NDT Terminology

There seems to be a tremendous need to become familiar with the NDT methods available.

There are several issues that need to be addressed when it comes to requesting nondestructive testing (NDT) namely type of inspection, type of discontinuity to be detected, probability of detection (POD), technical requirements, and personnel requirements.

“What’s in a name?” a famous phrase that can have so much meaning, or in some cases so little meaning. There seems to be a tremendous need, not just in the aviation maintenance community, but across the board, to become familiar with the NDT methods available and the limitation of each of those disciplines.

Over the years some terms such as “Zyglo,” “Magnaflux, and “X-ray,” have become commonplace although they are not exactly correct. Zyglo and Magnaflux are actually trade names, related to penetrant and magnetic particle inspections respectively. Just as “Xerox” many times is used to denote producing a facsimile so too are some trade names related to NDT. X-ray simply defines a specific form of electromagnetic radiation with a wavelength in the range between gamma and ultraviolet radiation, and is often confused with the proper term of “radiography.”

Select the Right Inspection

The point here is to highlight the need to be able to provide the appropriate technical information for requested inspections. It isn’t uncommon for maintenance processes to request an NDT inspection, and not know what type of inspection is needed. An example of such would be a request made for an eddy current inspection to be performed on a carbon-fiber panel (without conductive mesh); one problem exists, it’s not conductive; it can’t be done. Additionally, typically the inspection required for laminar composites is sonic or ultrasonic. There are many other scenarios where this occurs. An excellent source is available to help individuals select the appropriate NDT discipline — “Choosing NDT Applications, Costs, and Benefits of Nondestructive Testing in Your Quality Assurance Program” (American Society for Nondestructive Testing, 1987) — a long title but an invaluable tool for the NDT user and in some cases the practitioner.

Methods and Certification

It’s been stated on many occasions that NDT is not a panacea, and no single method will provide all the possible relevant data for a specific application. This is only compounded when an inappropriate NDT test is requested, typically due to lack of knowledge. Generally it isn’t up to the NDT technician to select the appropriate discipline to be used. Several issues need to be considered in determining the proper NDT method which include the following:

a. Minimum size of discontinuity to be detected

b. Type of material being tested

c. Fabrication process

d. Orientation of discontinuity

e. Direction of propagation

f. POD (probability of detection)

Another issue that is often overlooked is the certification and qualification of the personnel performing the NDT tasks. In most circumstances the certification requirements are listed in the governing technical documentation. As an example, it is not uncommon for personnel to be required to be certified in accordance with ASNT’s SNT-TC-1A as a minimum Level II technician in order to perform and evaluate the results of any inspection. Often other certification requirements come into play (NAS 410, ATA 105, etc.); however it is up to the user to determine that the certifications are appropriate to the task requested. There are considerable differences in the certification program as the following shows:

The Aerospace Industries Association’s National Aerospace Standard for Certification and Qualification for Nondestructive Test Personnel, NAS 410, requires 80 hours total formal classroom training for Level II certification in ultrasonics, and 1,600 hours of experience.

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