1. DEPARTMENT OF MECHANICAL ENGINEERING 3RD SEMESTER MECHANICAL MATERIAL SCIENCE & METALLURGY YEAR-2014

2. “Eddy Current Testing”
PREPARED BY
GUIDED BY:
PROF.D.K. PATEL
24 Nov 1998

3. Eddy Current Testing
Electrical currents are generated in a conductive material by an induced alternating magnetic field. The electrical currents are called eddy currents because the flow in circles at and just below the surface of the material. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the material's conductive and permeability properties, can be detected with the proper equipment.
Eddy current testing can be used on all electrically conducting materials with a reasonably smooth surface.
The test equipment consists of a generator (AC power supply), a test coil and recording equipment, e.g. a galvanometer or an oscilloscope
Used for crack detection, material thickness measurement (corrosion detection), sorting materials, coating thickness measurement, metal detection, etc.

4. Principle of Eddy Current Testing (I)
When a AC passes through a test coil, a primary magnetic field is set up around the coil
The AC primary field induces eddy current in the test object held below the test coil
A secondary magnetic field arises due to the eddy current

5. Mutual Inductance (The Basis for Eddy Current Inspection)
The magnetic field produced by circuit 1 will intersect the wire in circuit 2 and create current flow. The induced current flow in circuit 2 will have its own magnetic field which will interact with the magnetic field of circuit 1. At some point P on the magnetic field consists of a part due to i1 and a part due to i2. These fields are proportional to the currents producing them.
The flux B through circuits as the sum of two parts.
B1 = L1i1 + i2M
B2 = L2i2 + i1M
L1 and L2 represent the self inductance of each of the coils. The constant M, called the mutual inductance of the two circuits and it is dependent on the geometrical arrangement of both circuits.

6. Principle of Eddy Current Testing (II)
The strength of the secondary field depends on electrical and magnetic properties, structural integrity, etc., of the test object
If cracks or other inhomogeneities are present, the eddy current, and hence the secondary field is affected.

7. Principle of Eddy Current Testing (III)
The changes in the secondary field will be a ‘feedback’ to the primary coil and affect the primary current.
The variations of the primary current can be easily detected by a simple circuit which is zeroed properly beforehand
The bridge circuit here is known as the Maxwell-Wien bridge (often called the Maxwell bridge), and is used to measure unknown inductances in terms of calibrated resistance and capacitance.
Calibration-grade inductors are more difficult to manufacture than capacitors of similar precision, and so the use of a simple "symmetrical" inductance bridge is not always practical. Because the phase shifts of inductors and capacitors are exactly opposite each other, a capacitive impedance can balance out an inductive impedance if they are located in opposite legs of a bridge, as they are here.
Unlike this straight Wien bridge, the balance of the Maxwell-Wien bridge is independent of source frequency, and in some cases this bridge can be made to balance in the presence of mixed frequencies from the AC voltage source, the limiting factor being the inductor's stability over a wide frequency range.
In the simplest implementation, the standard capacitor (Cs) and the resistor in parallel with it are made variable, and both must be adjusted to achieve balance. However, the bridge can be made to work if the capacitor is fixed (non-variable) and more than one resistor is made variable (at least the resistor in parallel with the capacitor, and one of the other two). However, in the latter configuration it takes more trial-and-error adjustment to achieve balance as the different variable resistors interact in balancing magnitude and phase.
Another advantage of using a Maxwell bridge to measure inductance rather than a symmetrical inductance bridge is the elimination of measurement error due to mutual inductance between two inductors. Magnetic fields can be difficult to shield, and even a small amount of coupling between coils in a bridge can introduce substantial errors in certain conditions. With no second inductor to react within the Maxwell bridge, this problem is eliminated.

8. Eddy Current Instruments
Voltmeter
Coil's
magnetic field
Coil
Eddy current's
magnetic field
Eddy
currents
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.
While it might actually be possible to detect some types of defects with this type of an equipment, most eddy current instruments are a bit more sophisticated. In the following pages, a few of the more important aspects of eddy current instrumentation will be discussed.
Conductive
material

9. Depth of Penetration
Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and flow is limited to the area of the inducing magnetic field. Eddy currents concentrate near the surface adjacent to an excitation coil and their strength decreases with distance from the coil as shown in the image. Eddy current density decreases exponentially with depth. This phenomenon is known as the skin effect.
Skin effect arises when the eddy currents flowing in the test object at any depth produce magnetic fields which oppose the primary field, thus reducing net magnetic flux and causing a decrease in current flow as depth increases. Alternatively, eddy currents near the surface can be viewed as shielding the coil's magnetic field, thereby weakening the magnetic field at greater depths and reducing induced currents.
The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic 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 (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.
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 ().

10. Three Major Types of Probes
The test coils are commonly used in three configurations
Surface probe
Internal bobbin probe
Encircling probe

11. Result presentation
The impedance plane diagram is a very useful way of displaying eddy current data. 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 and this energy loss shows up as resistance) and 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.
When a probe is placed on a magnetic material such as steel, something different happens. Just like with aluminum (conductive but not magnetic) eddy currents form which takes energy away from the coil and this shows up as an increase in the coils resistance. And, just like with the aluminum, the eddy currents generate their own magnetic field that opposes the coils magnetic field. However, you will note for the diagram that the reactance increase. 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 conductive will produce a change in the eddy current signal similar to that seen with aluminum.

12. Applications Crack Detection Material Thickness Measurements
Coating Thickness Measurements
Conductivity Measurements For:
Material Identification
Heat Damage Detection
Case Depth Determination
Heat Treatment Monitoring

13. Surface Breaking Cracks
Eddy current inspection is an excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known.
Successful detection 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.
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.
In the lower image, there is a flaw under the right side of the coil and it can be see that the eddy currents are weaker in this area.

14. Advantages of ET 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

15. Limitations of ET 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