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NDT Method Summary
No single NDT method will work for all flaw detection or measurement applications. Each of the
methods has advantages and disadvantages when compared to other methods. The table below
summarizes the scientific principles, common uses and the advantages and disadvantages for some
of the most often used NDT methods.
Penetrant
Testing
Magnetic Particle
Testing
Ultrasonic
Testing
Eddy Current
Testing
Radiographic
Testing
A magnetic field is
High frequency sound
established in a
waves are sent into a
component made from material by use of a
ferromagnetic material. transducer. The sound
The magnetic lines of waves travel through
force travel through the the material and are
material, and exit and received by the same
reenter the material at transducer or a second
the poles. Defects such transducer. The amount
as crack or voids
of energy transmitted or
cannot support as much received and the time
flux, and force some of the energy is received
the flux outside of the are analyzed to
part. Magnetic particles determine the presence
distributed over the
of flaws. Changes in
component will be
material thickness, and
attracted to areas of
changes in material
flux leakage and
properties can also be
produce a visible
measured.
indication.
Alternating electrical
current is passed
through a coil producing
a magnetic field. When
the coil is placed near a
conductive material, the
changing magnetic field
induces current flow in
the material. These
currents travel in closed
loops and are called
eddy currents. Eddy
currents produce their
own magnetic field that
can be measured and
used to find flaws and
characterize
conductivity,
permeability, and
dimensional features.
X­rays are used to
produce images of
objects using film or
other detector that is
sensitive to radiation.
The test object is
placed between the
radiation source and
detector. The thickness
and the density of the
material that X­rays
must penetrate affects
the amount of radiation
reaching the detector.
This variation in
radiation produces an
image on the detector
that often shows
internal features of the
test object.
Used to inspect
Used to locate surface
ferromagnetic materials and subsurface defects
(those that can be
in many materials
magnetized) for defects including metals,
that result in a transition plastics, and wood.
in the magnetic
Ultrasonic inspection is
permeability of a
also used to measure
material. Magnetic
the thickness of
particle inspection can materials and otherwise
detect surface and near characterize properties
surface defects.
of material based on
sound velocity and
Used to detect surface
and near­surface flaws
in conductive materials,
such as the metals.
Eddy current inspection
is also used to sort
materials based on
electrical conductivity
and magnetic
permeability, and
measures the thickness
of thin sheets of metal
Used to inspect almost
any material for surface
and subsurface defects.
X­rays can also be
used to locates and
measures internal
features, confirm the
location of hidden parts
in an assembly, and to
measure thickness of
materials.
Scientific
Principles
Penetrant solution is
applied to the surface of
a precleaned
component. The liquid
is pulled into surface­
breaking defects by
capillary action. Excess
penetrant material is
carefully cleaned from
the surface. A
developer is applied to
pull the trapped
penetrant back to the
surface where it is
spread out and forms
an indication. The
indication is much
easier to see than the
actual defect.
Main Uses
Used to locate cracks,
porosity, and other
defects that break the
surface of a material
and have enough
volume to trap and hold
the penetrant material.
Liquid penetrant testing
is used to inspect large
areas very efficiently
and will work on most
nonporous materials.
attenuation
measurements.
and nonconductive
coatings such as paint.
Main Advantages
Large surface areas or
large volumes of
parts/materials can be
inspected rapidly and at
low cost.
Large surface areas of
complex parts can be
inspected rapidly.
Depth of penetration for
flaw detection or
measurement is
superior to other
Can detect surface and methods.
subsurface flaws.
Parts with complex
Only single sided
geometry are routinely Surface preparation is access is required.
inspected.
less critical than it is in
penetrant inspection.
Provides distance
Indications are
information.
produced directly on
Magnetic particle
surface of the part
indications are
Minimum part
providing a visual image produced directly on the preparation is required.
of the discontinuity.
surface of the part and
form an image of the
Method can be used for
Equipment investment discontinuity.
much more than just
is minimal.
flaw detection.
Equipment costs are
relatively low.
Detects surface and
near surface defects.
Can be used to inspect
virtually all materials.
Test probe does not
need to contact the
part.
Detects surface and
subsurface defects.
Ability to inspect
Method can be used for complex shapes and
more than flaw
multi­layered structures
detection.
without disassembly.
Minimum part
Minimum part
preparation is required. preparation is required.
Disadvantages
Detects only surface
breaking defects.
Only ferromagnetic
materials can be
inspected.
Surface preparation is
critical as contaminants Proper alignment of
can mask defects.
magnetic field and
defect is critical.
Requires a relatively
smooth and nonporous Large currents are
surface.
needed for very large
parts.
Post cleaning is
necessary to remove
Requires relatively
chemicals.
smooth surface.
Surface must be
Only conductive
accessible to probe and materials can be
couplant.
inspected.
Extensive operator
training and skill
required.
Skill and training
required is more
extensive than other
technique.
Ferromagnetic materials Access to both sides of
require special
the structure is usually
treatment to address
required.
magnetic permeability.
Orientation of the
Surface finish and
Depth of penetration is radiation beam to non­
roughness can interfere limited.
volumetric defects is
with inspection.
critical.
Flaws that lie parallel to
Thin parts may be
the inspection probe
Field inspection of thick
difficult to inspect.
coil winding direction
section can be time
can go undetected.
consuming.
Requires multiple
Paint or other
operations under
nonmagnetic coverings Linear defects oriented
controlled conditions.
adversely affect
parallel to the sound
Skill and training
Relatively expensive
sensitivity.
beam can go
required is more
equipment investment
undetected.
extensive than other
is required.
Chemical handling
techniques.
precautions are
Demagnetization and
necessary (toxicity, fire, post cleaning is usually Reference standards
Possible radiation
necessary.
are often needed.
Surface finish and
hazard for personnel.
waste).
roughness may
interfere.
Reference standards
are needed for setup.
Penetrant
Testing
Magnetic Particle
Testing
Ultrasonic
Testing
Eddy Current
Testing
Radiographic
Testing