ASNT

requirements. Nondestructive testing should not be used to obtain quality at the end of manufacturing. This approach will ultimately increase production costs. When used properly, nondestructive testing saves money for the manufacturer. Rather than costing the manufacturer money, properly implemented nondestructive testing will add profits to manufacturing. NONDESTRUCTIVE TEST METHODS To optimize nondestructive testing, it is necessary first to understand the principles and applications of all the methods. The following section brief ly describes major methods and the applications associated with them. Visual Testing Principles . Visual testing (Figure 4) is the observa- tion of a test object, either directly with the eyes or indirectly using optical instruments, by an inspector to evaluate the presence of surface anomalies and the object’s conformance to specifi- cation. Visual testing is the first nondestructive test method applied to an item. The test procedure is to clear obstructions from the surface, provide adequate illumination and observe. A prerequisite necessary for competent visual testing of an object is knowledge of the manufacturing processes by which it was made, of its service history, and of its potential failure modes, as well as related industry experience. Applications . Visual testing is widely used on a variety of objects to detect surface anomalies associated with various structural failure mechanisms. Even when other nondestructive tests are performed, visual tests often provide a useful supplement. When the eddy current testing of process tubing is performed, for example, visual testing is often performed to examine the surface more closely. The following discontinuities may be detected by a simple visual test: surface disconti- nuities, cracks, misalignment, warping, corrosion, wear, and dents. Liquid Penetrant Testing Principles . Penetrant testing (Figure 5) reveals discontinuities open to the surfaces of solid and nonporous materials. Indications of a wide variety

of discontinuity sizes can be found, regardless of the configuration of the test object and regardless of discontinuity orientations. Liquid penetrants seep into various types of minute surface openings by capillary action. The cavities of interest can be very small, often invisible to the unaided eye. The ability of a given liquid to f low over a surface and enter surface cavities depends on the following conditions: cleanliness of the surface, surface tension of the liquid, configuration of the cavity, contact angle of the liquid, ability of the liquid to wet the surface, cleanliness of the cavity, and size of the surface opening of the cavity. Applications . The principal industrial uses of penetrant testing include post-fabrication testing, receiving testing, in-process testing and quality control, testing for maintenance and overhaul in the transportation industries, in-plant and machinery maintenance testing, and testing of large components. The following are some of the typically detected discontinuities: surface discontinuities, seams, cracks, laps, porosity, and leak paths.

Figure 4 Visual testing using a portable videoscope.

Figure 5 Liquid penetrant indication of cracking.

CHAPTER 1

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Part 1