316, 316L and 304 L)
• Detection of weld centre line in austenetic stainless steel welds at high temperature
• Measurement of coating thickness of SiC on carbon-carbon composites
• Sorting of materials based on electrical conductivity and magnetic permeability
• Characterisation of heat treated as well as degraded microstructures in alloys
• Non-contact detection of metallic objects, land mines, security metal detectors
Working on the principle of electromagnetic induction, eddy current technique is a widely used NDE technique for detection of surface and sub-surface damage. The attractive features of this technique include ease of operation, high sensitivity to tight cracks, versatility, extremely high testing speeds (up to 10 m/s), repeatability and reliability. This technique can detect wall thinning, cracks, pitting, stress corrosion cracking, hydrogen embrittlement, carburization, denting and crud deposits etc. This technique finds a lot of applications in engineering industry including material sorting, determination of hardness, heat treatment adequacy assessment, material property determination, coating thickness measurements, and detection of defects in tubes, rods, bars, multi-layer structures, discs, welds, blades and other regular as well as irregular geometries. Successful testing requires selection of proper instrument and probes, optimisation test frequency and use of reference calibration standards. When appropriate standards are used, not only detection of defects but also their sizing is possible using eddy current technique.