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Chapter 17: D. Testing Methods 183Figure 17.3 Example of an inspection using X-ray.CoilCoil’smagneticfieldPart III.D.1Eddy current’smagnetic fieldEddycurrentsConductivematerialFigure 17.4 Eddy current method.also be used to make electrical conductivity and coating thickness measurements(Larson 2000).Eddy current can also be used for monitoring metallic structures such as aircraftfuselage and aircraft structural components. Some disadvantages of thistype of testing are that it works only with conductive materials and the surface ofthe material must be accessible. Other limitations exist such as the depth of penetration,which is bounded to a certain extent, and that the finish of the productcan cause bad readings. With such limitations it still has lots of advantages such asportability and the ability to provide immediate feedback (Soleimani et al. 2006).

184 Part III: Inspection and TestPulser/receiverOscilloscope or flawdetector screenTransducerInitial pulseBack surfaceechoCrackechoCrack0 2 4 6 8 10 12PlateFigure 17.5 The general inspection principle for ultrasonic testing.Part III.D.1Ultrasonic. Ultrasonic testing (UT) uses high frequency sound waves to conductexaminations and make measurements. The pulse echo technique is accomplishedby introducing sound into the test object so that reflections (echoes) arereturned to a receiver from internal imperfections or from geometrical surfacesof the part. It is typically used to detect subsurface defects, or defects originatingfrom surfaces not accessible without disassembly or removal. It can also be usedto detect laminations, lack of fusion, and corrosion of various materials (AircraftNDT) (NDT-ED 2008a). Figure 17.5 shows the general inspection principle.Some of the advantages of ultrasonic inspection that are often cited include(NDT-ED 2008a):• It is sensitive to both surface and subsurface discontinuities.• The depth of penetration for flaw detection or measurement issuperior to other NDT methods.• Only single-sided access is needed when the pulse-echo techniqueis used.• It is highly accurate in determining reflector position and estimatingsize and shape.• Minimal part preparation is required.• Electronic equipment provides instantaneous results.• Detailed images can be produced with automated systems.• It has other uses, such as thickness measurement, in addition toflaw detection.As with all NDT methods, ultrasonic inspection also has its limitations, whichinclude (NDT-ED 2008):• Surface must be accessible in order to transmit ultrasound.

Chapter 17: D. Testing Methods 183

Figure 17.3 Example of an inspection using X-ray.

Coil

Coil’s

magnetic

field

Part III.D.1

Eddy current’s

magnetic field

Eddy

currents

Conductive

material

Figure 17.4 Eddy current method.

also be used to make electrical conductivity and coating thickness measurements

(Larson 2000).

Eddy current can also be used for monitoring metallic structures such as aircraft

fuselage and aircraft structural components. Some disadvantages of this

type of testing are that it works only with conductive materials and the surface of

the material must be accessible. Other limitations exist such as the depth of penetration,

which is bounded to a certain extent, and that the finish of the product

can cause bad readings. With such limitations it still has lots of advantages such as

portability and the ability to provide immediate feedback (Soleimani et al. 2006).

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