Magnetic
Particle Inspection
Magnetic
particle inspection is a non-destructive inspection method of detecting
invisible cracks and other defects in ferromagnetic materials, such as iron and
steel.
Nonmagnetic materials are not affected by
it. Small imperfections frequently progress to the point where they completely
ruin aircraft parts that are rapidly revolving, reciprocating, vibrating, and
subjected to other severe stresses. For the quick identification of such flaws
present on or near the surface, magnetic particle examination has proven to be
incredibly reliable. This type of examination shows the approximate size and
shape of the fault as well as its location.
The
part is magnetized before ferromagnetic particles are applied to the region to
be inspected during the inspection procedure. The ferromagnetic particles
(indicating medium) may be flushed over the part in a liquid that holds them in
suspension, submerged in the liquid, or sprinkled on the part's surface in the
form of dry powder. The inspection of aircraft parts is more frequently
conducted using the wet technique.
The
magnetic lines of force are disrupted and opposite poles occur on either side
of a discontinuity if one is present. In the magnetic field between the
opposing poles, the magnetized particles consequently create a pattern. An
"indication" is a pattern that roughly resembles the surface
projection of the discontinuity. A fracture, forging lap, seam, inclusion,
porosity, and other interruptions in a part's regular physical structure or
arrangement are examples of discontinuities. The functionality of a part may or
may not be impacted by a discontinuity.
Development of Indications
The
flux leakage at the discontinuity tends to shape the indicating medium into a
channel of higher permeability when a discontinuity in a magnetized material is
open to the surface and a magnetic substance (indicating medium) is present on
the surface. (The ease with which a magnetic flux can be established in a
specific magnetic circuit is referred to as permeability.) The indication is
still visible on the part's surface as a rough outline of the discontinuity
that lies directly beneath it because of the part's magnetism and the magnetic
particles' adhesion to one another. When the discontinuity is closed off from
the surface, the same action occurs, but because there is less flux leakage,
fewer particles are held in place, yielding a fainter and less clearly defined
signal.
There
might not be any flux leakage and no visible indication if the discontinuity is
very far below the surface.
Types of Discontinuities Disclosed
The
magnetic particle test typically picks up cracks, laps, seams, cold shuts,
inclusions, splits, tears, pipes, and voids among other sorts of discontinuities.
Each of these could have an impact on how reliable parts are in use.
The
solid metal actually separates or ruptures to generate cracks, splits, bursts,
tears, seams, voids, and pipes. Folds that have developed in the metal, known
as cold closes and laps, break up its continuity.
When metal is processed, impurities create inclusions, which are alien materials. For instance, they might be made up of foreign substances or pieces of furnace lining that were melted along with the base metal.
Because
they inhibit the joining or welding of neighbouring metal faces, inclusions
break the continuity of the metal.
Preparation of Parts for Testing
Before
any parts are tested, they must be thoroughly cleansed of any grease, oil, and
grime. Cleaning is crucial because any grease or other foreign matter might
cause magnetic particles to stick to the foreign matter when the suspension
drains from the part, producing irrelevant indications.
A
substantial layer of grease or foreign substance over a discontinuity may also
hinder the creation of a pattern there. Relying on the magnetic particle
suspension to clean the part is not advised. Cleaning by suspension is
ineffective because it is not comprehensive and because any foreign objects
that are removed from the part contaminate the suspension.
A
thorough cleaning is very important for the dry procedure. Grease or another
foreign substance traps the magnetic powder, producing irrelevant signals and
making it difficult to apply the indicating medium uniformly across the surface
of the item. Paraffin or another suitable nonabrasive material must be used to
block any small gaps and oil holes leading to internal channels or cavities.
Unless
the coatings are exceptionally heavy or the discontinuities to be detected are
unusually minute, coatings of cadmium, copper, tin, and zinc do not interfere
with the satisfactory performance of magnetic particle inspection.
In
general, plating with chromium and nickel does not affect the appearance of
cracks that are visible at the base metal's surface, but it does prevent the
appearance of fine discontinuities like inclusions. Nickel plating is more
effective than chromium plating at preventing the creation of indications
because it has a stronger magnetic field.
Effect of Flux Direction
It
is imperative that the magnetic lines of force flow roughly perpendicular to
the flaw in order to find it in a part. Since faults are likely to exist at any
angle to the primary axis of the part, it is important to create magnetic flux
in more than one direction. This necessitates the use of two distinct magnetizing
processes, known as circular magnetization and longitudinal magnetization.
The
induction of a magnetic field made up of concentric circles of force around and
inside the part is known as circular magnetization. This is done by putting an
electric current through the component and looking for flaws that are roughly
parallel to the axis of the component. The magnetic field is created during
longitudinal magnetization in a direction parallel to the component's long
axis. The part is inserted into a solenoid that has been energized by an
electric current to achieve this. The metal component subsequently transforms
into the electromagnet's core and becomes magnetized through induction due to
the magnetic field produced by the solenoid.
