is important to inspect the mechanical
damage and assess the extent of the
repair work. But in schedule
maintenance it is a difficult to finding
the defects rapidly, as the maintenance of aircraft must be accomplished
within scheduled time and same to be
released in time for commercial
operation. During aircraft maintenance
'NONDESTRUCTWE TESTING' (NDT) is the
most economical way of performing
inspection and this is the only way of
discovering defects. In simply we can
say, NDT can detect cracks or any other irregularities in the airframe
structure and engine components
which are obviously not visible to the
naked eye. Structures & different assemblies of
aircraft are made from various
materials, such as aluminium alloy,
steel, titanium and composite
materials. To dismantle the aircraft in
pieces and then examine each component would take a long time, so
the NDT method and equipment
selection must be fast and effective. In the present trend of NDT application
on aircraft 70-80% of NDT is
performed on the airframe, structure,
landing gears and the rest carried out
on engine & related components. In order to maintain the aircraft
defects free and ensure a high degree
of quality & reliability and as a part of
inspection programme, usually
following NDT methods are applied; 1) Liquid penetrant 2) Magnetic
particle, 3) Eddy current 4) Ultrasonic
5) Radiography (x-ray/gama ray) 6)
Visual/Optical 7) Sonic/Resonance 8)
Infrared Thermography. DIFFERENT NDT METHODS
1. Liquid Penetrant : Liquid penetrant testing is one of the
oldest of modern nondestructive
testing methods & widely used in
aircraft maintenance. Liquid penetrant
testing can be defined as a physical &
chemical nondestructive procedure designed to detect & expose surface
connected discontinuities in
'nonporous' engineering materials. The fundamental purpose of
penetrant testing is to increase the
visible contrast between a
discontinuity & its background. This is
achieved by treating the area with an
appropriately formulated liquid of high mobility & penetrating power
(which enters the surface cavities),
and then encouraging the liquid to
emerge from the developer, to reveal
the flaw pattern under white light
(when visible dye penetrants are used) or under ultraviolet light (when
fluorescent penetrants are used).
Evaluation also conduct with the aid of
3X to 5X magnification. The objective
of liquid penetrant testing is to provide
visual evidence cracks, porosity, laps, seams of other surface discontinuities
rapidly & economically with high
degree of reliability. Equipment : Various types of penetrant test units are used in aircraft
maintenance i) Portable Equipment : Penetrants
materials are available in 'Aerosol
spray cans' in small containers for
brush or wipe application. With these
aerosol can penetrant testing are
performed on installed parts on aircraft's, structure or in power plants ii) Stationary Test Equipment : This type
of equipment is most frequently used
in fixed installations, consists of a
series of modular work stations.
Typical stations are as follows: a) deep
tanks for penetrant b) emulsifier & developer c) a number of drain or
dwell areas d) a wash area with
appropriate lighting e) drying oven
and f) an inspection booth. iii) Small Parts Test Unit : These
inspection units designed for
processing aircraft small parts. The
units are smaller than the stationary
system. Small parts are loaded into
wire baskets & then processed through each of the stations. iv) Automated Test System : In this
penetrant testing process penetrant
application, washing, and drying are
automatic, but developer application,
the ultraviolet light inspection &
interpretation are manually performed by an inspector. Large aircraft
components are inspected in this
automatic system. Applications : Detection of surface detects or structural damage in all
materials of aircraft. Fluorescent
penetrants are used in critical areas for
more sensitive evaluation. Key Points : Fast & simple to use, inexpensive and easily transportable.
Can detect very small surface
discontinuity. Can be used on aircraft
or in the workshop. Frequently used
to confirm suspected defects. Area to
be cleaned before and after check. 2. Magnetic Particle : Magnetic particle testing is a sensetive
method of nondestructive testing for
surface breaking and some sub-
surface discontinuation in 'ferro-
magnetic' materials. The testing method is based on the
principle that magnetic flux in a
magnetised object is locally distorted
by the presence of discontinuity. This
distortion causes some of the
magnetic field to exit & re-enter the test object at the discontinuity. This
phenomenon is called magnetic flux
leakage. Flux leakage is capable of
attracting finely divided particles of
magnetic materials that in turn form an
'indication' of the discontinuity. Therefore, the test basically consists of
three operations : a) Establish a
suitable magnetic flux in the test
object by circular or longitudinal
magnetisation. b) Apply magnetic
particles in dry powder of a liquid suspension; and c) Examine the test
object under suitable lighting
conditions for interpreting &
evaluating the indications. Fluorescent or black oxide particles in
the aerosol cans are used during
critical areas of aircraft structure/
components inspection when using
either permanent or electromagnets.
