NDT – Non Destructive Testing
Magnetic Particle Inspection (MT)
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Magnetic Particle Inspection (MT) is one of the best-known and commonly used methods of NDT.
Its aim is to detect the presence of surface braking discontinuities (cracks) in the part under
inspection.
Only Ferromagnetic materials can be inspected by the MT method. This is because Ferromagnetic
materials develop strong internal magnetic fields when an electrical current is passed through
them. An electric current can be introduced in to the test part in several ways. It can be wrapped in
encircling coils and rods or the current can be applied directly with the use of the yoke producing a
magnetic field perpendicular to the current flow. When these internal magnetic fields encounter a
change in permeability (i.e. an open fissure/crack) the magnetic field is forced outside of the
materials surface, and produces flux leakage. This leakage will attract any other Ferromagnetic
materials that may be close to the leakage site.
Prior to any MT being carried out the part is cleaned of any loose scale, oil/grease, and then
covered with a very thin layer of background contrast paint (this is applied by painting or by
aerosol). The aim of the contrast paint is to make any defects or anomalies stand out, and help the
Inspector in locating the defect. Once the contrast paint is dry, particles with an affinity for leakage
fields are passed over the part, these Ferromagnetic particles are applied by aerosol i.e. wet or dry
powder form depending on the temperature or the part. These particles are highly visible against
the contrast paint. When the particles are attracted to the leakage field around the surface flaw,
they take the shape of the anomaly that has broken the magnetic field. The pattern of the particles
clearly shows the shape and contours of the anomaly, allowing for easy monitoring and recording
by the inspector.
Liquid Penetrant (PT)
• Liquid Penetrant Inspection (PT) is another common method of NDT, and
is solely used to detect surface breaking discontinuities, which are free
from debris that can limit the entrance of the dye (oil, grease and paint).
• This method of Inspection can be applied to Ferromagnetic and nonferromagnetic materials, however it is most commonly used in nonferromagnetic components, for example when MT is not practical. The
Liquid Penetrants used have a low viscosity and a high affinity for metallic
surfaces. The dyes are applied to the test part by aerosol spray or by being
submerged in a tank were the dye penetrates any surface breaking flaws.
• After a required dwell time the dye can be removed so no excess fluid
remains on the surface and a thin coating of a highly absorbent developer
is sprayed over the test area. The developer draws any entrapped dye out
of any cracks of fissures by capillary action and the dye spreads
throughout the developer surface magnifying the size of the indication.
The contrast in colour between the red penetrant and the white developer
plus the magnifying effect caused by the spreading of the dye leads to a
clearly visible indication.
Ultrasonic Inspection (UT)
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Ultrasonic Inspection (UT) is a method of NDT that is used to detect internal anomalies in a part,
which may contain welding, or stress defects that can be detrimental to the integrity of the
component.
It is also a commonly accepted method of checking the wall thickness of pipelines and vessels,
which are suspected of being corroded internally, when access is limited to one side of the
material. UT is very sensitive to critical defects in a material like cracks, welding defects, porosity,
lack of fusion and inclusions, which may weaken the weld. It is also very portable and can be used
on a wide range of materials. This method of inspection, though, is very reliant on having an
experienced and well-trained Inspector to interpret the indications they come across in field
conditions and to determine if the part is satisfactory or if remedial action is required.
UT uses very short duration sound pulses which when induced into a material reflects off different
media i.e. air interfaces and inclusions. The time for the reflection from these media are monitored
and compared against the known travel speeds for the given material. The careful measurement of
these pulse times becomes a measure of the distance the pulse has travelled and these are
monitored via signals on a visual display screen.
These signals may represent cracks, back wall echo and lack of fusion (common air interfaces), slag,
tungsten and copper (common welding inclusion). The tracking of these signal levels during the
inspection enables the inspector to gather information about the size, type and location of the
anomalies detected. During wall thickness monitoring of pipelines and vessels where access is
limited to one side only, the sound pulse reflects off the inner wall of the component. These signal
times become a measure of the distance travelled and any internal erosion/corrosion can be
detected due to a reduction in wall thickness (a faster signal time).
Eddy Current Inspection (ET)
• Eddy Current Inspection (ET) is one of the most widely applicable of
the Non-Destructive Inspection methods.
• It depends on measuring the changes in the impedance of a coil
due to change in the flow of eddy current in a conductor. Any
material change that affects the flow of the induced eddy current
sufficiently can be detected. Because so many things affect the
flow, eddy current inspection can be applied to a wide variety of
test situations. Principle areas of application are flaw detection,
material-thickness measurements, alloy sorting and the monitoring
of metallurgical conditions such as hardness and heat treatment.
Besides its versatility, the major advantages of eddy current
inspection are the speed at which tests can be performed and its
ability to inspect through painted coatings. It can be used also to
inspect both ferromagnetic and non-magnetic materials. The
principle disadvantages of the method is its limited depth of
inspection into the material or part being inspected.
Radiographic Inspection and
Interpretation
• Radiography (RT) and Radiographic Interpretation (RI) uses
X-rays or gamma-rays to produce an image of an object on
film.
• The image is usually natural-size. X-rays and gamma-rays
are very short wavelength electromagnetic radiation which
can pass through solid material, being partly absorbed
during transmission. Thus, if an X-ray source is placed on
one side of a specimen and a photographic film on the
other side, an image is obtained on the film of the
thickness variations in the specimen, whether these are
surface or internal. This is a well-established technique
which gives a permanent record and is widely used to
detect internal flaws in weldments and castings and to
check for mis-constructions in assemblies.