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LATEST TRENDS IN NONDESTRUCTIVE TESTINGWITH REFERENCE TO PAF
Department of Aerospace Engineering College of Aeronautical EngineeringPAF Academy, Risalpur
Sqn Ldr Farrukh Mazhar
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Sequence
Aircraft structure inspectionsNDT – DefinitionNeed for NDTNDT applicationNDT in PAFModern NDT techniquesConclusion
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Structure Inspection - Aim
Ensure safety and airworthinessDetecting manufacturing or service-
induced damageTo achieve this goal, a structure inspection
program has been developed for PAF fleet
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Structural Deterioration
CorrosionFatigue (cyclic loading)Fabrication defectsOperation and MaintenanceUnforeseen loading (overloads)Result into Failure or Fracture
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Inspection Levels
General Visual Inspection (GVI) During pre, tru or post flight
Detailed Visual Inspection (DET) During periodic inspection
Special Detailed Inspection (SDET) NDT of selected parts during periodic
inspections
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Technique that allows a component to be inspected for serviceability, without impairing its usefulness
i.e. Inspect or measure without doing harm
Definition of NDT
Uses of NDT Methods
Flaw detection and evaluation
Leak detection Location determinationDimensional
measurements
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Uses of NDT Methods
Structure and microstructure characterization
Material sorting and chemical composition determination
Mechanical and physical properties estimation
Stress (strain) and dynamic response measurements
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Aircraft Inspection• During manufacturing of aircraft
• To detects damage during operation of the aircraft
• A fatigue crack that started at the site of a lightning strike is shown below
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Jet Engine Inspection
•During engine overhaul•Completely disassembly, cleaning, inspection and reassembly
•Fluorescent penetrant inspection is shown to check engine parts
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A defect that went undetected in an engine disk was responsible for the crash of United Flight 232
Crash of United Flight 232
“Periodic inspection should be a systematic and complete examination of
the entire structure with particular attention given to the critical locations”
Critical Areas – Where to look
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Airframe Loading : Critical Locations
Non-destructive testing (NDT) is needed in order to ensure the integrity of the airframe.
Methods of NDT
Visual
Liquid Penetrant
Magnetic Particle
Eddy Current
Ultrasonic
X-ray
Microwave
Acoustic Emission
Thermography
Laser Interferometry
Replication
Flux Leakage
Acoustic Microscopy
Magnetic Measurements
Tap Testing
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Six Common NDT Methods in PAF
Visual Liquid Penetrant Magnetic Ultrasonic Eddy Current Radiography
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Visual Inspection - Types
Direct Visual Testing Remote Visual Testing
Tools for remote inspection include fiberscope & borescope
Portable video inspection unit with
zoom allows inspection of large tanks and vessels
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Remote Visual Testing – Borescopes
Flexible Borescopes Contains a bundle of optical fibers Also known as a fiberscope Used for inaccessible cavities Such as air inlets, combustion chamber,
compressor, turbine blades, seals and other inaccessible aircraft parts
Good Image quality is required
Remote Visual Testing – Borescopes
Video borescopesSimilar to the flexible borescope but uses
a miniature video cameraA display shows the camera viewMuch less costly and have potentially
better resolution Digital models have an integrated recorder
and images / video can be saved
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MODERN NDT TECHNIQUES
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Sensor Based Inspections : Principle
ExcitationSource
ExcitationSource
Signal / ImageProcessing
Signal / ImageProcessing
Signal / ImageRecognition
Signal / ImageRecognition
DisplayResult
DisplayResult
Input transducer
Measurement transducer
Modern NDT Techniques Acoustic emission testing (AE or AT) Electromagnetic testing (ET)
Alternating current field measurement (ACFM) Alternating current potential drop measurement
(ACPD) Direct current potential drop measurement (DCPD) Magnetic flux leakage testing (MFL) Remote field testing (RFT)
Ellipsometry Guided wave testing (GWT) Impulse excitation technique (IET) Infrared and thermal testing (IR)
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Modern NDT Techniques Laser testing
Profilometry Shearography
Optical microscopy Radiographic testing (RT)
Computed radiography Digital radiography (real-time)
Ultrasonic testing (UT) Electro Magnetic Acoustic Transducer (EMAT) Laser ultrasonics (LUT) Phased array ultrasonics Time of flight diffraction ultrasonics (TOFD)