Fluorescent particle inspection method is evaluated by black light (Black light
consists of a 100 watt mercury vapour
projection spot lamp equipped with a
filter to transmit wave length between
3200 to 3800 Angstrom unit and
absorb substantially all visible white light). Equipment : Following types of equipments are used for magnetic
particle inspection : i) Stationary magnetic flux machines
(using FWDC, HWDC AC) : Fixed cabinet
with fluid suspension circulation &
delivery system, adjustable position of
coils, head stock & moveable tail stock
used for, inspecting still parts removed from engine and aircraft. ii) Mobile portable magneticflux
machine : Hand carried or dolly
transported with limited of current
facility. iii)Electromagnet yokes(adjustable) :
Suitable for inspecting irregular
shaped parts for surface defects. iv) Permanent magnet : It is used in
isolated critical area of small & large
parts in aircraft. Applications : Simple in principle, easily portable. Fast and effective for
surface & subsurface defects in
ferromagnetic materials of any shape,
removed from engines, pumps,
landing gear, gear boxes, shafts,
shock struts etc. Widely used for bolts inspection. Key Points : Only suitable for ferro- magnetic materials. Demagnetisation
procedure is required. Positional
limitations - a magnetic field is
directional & best results must be
oriented perpendicular to the
discontinuity. 3. Eddy Current : Eddy current tests are important test &
widely used method within the broad
field of Nondestructive materials &
evaluation. This method is particularly
well suited for the detection of service
induced cracks usually caused either by fatigue or by stress corrosion. Eddy
current inspection can be performed
with a minimum of part preparation
and a high degree of sensitivity. Eddy currents are electrical currents
induced in a conductor of electricity by
reaction with alternating magnetic
field. Eddy currents are circular &
oriented perpendicular to the direction
of the applied magnetic field. The a) electrical conductivity b) magnetic
permeability c) geometry and d)
homogeneity of the test object, all
affects the induced currents. The electrical conductivity & magnetic
permeability of a material is influenced
by its chemistry & heat treat condition.
Mixed lots of materials or parts
subjected to fire or excessive heat
damage can be quickly & easily separated (conductivity testing).
Changes in the geometry &
homogeneity of the test object will
change the magnitude & distribution
of the eddy currents. By monitoring
these changes, the presence of cracks & other flaws can be detected. The eddy current inspection system
basically consists of five functions : a)
Oscillator b) Test coil absolute or
differential c) Bridge circuit d) Signal
processing circuits e) Read out or
display. Equipment : Usually for aircraft eddy current inspection following test
instruments are used 1) Meter display instrument - It
comprises a graduated scale in
milliampers of moveable meter needle.
The amplitude of needle movement in
proportional to the impedance of the
test circuit. 2) Impedance plane display
instrument - It features a 'flying dot'
on a CRT, LCD or video display. The
position of flying dot indicates the
impedance of the test circuit, but also
displays effect of both resistance & reactance presenting both phase and
amplitude information. 3) Linear time base display instrument
- It is usually used with rotating open
hole probe scanners. The 'horizontal
position' of the signal on the display
indicates sensor clock pPosition in the
hole & the 'vertical peak' of the signal indicates amplitude of response. 4) Bargraph display instrument - It
features on LCD read out bar scale
graduated in voltage sensitive
increments. The position of the display
indication is adjustable from one bar
to full scale. Compatibility with the instrument &
material selection different types of
probes are used Such are i) High
frequency surface & bolt hole probes
ii) High frequency special probes
(counter sink plug & shaped) iii) Low frequency probe (spot encircling &
shaped) iv) Sliding probe (driver/
receiver). Applications : Eddy current test is used to detect surface & subsurface
defects, corrosion in aircraft structures,
fastener holes and bolt holes. Surface
detects and conductivity testing by
high frequency and sub-surface
detects by low frequency methods. Routine eddy current inspection is
carried out on aircraft under carriage
wheel hubs for cracks also used to
detect cracks in different tubes, tublar
components of aircraft & engine. Key Points : Only applicable to conductive materials (ferrous, non
ferrous & austenitic components).
