BMFB 4283 NDT & FAILURE ANALYSIS

1. BMFB 4283NDT & FAILURE ANALYSIS Lectures for Week 6 Prof. QumrulAhsan, PhD Department of Engineering Materials Faculty of Manufacturing Engineering

2. Issues to address 6.0 Eddy Current 6.1 Introduction 6.2 Electromagnetic Principles 6.3 Equipment and Testing Techniques 6.4 Inspection 6.5 Application

3. Introduction • This module is intended to present information on the NDT method of eddy current inspection. • 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.

4. 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

5. Electromagnetic Induction • 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.

6. Current Flow 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!

7. 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

8. 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. • Eddy currents flowing in the material will generate their own “secondary” magnetic field which will oppose the coil’s “primary” magnetic field.

9. 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.

10. Generation of Eddy Currents (cont.) • The eddy currents produce their own magnetic fields that interact with the primary magnetic field of the coil. • By measuring changes in the resistance and inductive reactance of the coil, information can be gathered about the test material. • This information includes the electrical conductivity and magnetic permeability of the material, the amount of material cutting through the coils magnetic field, and the condition of the material (i.e. whether it contains cracks or other defects.)

11. Generation of Eddy Currents (cont.)

12. Generation of Eddy Currents (cont.) Depth Standard Depth of Penetration (Skin Depth) 1/e or 37 % of surface density Depth Eddy Current Density Eddy Current Density Low Frequency Low Conductivity Low Permeability High Frequency High Conductivity High Permeability 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. 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 (δ). Although eddy currents penetrate deeper than one standard depth of penetration, they decrease rapidly with depth. At two standard depths of penetration (2 δ), eddy current density has decreased to 1/e squared or 13.5% of the surface density. At three depths (3 δ), the eddy current density is down to only 5% of the surface density.

13. Inspection Data • There are three characteristics of the specimen that affect the strength of the induced eddy currents. • The electrical conductivity of the material • The magnetic permeability of the material • The amount of solid material in the vicinity of the test coil. • 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.

14. R Test Coil ~ XL 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 (XL). • Definitions: • Resistance - The opposition of current flow, resulting in a change of electrical energy into heat or another form of energy. • Inductive Reactance (XL) - 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.

15. Lift off in Eddy Current • The distance that the coil is from the conductive material is called liftoff, and this distance affects the mutual-inductance of the circuits. With increase of frequency will give wider angle (i.e diff in 20 kHz and 2 MHz Fig. 1.a) Impedance plane diagram representation in conventional eddy current testing, b) zoomed-in and rotated area on the operating point; the lift-off variation is kept along the horizontal axis.

16. Phase Lag in Eddy Current Phase lag can be calculated with the following equation.  • Phase lag is to obtain information about the depth of a defect within a material.  • Phase lag is the shift in time between the eddy current response from a disruption on the surface and a disruption at some distance below the surface.  • Disruptions in the eddy currents away from the surface will produce more phase lag than disruptions near the surface. • Both the signal voltage and current will have this phase shift or lag with depth • discontinuities that have a significant dimension normal to the surface, will produce an angle that is based on the weighted average of the disruption to the eddy currents at the various depths along its length.

17. Impedance Plane (eddy scope) the strength of the eddy currents and the magnetic permeability of the test material cause the eddy current signal on the impedance plane to react in a variety of different ways. • If the eddy current circuit is balanced in air and then placed on a piece of aluminum, • the resistance component will increase (eddy currents are being generated in the aluminum and this takes energy away from the coil, which shows up as resistance) • the inductive reactance of the coil decreases (the magnetic field created by the eddy currents opposes the coil's magnetic field and the net effect is a weaker magnetic field to produce inductance). • If a crack is present in the material, fewer eddy currents will be able to form and the resistance will go back down and the inductive reactance will go back up. • Changes in conductivity will cause the eddy current signal to change in a different way.

18. Impedance Plane (eddy scope) the strength of the eddy currents and the magnetic permeability of the test material cause the eddy current signal on the impedance plane to react in a variety of different ways. • When a probe is placed on a magnetic material such as steel, something different happens. • eddy currents form, taking energy away from the coil, which shows up as an increase in the coils resistance. • the eddy currents generate their own magnetic field that opposes the coils magnetic field. • the reactance increases. This is because the magnetic permeability of the steel concentrates the coil's magnetic field. This increase in the magnetic field strength completely overshadows the magnetic field of the eddy currents. • The presence of a crack or a change in the conductivity will produce a change in the eddy current signal

19. 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

20. Equipment • Eddy current instruments can be purchased in a large variety of configurations. Both analog and digital instruments are available. Instruments are commonly classified by the type of display used to present the data. The common display types are analog meter, digital readout, impedance plane and time versus signal amplitude. Some instruments are capable of presenting data in several display formats. • The most basic eddy current testing instrument consists of an alternating current source, a coil of wire connected to this source, and a voltmeter to measure the voltage change across the coil. An ammeter could also be used to measure the current change in the circuit instead of using the voltmeter.

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

22. 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.

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

24. Analog Meters (cont.) The display read-out found on most analog instruments is typically either a calibrated or uncalibrated display. • Calibrated displays have an inherent scaling factor which correlates to the property the instrument is designed to measure such as conductivity. • Uncalibrated displays are typically more flexible in the variety of different tests they can perform. These types of instruments, however, require the use of data extrapolation techniques if quantitative data is desired.

