NDT Lecture NDT Composites 2012 (1)
Transcript of NDT Lecture NDT Composites 2012 (1)
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NDT of Composite Materials
Lecture 9
Introduction to Composite Materials
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NDT methods for composites
Dr A De Silva School of Engineering & Built Environment
COMPOSITE MATERIALSWhat are composites?
Definitions & Classifications
Why are composites used instead of metals, y pceramics, or polymers?
How do composite materials differ from other materials?What are the constituent materials, and how do their properties compare?
What are some typical applications?
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Materials perspective
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Definitions and Terms of CompositesCOMPOSITE
Composites are combinations of two (or more) materials, usuallly:
the reinforcing (dispersed) phase,
REINFORCEMENT
(DISPERSED PHASE)
MATRIX
fibers, sheets, or particles, and is embedded in the other materials called
the matrix phase.
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Create a hybrid or a composite by:
Combining anymaterial with any othermaterial to take advantagegof their mutual properties
Design the material tomeet the requirements!
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Classification
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Matrix phaseStructures cannot be made from fibres alone -the high properties of fibres are not realisable in practicepractice A matrix is required to:
hold reinforcement in correct orientationprotect fibres from damagetransfer loads into and between fibres
http://www.carlosantulli.net/aim2001.pdf7
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Reinforcing Phase
Reinforcing phase is added to strengthen weak materialsto strengthen weak materialsto toughen brittle materials
Reinforcing phase ParticlesWhiskersFibres (Continuous Long Short)Fibres (Continuous, Long, Short)Flakes, sheetsWoven assemblies
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Matrices
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Fibres
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Structural composites
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Properties of Composites
If the composite is designed and fabricated correctly it combines the strength of thecorrectly, it combines the strength of the reinforcement with the toughness of the matrix to achieve a combination of desirable properties not available in any single conventional material. The downside is that such composites are often more expensive than conventional materials
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Addition of properties:
GLASS + POLYESTER = GRP(strength) (chemical resistance) (strength and(strength) (chemical resistance) (strength and
chemical resistance)
Unique properties:
GLASS + POLYESTER = GRP(brittle) (brittle) (tough!)
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Specific Strength/Stiffness
Specific properties are properties that have been divided by thebeen divided by the material density. This chart is useful for identifying materials for components which require high stiffness and/or strength combined with low weight. Composites provide a means of achieving good specific properties.
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Why are composites used in engineering?
Weight saving (high specific properties)C i i tCorrosion resistanceFatigue propertiesManufacturing advantages:- reduced parts count- novel geometries- low cost toolingDesign freedoms- continuous property spectrum- anisotropic properties
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Why aren’t composites used more in engineering?
High cost of raw materialsLack of design standardsFew ‘mass production’ processes availableProperties of laminated composites:- low through-thickness strength- low interlaminar shear strengthNo ‘off the shelf’ properties - performance depends on quality of manufacture
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Applications
Aerospace EEnergy
AutomobileRecreationStructures
M iMarineElectronics
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Composites in airplanes
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Airbus - composites
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Energy:Composite – wood, glass, carbon?
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Offshore oil industry: flexible pipes
CompositeArmor Layers
(W ll t I )
Advantages of Polymer Composite material in flexible pipes30% weight reduction - greater depths, lower deck loadsCorrosion resistance - longer life, more fluid options
(Wellstream Inc.)
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Automotive
The automotive industry has long recognized the potential of advanced composites and carbon fiber to help it meet the i i d f i d f t d d i iincreasing need for improved safety, reduced emission, improved fuel consumption, lower weight and demanding cost reductions.
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Automotive- Composite Leaf Springs
Leading car and truck designers are achieving significant weight and cost savings by building leaf springs in composites,
d i i f d t f th t i land gaining performance advantages from the material. Composite leaf springs weigh up to 60% less than their steel counterparts. These weight savings save costs by providing the driver or operator with a greater loading capacity and/or reduced fuel consumption.
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Automotive - Motor sport
The composite carbon body shell, which forms the basis ,of most modern day racing cars, provides the safest yet most efficient framework through a combination of stiffness, lightness and strength.Combination of stiffnessCombination of stiffness, weight reduction, corrosion resistance, impact/damage resistance and occupant crash protection.
