NDT in aircraft

transcript

LATEST TRENDS IN NONDESTRUCTIVE TESTINGWITH REFERENCE TO PAF

Department of Aerospace Engineering College of Aeronautical EngineeringPAF Academy, Risalpur

Sqn Ldr Farrukh Mazhar

Sequence

Aircraft structure inspectionsNDT – DefinitionNeed for NDTNDT applicationNDT in PAFModern NDT techniquesConclusion

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

Structural Deterioration

CorrosionFatigue (cyclic loading)Fabrication defectsOperation and MaintenanceUnforeseen loading (overloads)Result into Failure or Fracture

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

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

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

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

Jet Engine Inspection

•During engine overhaul•Completely disassembly, cleaning, inspection and reassembly

•Fluorescent penetrant inspection is shown to check engine parts

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

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

Microwave

Acoustic Emission

Thermography

Laser Interferometry

Replication

Flux Leakage

Acoustic Microscopy

Magnetic Measurements

Tap Testing

Six Common NDT Methods in PAF

Visual Liquid Penetrant Magnetic Ultrasonic Eddy Current Radiography

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

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

MODERN NDT TECHNIQUES

Sensor Based Inspections : Principle

ExcitationSource

Signal / ImageProcessing

Signal / ImageRecognition

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)

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)

Wire Rope Inspection

Electromagnetic devices and visual inspections are used to find broken wires and other damage to the wire rope

Wire Rope NDT Technology

Uses the magnetic properties of the steel wire rope

The principle of operation employs:

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”

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

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

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

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

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

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

Phased Array Scan

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

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

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

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

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

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.

Fuselage inspection of Boeing 737

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

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

Received and reference signals are compared

Flaws, conductivity, and dimensional changes can be measured

Depth of a part can be measured

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

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

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

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.

Structural Health Monitoring

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

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

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

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

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

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

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

Questions?