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Wire Rope Inspection
Electromagnetic devices and visual inspections are used to find broken wires and other damage to the wire rope
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Wire Rope NDT Technology
Uses the magnetic properties of the steel wire rope
The principle of operation employs:
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Measurements of EM fields near the surface to detect local defects
Measurements of changes in magnetic flux passing through the rope to evaluate cross section
Wire Rope NDT Readings compared with new wire “signature” Monitoring the rate of degradation of a rope A good rope will show a reproducible “signature”
trace
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Acoustic Emission (AE)
External stimuli, like mechanical loading, generate elastic waves in the material
As stress waves are generated when there is a rapid release of energy in a material, or on its surface
Used for part inspection, process monitoring etc
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Acoustic Emission Phenomena
Related to an irreversible release of energy
Can be generated from friction, cavitations and impact
Ranges from 1kHz, up to 100 MHz frequencies
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Use in Non-destructive Testing
AE uses ultrasonic regime ( 100kHz and 1MHz)
Uses passive sensor which monitors acoustic emissions produced
Used to study the formation of cracksGroup of transducers record signals and
locate area of defect originApplications in process monitoring
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Phased Array (PA) Ultrasonics
Phased Array (PA) ultrasonics has applications in industrial nondestructive testing
The PA image shows the defects hidden inside a structure or weld
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Phased Array (PA) Ultrasonics
PA probe consists of many small elements pulsed separately
Ultrasonic ripple are send as multiple waves to make a single wave front travelling at a set angle
Weld examination by phased array
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Features of Phased Array
Produce a steerable, tightly focused, high-resolution beam
Produces an inside image of the objectPA instruments and probes are more
complex and expensiveMore experience and training than
conventional technicians
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Phased Array Scan
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Ultrasonic Imaging
Gray scale image produced using the sound reflected from the front surface of the coin
Gray scale image produced using the sound reflected from the back surface of the coin (inspected from “head’s” side)
High resolution images can be produced by plotting signal strength or time-of-flight using a computer-controlled scanning system
Infrared and Thermal Testing
Principle electromagnetic radiation emission as a function the temperature
Thermographic inspection NDT of parts through the surface imaging of the thermal patterns
Used for monitoring thermal changes Infrared Thermography mapping of thermal
patterns, on the surface of objects using infrared Resolution few hundredths of a degree Celsius Radiation is detected, processed and displayed
on a computer display
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Thermographic Testing
Measures surface temperatures only But surface temperatures are dependent
upon the subsurface conditionsEnergy flows can be slowed down by the
insulating effectsDetect hidden thermal insulation faults and
subsurface targets such as hot air leaks
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Thermographic Testing
Two methods are used : Passive, in which the features of interest are
naturally at a higher or lower temperature than the background
Active, in which an energy source is required to produce a thermal contrast
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Active Thermography Excitations
Thermographic TestingEnergy sources induces a thermal contrast
between defective and non-defective zones Energy cannot pass through a flawExternal excitation
photographic flashes (for heat pulsed stimulation) or
halogen lamps (for periodic heating) Internal excitation
Mechanical oscillations, with a sonic or ultrasonic transducer
Burst and amplitude modulated stimulations
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Thermographic Testing : Features
Thermographic inspection is safe, nonintrusive and noncontact, allowing the detection of relatively shallow subsurface defects
Large area can be inspectedFast and time savingFriction in moving parts like control links,
cables, surfaces hinges etc.Hidden hydraulic, hot /cold air, fuel leak
can be detected
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Thermographic Testing : Features
Looks for “hot spots” in electrical equipment, showing high resistance areas
Inspecting composite or honeycomb aircraft structural components
This method is reliable and cost effective
This thermogram shows a fault with an industrial electrical fuse block.