Calibration standards & trained
operator required. Fast & portable.
Spacial probes required for variation
of materials and accessibility. 4. Ultrasonic : Sound with a frequency above the
limit of audibility is called 'ultrasonic'. It
ranges with a frequency of 0.2 MHz to
800 MHz. Ultrasonic inspection provides a
sensitive method of nondestructive
testing in most materials, metallic,
nonmetallic, magnetic or nonmagnetic.
It permits the detection of small flaws
with only single surface accessibility and is capable of estimating location &
size of the defect Providing both
surfaces are parallel, ultrasonics may
be used for thickness measurement,
where only one surface is accessible.
The effective result of an ultrasonic test is heavily dependent on subject
surface condition, grain size &
direction and acoustic impedance.
Ultrasonic techniques are very widely
used for the detection of internal
defects in materials. Ultrasonic inspection operates on the
principle of 'transmitted' & 'reflected'
sound wave. Sound has a constant
velocity in a given substance;
therefore, a change in the acoustical
impedance of the material causes a change in the sound velocity at that
point producing an echo. The distance
of the acoustical impedance (flaw) can
be determined if the velocity of the
sound in the test material, and the time
taken for the sound to reach & return from the flaw is known. Ultrasonic inspection is usually
performed with two techniques (i)
Reflection (Pulse echo) technique (ii)
Through transmission technique.
'Pulse echo' technique is most widely
used in aircraft maintenance inspection. Equipment : The ultrasonic flaw detection equipment comprises with
the following basic elements : (i)
Cathode ray oscilloscope (ii) Timing
Circuit (iii) Rategenerator (iv) RF pulser
(v) Amplifier & (vi) Transducer (search
unit) Acoustic energy (transmitted or
reflected) are presented, displayed or
recorded in four ways. i) A-Scan : The basic components of
'pulse echo' system. Employs a stand
video, cathode ray tube or LCD display.
Display discontinuity depth and
amplitude of signal. Most commonly
used in aircraft inspection ii) B-Scan : It displays discontinuity
depth and distribution in 'cross
sectional view'. Means of presentation
recording paper and computer
monitor. iii) C-Scan : It displays discontinuity
distribution in 'flat plan view'.
Recording paper & computer monitor
required for presentation. iv) Digital Readout : It displays a
ultrasonic time of flight information in
digital format representing sound
velocity thickness readings. Applications : Used for detection of surface & subsurface detects in welds,
forging, casting main structural fittings
of landing gear legs & engine
attachments. Bolts in critical areas,
aircraft structure joints & pylon. Also
checks adhesive bond quality of lap joints & composite structure. Used for
thickness measurement after damage
or corrosion removal. Key Points : Fast, dependable & portable. Results are immediately
known. Calibration standards &
trained operator required.
Discontinuity orientation of test object
must be known to select wave mode. 5. Radiography : Radiography is one of the oldest and
widely used nondestructive testing
methods. A radiograph is a
photographic record produced by the
passage of electromagnetic radiation
such as x-rays or gamma rays through an object onto a film. When
film is exposed to x-rays, gamma rays
or light an invisible change called a
'latent image' is produced in film
emulsion. The areas so exposed
become darker when the film is immersed in a developing solution.
After development the film is rinsed to
stop development. The film is next put
into a fixing bath and then washed to
remove the fixer. Finally dried so that it
may handled for interpretation and record. X-ray : Three things required to
generate x-rays, a source of electrons,
a means of propelling electrons at
high speeds and target materials.
