Intensity And distribution Of Eddy Currents

Figure 4-10. Relative Intensity of Eddy Currents with Variations in Lift-Off

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 4-14 N=(D₀/d_i)² Where: N = Fill factor D_o = Outside diameter of test part d_i = Inside diameter of coil For example, if an encircling coil with an internal diameter of 2.25 inches were used to inspect 2.00-inch diameter rod, the fill factor would be: N=(D₀/d_i)²= (2.00 / 2.25)²= (0.889)² = 0.79. 4.2.2.2.4.1 For internal coils, electromagnetic (inductive) coupling is determined by the air gap between the external diameter of the coil and the internal diameter being inspected. Fill-factor is again calculated using the above formula, but in this case "d_i" is the inside diameter of the part and "D_o" is the outside diameter of the coil placed in the part. For example, if a coil with an external diameter of 1.5 inches is used to inspect tubing with an internal diameter of 1.6 inches, the fill factor is given by: N = (1.5/1.6)² = (0.9375)² = 0.88. 4.2.2.2.5 Coil Current. With all other factors constant, an increase in current flowing through the coil results in a higher magnetic field strength (H) applied to the inspection part in accordance with the following equation: H = 4 (3.14) N I/10 L = 4 (3.14) n I/ 10 Where: H = Magnetic field along axis of coil (oersteds) N = Number of turns I = Coil current (amperes) L = Length of coil (cm) n = Number of turns per unit length = N / L 4.2.2.2.6 Temperature. The temperature at which an inspection is performed affects both the electrical conductivity and the ferromagnetic properties of the inspection article. Electrical conductivity generally decreases with increasing temperature, and conversely increases with decreasing temperatures. The reduction at higher temperatures occurs because of the scattering of conduction electrons by atoms moving with increased thermal oscillations. Temperature effects on the ferromagnetic properties of a material are generally negligible with one exception. Above a specific temperature called the Curie temperature, (about 1400 F) ferromagnetic properties disappear. However, rarely is eddy current inspection performed above this temperature. Because of the thermal effects on conductivity, increasing temperature of the inspection article slightly decreases the intensity of eddy currents at the surface of a part and slightly increases the depth of penetration. Temperature variations also affect the inductance of the coil. The main concern is that temperature changes that occur during the eddy current test can produce responses that could either mask or be mistaken for flaws or variability in properties of interest such as coating thickness or conductivity. Therefore, during inspections, time should be allowed for the test system and the test part to stabilize to the ambient temperature. 4.2.3 Intensity And distribution Of Eddy Currents. 4.2.3.1 Intensity at Surface. The magnitude of the eddy currents at the surface of the inspection part is related to a combination of variables. One of these variables, the test system, is discussed in section 4.4. The other test variables and their influence are summarized in Table 4-4. In general, any change in a parameter that would tend to increase the intensity of surface eddy currents

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