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Overview of NDT

 

It is fact that there are inherent flaws in materials due to crystal lattice imperfections and dislocations however microscopic they may be. Manufacturing processes such as welding, casting, forging, surface treatment, etc. may cause further flaws or defects.

 

Materials are used under various conditions of stress, fatigue and corrosion, which may create additional defects or aggravate present ones. It has been established that most material failures occur because these defects reach dangerous proportions such that remaining parts of the materials could not withstand the stress they are subjected to, thus become ductile or brittle.

 

There is, therefore, a need to detect these flaws and evaluate them in terms of their nature, size and location. Further steps should be to assess (a) how severe and dangerous the flaws are in their present state, (b) whether they need to be removed by repairing the tested component, (c) if the component be scrapped, or (d) with known flaws, if the product can be allowed to go into service. These are done through inspection and testing.

 

One method of inspection is to subject the material or weld to destructive tests, which would provide information about the performance of that test object. The disadvantage of destructive testing is that, as the name implies, the test object is destroyed in the process. Therefore, testing methods have been developed to provide the information required of the test object without rendering it unfit for service.

 

These methods are referred to as non-destructive tests because they permit evaluation of the material or component without destroying it. Destructive testing of parts can be expensive and assumes that the untested parts are of the same quality as those tested. Non-destructive tests give indirect yet valid results and, by definition, leave the test object fit for its intended use.

 

There are a variety of NDT methods that can be used to evaluate the materials, components as well as completed welds. All NDT methods share several common elements.

 

These elements are:

 

• some source of probing energy or some type of probing medium
• discontinuity that must cause a change or alteration of the probing medium
• some means of detecting the change
• some means of indicating the change

• some means of observing and/or recording this indication so that an interpretation can be made.

 

The suitability of any NDT method for a given application will be determined by considering the above elements. The source of the probing energy or probing medium must be suitable for the test object and for detecting the defect or discontinuity sought. If present, a defect or 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 form some indication or otherwise be recorded. Finally, this indication must be reviewed in order for it to be interpreted and classified.

 

Various NDT methods have been developed, each one having advantages and limitations making it more or less appropriate for a given application. With the variety of NDT methods available, it is important to select the method that will provide the necessary results. A combination of different NDT tests may be applied to provide assurance that the material or component is fit for use.

 

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