When high speed electrons interact
with matter (the nucleus of the target material), their energy is provided, it is
high enough, converted to x-ray
energy. Typical x-ray equipment is consists of
following features : i) Tube envelope
ii) Cathode of the x-ray tube iii) Anode
of the x-ray tube iv) Focal spot (size of
the radiation focal spot) v) X-ray beam
configuration vi) Accelerating potential (the operating voltage -
difference in electrical potential
between the cathode and anode) Gamma-ray : Gamma-rays are the
emissions from the disintegrating
nuclei of radioactive substances. Two
most commonly used 'isotopes' for
performing industrial inspections are
Iridium-192 and Cobalt-60. But in aircraft maintenance during gamma-
radiography Iridium- 192 is usually
used. Isotopes of Radium-226 and
Cesium- 13 7 are available but are not
generally used for aircraft
radiography. Gamma-ray radiography has the advantages of simplicity of
apparatus, compactness of the
radioactive sources and independence
from outside electrical source. Applications : Considering the penetration and absorption capability
of x-radiation, radiography is used to
inspect a variety of nonmetallic parts;
for porosity, water entrapment,
crushed core, cracks and resin rich/
straved conditions; and metallic products; such as welds, castings and
forging as well as locating
discontinuities in fabricated structural
assemblies such as cracks, corrosion,
inclusions, debris, loose fittings, rivets,
out of round holes & thickness variations. Gamma ray radiography is
usually used for detection of internal
flaws of aircraft structure (steel &
titanium) and engine components
which require higher energy levels or
other assemblies where access is difficult. Key Points : Radiation hazard, aircraft must be clean of all personnel. Trained
operator, film processing & viewing
equipments required. Crack point
must be nearly paralleled to X-ray
beam. Eliminates many disassembly
requirement. Provides permanent records of findings. Accessibility
required in both sides of the test
specimen. 6. Visual/Optical : Visual inspection is probably the most
widely used of all the nondestructive
tests. It is simple, easy to apply, quickly
carried out and usually low in cost.
The basic principle used in visual
inspection is to illuminate the test specimen with light and examine the
specimen with the eye. In many
instances aids are used to assist in the
examination. This method is mainly used i) to
magnify defects which can not be
detected by the unaided eye, ii) to
assist in the inspection of defects and
iii) to permit visual checks of areas not
accessible to unaided eye. Equipment : Visual and Optical tests are carried out in aircraft maintenance
with following equipment: i) Magnifying Glass - Generally consists
of a single lens for lower power
magnification and double or multiple
lenses for higher magnification. ii) Magnifying Mirror - This one is a
concave reflective surface, such as a
dental mirror may be used to view
restricted areas of aircraft not
accessible with a magnifying glass. iii) Microscope - It is a multiple element
magnifier, providing very high power
magnification, is used for the
inspection of parts removed from the
aircraft. Some portable units are also
used to evaluate suspected indications found on the aircraft. iv) Borescope - Borescope is a
precision optical instrument with
builtin illumination. Borescopes
sometimes called 'endoscopes' or
'endoprobes', which consists with
superior optical systems and high intensity light sources, some
broescopes provides magnification
option, zoom controls or accessories. v) Flexible Fibre Optic Borescope -
Permits manipulation of the instrument
around camers and through passages
with several directional changes.
Woven stainless steel sheathings
protects the image relay bundle during repeated flexing and
manoeuvring. The working lengths
are normally 60 to 365 cm with
diameters from 3 to 12.5 min. vi) Video Imagescope - The video
Imagescope is similar to a Fibrescope
with the exception that video camera
and its connections have replaced the
image bundle and a TV monitor has
replaced the eyepiece. This image may be magnified for precise viewing. The
field of vision is up to 90 degree and
probe tip has four way articulation.
Presently the smallest diameter is 9.5
mm with working length up to 100
feet. Applications : Detection of surface defects or structural damage in all
materials. Optical instruments are used
for visual checks of internal areas and
for deep holes and bores of aircraft
structure, landing gears etc. Widely
used to monitor engine components, such as, turbine wheels and nozzles,
compressor vanes and blades
combustion cans without opening the
engine. 'Borescopes', 'fibrescopes' and
'video imagescopes' are most
important optical aids in remote - visual inspection, which area is
normally inaccessible. Key Points : Simple to use in areas where other methods are impractical.
Accessibility required. Reliability
depends upon the experience of the
operator. 7. Sonic /Resonance : Sonic and resonance testing methods
are used primarily for the detection of
separations between layers of
laminated structures. Sonic and Resonance testing is
effective for detection of crushed core
or debonds in adhesive bonded
honeycomb, impact damage and
delimitations in composite structures
and exfoliation corrosion. The tap test method has demonstrated
the ability to detect cracks, corrosion,
impact damage and debonding. The
sonic testing instrument operate in the
audio or near audio frequency range. Resonance testing instruments may
operate either or both the sonic or
ultrasonic frequency range. Different
methods of transmitting and receiving
energy have been developed.
