ME367 NON-DESTRUCTIVE TESTING MODULE- 6 - ECT Sukesh O P, AP-ME, JECC

ME357 Non-Destructive Testing 16-Oct-18 SUKESH O P/ APME/JECC 2 Introduction to NDT- Visual Inspection- Liquid Penetrant Inspection- Magnetic Particle Inspection- Ultrasonic Testing- Radiography Testing- Eddy Current Testing.

MODULE-6 (20%) 16-Oct-18 SUKESH O P/ APME/JECC 3 physics aspects of ECT like conductivity, permeability, resistivity, inductance, inductive reactance, impedance impedance plane diagram in brief relation between frequency and depth of penetration in ECT Eddy Current Testing (ECT) - Principle, physics aspects of ECT like conductivity, permeability, resistivity, inductance, inductive reactance, impedance-Field factor and lift of effect, edge effect, end effect, impedance plane diagram in brief, depth of penetration of ECT, relation between frequency and depth of penetration in ECT - equipment's and accessories, various application of ECT such as conductivity measurement, hardness measurement, defect detection-coating thickness measurement, advantages and limitations of eddy current testing

16-Oct-18 SUKESH O P/ APME/JECC 4  Eddy current inspection is one of several methods that use the principal of “electromagnetism” as the basis for conducting examinations. Several other methods such as Remote Field Testing (RFT), Flux Leakage and Barkhausen Noise also use this principle.

16-Oct-18 SUKESH O P/ APME/JECC 5  Eddy currents  Eddy currents are defined as oscillating electrical currents induced in a conductive material by an alternating magnetic field, due to electromagnetic induction.  ECT  ECT is used for sorting materials, measurement and control of dimensions of tubes, sheets and rods, coating thickness and pre-service and in-service examination of heat exchanger tubes for detection of defects.  Making use of electromagnetic induction to defect amd characterize surface and sub-surface flaws in conductive material.

Properties of eddy currents 16-Oct-18 SUKESH O P/ APME/JECC 6  Eddy current density decreases with the depth exponentially. This phenomenon is known as the skin effect.  Eddy current are closed loops of induced current circulating in planes perpendicular to the magnetic flux.  Eddy currents normally travel parallel to the coil’s windings and the flow is limited to the area of inducing magnetic field and perpendicular to the axis of the coils flux field.

Electromagnetic Induction 16-Oct-18 SUKESH O P/ APME/JECC 7  Eddy currents are created through a process called electromagnetic induction.  When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor.  This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero.

Electromagnetic Induction 16-Oct-18 SUKESH O P/ APME/JECC 8

Generation of Eddy Currents 16-Oct-18 SUKESH O P/ APME/JECC 9

Generation of Eddy Currents 16-Oct-18 SUKESH O P/ APME/JECC 10

Generation of Eddy Currents 16-Oct  Alternating current is allowed to flow in the coil at a frequency chosen by the technician for the type of test involved. SUKESH O P/ APME/JECC

Generation of Eddy Currents 16-Oct-18 SUKESH O P/ APME/JECC 12  A dynamic expanding and collapsing magnetic field forms in and around the coil as the alternating current flows through the coil.

Generation of Eddy Currents 16-Oct-18 SUKESH O P/ APME/JECC 13  When an electrically conductive material is placed in the coil’s dynamic magnetic field electromagnetic, induction will occur and eddy currents will be induced in the material.

Generation of Eddy Currents 16-Oct-18 SUKESH O P/ APME/JECC 14 Eddy currents flowing in the material will generate their own “secondary” magnetic field which will oppose the coil’s “primary” magnetic field.

Generation of Eddy Currents 16-Oct-18 SUKESH O P/ APME/JECC 15  This entire electromagnetic induction process to produce eddy currents may occur from several hundred to several million times each second depending upon inspection frequency.

Generation of Eddy Currents 16-Oct-18 SUKESH O P/ APME/JECC 16

Principle of EDDY CURRENT TESTING 16-Oct  An alternating current (A.C) of frequency 1kHz – 2MHz is made to flow in a coil which in turn, produces an alternating magnetic field around it.  This coil when brought close to the electrically conducting surface of a metallic material to be inspected, induces an eddy current flow in the material due to electromagnetic induction.  These eddy currents are generally parallel to the coil winding. The presence of any defect or discontinuity in the material disturbs the eddy current flow. SUKESH O P/ APME/JECC

16-Oct-18 SUKESH O P/ APME/JECC 18

LIFT OFF EFFECT 16-Oct-18 SUKESH O P/ APME/JECC 19  The distance between a surface coil and the test surface is called proximity or lift off.  Since flux density decreases exponentially with distance from the test coil, the amount of lift or separation between the coil and test specimen has a significant effect on sensitivity.  Close coupling increases the sensitivity to lift off effect, noise due to probe nobbles, when encircling coils are used. Lift off is equivalent to fill factor.