Long
components must be longitudinally magnetized, which requires moving the
solenoid along the part. This is required to guarantee sufficient field
strength along the whole length of the part.
Solenoids
accommodate pieces or sections up to 30 inches in length by producing effective
magnetization for about 12 inches from each end of the coil. The part can be
given longitudinal magnetization similar to that produced by a solenoid by
having a flexible electrical conductor wrapped around it. The coils adhere more
closely to the shape of the part, resulting in a little more uniform
magnetization, even though this method is less convenient. When normal
solenoids are unavailable, the flexible coil approach can also be utilized for
big or atypically shaped items.
Effect of Flux Density
The
flux density or field strength at the surface of the component when the
indicating medium is applied also affects how well the magnetic particle
inspection works. Due to larger flux leakages at discontinuities and the
associated enhanced production of magnetic particle patterns, the test becomes
more sensitive as the flux density in the part increases.
Excessively
high flux densities may provide irrelevant indicators, including patterns in
the material's grain flow. The detection of patterns originating from major
discontinuities is hindered by these signals. Therefore, it is essential to use
a field strength that is both strong enough to disclose any potentially
damaging discontinuities and weak enough to avoid producing confusing or
irrelevant indicators.
Magnetizing Methods
When
a part is magnetized, its field strength rises to a maximum for the specific magnetizing
force and stays there for the duration of the magnetizing force.
Depending
on the magnetic characteristics of the material and the geometry of the part,
the field strength reduces to a lower residual value after the magnetizing
force is removed. Depending on the part's magnetic properties, either the
continuous method or the residual method is employed to magnetize it.
The
part is magnetized and the indicating medium is applied while the magnetizing
force is maintained in the continuous inspection method. Thus, the part's
available flux density is at its highest. The magnetizing force and material
permeability of the part's construction directly affect the flux's maximum
value.
Practically
all circular and longitudinal magnetization processes can be done using the
continuous approach. In particular when looking for subsurface discontinuities,
the continuous technique offers more sensitivity than the residual procedure.
The continuous approach is more frequently utilized because of the highly
critical nature of aviation parts and assemblies and the requirement for
subsurface inspection in many applications. Since the continuous approach shows
more insignificant discontinuities than the residual procedure, the discontinuities
disclosed by this procedure must be carefully and intelligently interpreted and
evaluated.
Following
the removal of the magnetizing force, the part is magnetized and the indicating
medium is applied in the residual inspection operation. When magnetization is
achieved by flexible coils wrapped around the part, this approach relies on the
residual or permanent magnetism in the part and is more practical than the
continuous procedure. The residual method is often only applied to steels that
have undergone heat treatment for stressed applications.
Identification of Indications
It might be challenging to accurately assess the nature of indicators based solely on observation, notwithstanding how critical this judgement is. Shape, accumulation, width, and outline sharpness serve as the indicators' primary identifying characteristics. As opposed to assessing the severity of discontinuities, these features are more useful in identifying different types of discontinuities. In order to fully assess the significance of an indicated discontinuity, careful study of the magnetic particle pattern must always be incorporated.
The
signs created by cracks that are exposed to the surface are the easiest to
spot. These cracks include grinding cracks, shrink cracks in welds and
castings, fatigue cracks, heat-treat cracks, and cracks caused by heat
treatment.
Magnaglo Inspection
The
previous method is comparable to the magnaglo inspection, but it varies in that
it uses a fluorescent particle solution and conducts the inspection under a
black light. The ability to discern minor flaw signals is made possible by the
neon-like glow of flaws, which increases inspection efficiency. On gears,
threaded parts, and aircraft engine components, this technique works well.
Magnaglo paste (mag particle) is combined with a light oil in a reddish-brown
liquid spray or bath at a ratio of 0.10 to 0.25 ounces of paste per gallon of
oil. The component needs to be demagnetized and cleaned with a cleaning agent
after inspection.
Magnetizing Equipment
Unported, non-portable, fixed-purpose unit Direct current (DC) is supplied for wet, continuous, or residual magnetization processes by a stationary, all-purpose unit.
It
can be powered with rectified AC as well as DC and can use circular or
longitudinal magnetization. The electrical terminals for circular magnetization
are provided by the contact heads. The contact plate of one head, which is
fixed in place, is mounted on a shaft and is supported longitudinally by a
pressure spring. The spring keeps the plate in its extended position until
pressure from the movable head that is communicated via the work pushes it
back.
A
switch controls a moving head that is powered by an electric motor and slides
along longitudinal guides. The spring applies pressure to the ends of the work
to guarantee proper electrical contact while also allowing the motor-driven
head to overflow sufficiently to prevent jamming.
When
the spring is sufficiently compressed, a plunger-operated switch in the fixed
head shuts off the forward motion circuit of the moveable head motor. The
contact plate may occasionally be set up to be moved by an air ram, whereas the
moveable head in some systems is hand-controlled. Both contact plates have
numerous fittings installed to help support the work.
