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| Simulated cine images of waves produced by magnetic resonance elastography. |
INTRODUCTION TO NONDESTRUCTIVE TESTING
Definition of Nondestructive Testing
Purposes of Nondestructive Testing
Rapid Growth and Acceptance of Nondestructive Tests
Classification of NDT Methods
Classification of NDT Methods (cont'd)DEFINITION OF NONDESTRUCTIVE TESTING
Nondestructive testing (NDT) has been defined as comprising those test methods used to examine an object, material or system without impairing its future usefulness. The term is generally applied to nonmedical investigations of material integrity.
Strictly speaking, this definition of nondestructive testing does include noninvasive medical diagnostics. Ultrasound, X-rays and endoscopes are used for both medical testing and industrial testing. In the 1940s, many members of the American Society for Nondestructive Testing (then the Society for Industrial Radiography) were medical X-ray professionals. Medical nondestructive testing, however, has come to be treated by a body of learning so separate from industrial nondestructive testing that today most physicians never use the word nondestructive.
Nondestructive testing is used to investigate the material integrity of the test object. A number of other technologies - for instance, radio astronomy, voltage and amperage measurement and rheometry (flow measurement) - are nondestructive but are not used to evaluate material properties specifically. Nondestructive testing is concerned in a practical way with the performance of the test piece - how long may the piece be used and when does it need to be checked again? Radar and sonar are classified as nondestructive testing when used to inspect dams, for instance, but not when they are used to chart a river bottom.
What Is Not Nondestructive Testing?
Nondestructive testing asks "Is there something wrong with this material?" Various performance and proof tests, in contrast, ask "Does this component work?" This is the reason that it is not considered nondestructive testing when an inspector checks a circuit by running electric current through it. Hydrostatic pressure testing is usually proof testing and intrinsically not nondestructive testing. Acoustic emission testing used to monitor changes in a pressure vessel's integrity during hydrostatic testing is nondestructive testing.
Another gray area that invites various interpretations in defining nondestructive testing is that of future usefulness. Some material investigations involve taking a sample of the inspected part for testing that is inherently destructive. A noncritical part of a pressure vessel may be scraped or shaved to get a sample for electron microscopy, for example. Although future usefulness of the vessel is not impaired by the loss of material, the procedure is inherently destructive and the shaving itself - in one sense the true "test object" - has been removed from service permanently.
The idea of future usefulness is relevant to the quality control practice of sampling. Sampling (that is, the use of less than 100 percent inspection to draw inferences about the unsampled lots) is nondestructive testing if the tested sample is returned to service. If the steel is tested to verify the alloy in some bolts that can then be returned to service, then the test is nondestructive. In contrast, even if spectroscopy used in the chemical testing of many fluids is inherently nondestructive, the process is destructive if the test samples are discarded after testing.
Hardness testing by indentation provides an interesting test case for the definition of nondestructive testing. Hardness testing machines look somewhat like drill presses. The applied force is controlled as the bit is lowered to make a small dent in the surface of the test piece. Then the diameter or depth of the dent is measured. The force applied is correlated with the dent size to provide a measurement of surface hardness. The future usefulness of the test piece is not impaired, except in rare cases when a high degree of surface quality is important. However, because the piece's contour is altered, the test is rarely considered nondestructive. A nondestructive alternative to this hardness test could be the use of electromagnetic nondestructive testing.
Discontinuity Detection
Nondestructive testing is not confined to crack detection. Other discontinuities include porosity, wall thinning from corrosion and many sorts of disbonds. Nondestructive material characterization is a growing field concerned with material properties including material identification and microstructural characteristics - such as resin curing, case hardening and stress - that have a direct influence on the service life of the test object. Nondestructive testing has also been defined by listing or classifying the various methods. This approach is practical in that it typically highlights methods in use by industry.
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