25. Portable Eddy Scopes

26. Portable Eddy Scopes (cont.) Portable eddy scopes are another category of instrumentation and they present the inspection data in the form of an impedance plane diagram. • On the impedance diagram, the total impedance is displayed by plotting its resistance component and inductive reactance component at 90 degrees to each other. • This is beneficial for both separation and identification of test variables that can effect inspection results.

27. Portable Eddy Scopes (cont.) Modern eddy scopes are usually digital based instruments which can often be purchased as either a single or dual frequency tester. Dual frequency instruments are capable of sequentially driving a probe at two different inspection frequencies.

28. Portable Eddy Scopes (cont.) Digital scopes often have an RS232 (serial) connection for interfacing with a serial printer or computer as well as provisions for output of signals to recording devices such as a strip-chart recorder. In addition, these instruments contain a small amount of RAM so that equipment settings as well as screen presentations can be stored for later reference.

29. Multi-Frequency Eddy Current Instruments

30. Multi-Frequency Eddy Current Instruments (cont.) • Multi-Frequency instruments usually refer to equipment that can drive inspection coils at more than two frequencies either sequentially (multiplexing) or simultaneously. • This type of instrumentation is used extensively for tubing inspection in the power generation, chemical and petrochemical industries. • These instruments are often capable of being computer networked and may have as many as four probes attached to them at one time.

31. Multi-Frequency Eddy Current Instruments (cont.) Advantages of Multi-frequency inspections: • Allows increased inspection information to be collected from one probe pulling. • Provides for comparison of same discontinuity signal at different frequencies. • Allows mixing of frequencies which helps to reduce or eliminate sources of noise. • Often improves detection, interpretation and sizing capabilities of discontinuities.

32. Multi-Frequency Eddy Current Instruments (cont.) Screen of multi-frequency instrument during inspection.

33. Eddy Current Probes

34. 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) • Probe type (absolute, differential, reflection or hybrid)

35. Eddy Current Probes (cont.) Surface probes are coils that are typically mounted close to one end of a plastic housing. As the name implies, the technician moves the coil end of the probe over the surface of the test component.

36. Eddy Current Probes (cont.) Some surface probes are specifically designed for crack detection of fastener holes. These include sliding probes, ring probes and hole probes.

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

38. Eddy Current Probes (cont.) Inside Diameter (I.D.) probes, also known as bobbin probes, are coils that are usually wound circumferentially around a plastic housing. These probes are primarily designed for inspection inside of tubular materials.

39. Eddy Current Probes (cont.) Outside Diameter (O.D.) probes are coils that are wound the circumference of a hollow fixture. The coil is designed such that the test part is ran through the middle of the coil. These probes can be used to inspect bars, rods as well as tubes.

40. Eddy Current Probes (cont.) • Absolute probes generally have a single test coil that is used to generate the eddy currents and sense changes in the eddy current field • AC is passed through the coil and this sets up an expanding and collapsing magnetic field in and around the coil. • When the probe is positioned next to a conductive material, the changing magnetic field generates eddy currents within the material. • The generation of the eddy currents take energy from the coil and this appears as an increase in the electrical resistance of the coil. • The eddy currents generate their own magnetic field that opposes the magnetic field of the coil and this changes the inductive reactance of the coil. • By measuring the absolute change in impedance of the test coil, information can be gained about the test material.

41. Eddy Current Probes (cont.) • Absolute coils can be used for • flaw detection, • conductivity measurements, • liftoff measurements • thickness measurements. • Since absolute probes are sensitive to things such as conductivity, permeability liftoff and temperature, steps must be taken to minimize these variables when they are not important to the inspection being performed. • It is very common for commercially available absolute probes to have a fixed "air loaded" reference coil that compensates for ambient temperature variations.

42. Eddy Current Probes (cont.) • Differential Probes • Differential probes have two active coils usually wound in opposition • When the two coils are over a flaw-free area of test sample, there is no differential signal developed between the coils since they are both inspecting identical material. • However, when one coil is over a defect and the other is over good material, a differential signal is produced. • They have the advantage of being very sensitive to defects yet relatively insensitive to slowly varying properties such as gradual dimensional or temperature variations. • Probe wobble signals are also reduced with this probe type. • There are also disadvantages to using differential probes. • Most notably, the signals may be difficult to interpret. For example, if a flaw is longer than the spacing between the two coils, only the leading and trailing edges will be detected due to signal cancellation when both coils sense the flaw equally.

44. Eddy Current Probes (cont.) • Reflection Probes • Reflection probes have two coils similar to a differential probe • one coil is used to excite the eddy currents and the other is used to sense changes in the test material. • Probes of this arrangement are often referred to as driver/pickup probes. • The advantage of reflection probes is that the driver and pickup coils can be separately optimized for their intended purpose. • The driver coil can be made so as to produce a strong and uniform flux field in the vicinity of the pickup coil, while the pickup coil can be made very small so that it will be sensitive to very small defects.

45. Reference Standards

46. 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.

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

48. Reference Standards (cont.) Crack Standards:

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

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