Pushing
a button on the unit's front will shut off the magnetizing circuit. It is
programmed to open automatically, often after 0.5 seconds. By using the
rheostat, the magnetizing current's intensity can be manually adjusted to the
appropriate value or boosted to the unit's maximum capacity by using the
rheostat short circuiting switch. The ammeter displays the current being used.
The solenoid, which moves in the same guide rail as the movable head and is
coupled to the electrical circuit by way of a switch, produces longitudinal
magnetization.
Through
a nozzle, the suspension is applied to the piece of art. A nonmetallic grill
allows the suspension to exit the work and flow into a collecting pan that goes
back to the sump. A pushbutton switch controls the circulating pump.
Portable General Purpose Unit
It
is often necessary to conduct magnetic particle inspections in places without
established general purpose equipment or to inspect aircraft construction
components without removing them from the aircraft. It is especially helpful
for examining engine mounts and landing gear that may have cracked while in
use. Both AC and DC magnetization is provided by portable equipment.
While
providing a source of current for magnetizing and demagnetizing, this device is
unable to sustain work or apply suspension. It uses 200 volts of 60 cycle AC
power and has a rectifier for creating DC as needed.
With
the help of prods or contact clamps, the magnetizing current is provided
through the flexible cables. The cable terminals might have prods or contact
clamps installed. Either the prods or the clamps can be used to produce
circular magnetization.
Wrapping
the cable around the part results in the development of longitudinal magnetism.
An eight-point tap switch regulates the magnetizing current's intensity, and an
automatic cutoff similar to that found in fixed general purpose units controls
how long it is applied.
This portable device provides high amperage, low-voltage AC for demagnetizing purposes. Through the part and a current reducer, the AC is gradually lowered for demagnetization.
It
is occasionally impossible to utilise contact clamps when testing huge
structures with flat surfaces where current must travel through the component.
Contact prods are employed in these circumstances.
Both
the fixed general purpose unit and the portable device can be used with prods.
The part or assembly being tested may be supported or fastened above the
conventional unit, with the excess suspension hosed over the area and draining
into the tank. The dry method can also be applied.
Prods
are firmly pressed up against the surface being examined. High-amperage currents
have a propensity to burn the contact points, but with the right precautions,
such burning is typically minimal. Burning that is barely perceptible is
typically appropriate for situations where mild magnetization is acceptable.
Indicating Mediums
Wet
and dry materials make up the majority of the indicating medium varieties used
for magnetic particle examination. Any signalling medium must first and
foremost offer reliable signs of components' discontinuities.
The
contrast that a specific signifying media offers on the backdrop or portion
surface is crucial. For the wet technique, black and red, and for the dry
procedure, black, red, and grey, are the colours that are most frequently
utilised.
The
indicating medium must have a high permeability and a low retentivity for
satisfactory performance. High permeability guarantees that the material will
only be drawn to flux leakage induced by discontinuities with the least amount
of magnetic energy. Low retentivity makes sure that the magnetic particles'
movement is not restricted by the magnetization and attraction of the particles
to one another.
Demagnetizing
If
the part is to be put back into service, any residual permanent magnetism
discovered after examination needs to be eliminated by a demagnetization
procedure. To avoid magnetized parts from drawing steel particles from
operational wear or unintentionally left in the system, filings, grindings, or
chips, parts of operating mechanisms must be demagnetized. Such particles can
build up on a magnetized item and score bearings or other working parts.
Demagnetizing some airframe components is necessary to prevent instrument
damage.
It's
not usually necessary to demagnetize in between magnetizing processes unless
past experience shows that doing so reduces the effectiveness for a given
application. Several methods can be used to demagnetize an object. For aviation
parts, it is practical to subject the component to a magnetizing force that is
continuously changing direction while also steadily losing strength. The
magnetization of the component also lowers as the decreasing magnetizing force
is applied first in one direction and then the other.
Standard Demagnetizing Practice
The
usage of an AC-energized solenoid coil is the fundamental process for creating
a reversing and gradually decreasing magnetizing force in a part. The part's
magnetism steadily decreases as it is pushed away from the solenoid's
alternating field.
It
is used a demagnetizer whose size is close to that of the work. Small pieces
are held as close to the coil's inner wall as feasible for maximum efficacy.
Parts that are difficult to demagnetize are repeatedly passed slowly through
the demagnetizer while also being tumbled or turned in different directions.
There is relatively little practical demagnetization accomplished by leaving a
part in the demagnetizer with the current running.
Moving the component gradually out of the coil and away from the magnetizing field strength is the most efficient operation in the demagnetizing process. Up until it is one or two feet away from the demagnetizer, the part is withdrawn while being held directly across from the opening. As the part may be remagnetized if current is removed too soon, the demagnetizing current is not turned off until the part is one or two feet from the opening. Another method used with portable devices is to run AC through the component that has to be demagnetized while reducing the current until it is zero.
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