ASNT

Applications . The principal industrial uses of radiographic testing involve testing of castings and weldments, particularly where there is a critical need to ensure freedom from internal discontinuities. Radiographic testing is often specified for castings, forgings, and for weldments. The method can also be used on forgings and mechanical assemblies, although with mechanical assemblies radiographic testing is usually limited to testing for conditions and proper placement of components. Radiographic testing is used to detect voids, inclusions, lack of fusion, cracks, corrosion, porosity, leak paths, missing or incomplete Principles . Based on electromagnetic induction, eddy current testing is the best known of the techniques in the electromagnetic test method. Eddy current testing is used to identify or differen- tiate among a wide variety of physical, structural, and metallurgical conditions in electrically conductive ferromagnetic and nonferromagnetic metals and metal test objects. The method is based on indirect measurement and on correlation between the instrument reading and the structural characteristics and serviceability of the test objects. With a basic system, the test object is placed within or next to an electric coil in which high-frequency alternating current is f lowing. This excitation current establishes an electromagnetic field around the coil. This primary field causes eddy currents to f low in the test object because of electromagnetic induction (Figure 9). Inversely, the eddy currents affected by all characteristics (conductivity, permeability, thickness, discontinuities, and geometry) of the test object create a secondary magnetic field that opposes the primary field. This interaction affects the coil impedance and can be displayed in various ways. Eddy currents f low in closed loops in the test object. Their two most important characteris- tics, amplitude and phase, are inf luenced by the arrangement and characteristics of the instrumen- tation and test object. For example, during the test of a tube, the eddy currents f low symmetrically in the tube when discontinuities are not present. However, when a crack is present, then the eddy components, and debris. Eddy Current Testing

Figure 7 Infrared thermography of automatic transfer switches for emergency diesel generator. Hot spots appear bright in thermogram (inset).

Radiation source

Test object

Void

Discontinuity images

Image plane

Figure 8 Representative setup for radiographic testing.

CHAPTER 1

12

Part 1