Nondestructive Testing (NDT)

Nondestructive Testing (NDT)

One of the purposes of an effective quality control program is to determine the suitability of a given base metal or a weld to perform its intended service.

One way to judge that suitability is to subject the base metal or weld to destructive tests which would provide information about the performance of that test object. The major disadvantage of such an approach is that, as the name implies, the test object is destroyed in the process.

Therefore, a number of tests have been developed to provide an indication of the acceptability of the test object without rendering it unusable for service.

These various tests are referred to as nandestructìve tests, because they permit the nondestructive evaluation (NDE) of the metal or component.

Even the destructive testing of a given percentage of parts can be expensive and assumes that the untested parts are of the same quality as those tested.

Nondestructive tests yield indirect, yet still valid, results and, by definition, leave the test object unchanged and ready to be placed in service if acceptable.

As mentioned, there are numerous nondestructive tests used to evaluate the base metals to be joined as well as the completed welds.

The most common test methods will be discussed, noting both their advantages and limitations, and applications.

However, all of these nondestructive test methods share several common elements.

These essential elements are summarized below:

(1) A source of probing energy or medium
(2) A discontinuity must cause a change or alteration of the probing energy
(3) A means of detecting this change
(4) A means of indicating this change
(5) A means of observing or recording this indication so

that an interpretation can be made The suitability of a particular nondestructive test for a given application will be determined by considering each of these factors. The source of the probing energy or probing medium must be suitable for the test object and the discontinuity of concern.

If present, a discontinuity must then be capable of somehow modifying or changing the probing medium.

Once changed, there must be some way of detecting these changes. These changes to the probing medium by the discontinuity must create an indication or otherwise be recorded.

Finally, this indication must be preserved somehow so that it can be interpreted.

As each of these nondestructive test methods are discussed, it is important to understand how they each provide the essential elements. This will aid in deciding which nondestructive test method is best suited for a particular application.

Over the years, numerous nondestructive test methods have been developed. Each one has associated with it various advantages and limitations making it more or less appropriate for a given application.

With the number of test methods available, it is important to select that method which will provide the necessary results.

In many cases, several different tests may be applied to provide adequate assurance that the material or component is satis factory.

Since so many tests exist, it would be difficult to mention each one in the context of this course.

Therefore, we will concentrate on those more common nondestructive test methods which are commonly used for the evaluation of base metals and weldments.

Those test methods to be discussed are noted, with their abbreviations in parentheses.

(1) Penetrant (PT)
(2) Magnetic Particle (MT)
(3) Radiographic (RT)
(4) Ultrasonic (UT)
(5) Eddy Current (ET)

While the welding inspector is usually not called upon to perform these tests, other than visual testing covered previously, it is important that he or she has a basic understanding of these other tests for several reasons.

First, the welding inspector should be aware of the advantages and limitations of these methods.

This will assist in deciding which test might be used to provide some additional information about the apparent quality of a material or weld. In that way, visual evaluation can be further substantiated by some additional testing.

Knowledge of the advantages and limitations will also help in determining if the nondestructive testing specialist doing the actual testing is applying the test properly.

Since the welding inspector may be called on to monitor the performance of, or maintain records about these tests, this knowledge should aid in understanding the results.

As each of the various tests is discussed, there will be a description of the advantages and limitations, as well as the basic principles of operation.

The necessary equipment for each test will also be discussed, and there will also be mention of some of the typical applications of each of the methods.



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