Electromagnetic Testing Chapter 3- Electromagnetic Testing

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3.18 ELECTROMECHANICAL ACOUSTIC TRANSDUCERS (EMATS) Weld Inspection Systems, Inc. (WIS, Inc.) applies EMAT technology to the nondestructive evaluation (NDE) needs of industry. EMATs were developed in the 1970s and have become the technique of choice for many real-time, high-speed, noncontacting, ultrasonic analyses in conductive metals. The EMATs system requires specialized high-current, high-voltage electronics. Standard ultrasonic techniques have traditionally been used to detect cracks, thin spots, corrosion, disbonds, laminations, blowholes, and voids. The ultrasonic method has also been used to inspect welds and measure remaining wall thickness. Charlie Chong/ Fion Zhang
EMATs overcome many common ultrasonic problems associated with UT couplants by using the interaction between eddy currents and magnetic fields to generate UT waves directly in the test piece.When static or pulsed magnetic fields are applied to the test piece, the driven EMAT circuit induces eddy currents in the test piece. The interaction of the eddy currents with the magnetic field induces stresses in the test piece, which generates ultrasonic waves that are detected by the transducer. A high-frequency focused-beam EMAT or flaw detection on the ID of a cylindrical part is shown in Figure 3.43. In this case, the transducer generates angle beam shear waves. Charlie Chong/ Fion Zhang
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Electromagnetic Testing Chapter 3-
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Submarine Applications Charlie Chon
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Fion Zhang at Shanghai 6th April 20
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IVONA TTS Capable. Charlie Chong/ F
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Chapter 3. ELECTROMAGNETIC TESTING
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Barkhausen effect The Barkhausen ef
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Barkhausen Noise Charlie Chong/ Fio
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Figure 3.1 Eddy current principle.
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Figure 3.2 Schematic diagram of bas
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Figure 3.3 The effect of conductivi
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Figure 3.3 The effect of conductivi
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Another important influence on coil
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If the conductivity of the material
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Figure 3.5 The effect of a crack on
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FIGURE 8-49 Conductivity curve: (a)
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So far, we have described how eddy
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Figure 3.6 The effects of (a) condu
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3.1.2 Limiting Frequency f g of Enc
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Figure 3.7 The effect of degree of
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Charlie Chong/ Fion Zhang
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Eddy Current Heating Charlie Chong/
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Eddy Current Heating Charlie Chong/
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Eddy currents weaken the original m
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3.2 MAGNETIC FLUX LEAKAGE THEORY Wh
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3D Magnetic Field Charlie Chong/ Fi
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3D Magnetic Field Fig.1 THE PRINCIP
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Based on what we have learned about
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Charlie Chong/ Fion Zhang Figure 3.
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Magnetic Field - Simple Semi Ellipt
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Flux leakage sensors have small dia
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The frequency of the alternating fi
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Figure 3.14 Rotating magnet arrange
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3.3 EDDY CURRENT SENSING PROBES For
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As the probe moves over the defect,
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Figure 3.17 Differential surface co
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Figure 3.19 shows an encircling coi
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The application of eddy current sen
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Baffles and tube sheets. Charlie Ch
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Another important consideration for
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3.4.1 Induction Coils For an induct
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If the diameter of a surface coil i
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3.4.2 Hall Effect Sensors Hall effe
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Figure 3.21 illustrates the Hall ef
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Hall voltage Charlie Chong/ Fion Zh
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Since a 1 gauss (G) field produces
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Figure 3.22 Effect of magnetic fiel
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3.5 FACTORS AFFECTING FLUX LEAKAGE
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Defect & Flux Distribution Charlie
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The distance between adjacent defec
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3.6 SIGNAL-TO-NOISE RATIO The signa
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With flux leakage testing, signal-t
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As shown in Eq. (3.6), test frequen
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The characteristic frequency for a
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Charlie Chong/ Fion Zhang
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Figure 3.23 Magnetization or magnet
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By taking the product of B and H fo
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The rate of change of the opposing
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Figure 3.25 Encircling coils establ
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Figure 3.27 DC-MFL sensor illustrat
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Fill factors apply only to encircli
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For best results with encircling co
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3.11 INSTRUMENT DESIGN CONSIDERATIO
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Charlie Chong/ Fion Zhang Figure 3.
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TABLE 3.2. Comparison of Instrument
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Some general categories of equipmen
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Figure 3.29 UniWest-454 EddyView mu
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Scanner operation uses a separate c
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Figure 3.30 E-Lab model US-450 full
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Page 155 and 156: It is possible to provide 100% eddy
Page 157 and 158: Figure 3.33 ET testing of hot coppe
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Page 161 and 162: 3.14 MAGNETIC FLUX LEAKAGE TESTING
Page 163 and 164: ASTM E 570 describes magnetic flux
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Page 179 and 180: 3.17.1 Introduction In 1911, Dr. He
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Page 187 and 188: Figure 3.38 A comparison of Barkhau
Page 189 and 190: Figure 3.39 Three-channel Rollscan
Page 191 and 192: The appropriate depth of measuremen
Page 193 and 194: 3.17.5 Calibration and Testing Bark
Page 195 and 196: Comparative studies by Kirsti Titto
Page 197 and 198: 3.17.6 Current Applications Barkhau
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Page 225 and 226: Other applications include: • Ins
Page 227 and 228: Eddy Current Testing at High Temper
Page 229 and 230: Good Luck Charlie Chong/ Fion Zhang
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