NDT Terminology

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

The Air Transport Association of America Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods, ATA 105, requires only 40 hours total formal classroom training for Level II certification in ultrasonics and 1,440 hours of experience.

Again, “Level II” can be misleading if one isn’t aware of the certification requirements.

Additionally there are different requirements for the physical (eye) exams. It should be evident that there is a need to assure proper certification so that it meets the intended requirements.

A competent Level II technician should always request the appropriate technical data — which provides direction related to the task, and will in most cases be able to determine if the requested task is applicable to the technical documentation.

Probability of Detection (POD)

Probability of detection (POD) is another issue that is of concern and should be considered prior to the inspection process. This is not an issue that would be determined by the practitioner nor the user, but by the developers of the actual testing requirements, i.e. engineering or manufacturing. This value is very important in determining the proper NDT method. The objective is to have a high POD and a high confidence level. Basically we want to be as certain as possible that the inspection is capable of detecting the discontinuity of interest. A random, “shoot-from-the-hip” approach is obviously not the correct way to tackle an inspection problem.

Not too long ago a client contacted the company to schedule an “NDT inspection” on the fuselage of an aircraft. When questioned as to what kind of “NDT inspection,” the client had no response — he would have to get back to us. Later, came the information, it was an eddy current test. When the technician showed up with the eddy current unit, various probes, and several reference standards — it was to no avail. The actual inspection requirement was for bond testing on a composite panel.

This wasn’t the client’s fault completely, the service company didn’t request enough information, and had it been an eddy current test, was it high, medium, or low frequency? What was the inspection for — conductivity, flaw detection?

Hopefully it can be seen that there is a need to use the appropriate terminology related to NDT, and that there are several considerations in selecting a specific NDT method. It is usually a good idea to confer with someone experienced in the field; usually an NDT Level III or an experienced engineer can provide sound guidance if there are any questions.

The above chart is a sample quick reference only and does not purport to address all processes, materials, or conditions. Visual testing (VT) can be performed on any component when inspecting for surface discontinuities, and should always be included when applicable. Additionally the chart doesn’t detail specific techniques that may be required for a particular NDT method.

It should be realized that no one single NDT process is capable of providing every bit of pertinent data related to a specific inspection, and the chart provides the most common methodology for detecting the most common types. Therefore it is necessary for all involved in the NDT process — buyer to practitioner — to make every effort to use the proper terminology in an effort to become more efficient in the inspection process.

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