EDDY CURRENT TESTING

Transcription

1 EDDY CURRENT TESTING

2 Introduction Eddy current inspection is a method that use the principal of electromagnetism as the basis for conducting examinations. Eddy Current NDT is a technique that can test metals for flaws either during the manufacturing process or as a consequence of age or environment. It is a highly sensitive technique and completely Non-Destructive. (No damage to test object) It is quick, safe and cost-effective to use.

3 Outline Electromagnetic induction Generation of eddy currents Inspection applications Equipment utilized in eddy current inspection Probes/Coils Instrumentation Reference standard Advantages and Limitations Glossary of Terms

4 Electromagnetic Induction Eddy currents are created through a process called electromagnetic induction. Theory Electromagnetic Induction Current passing through a coil creates a magnetic field A moving magnetic field would induce a voltage in an electrical conductor 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.

5 Electromagnetic Induction (cont.) If another electrical conductor is brought into the proximity of this changing magnetic field, the reverse effect will occur. Magnetic field cutting through the second conductor will cause an induced current to flow in this second conductor. Eddy currents are a form of induced currents! Current Flow

6 Generation of Eddy Currents Eddy currents are induced electrical currents that flow in a circular path. They get their name from eddies that are formed when a liquid or gas flows in a circular path around obstacles when conditions are right. Test Probe Eddy Currents

7 Generation of Eddy Currents (cont.) In order to generate eddy currents for an inspection a probe is used. Inside the probe is a length of electrical conductor which is formed into a coil.

8 Generation of Eddy Currents (cont.) Alternating current is allowed to flow in the coil at a frequency chosen by the technician for the type of test involved.

9 Generation of Eddy Currents (cont.) A dynamic expanding and collapsing magnetic field forms in and around the coil as the alternating current flows through the coil.

10 Generation of Eddy Currents (cont.) 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.

11 Generation of Eddy Currents (cont.) Eddy currents flowing in the material will generate their own secondary magnetic field which will oppose the coil s primary magnetic field.

12 Generation of Eddy Currents (cont.) This entire electromagnetic induction process to produce eddy currents may occur from several hundred to several million times each second depending upon inspection frequency.

13 Eddy Current Theory Testing When an AC current flows in a coil in close proximity to a conducting surface, the magnetic field of the coil will induce circulating(eddy) currents in that surface. The magnitude and phase of the eddy currents will affect the loading on the coil and thus its impedance

14 Eddy Current Testing-Basic Training Eddy Current Theory - Testing

15 Eddy Current Theory - testing

16 Eddy Current Testing-Basic Training A deep crack in the surface below the coil will interrupt or reduce the eddy current flow, thus decreasing the loading of the coil and increasing its effective impedence By monitoring the voltage across the coil we can detect changes in the test material

17 Eddy Current Theory Testing Cracks MUST interrupt the surface eddy current flow to be detected

18 Inspection Data Factors affecting Eddy Current response The electrical conductivity of the material The magnetic permeability of the material Frequency Geometry Proximity / Lift-off Depth of penetration

19 Factors affecting Eddy Current response Material conductivity - Greater the conductivity greater the eddy current flow - conductivity depends on material composition, heat treatment, work hardening etc

20 Factors affecting Eddy Current response Permeability Described as the ease with which a material can be magnetised For nfe metals and austenitic S.Steel, the permeability (m r )is 1 (as for free space) For Fe metals the value of m r may be several hundred, thus influencing the eddy current response Permeability may vary within a metal part due to localised stresses, heating effects etc

21 Factors affecting Eddy Current response Frequency Eddy current response is greatly affected by the test frequency, but this property can be controlled

22 Factors affecting Eddy Current response Geometry Curvature,edges,grooves etc will affect the eddy current response When the material thickness is less than the effective depth of penetration, this will also affect the eddy current response

23 Eddy Current Testing-Basic Training Factors affecting Eddy Current response Proximity / Lift-off The closer the probe coil to the surface, greater the effect on the coil. This has two main effects 1. Lift off signal as the probe is moved on and off the surface 2. A reduction in sensitivity as the coil to product spacing increases

24 Eddy Current Testing-Basic Training Factors affecting Eddy Current response Depth of Penetration The eddy current density is greatest on the surface of the metal and declines with depth Depth of penetration -decreases with an increase in frequency -decreases with an increase in conductivity -decreases with an increase in permeability

25 Eddy Current Testing-Basic Training Factors affecting Eddy Current response Depth of Penetration The eddy current density is greatest on the surface of the metal and declines with depth Depth of penetration -decreases with an increase in frequency -decreases with an increase in conductivity -decreases with an increase in permeability

26 Factors affecting Eddy Current response

27 Effective Depth of Penetration It is defined as three times the standard depth, where eddy current density has fallen to 3-5% of the surface value The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (d). The word 'standard' denotes plane wave electromagnetic field excitation within the test sample (conditions which are rarely achieved in practice). Although eddy currents penetrate deeper than one standard depth of penetration, they decrease rapidly with depth. At two standard depths of penetration (2d), eddy current density has decreased to 1/e squared or 13.5% of the surface density. At three depths (3d), the eddy current density is down to only 5% of the surface density.