EDGE EFFECT 16-Oct-18 SUKESH O P/ APME/JECC 20  This refers to the effect that the components edge or shape changes in geometry due to the eddy currents.  This can be neglected by placing a balancing probe near to edge and scanning at the distance.  Edge effect is phenomenon that occurs when an inspection coil is at the end of the test piece.  In order to avoid confusion with flaws, inspection is limited near the edges.

FILL FACTOR 16-Oct-18 SUKESH O P/ APME/JECC 21  It is number which measures how well the test piece fills the coil in external encircling probes,  Fill factor= ie,  Fill facor is the ratio of the cross sectional area of the test piece and area of the coil section.  It is necessary that the coil wires be as close as possible to the test piece, in order to have a greater response potential to cracks. Fill factor should be as near as unity

END EFFECT 16-Oct-18 SUKESH O P/ APME/JECC 22  End effect is defined as the disturbance of the magnetic field eddy current distribution, impedance due to proximity of the coil to an abrupt change in geometry.  The end effect is common for cylindrical parts being inspected with encircling or inside diameter bobbin coils.

DEPTH OF PENETRATION 16-Oct-18 SUKESH O P/ APME/JECC 23  Eddy current concentrates near to the surface adjacent to an excitation coil and their strength decreases with distance from the coil.  Skin effect arises when the eddy current flowing in the test object at any depth produces magnetic fields which oppose the primary field, thus reducing the net magnetic flux and causing a decrease in eddy current flow as the depth increases.  It is mathematically convenient to define the standard depth of penetration where the eddy current is 37% of its surface value.

APPLICATIONS OF ECT 16-Oct-18 SUKESH O P/ APME/JECC Detection of discontinuities / surface breaking cracks. 2. Conductivity measurement. 3. Inspection of tube. 4. Thickness measurement.

1. Detection of discontinuities 16-Oct-18 SUKESH O P/ APME/JECC 25  Defects such as cracks are detected when they disrupt the path of eddy currents and weaken their strength.  The sensitivity of ECT to detect surface discontinuities depends on the factors such as type of material, surface finish and condition of material, the design of the probe.  For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity.  For subsurface flaws, lower frequencies are preferred to set the required depth of penetration and this results in less sensitivity.

16-Oct-18 SUKESH O P/ APME/JECC 26 To test surface cracks, pancake probe or surface probe is preferred. 1. Selection of instrument and probe: the instrument includes the basic devices such as oscillator, probe/coil, signal processing and display. To test surface cracks, pancake probe or surface probe is preferred. High frequency is used for surface cracks and low frequency is used to locate deeper cracks. 2. Selection of frequency to produce the desired depth of penetration: depth of penetration depends on frequency, conductivity and permeability. High frequency is used for surface cracks and low frequency is used to locate deeper cracks.

16-Oct-18 SUKESH O P/ APME/JECC 27  3. placing the coil probe and setting the instrument to Null point: place the coil(probe) on the surface of the component to be tested and set the instrument to null point.  4. scanning the surface by moving the probe in a pattern : when the probe is moved over the surface of the specimen, eddy current strength is altered due to presence of crack.  Depending on the density and phase lag of interrupted eddy current circulation, signal response is displayed through impedance plane.

SUKESH O P/ APME/JECC 16-Oct Monitoring the signal for a change in impedance: with increasing depth of crack from the test surface, the impedance locations are:  Never to the surface  Crack at greater depth The direction of movement of the signal curve reveals the possible discontinuity in the material.