Basically, each technique introduces a pressure wave into the specimen and
then detects the resonant, transmitted
or reflected wave. Generally following acoustic
mechanical principles are used to
evaluate the damping characteristics
of the specimen. a.Resonance test method : This test
works well for many unbonds and
deliminated. b.Pitch/catch swept test method : This
test is best detecting unbonds and
deeper defects. c.Pitch/catch impulse test method : In
this method the joints not testable by
swept method, can be tested
satisfactorily by this mode. d.MIA(Mechanical Impedance
Analysis) test method : This method
works well on unbonds crushed core
and defects on the inside of composite
structure. e.Eddy sonic harmonic test method : It
is capable of detecting both near side
and far side disbond. f.Tap test : Tap test is a manual
method. Tap testing is a common and
inexpensive type of inspection. In this
procedure the inspector taps the
surface of the test structure and
evaluate the sound generated. The inspector either listens directly to the
sound or uses specially designed
receiver to analyse the sound and
compare the response with defect free
part. Application : To examine bonding exists between honeycomb, detect
delaminations in composite laminates.
Large structures such as, fairings, cowl
and wing trailing edge, rudder, flaps,
ailerons, elevators etc. are made from
composites and honeycomb materials. Tap testing is limited to detection of
disbonds or voids between
upperfacing sheet and adhesive. It will not detect disbond or voids at 2 nd or 3 rd layer bondlines, such as doubler areas. It is limited to the detection of
delaminations, approximately 25 mm
(1 inch) in dia or greater, located less
than 1.3 mm (0.05 inch) below the
surface being examined. Key points : Loses sensitivity with increasing material thickness. Electrical
source and reference standards
required. Infrared Thermography : Infrared and thermal methods for
nondestructive are based on the
principle that heat flow in a material is
altered by the presence of some types
of anomalies. These changes in heat
flow cause localized temperature differences in the material. The
imaging or study of such thermal
patterns is known as 'thermography'.
The terms 'infrared' and 'thermal' are
used interchangeably in some
contexts. Thermal refers to the physical phenomenon of heat,
involving the movement of molecules.
Infrared (below the colour red)
denotes radiation between the visible
and microwave regions of the
electromagnetic spectrum. The intensity and frequency/
wavelength of the radiation can be
correlated closely with the heat of the
radiator. it follows that radiation
sensors can be used to tell us about
the physical condition of the test object. This is the basis of the
technology of 'thermography'. Equipment : A thermal imager basically consists of a detector, a
scanning system, an optical system &
video display unit. The majority of
cameras function like a television
camera and their output is a video
signal which is proportional to the output signal of the detector.
Subsequently, this passes on to a
signal treatment and visualization
system which assigns to each level a
grey tone in an scale or false colour. In
this way, an image can be obtained on a TV monitor which represent the
distribution of temperatures
throughout all the field of viewor
printed out as colour graphics. Applications : Used to detect certain voids, inclusions, debonds, liquid
ingress or contamination, foreign
objects and damaged or broken
structural assemblies. Infrared
thermography also been chosen for
quick operational use and the reliability of defection 'liquid
contamination' in the composite
sandwich in compared to x-ray
method. Detection of thermal
overheating in electrical & hydraulic
system. Specially thermographic inspection on aircraft structures are
carried out to detect following defects :
(i) Composite laminate parts - for
delamination debonding or foreign
objects (ii) Composite sandwich parts -
for debonding and liquid contamination. (iii) Metallic bonded
parts - for debonding of corrosion on.
iv) Metallic sandwich parts - for liquid
contamination, debonding of
corrosion. Key points : This method shows temperature changes which can
indicate defects. Required trained
operator. Transportable & reference
standards may be required. CONCLUSION Probably the aerospace industry is
leading in the world for innovation of
new materials and fabrication
technique regularly to improve safety,
efficiency & reduce cost. At the same
time inspection techniques are also being developed to monitor their
integrity. For instance, with increasing
use of composites in latest commercial
aircraft construction has motivated
rapid development in ultrasonic
technique C scan presentation. It can detect defects deep inside composites
producing 3 dimensional images of
the structures & any irregularities
within the test item. Only with appropriate applications of
nondestructive testing techniques can
bring the benefits of advanced
materials science be fully utilized.
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