28 Standard depth penetration, d d f 1 r Where: δ or d = Standard Depth of Penetration (mm) f = Test Frequency (Hz) μ r = Relative Permeability = Electrical Conductivity

29 Depth Depth Generation of Eddy Currents (cont.) Eddy currents are strongest at the surface of the material and decrease in strength below the surface. The depth that the eddy currents are only 37% as strong as they are on the surface is known as the standard depth of penetration or skin depth. This depth changes with probe frequency, material conductivity and permeability. Standard Depth of Penetration (Skin Depth) Eddy Current Density High Frequency High Conductivity High Permeability 1/e or 37 % of surface density Eddy Current Density Low Frequency Low Conductivity Low Permeability

30 Since the sensitivity of an eddy current inspection depends on the eddy current density at the defect location, it is important to know the strength of the eddy currents at this location. When attempting to locate flaws, a frequency is often selected which places the expected flaw depth within one standard depth of penetration. This helps to assure that the strength of the eddy currents will be sufficient to produce a flaw indication. Alternately, when using eddy currents to measure the electrical conductivity of a material, the frequency is often set so that it produces three standard depths of penetration within the material. This helps to assure that the eddy currents will be so weak at the back side of the material that changes in the material thickness will not affect the eddy current measurements.

31 Inspection Information about the strength of the eddy currents within the specimen is determined by monitoring changes in voltage and/or current that occur in the coil. The strength of the eddy currents changes the electrical impedance (Z) of the coil.

32 Inspection Data (cont.) Impedance (Z) in an eddy current coil is the total opposition to current flow. In a coil, Z is made up of resistance (R) and inductive reactance (X L ). ~ R X L Test Coil Definitions: Resistance - The opposition of current flow, resulting in a change of electrical energy into heat or another form of energy. Inductive Reactance (X L ) - Resistance to AC current flow resulting from electromagnetic induction in the coil. Impedance (Z) - The combined opposition to current flow resulting from inductive reactance and resistance. In an AC coil, induction from the magnetic field of one loop of the coil causes a secondary current in all other loops. The secondary current opposes the primary current.

33 Inspection Applications One of the major advantages of eddy current as an NDT tool is the variety of inspections that can be performed.

34 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. Magnetic Field From Probe Test Material Eddy Currents

35 Material Thickness Measurement (cont.) Eddy current inspection is often used in the aviation industries to detect material loss due to corrosion and erosion.

36 Material Thickness Measurement (cont.) Eddy current inspection is used extensively to inspect tubing at power generation and petrochemical facilities for corrosion and erosion.

37 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

38 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.

39 Successful detection of surface breaking and near surface cracks requires: A knowledge of probable defect type, position, and orientation. Selection of the proper probe. The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw. Selection of a reasonable probe drive frequency. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity. For subsurface flaws, lower frequencies are necessary to get the required depth of penetration and this results in less sensitivity. Ferromagnetic or highly conductive materials require the use of an even lower frequency to arrive at some level of penetration. Setup or reference specimens of similar material to the component being inspected and with features that are representative of the defect or condition being inspected for.

40 The basic steps in performing an inspection with a surface probe are the following: Select and setup the instrument and probe. Select a frequency to produce the desired depth of penetration. Adjust the instrument to obtain an easily recognizable defect response using a calibration standard or setup specimen. Place the inspection probe (coil) on the component surface Scan the probe over part of the surface in a pattern that will provide complete coverage of the area being inspected. Care must be taken to maintain the same probe-to-surface orientation as probe wobble can affect interpretation of the signal. In some cases, fixtures to help maintain orientation or automated scanners may be required. Monitor the signal for a local change in impedance that will occur as the probe moves over a discontinuity.

41 A simple eddy current probe near the surface of a calibration specimen. The probe is scanned over the surface of the specimen and the signal responses from surface breaking crack with the signals from the calibration notches (A, B, C) will be displayed.

42 50 KHz 300 KHz For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity.

43 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.) 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. Nonconductive Coating Conductive Base Metal Eddy Currents

44 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

45 Conductivity Measurements Boeing employees in Philadelphia were given the privilege of evaluating the Liberty Bell for damage using NDT techniques. Eddy current methods were used to measure the electrical conductivity of the Bell's bronze casing at a various points to evaluate its uniformity.

46 Equipment Equipment for eddy current inspection is very diversified. Proper equipment selection is important if accurate inspection data is desired for a particular application. As a minimum, at least three basic pieces of equipment are needed for any eddy current examination: Instrumentation Probes Reference Standards

47 Instrumentation - Meters Meters are typically the simplest form of eddy current instrumentation. The two general categories of meters are digital and analog.

48 Digital Meters Digital meters are typically designed to examine one specific attribute of a test component such as conductivity or nonconductive coating thickness. These meters tend to have slightly higher accuracy than analog devices.

49 Analog meters can be used for many different inspection applications such as crack detection, material thickness measurements, nonconductive coating measurements or conductive coating measurements. Analog Meters

50 Portable Eddy Scopes

51 Eddy Current Probes

52 Eddy Current Probes (cont.) Probes selection is critical to acquiring adequate inspection data. Several factors to consider include: Material penetration requirements (surface vs. subsurface) Sensitivity requirements Type of probe connections on eddy current instrument (many variations) Probe and instrument impedance matching (will probe work with instrument) Probe size (smaller probes penetrate less)

53 Eddy Current Probes (cont.) Surface probes can be very small in size to allow accessibility to confined areas. Finger Probe

54 Reference Standards

55 Reference Standards (cont.) In order to give the eddy current inspector useful data while conducting an inspection, signals generated from the test specimen must be compared with known values. Reference standards are typically manufactured from the same or very similar material as the test specimen. Many different types of standards exist for due to the variety of eddy current inspections performed. The following slides provide examples of specific types of standards.

56 Reference Standards (cont.) Material thickness standards used to help determine such things as material thinning caused by corrosion or erosion.

57 Reference Standards (cont.) Crack Standards:

58 Reference Standards (cont.) ASME Tubing Pit Standard:

59 Reference Standards (cont.) Nonconductive coating (paint) standard with various thickness of paint on aluminum substrate.

60 Advantages of Eddy Current Inspection 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

61 Limitations of Eddy Current Inspection 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 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

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