ASNT

current f low is impeded and changed in direction, causing significant changes in the associated electromagnetic field. Applications . An important industrial use of eddy current testing is on heat-exchanger tubing. For example, eddy current testing is often specified for thin-wall tubing in pressurized water reactors, steam generators, turbine condensers, and

air-conditioning heat exchangers. Eddy current testing is also used in aircraft maintenance. The following are some of the typical material characteristics that may affect conductivity and be evaluated by eddy current testing: cracks, inclusions, dents, and holes; grain size; heat treatment; coating and material thickness; composition, conductivity, or permeability; and alloy composition. Magnetic Particle Testing Principles . Magnetic particle testing (Figure 10) is a method of locating surface and near-surface discontinuities in ferromagnetic materials. When the test object is magnetized, discontinuities that lie in a direction generally transverse to the direction of the magnetic field will cause a magnetic f lux leakage field to be formed at and above the surface of the test object. This leakage field and hence, a discontinuity, is located with fine ferromagnetic particles applied over the surface, with some of the particles drawing together over the discontinuity and tracing it. This outline generally indicates the discontinuity’s location, size, shape, and extent. Magnetic particles are applied over a surface as dry particles or as wet particles in a liquid carrier such as water or oil. Applications . The principal industrial uses of magnetic particle testing include final, receiving and in-process testing; testing for quality control; testing for maintenance and overhaul in the transportation industries; testing for plant and machinery maintenance; and testing of large components. Some discontinuities typically detected are surface discontinuities, seams, cracks, and laps. Ultrasonic Testing Principles . In ultrasonic testing (Figure 11), beams of acoustic waves at a frequency too high to hear are introduced into a material for the detection of surface and subsurface discontinuities. These acoustic waves travel through the material with some energy loss (attenuation) and are ref lected and refracted at interfaces. The echoes are then analyzed to define and locate discontinuities. Applications . Ultrasonic testing is widely used in metals, principally for thickness measurement and discontinuity detection. This method can be used

Primary electromagnetic field

Direction of primary alternating current

Coil in eddy current probe

Induced field

Induced field

Direction of eddy current

Conducting test object

Eddy current intensity decreases with increasing depth

(a)

(b)

Figure 9 Electromagnetic testing: (a) representative setup for eddy current test; (b) in-service detection of discontinuities.

CHAPTER 1

13

Part 1