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INTRODUCTION TO PENETRANT TESTING (PT)

Development of Method
Area of Application
Limitations of Penetrant Testing
Training & Certification

Development of Method

The liquid-penetrant test (commonly known as Penetrant Test) is one of the oldest methods of nondestructive testing. It is based on the old oil-and-whiting process, formerly widely employed on steel parts, particularly in the railroad industry.

Area of Application

Because the method relies on a penetrant’s seeping into a discontinuity, it is obvious that it is applicable only to surface defects or to subsurface defects with surface openings.

In practice, the liquid-penetrant process is relatively simple. Equipment generally is simpler and less costly than that for most other nondestructive-inspection methods. Establishment of procedures and standards for inspection of specific parts or products is usually less difficult than for the more highly sophisticated inspection methods.

The liquid-penetrant method does not depend on ferromagnetism, and the arrangement of the discontinuities is not a factor. The penetrant method is good not only for detecting surface flaws in non-magnetic metals, but also for revealing surface flaws in a variety of other non-magnetic materials.

It is applicable to all homogeneous materials except those of a generally porous nature where penetrant would seep into and leak from the entire surface.

Penetrant testing is therefore best adapted to inspection of all types of surface cracks, porosity, laminations, and lack of bond at exposed edges of joined materials, and of leaks in tubing, tanks, welds, and the like. It has been used with excellent success on ferrous and nonferrous metals, ceramics, powdered-metal products, ceramics, and glass, as well as on some plastic and synthetic materials.

Limitations of Penetrant Testing (PT)?

The major limitation of liquid-penetrant inspection is that it can detect only imperfections that are open to the surface. Another factor that may inhibit the effectiveness of liquid penetrant inspection is the surface roughness of the object being inspected. Extremely rough or porous surfaces are likely to produce false indications. The process generally is not well suited to inspection of low-density powder metallurgy parts or other porous materials, because the penetrant enters the pores and thus registers each pore as a defect.

Training & Certification

It is recognised that the effectiveness of nondestructive testing depends on the capabilities of the personnel who are responsible for, and perform NDT. Thus all customers will require an assurance that NDT personnel whose specific jobs require appropriate knowledge of the technical principles underlying the nondestructive tests they perform, witness, monitor, or evaluate be qualified and certified.

The American Society for Nondestructive Testing recommends the use of the documents “recommended practice no. SNT-TC-1A”. This document provides the employer with the necessary guidelines to properly qualify and certify the NDT technician in all methods. To comply with this document the employer must establish a “written practice” which describes in detail how the technician will be trained, examined and certified.

Current edition of SNT-TC-1A may be referred to determine the recommended number of hours of classroom instruction and months of experience necessary to be certified as a radiographic testing technician. Certification of NDT personnel always rests with the employer and is usually at three levels.

Level I - is qualified to perform specific calibrations, specific tests, and specific evaluations.

Level II - is qualified to set up and calibrate equipment and to interpret and evaluate results with respect to codes, standards and specifications. Must be able to prepare written instructions and report test results.

Level III - must be capable and responsible for establishing techniques, interpreting codes, and designating the test methods and techniques to be used . Must have a practical background in the technology and be familiar with other commonly used methods of NDT.

The SNT-TC-1A document recommends that level I and level II NDT technicians be examined in the following areas:

A. General examination.
B. Specific examination.
C. Practical examination.

The SNT-TC-1A document recommends that NDT level III personnel be examined in the following areas:

A. Basic examination.
B. Method examination.
C. Specific examination

 

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