Machinery's Handbook, 31st Edition
Nondestructive Testing 1629 Penetrant Testing (PT): Widely used to reveal surface discontinuities on numerous ma- terials—all metals (including aluminum, titanium, magnesium, and other nonferrous compounds), glass, ceramics, and various composites—penetrant testing works on the principle of capillary attraction. This surface tension phenomenon permits liquids to be drawn into tight crevices, regardless of orientation or gravitational forces. After the test area is cleaned, a dye concentrate is applied to and left on the material for a specific (dwell) time. When the penetrant is removed, the part usually is dried, and a developer is applied. The developer acts as an absorbent, drawing the remaining penetrant out of discontinuities open to the surface. Observing the test surface under appropriate lighting immediately after application of the developer assists proper interpretation of indications, though the final evaluation cannot be performed before the minimum devel - oper dwell time has elapsed. Utilizing the appropriate method as shown in Table 2, very large parts can be tested, or numerous small parts, such as turbine and compressor blades, can be processed simultaneously. Table 2. Classification of Penetrant Materials and Processes Type I Fluorescent penetrants May be used with all developers Type II Visible (color contrast) penetrants Nonaqueous and aqueous developers Method A Water-washable visible dye penetrants Water-washable fluorescent penetrants Method B Post-emulsified visible or fluorescent Lipophilic emulsifier (oil-based) Method C Solvent removable Visible or fluorescent penetrants Method D Post-emulsified fluorescent penetrants Hydrophilic emulsifier (water-based) Developers Dry powder, nonaqueous Water suspended, water soluble Note: Sensitivity levels available are 1, 2, 3, and 4 (4 is the highest). PT is versatile enough to be used in the field with or without electricity, is relatively low cost, and is easy to use. However, it is time-consuming and usually requires additional cleaning of the part after testing is complete. Eddy Current Testing (ET): This sophisticated NDT method can be used to detect conductivity variations; discontinuities; material thickness, including plating or cladding on a base material; and spacing between conductive layers. Based on the principle of electromagnetism, ET uses the inductive properties of alternating current flowing through a primary coil to develop an alternating magnetic field. Interruption of this magnetic field causes an impedance change in the coil, recorded as a phase change by the instrumentation. In theory, ET is much more complex; it was not until the early 1980s that explanatory material was produced that made this process understandable to laypersons. While ET is a high-speed process, it has a limited depth of penetration, depending on such factors as the test frequency, type of coil, and conductivity of materials being tested. Other NDT Methods.— In addition to the five primary NDT methods discussed above, there are six other methods recognized by the American Society of Nondestructive Test- ing (ASNT), including visual testing. Each has unique advantages and limitations. Visual Testing (VT): Being the first inspection, visual acuity examination generally is required for NDT certification. Many factors can affect VT, including cleanliness, brightness, glare, surface condition, shape, size, and temperature of an object. While the most important instrument in visual testing is the human eye, direct visual aids, such as light sources, magnifiers, cameras, and measuring devices are commonly used to verify conformance to industry codes, standards, and specification requirements.
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