TO NONDESTRUCTIVE TESTING
Definition of Nondestructive Testing
Methods of NDT
What is Not Non Destructive Testing
OF NONDESTRUCTIVE TESTING
Nondestructive testing (NDT) has been
defined as comprising those test methods used to examine
an object, material or system without impairing its future
usefulness. The term is generally applied to nonmedical
investigations of material integrity.
Strictly speaking, this definition of
nondestructive testing does include noninvasive medical
diagnostics. Ultrasound, X-rays and endoscopes are used
for both medical testing and industrial testing. In the
1940s, many members of the American Society for Nondestructive
Testing (then the Society for Industrial Radiography) were
medical X-ray professionals. Medical nondestructive testing,
however, has come to be treated by a body of learning so
separate from industrial nondestructive testing that today
most physicians never use the word nondestructive.
Nondestructive testing is used to investigate
the material integrity of the test object. A number of other
technologies - for instance, radio astronomy, voltage and
amperage measurement and rheometry (flow measurement) -
are nondestructive but are not used to evaluate material
properties specifically. Nondestructive testing is concerned
in a practical way with the performance of the test piece
- how long may the piece be used and when does it need to
be checked again? Radar and sonar are classified as nondestructive
testing when used to inspect dams, for instance, but not
when they are used to chart a river bottom.
Methods of Non Destructive Testing?
UT- Ultrasonic Testing
MT- Magnetic Particle Testing
What Is Not Nondestructive Testing?
Nondestructive testing asks "Is there
something wrong with this material?" Various performance
and proof tests, in contrast, ask "Does this component work?"
This is the reason that it is not considered nondestructive
testing when an inspector checks a circuit by running electric
current through it. Hydrostatic pressure testing is usually
proof testing and intrinsically not nondestructive testing.
Acoustic emission testing used to monitor changes in a pressure
vessel's integrity during hydrostatic testing is nondestructive
Another gray area that invites various
interpretations in defining nondestructive testing is that
of future usefulness. Some material investigations involve
taking a sample of the inspected part for testing that is
inherently destructive. A noncritical part of a pressure
vessel may be scraped or shaved to get a sample for electron
microscopy, for example. Although future usefulness of the
vessel is not impaired by the loss of material, the procedure
is inherently destructive and the shaving itself - in one
sense the true "test object" - has been removed from service
The idea of future usefulness is relevant
to the quality control practice of sampling. Sampling (that
is, the use of less than 100 percent inspection to draw
inferences about the unsampled lots) is nondestructive testing
if the tested sample is returned to service. If the steel
is tested to verify the alloy in some bolts that can then
be returned to service, then the test is nondestructive.
In contrast, even if spectroscopy used in the chemical testing
of many fluids is inherently nondestructive, the process
is destructive if the test samples are discarded after testing.
Hardness testing by indentation provides
an interesting test case for the definition of nondestructive
testing. Hardness testing machines look somewhat like drill
presses. The applied force is controlled as the bit is lowered
to make a small dent in the surface of the test piece. Then
the diameter or depth of the dent is measured. The force
applied is correlated with the dent size to provide a measurement
of surface hardness. The future usefulness of the test piece
is not impaired, except in rare cases when a high degree
of surface quality is important. However, because the piece's
contour is altered, the test is rarely considered nondestructive.
A nondestructive alternative to this hardness test could
be the use of electromagnetic nondestructive testing.
Nondestructive testing is not confined
to crack detection. Other discontinuities include porosity,
wall thinning from corrosion and many sorts of disbonds.
Nondestructive material characterization is a growing field
concerned with material properties including material identification
and microstructural characteristics - such as resin curing,
case hardening and stress - that have a direct influence
on the service life of the test object. Nondestructive testing
has also been defined by listing or classifying the various
methods. This approach is practical in that it typically
highlights methods in use by industry.
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