Non-Destructive Testing of Materials IM 714 Dr Yehia M. Youssef 1
Electromagnetic Testing Methods Eddy current testing involves the use of alternating magnetic fields and can be applied to any conductor. Leakage flux testing involves the use of a permanent magnet, DC or AC electromagnetic fields, and can be applied only to ferromagnetic materials. In eddy current testing, the alternating magnetic field sets up circulating eddy currents in the test part. Any parameter that affects the electrical conductivity of the test area can be detected with the eddy currents. With the flux leakage technique, any discontinuity that produces lines of leakage flux in the test area can be detected. 2
Electromagnetic Testing Methods The magneto-elastic technique is used for characterizing and determining the amount of residual stress in magnetic materials by measuring magnetic or Barkhausen noise. These techniques can be combined with other methods, such as ultrasonic testing and laser dimensional measuring to achieve multifunction highspeed testing of oil field drilling pipes and other piping systems that are subject to stringent overall quality requirements. High-speed automatic testing is possible using multiple NDT methods because they can be electronically gated & discriminated to evaluate a large number of variables simultaneously with PCs. 3
Eddy Current Inspection is based on the principles of electromagnetic induction and is used to identify or differentiate among a wide variety of physical, structural, and metallurgical conditions in electrically conductive ferromagnetic and non-ferromagnetic metals and metal parts. Eddy current inspection can be used to: Measure or identify such conditions and properties as electrical conductivity, magnetic permeability, grain size, heat treatment condition, hardness, and physical dimensions. Detect seams, laps, cracks, voids, and inclusions. 4
Sort dissimilar metals and detect differences in their composition, microstructure, and other properties. Measure the thickness of a nonconductive coating on a conductive metal, or the thickness of a nonmagnetic metal coating on a magnetic metal. Because eddy currents are created using an electromagnetic induction technique, the inspection method does not require direct electrical contact with the part being inspected. The eddy current method is adaptable to high-speed inspection and, because it is non-destructive, can be used to inspect an entire production output if desired. 5
The method is based on indirect measurement, and the correlation between the instrument readings and the structural characteristics and serviceability of the parts being inspected must be carefully and repeatedly established. 6
When an alternating current is used to excite a coil, an alternating magnetic field is produced and magnetic lines of flux are concentrated at the centre of the coil. Then, as the coil is brought near an electrically conductive material, the alternating magnetic field penetrates the material and generates continuous, circular eddy currents as shown in Figure 3.1. Larger eddy currents are produced near the test surface. As the penetration of the induced field increases, the eddy currents become weaker. The induced eddy currents produce an opposing (secondary) magnetic field. 7
This opposing magnetic field, coming from the material, has a weakening effect on the primary magnetic field and the test coil can sense this change. In effect, the impedance of the test coil is reduced proportionally as eddy currents are increased in the test piece. A crack in the test material obstructs the eddy current flow, lengthens the eddy current path, reduces the secondary magnetic field, and increases the coil impedance. 8
Figure 3.1 Eddy current principle. Primary field of test coil enters the test part, generates eddy currents that generate second field. Strength of the eddy currents decreases with depth of penetration. 9
If a test coil is moved over a crack or defect in the metal, at a constant clearance and constant rate of speed, a momentary change will occur in the coil reactance and coil current. This change can be detected, amplified, and displayed by an eddy current flaw detector. Changes in magnetic flux density may also be detected by Hall effect devices, amplified, and displayed on PCs and laptop computers. A block diagram of a simple eddy current tester is shown in Figure 3.2. As shown in the figure, an AC generator is used to drive the test coil. 10
As the test coil passes over various defects, the coil impedance and AC voltage changes. The AC voltage is converted to DC voltage by a diode rectifier and compared to a stable DC voltage of opposite polarity produced by a battery. With the meter properly zeroed at the start, changes in coil voltage can be measured. The block diagram represents the most elementary form of eddy current instrument. As such, it would not be capable of detecting minute discontinuities that can be reliably detected with today s more sophisticated instruments. 11
Figure 3.2 Schematic diagram of basic eddy current instrument. 12
Figure 3.3 shows an eddy current test coil located at distance A above a conductive material. The coil is considered to be an ideal coil with no resistive losses. The impedance of the coil in the complex plane shown is a function of the conductivity of the material at distance A. If the material in the figure was an insulator, its conductivity (the reciprocal of resistivity) would be infinite. The coil s reactance would remain unchanged at point P1. However, if the material is a conductor, eddy current losses will occur. 13
Figure 3.3 The effect of conductivity on coil impedance. 14
Principal elements of a typical system for eddy current inspection of bar or tubing. 15