Crack Detection Crack detection is one of the primary uses of eddy current inspection. Cracks cause a disruption in the circular flow patterns of the eddy currents and weaken their strength. This change in strength at the crack location can be detected. Magnetic Field From Test Coil Magnetic Field From Eddy Currents Eddy Currents Crack 16-Oct-18 SUKESH O P/ APME/JECC 29

Crack Detection (cont.) Eddy current inspection is exceptionally well suited for the detection of cracks, with an especially high sensitivity to detection of surface breaking cracks. 16-Oct-18 SUKESH O P/ APME/JECC 30

Crack Detection (cont.) Eddy current inspection of “bead seat” area on aircraft wheel for cracks using special probe that conforms to the shape of the rim. 16-Oct-18 SUKESH O P/ APME/JECC 31

Crack Detection (cont.) Loading points, such as fastener holes, are high stress areas and often the site of service induced fatigue cracking. Rotating probe guns can be used to inspect a large number of holes in a short period of time. The photo on the right is a waterfall plot of the cross section of a fastener hole. Each horizontal line represents one rotation of the probe gun. A vertical signal indicates a crack. 16-Oct-18 SUKESH O P/ APME/JECC 32

2. Conductivity Measurement 16-Oct-18 SUKESH O P/ APME/JECC 33  The value of the electrical conductivity of a material depends on several factors such as chemical composition and the stress state of its crystalline structure.  Electrical conductivity information can be used for sorting metals, checking for proper heat treatment and inspecting for heat damage.  For non magnetic materials, the change in impedance of the coil can be correlated directly to the conductivity of the material.

Monitoring Conductivity and Permeability Variations Eddy current inspection is sensitive to changes in a material’s electrical conductivity and magnetic permeability. This “sensitivity” allows the inspection method to be used for such inspection procedures as: Material Identification Material Sorting Determination of heat damage Cladding and plating thickness measurement Heat treatment monitoring 16-Oct-18 SUKESH O P/ APME/JECC 34

3. Inspection on Tube 16-Oct-18 SUKESH O P/ APME/JECC 35  ECT is often used to detect corrosion, erosion, cracking and other changes in tubing.  Heat exchangers and steam generators, which are used in power plants, have many tubes which must be prevented from leaking.  A technique that is often used involves feeding a differential bobbin probe into the individual tube of the heat exchanger.  When the corrosion is on the outside surface of the tube, the depth of corrosion is indicated by a shift in the phase lag.

Material Thickness Measurement  Thickness measurements are possible with eddy current inspection within certain limitations.  Only a certain amount of eddy currents can form in a given volume of material.  Therefore, thicker materials will support more eddy currents than thinner materials.  The strength (amount) of eddy currents can be measured and related to the material thickness. Eddy Currents Magnetic Field From Probe Test Material 16-Oct-18 SUKESH O P/ APME/JECC 36

Material Thickness Measurement (cont.) Eddy current inspection is often used in the aviation industries to detect material loss due to corrosion and erosion. 16-Oct-18 SUKESH O P/ APME/JECC 37

Material Thickness Measurement (cont.) Eddy current inspection is used extensively to inspect tubing at power generation and petrochemical facilities for corrosion and erosion. 16-Oct-18 SUKESH O P/ APME/JECC 38 SUKESH O P/ APME/JECC

Nonconductive Coating Measurement Nonconductive coatings on electrically conductive substrates can be measured very accurately with eddy current inspection. (Accuracy of less that one mil is not uncommon.) Conductive Base Metal Nonconductive Coating Eddy Currents The coating displaces the eddy current probe from the conductive base material and this weaken the strength of the eddy currents. This reduction in strength can be measured and related to coating thickness. 16-Oct-18 SUKESH O P/ APME/JECC 39 SUKESH O P/ APME/JECC

Nonconductive Coating Measurement (cont.) The photo to the left shows an aircraft panel paint thickness inspection. On the right, the display of a digital eddy current inspection instrument shows the different signals obtained by measuring eight different thicknesses of paint on aluminum. Increasing paint thickness 16-Oct-18 SUKESH O P/ APME/JECC 40

Sensitive to small cracks and other defects Detects surface and near surface defects Inspection gives immediate results Equipment is very portable Method can be used for much more than flaw detection Minimum part preparation is required Test probe does not need to contact the part Inspects complex shapes and sizes of conductive materials Advantages of ECT 16-Oct-18 SUKESH O P/ APME/JECC 41

Only conductive materials can be inspected Surface must be accessible to the probe Skill and training required is more extensive than other techniques Surface finish and roughness may interfere Reference standards needed for setup Depth of penetration is limited Flaws such as delaminations that lie parallel to the probe coil winding and probe scan direction are undetectable Limitations of ECT 16-Oct-18 SUKESH O P/ APME/JECC 42