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Structures:FRP BRIDGE ENCLOSURES
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Structures:FRP MOSQUEDOMESDOMES
PHOTOS COURTESY OF NORTHSHORE COMPOSITES
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FRP OBSERVATION
GLASGOW SCIENCE CENTRE
FRP OBSERVATION CABIN & CARBON FIBREMAST
Photo - Carrillion 28Dr A De Silva
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Sports Equipment
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NDT of Composites Materials/StructuresNDT is an essential part of composite manufacturing and usage Requires trained and experienced inspectorsRequires trained and experienced inspectors.NDT methods are used in:
Pre-production for process developmentDuring production to provide process information and ensure part conformanceIn service to maintain
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In service to maintain
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NDT Utilisation - Airplane Product Life CyclePre-production for process development
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NDT Utilisation - Airplane Product Life CycleDuring production- to provide process information and ensure part conformance
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Airplane Product Life Cycle In service to maintain a safe fleet
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Composite failure modes
Delaminations:Separation of the layers of material in a laminate, either p ylocal or covering a wide area.
Disbond/Unbond:An area within a bonded interface between two adherents in which adhesion failure or separation has occurred.
Porosity:A condition of trapped air, gas or vacuum within a solid material.
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Crack:An actual separation of material visible on opposite surfaced of the part, and extending through the thickness. A fracture.
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Defects in Composites
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Defects in laminates
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Sandwich panel defects
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NDT methods for composites
Visual Inspection: Using the eye alone or in conjunction with aidsconjunction with aidsUltrasonic Testing: Most Common NDT Method for the inspection of CompositesRadiography: Bond Testing: Single sided Bond Testing utilizing Pitch Catch or Resonance testing modes
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Pitch Catch or Resonance testing modesTap Testing: Oldest and simplest method of Bond Testing.
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NDT Methods
Traditional MethodsEddy current
Non-traditional MethodsThermographyEddy current
RadiographyUltrasonicPenetrantMagnetic particle
ThermographyShearographyBond Testing
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Eddy currentAdvantages
Highly portableModerate cost
DisadvantagesSuitable for inspection of conductive materials onlyS f t b iblImmediate results
Sensitive -to small imperfectionsMinimum part preparation required
Surface must be accessible to probeUsually no permanent recordManual tests require high degree of operator skill to properly use test equipment
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properly use test equipmentDefect Types: Cracks, corrosion, conductivity, coating measurement
Seldom used for composite inspection
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Radiography - Film
AdvantagesWell proven
DisadvantagesRadiation safety hazardp
Permanent record of test resultsMinimum part preparation required
yModerately expensiveWet-chemical processing required to developProper interpretation of results may require high degree of operator skill
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Defect Types: Voids, foreign material, internal damage, core conditions, water ingestion
Non-film Radiography
AdvantagesLower recurring costs
DisadvantagesRadiation safety hazardLower recurring costs
Shorter cycle timeElimination of chemical processingPortability
Radiation safety hazard (reduced from film)Higher initial costProper interpretation of results may require high degree of operator skill
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Defect Types: Voids, foreign material, internal damage, core conditions, water ingestion
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Non-Film Radiography
Real-time (a.k.a. radioscopy)Digital RadiographyDigital Radiography
Storage Phosphor: Use a flexible “imaging plate” to produce a latent image that must be subsequently scanned by a laser and converted into a video imageLinear Diode Array: Uses an x-ray sensitive photodiode array in place of film.Flat Panel Detectors: Consist of a coated Thin Film
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Flat Panel Detectors: Consist of a coated Thin Film
Transistor (TFT) array that captures and converts X-Ray energy into electronic signals
Real Time Radiography -Principle of Operation
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Real Time Radiography
AdvantagesInexpensive
DisadvantagesAnalogue deviceInexpensive
Well establishedInstantaneous “real-time” (dynamic) imaging
Analogue deviceDetector blooming (washout)Limited bit depth (Typically 4- 5 bits usable) = limited
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usable) limited sensitivityMost applications require magnification
Real Time Radiography
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Linear Diode Arrays – Principle of Operation
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Linear Diode Arrays
AdvantagesRobust design
DisadvantagesRequires movementRobust design
