ASNT

Nondestructive Testing PART 1

Nondestructive testing (NDT) comprises those test methods used to examine or inspect a part, material, structure, or system without impairing its future usefulness (ASNT 1996). The term is applied to nonmedical investigations of material integrity. Nondestructive testing asks: “Is this material fit for service?” Various performance and proof tests, in contrast, ask: “Does this component work?” For example, circuit checking by running electric current or hydrostatic pressure proof testing is not considered NDT. Some material investigations involve taking a sample of the inspected part for testing that is inherently destructive. A noncrit- ical portion of a product may be removed and destructively tested mechanically or analytically. For example, a pressure vessel may be scraped or shaved to get a sample for electron microscopy. Although future usefulness of the vessel is not impaired by the loss of material, the procedure is inherently destructive and the shaving itself has been removed from service permanently. NDT methods are important tools employed in quality assurance and quality control procedures. NDT is not confined to discontinuity detection such as cracks, porosity, inclusions, or disbonds. Other issues include dimension measurements, such as wall thinning from corrosion, wear or environ- ment, leak detection, and strain measurements. Nondestructive material characterization is an important field concerned with material properties, including material identification and microstruc- tural characteristics that have a direct inf luence on the service life of the test object.

Nondestructive testing has also been defined by listing or classifying the various methods (ASNT 1996; Wenk 1987; DOD TM 55-1500-335-23). This approach is practical in that it typically highlights methods in use by industry. PURPOSES OF NONDESTRUCTIVE TESTING Since the 1920s, the art of testing without destroy- ing the test object has developed from a laboratory curiosity to an indispensable tool of production and in-service inspection. No longer is visual examination of materials, parts, and complete products the principal means of determining adequate quality. Nondestructive tests in great variety are in worldwide use to detect variations in structure, minute changes in surface finish, the presence of cracks or other physical discontinuities, to measure the thickness of materials and coating, and to determine other characteristics of industrial products. The various nondestructive testing methods are covered in detail in the literature, but it is always wise to consider objectives before selecting a method. What is the use of nondestructive testing? Why do thousands of industrial concerns buy the testing equipment, pay the subsequent operating costs of the testing, and even reshape manufac- turing processes to fit the needs and findings of nondestructive testing? Modern nondestructive tests are used (1) to ensure product integrity and, in turn, reliability;

CHAPTER 1

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Part 1