The coil will signal this change by increases in resistive losses with a simultaneous decrease in reactance, and the operating point of the system will shift to P2. When the conductivity of the material approaches infinity (a superconductor), the resistive losses will again approach zero. With very highly conductive materials, eddy current flow will be very high and the strong secondary field will reduce the reactance of the coil to point P3. Since the complex plane approaches a semicircle as conductivity varies from zero to infinity, it can be concluded that the conductivity of a material has the greatest effect on coil impedance. 16
Coil impedance is dependent on the vector sum of the coil s inductive reactance and the test part s resistance to the eddy current field. Another important influence on coil impedance is the clearance or lift-off between the coil and the conductive material surface. At great distances above the surface, the field of the coil does not reach the surface of the test piece or induce eddy currents in it. In this case, coil impedance remains unchanged regardless of any conductivity changes in the material. 17
However, as the coil approaches the surface in the stepwise fashion illustrated in Figure 3.4, stronger eddy currents are induced in the material, producing the family of impedance plane curves shown. If A is held constant and conductivity varies, a circular curve is produced. As A approaches zero, the diameter of the circle increases. Due to the need for a wear surface, geometry, and finiteness of the coil, A cannot be actually zero. 18
Figure 3.4 The effect of liftoff or probe clearance on coil impedance. 19
If the conductivity of the material is held constant and A is changed, the straight line from point P1 to A o is generated. When attempting to measure changes in conductivity, changes in spacing or lift-off are highly undesirable. In order to minimize variations in lift-off, eddy current coils may be recessed a short distance into the eddy current probe head, and the probe head may be spring loaded to maintain surface contact. However, since the lift-off effect is linear over a limited probe clearance range, eddy current probes can be designed to measure nonconductive coating thickness over uniformly conductive materials. 20
Coil impedance can be calculated for any known combination of conductivity and probe clearance. In many cases, we do not want to measure the effect of probe clearance or conductivity on coil impedance. Instead we want to locate and measure the effect of discontinuities on coil impedance and probe output. Figure 3.5 shows the effect that cracks and defects have on coil impedance. When the coil passes over a crack, the impedance of the coil varies by the value shown by the vector point P1. A significant change in vector direction occurs and the vector points toward P o when probe clearance changes. 21
2 2 Z= R + X L Z: Impedance XL: Inductive Reactance R: Arc Resistance of the wire Figure 3.5 The effect of a crack on coil impedance. 22
The relationship shown at point P1 applies to a specific value of conductivity. If the conductivity value decreases to point P2, vector direction differences are less significant and it is harder to differentiate between the impedance change caused by the crack and the impedance change that is caused by probe clearance. The planar diagram shows that it is more difficult to distinguish between defect indications and lift-off indications with low conductivity materials. 23
So far, we have described how eddy current resistance (heating) losses, conductivity, probe spacing, and defects affect coil impedance; no mention has been made of the effect of frequency on coil impedance. We know that conductive (inductive) reactance and impedance of the coil are affected by test coil frequency in accordance with Eq 3.1: X L =2πfL. (3.1) where X L = the inductive reactance of the coil in ohms (Ω) f = test frequency in Hertz (Hz) L = coil inductance in Henrys (H) 24
Equation (3.1) shows that both inductance and frequency directly affect coil impedance. Thus, conductivity and frequency have exactly the same effect on coil impedance. Figure 3.6 shows the effect of holding frequency constant and varying conductivity and vice versa. Assuming that material conductivity is reasonably constant, we can use the frequency relationship to our advantage. 25
For a particular material conductivity, a test coil frequency may be selected that will create a favourable operating point for detecting flaws while differentiating against non-relevant indications. The frequency f g is the limiting frequency or the point where further increases in frequency will not increase the ohmic losses in the test material. When material conductivity is known, optimum test coil operating frequency can be calculated or determined experimentally. 26
Figure 3.6 The effects of (a) conductivity and (b) frequency on coil impedance. 27
Encircling coils are used more frequently than surface-mounted coils. With encircling coils, the degree of filling has a similar effect to clearance with surface-mounted coils. The degree of filling is the ratio of the test material cross-sectional area to the coil cross-sectional area. Figure 3.7 shows the effect of degree of filling on the impedance plane of the encircling coil. For tubes, the limiting frequency (point where ohmic losses of the material are the greatest) can be calculated precisely from Eq. (3.2): 28
where f g = limiting frequency f g σ = conductivity = d i = inner diameter 5056 σd wμ (3.2) w = wall thickness μ = relative permeability For most applications, two coils are employed the primary (field) coil generates the eddy currents and the secondary (pickup) coil detects the change in coil impedance caused by the changes in conductivity and permeability. i 29