FastHigh dynamic rangeNo bloomingDigital imaging
Requires movement between part and detectorResolution can be limitedLow kV applications
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Can be tailored to application
Low kV applications only
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Linear Diode Arrays
X- ray LDA image of honeycomb structure with
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yexcess resin
Thermography Methods
Liquid Crystal and Electronic ThermographyPulsed Thermography (Thermal Wave Imaging)Ultrasonic Thermography (ThermoSonix)
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Liquid Crystal Thermography -Principle of Operation
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Electronic Thermography -Principleof Operation
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Liquid Crystal and Electronic ThermographyAdvantages
Real time imagingDisadvantages
Not effective for panelsReal time imagingNon contact (electronic)Remote inspectionLow non-recurring costs
Not effective for panels with metal skinsLiquid crystal method requires intimate contact
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Defect Types: Water in honeycomb structure
Liquid Crystal and Electronic Thermography
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Liquid Crystal and Electronic Thermography
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Pulsed Thermography -Principleof Operation
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Pulsed Thermography
AdvantagesRapid wide area technique
DisadvantagesHigh equipment costRapid, wide area technique
Real time imaging, Noncontact, remote inspectionVersatile - New applications being approved and developed
High equipment costApplication base still limited
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Defect Types: Moisture in composites, metal skin to metal doubler disbonds
Pulsed Thermography
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Pulsed Thermography
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Ultrasonic NDT
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Pulse Echo Ultrasonics
AdvantagesSingle sided inspection
DisadvantagesMay be difficult to resolveSingle sided inspection
Identifies defect depth relative size of defectReasonably high resolution depending on equipmentSensitive to a wide range of foreign materials
May be difficult to resolve defects near surface defectsMay be ineffective with porosityMay not be sensitive to off angle defects
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gMay not be effective with sandwich structures.
Defect Types: Delaminations, foreign material, porosity (loss of back)
Ultrasonic scanning
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C Scan porosity
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Pulse Echo UltrasonicsAutomated Pulse Echo Scanner for 777 empennage skins
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Through Transmission Ultrasonics
AdvantagesComplete part inspection
DisadvantagesRequires access to bothComplete part inspection
with single scanLarge dynamic range availableRelatively fast inspectionIrregular part contours
Requires access to both sides of partDoes not identify defect type or depthNot as sensitive to foreign materials as other methods
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Defect Types: Delaminations and voids, porosity (equipment dependant) foreign materials (equipment dependant)
Through Transmission UltrasonicsAutomated Thru-transmission Ultrasonic Scanner for 737 Engine Cowls
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Through Transmission UltrasonicsThrough Transmission Scan Image
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Bondtesters
Bondtesters operate in the acoustic- mechanical mode and are used for single- sided bond inspectionsBondtesters are classified by their operating frequency
High frequency (100 - 500 kHz)Ultrasonic resonant transducer
Low frequency (7 - 80 kHz)Mechanical impedanceEddy SonicVelocimetric methods
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Velocimetric methodsAudible
Tap test (resonance)
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Resonant Transducer Bondtesters –Principle of Operation
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Resonant Transducer BondtestersAdvantages
Single sided inspectionDisadvantages
Limited defect typesSingle sided inspectionInspect an area of a part at one timeFast data acquisition
Limited defect typesLimited sizing capability
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Defect Types: Delaminations
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Resonant Transducer Bondtesters
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Velocimetric Bondtesters
Velocimetric bondtesters measure stiffness changes of the structure through ultrasonic mechanicalof the structure through ultrasonic mechanical vibrations. The transducer produces a flexural (bending) wave mode
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Velocimetric Bondtesters
AdvantagesSingle sided inspection
DisadvantagesLimited defect typesSingle sided inspection
Inspect an area of a part at one timeFast data acquisition
Limited defect typesPoor sizing accuracy
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Defect Types: Skin to core disbonds, interply delaminations
Velocimetric Bondtesters
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Tap Testers – Principle of Operation
Tap testing is a process of evaluating a structure by lightly tapping (structure vibrated) with a metallic disk, rod, or an
t ti t t t i t t Th t i it t t lautomatic tap tester instrument. The taping excites structural resonances that will vary with how the energy is transferred into the part.