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Fuselage inspection of Boeing 737
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Digital Radiography
Digital X-ray sensors are used instead of traditional photographic film
Time efficient due to through bypassing chemical processing
Digital processing and transferable enhance images
Also less radiation required Typically there are two variants of digital image
capture devices Flat Panel detectors (FPDs) High Density Line Scan Solid State detectors
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Digital Radiography
Pulsed Eddy Current Inspection
Step function voltage is used to excite the probe instead of sinusoidal AC current
A step function voltage contains a continuum of frequencies
The electromagnetic response to different frequencies can be measured
Depth information can be obtained
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Pulsed Eddy Current Inspection
Received and reference signals are compared
Flaws, conductivity, and dimensional changes can be measured
Depth of a part can be measured
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Pulsed Eddy Current Inspection
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Shearography
Shearography is an optical nondestructive testing method
Uses include aerospace, space, wind rotor blades, automotive and materials
Advantages include the large area testing capabilities, non-contact properties and its good performance on composites and honey-comb materials
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Principle of OperationAn interferometric image of the surface is
taken and stored it in a computerImage is a unique footprint of the surfaceThe material is then stressed with a small
amount of thermal loadThe material tries to expand when heated
up, and weak spots will expand moreSecond interferometric image is taken
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Principle of Operation By subtracting the two images a shearogram
is created The defects will be seen as fringe patterns
resembling a pair of “hills” or a pair of "bulls-eyes"
The size of the defects can be quantified by measuring how large this fringe pattern is
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Shearography
A primitive shearography setup; Two physical points on test object will be
projected on to one point on the CCD chip to record a interferometric footprint. The tested surface is illuminated with a monochromatic light, typical 650 nm.
The primitive shearography principle. A shearography image is recorded at
unloaded state and one image is recorded in the loaded state.
Thereafter they are subtracted and in the result defects can be detected.
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Structural Health Monitoring
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Continuous monitoring of structures using integrated or applied sensors
Aimed at assuring structural integrity of the aircraft, by detecting damages resulting from fatigue, corrosion, excessive loads, impact ...
This does not imply knowing the status of the structure in real-time
What is SHM54
After normal or exceptional events, maintenance can be planned at next appropriate inspection
Systems are available for aircraft condition monitoring - mostly for loads (accelerations, flight parameters, etc.) and enable decisions to be made based on actual flight load levels
What is SHM55
SHM Advantage
In terms of life cycle cost, a US DoD study attributed that 27% of the total cost of an aircraft being maintenance
related with structural inspection being a significant driver of this cost suggesting
SHM could save up to 44% of current inspection time on modern fighter aircraft
SHM will improve reliability since structures are monitored directly, measuring the effect of damage
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SHM Advantage
SHM has many advantages: No access to the inspection area necessary Safe inspection of hazardous areas Eliminating time consuming setup Sensors used in the inspection are integral to
the structure Automated process - no human factors
influence on inspection POD Interrogating many locations or wide field at
once - significant time saving
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Structural Health MonitoringWith SHM new possibilities exist which can be used to ensure the structure integrity: Damages Loads/Strains Flight parameters and conditions Environmental conditions Production parameters
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Possibilities for NDT
Visual Inspection (VI)• More than 95% of all NDI inspection are done visually
Non-Destructive Testing (NDT)• Performed where VI is not sensitive enough or damages
are not visible on the surface Structural Health Monitoring/Management
• Sensor permanently attached / embedded in the structure
• Information on structural events or states to arbitrary times available
• Automated assessment and prognostic of the health of aircraft structuresThe optimum solution for structural inspections should be chosen
out of these 3 options The optimum solution for structural inspections should be chosenout of these 3 options
NDT in PAFFrom visual inspections to more detailed
technique based on penetrant inspection, eddy currents, ultrasonics, x-rays, etc…
Inspection intervals are generally OEM specified or self experience based
These Inspections result in downtime and significant effort
Monitoring activity comes at a considerable cost and accounts a large maintenance man-hours for commercial aircraft
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Conclusions
NDT plays key role in safe operation and especially in damage tolerant design / operations of aircraft structures
Furthermore NDT is “enabler” for reliable introduction of new materials, technologies and design concepts
We have to realize that adopting new NDT techniques and more focus on training can improve flight safety
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Conclusions
Enhanced training On job expertise Developing NDT trade or screening NDT
trained specialists Developing and managing NDT pool
through sub NDT classifications Acquiring latest and modern equipment Education everyone about NDT
Advantages and Hazards
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Conclusions
Including introductory NDT course in PPT curriculum
Thorough scrutiny of inspection procedures from NDT Perspective
Ensuring availability of personnel protecting equipment especially in RT and PT
Making a PAF level certifying, training and regulation body
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Questions?
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