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INTRODUCTION
TO NON DESTRUCTIVE TESTING
The
aim of this section is to provide a general overview of the fundamental
nature of NDT, including its role in the engineering, construction and
maintenance of in-service industrial systems and components. It has been
written to acquaint non NDT personnel looking for a new career. The
emphasis will be on the more common forms of NDT and especially those
techniques used in conjunction with rope access. It will cover the basic
physical concepts of various testing techniques, how they are selected,
who uses them, their capabilities and their limitations.
Inspection
of materials, components, and assemblies is often required in order
to determine if they comply with design specifications or are defective.
Of utmost concern for newly manufactured or constructed items is their
ability to withstand the mechanical stresses and chemical attack to
which they are expected to be exposed. For items that are already
in service, the question is whether or not the item will continue
to function satisfactorily in its service environment until the next
scheduled inspection. NDT by virtue of its ability to detect flaws,
measure dimensions, and assess material characteristics, has become
a primary tool in addressing the predictions of component performance
and remaining service life. The selection of an appropriate NDT method
for a specific application is based upon the physical characteristics
of the material and the discontinuities of most interest, including
the probable location, orientation, size, and morphology. The assumption
is that if an item is free from unacceptable flawed conditions, it
should meet performance expectations, that is, carry the load, withstand
the vibration, or maintain dimensional integrity.
The
NDT approach to materials testing is ‘non-destructive’ in that each
inspected item is expected to be placed in service. This is in stark
contrast to the destructive methods of materials evaluation where
sacrificial specimens are required. In destructive testing, specially
prepared specimens are stressed in a manner that simulates the stresses
to which the item may be subjected and at levels exceeding the design
limits.
The
destructive approach results in specimens fracture, distortion, of some
other form of degradation, and thus is applied on only a sampled basis.
NDT
is most commonly used where component failure may have catastrophic
consequences, such as in aircraft, electric power plants and
petrochemical plants, as well as gas lines and offshore installations.
The periodic inspection of components in these fields determines if they
are suitable for continued service. The advantages of NDT for in service
applications include safety evaluation with a minimum of system down
time, early detection of potentially dangerous and costly unscheduled
service interruption and the ability to monitor degradation processes
from which realistic and cost effective re-inspection schedules can be
developed.
The
five main methods of NDT described in this brief are:
- Magnetic
Particle Inspection
- Liquid
Penetrant Inspection
- Ultrasonic
Inspection
- Eddy
Current Inspection
- Radiographic
Inspection and Interpretation
MAGNETIC
PARTICLE INSPECTION
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 attached 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 INSPECTION
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 non-ferromagnetic materials, however it is most commonly used
in non-ferromagnetic 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 soak 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
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 eroded 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 included 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
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.
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