Nondestructive Testing Machinery's Handbook, 31st Edition
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Nondestructive Testing Nondestructive testing (NDT) methods are used to examine a material, component, or assembly to measure thickness, determine structure, and/or search for surface or internal cracks, material flaws, or other nonhomogeneities. “Nondestructive” refers to a method of examination that does not affect or impair the use of the subject for its intended purpose. NDT is particularly useful in the welding process. Nondestructive Testing Methods.— The primary NDT methods, introduced below, in- clude ultrasonic, radiography, magnetic particle, penetrant, and eddy current. Excellent ref- erences, including books and standards, as well as certification and qualification programs, provide in-depth information on the principles and applications of each approach. Ultrasonic Testing (UT): The workhorse of NDT, ultrasonic testing utilizes high-fre- quency sound waves. An electronic pulser/receiver drives a transducer, which sends ul- trasonic sound waves through a test subject. These waves travel at the same distinct speed for any given material or medium that has matter (see Table 1). The time and distance that sound waves travel through a substance and echo off a reflecting surface can be ana - lyzed to determine medium characteristics, measure thickness, and detect and evaluate flaws. Table 1. Material Velocities
The calibration of the instrumentation is critical and must be carried out with reference blocks of the same material and velocity. This allows accurate measurement of the time and distance that a sound wave has traveled to and from a reflecting surface. Whether measuring a backwall for thickness or locating the depth and size of discontinuities, the accuracy of test results is only as reliable as the calibration. For the plate shown in Fig. 1, calculation of the angle, thickness, and sound path distance are required; these are ob- tained with formulas frequently used for ultrasonic testing.
Fig. 1. Shear Wave Welding Inspection Standard formulas use the refracted angle, thickness, and sound path distance to calcu- late the flaw depth and location. Other formulas are used to calculate acoustic impedance, nearfield, wavelength, and angular relationships. For instance, a formula developed from Snell’s law is used to calculate incident and refracted angles. Radiographic Testing (RT): Favored by many industries, due to the insight provided, radiographic testing is another method of detecting subsurface discontinuities and other material variations. It involves transmission of radiation through the material being tested, with the results recorded on photographic film or the digital equivalent. In general, areas of reduced thickness or lower-density material absorb less radiation, and more is
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