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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