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Tap Testers
AdvantagesLow Cost
DisadvantagesSomewhat unreliableLow Cost
Relatively simple to useSomewhat unreliableLimited to defects 3 to 4 plies deepLimited to defects larger than 1 inch diameterResults affected by environmental noiseDefect Types: Skin to core
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Results affected by hearing ability of inspector
Defect Types: Skin to core disbonds, interply delamination
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Acoustic Emission Testing
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Activity of AE Sources in Structural Loading
AE SourcesNon-metallic inclusionsCracks
Frequency range100 – 500kHz
ActivityPrimary activities
P t h i t i l ( k th)Permanent changes in material (crack growth)Secondary activities
E.g., crack surface rubbingNoise in AE testing
Friction, impacts, …
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AE Signal Features
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AE Signal FeaturesAmplitude, A, is the greatest measured voltage in a waveform measured in decibels (dB). An important parameter in AE inspection because it determines the detectability of the signal. Signals with amplitudes below the operator-defined, minimum threshold will not be recorded. Rise time, R, is the time interval between the first threshold crossing and the signal peak This parameter is related to the propagation of the wave betweensignal peak. This parameter is related to the propagation of the wave between the source of the acoustic emission event and the sensor. Rise time is used for qualification of signals and as a criterion for noise filter.Duration, D, is the time difference between the first and last threshold crossings. Used to identify different types of sources and to filter out noise. Like counts (N), this parameter relies upon the magnitude of the signal and the acoustics of the material. MARSE, E, sometimes referred to as energy counts, is the measure of the area under the envelope of the rectified linear voltage time signal from the transducer. It is useful as the energy of the emission can be determined. MARSE is also sensitive to the duration and amplitude of the signal but doesMARSE is also sensitive to the duration and amplitude of the signal, but does not use counts or user defined thresholds and operating frequencies. MARSE is regularly used in the measurements of acoustic emissions. Counts, N, refers to the number of pulses emitted by the measurement circuitry if the signal amplitude is greater than the threshold. Depending on the magnitude of the AE event and the characteristics of the material, one hit may produce one or many counts. While this is a relatively simple parameter to collect, it usually needs to be combined with amplitude and/or duration measurements to provide quality information about the shape of a signal.
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AE Source Location Techniques
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AE Source Location Techniques
∆T approach
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Good and Ambiguous Locations
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Lamb Wave Modes in Plates
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AET
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AE signals
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AE SummaryAcoustic Emission differs from other NDT techniques: Firstly, instead of supplying energy to the object under examination, AET simply listens for the energy released by it. AE tests are often performed on structures while in operation, as this provides adequate loading for propagating defects and triggering acoustic emissions. S dl AET d l ith d i h iSecondly, AET deals with dynamic processes, or changes, in a material. only active features (e.g. crack growth) are highlighted. The ability to discern between developing and stagnant defects is significant. However, it is possible for flaws to go undetected altogether if the loading is not high enough to cause an acoustic event. Furthermore, AE testing usually provides an immediate indication relating to the strength or risk of failure of a component. Other advantages of AET include fast and complete volumetric inspection using multiple sensors, permanent sensor mounting for process control, and no need to disassemble and clean a specimenand no need to disassemble and clean a specimen.Unfortunately, AE systems can only qualitatively gauge how much damage is contained in a structure. For quantitative results about size, depth, and overall acceptability of a part, other NDT methods (often ultrasonic testing) are necessary. Another drawback of AE stems from loud service environments which contribute extraneous noise to the signals. For successful applications, signal discrimination and noise reduction are crucial.
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Shearographic head(Laser + optical system)
Stimulating load
Laser beam
Laser ShearographyThe basic principles of the measurement method are interferometry and correlation techniques
Image processing
Object
Defect
Laser beam
Birefringentelement Polarizer
Loading: Thermal
correlation techniques
element
Illuminatedobject
Polarizer
Lens
CCD Image processing
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Laser ShearographyThe defects become visible on measuring their inhomogeneous deformation gradients. Depending on the spatial distribution of deformation the defect or the structure itself can be characterised. The rotor measurements show a dis-bond (left), a crack (right) and aThe rotor measurements show a dis bond (left), a crack (right) and a structural transition (centre). Compare fringe density anddistribution.
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DAMAGE TYPE DETECTED – Adhesive craking under impact
Cross section Adhesive cracks Honeycomb Skin
Shearography-Capabilities
Top view
Visible impacted area
Multiple adhesive cracks
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Shearography - Capabilities
Example of vacuum loading
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Comparison with Ultrasonics
US C-ScanImpacted Monolithc
(CFRP)Displacement field
Corrosion DetectionAluminium Alloys
CF C-ScanDisplacement field
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Benefits of Shearography
Contact-lessGlobal method, Good sensitivity,Real time inspection, Quicker than most of the NDI methods, Working on all type of materials and structures, Numerical acquisition give rise to efficient followup,New type of damage are now detectable withoutopening the structure…
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Selecting the NDT method
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NDT Summary
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