Eddy currents are generated in the material in accordance with Maxwell s law, which states that every part of an electric circuit is acted on by a force tending to move it in such a direction as to enclose the maximum amount of magnetic flux. Furthermore, according to Lenz s law, these eddy currents must flow in the opposite direction to the current in the field coil. The magnitude of the eddy current depends on frequency of the field current, conductivity and permeability of the test material, and geometry of the test part. 30
Because of the skin effect (eddy current heating), the depth of penetration of eddy currents is relatively small and can be calculated from Eq. (3.3): d where d p = depth of penetration f = frequency σ = conductivity μ= permeability p = 1 πfσμ (3.3) 31
Standard depths of penetration as a function of frequencies used in eddy current inspection for several metals of various electrical conductivities 32
Eddy currents weaken the original magnetic field in the interior of the material while strengthening the magnetic field outside the material, which is in opposition to the test coil s magnetic field. If a defect is present in the sample, the magnetic field just outside the defect region is reduced and the magnetic flux through the test coil and the test coil voltage increases. Figure 3.8 shows a hypothetical defect dipole that can be used to illustrate the effect a defect has on the test coil. 33
Figure 3.8 Simulation of a defect by a hypothetical defect dipole. 34
Most defects may be thought of as an infinite series of magnetic dipoles. The single dipole current path is represented by the infinitely small circular current whose direction is indicated by x going into the paper and going out of the paper. Eddy currents generated by the test coil are diverted by the magnetic field of the dipole; the external magnetic field is weakened, the magnetic field of the coil is strengthened, and the coil voltage is increased. 35
Advantages and Limitations Eddy current inspection is extremely versatile, which is both an advantage and a disadvantage. The advantage is that the method can be applied to many inspection problems provided the physical requirements of the material are compatible with the inspection method. In many applications, however, the sensitivity of the method to the many properties and characteristics inherent within a material can be a disadvantage; some variables in a material that are not important in terms of material or part serviceability may cause instrument signals that mask critical variables or are mistakenly interpreted to be caused by critical variables. 36
Figure 3.16 Absolute and differential (self-comparison) encircling and surface coils. Courtesy of Institut Dr. Foerster. 37
Figure 3.17 Differential surface coil showing that primary and detector coils share the same primary field. Note that crack distorts the eddy current path. Courtesy of Institut Dr. Foerster. 38
Figure 3.18 Conventional and transmission-type eddy current methods. Courtesy of Institut Dr. Foerster. 39
Figure 3.19 Encircling coil arrangement. Courtesy of Institut Dr. Foerster. 40
Flux Leakage Sensing Probes Flux leakage sensors are designed to measure the leakage flux emanating from the surface discontinuities in magnetized ferromagnetic materials. The ferromagnetic material can be continuously magnetized or contain a residual magnetic field. The majority of these sensors are inductive coil sensor or solid-state Hall effect sensors. Magnetic powder, magnetic diodes, and transistors, whose output current or gain change with magnetic field intensity, and to a lesser extent, magnetic tape systems can also be used. 41
Flux Leakage Sensing Probes Figure 3.20 Bobbin-type differential coil for scanning inner surface. 42
Flux Leakage Sensing Probes For an induction coil to detect a magnetic field, the magnetic field must be alternating or pulsating, or the coil must be moved through the magnetic field at a reasonable rate of speed. Various absolute and differential surface and encircling coil arrangements were illustrated in Figures 3.16 through 3.19. Figure 3.20 illustrates the design and coil arrangement of a differential-type inside or annular coil. 43
Flux Leakage Sensing Probes Some coil parameters that would affect the coil s ability to pick up or detect small leakage flux fields are: Coil diameter Coil length Number of turns of wire Permeability of core material Coil orientation 44
s Figure 3.32 Eddy current techniques commonly used by Institut Dr Foerster for the detection of flaws in wires, bars, pipes, and tubing. Courtesy of Institut Dr. Foerster. 45
Eddy Current Applications Applications involving physical parameters first and permeability last are: 1.Detect and determine the severity of various surface cracks (stress, hardening, grinding, etc.), weld seams, laps, pits, scabs, porosity, voids, inclusions, and slivers. 2.Determine seam and seamless tubing integrity by measurement of % OD wall loss, inter-granular corrosion, seam cracks, splits, and so on. 3.Measure flaws in graphite composites, aluminium, and titanium. 4.Detect and measure flaws in fastener holes. 5.Measure coating and plating thickness. Measure nonconductive coatings on conductive materials. 46
Eddy Current Applications 6. Measure nonmagnetic conductive sheet thickness. Measure dimensional differences in machined, formed, or stamped parts. 7. Determine the integrity of wire cable. Detect and locate broken strands. 8. Detect wanted or unwanted metals in nonmagnetic materials. There is a broad range of metal detectors or treasure finders that can be placed in this category. 9. Determine metal powder mixture ratios and the degree of sintering in metal powder parts. 10. Determine the hardness and depth of case hardening in bearing rings and other parts. 47