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?

Dr A De Silva School of Engineering & Built Environment 2

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Materials perspective


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!


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


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


Fibres


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Structural composites


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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School of Engineering & Built Environment

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.


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.


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


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


In service to maintain

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NDT Utilisation - Airplane Product Life CyclePre-production for process development


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


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.


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


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


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


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


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


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


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


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


usable) limited sensitivityMost applications require magnification

Real Time Radiography


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Linear Diode Arrays – Principle of Operation


Linear Diode Arrays

AdvantagesRobust design

DisadvantagesRequires movementRobust design

FastHigh dynamic rangeNo bloomingDigital imaging

Requires movement between part and detectorResolution can be limitedLow kV applications


Can be tailored to application

Low kV applications only

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Linear Diode Arrays

X- ray LDA image of honeycomb structure with


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


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


Defect Types: Water in honeycomb structure

Liquid Crystal and Electronic Thermography


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Liquid Crystal and Electronic Thermography


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


Defect Types: Moisture in composites, metal skin to metal doubler disbonds

Pulsed Thermography


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Pulsed Thermography


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


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


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


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


Velocimetric methodsAudible

Tap test (resonance)

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Resonant Transducer Bondtesters –Principle of Operation


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


Defect Types: Delaminations

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Resonant Transducer Bondtesters


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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AdvantagesSingle sided inspection

DisadvantagesLimited defect typesSingle sided inspection

Inspect an area of a part at one timeFast data acquisition

Limited defect typesPoor sizing accuracy


Defect Types: Skin to core disbonds, interply delaminations



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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.


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


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


ndt lecture ndt composites 2012 (1)

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