NonDestructiveEvaluation(2)

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Non‐Destructive Evaluation and Testing

5/12/2013 1CompositesIves De Baere and Joris Degrieck – 2013‐2014

NDT&E – what and why?

• Detection and assessment of damage can be done in two ways: destructive and none destructivedestructive

• Destructive: by means of mechanical characterization (e.g. compression after impact, residual strength, residual stiffness)

• Non‐destructive has the advantage that the method itself does not introduce extra damage


method itself does not introduce extra damage (e.g. no need for cutting of samples) so that when damage is negligible/limited, the structure can be put in service again.

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NDT&E ‐ How

There are a number of possibilities. Depending on the type, some are only possible on‐line, some are only possible off line and some can be used in bothonly possible off‐line and some can be used in both cases

• Visual inspection (both)

• Thermography (both)

• Radiography (off‐line)

h ( ff l )


• Micro‐tomography (off‐line)

• Acoustic emission (on‐line)

• Ultrasonic (off‐line)

Visual inspection

• Still of great value, most types of damage are visible with the naked eye

• Visual inspection might be the first step in the NDE, to narrow down the search area or to decide whether to do a more detailedthe search area or to decide whether to do a more detailed inspection

• Can be on macro scale, but also on micro scale (optical microscopy, Scanning Electron Microscopy)

SEM image to determine fibre matrix


SEM image to determine fibre matrix interface bad bonding(CETEX material)

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Thermography ‐ principle

• Determining temperature distributions over a structure– Heat transport will be influenced by existing damage– Heat might be generated due to internal friction and damage

thgrowth

• Heat can be generated by– Laser generates heat pulses (IR‐radiation) which are partially

absorbed or transmitted and partially reflected– Microwave heating– Eddy currents, due to magnetic fields

• Thermografic camera’s are used to visualize the surface


temperatures

Thermography ‐ examples

Air inclusions caused by quality

Impacted area on a composite and corresponding thermogram

y q ydefects during manufacturing


Thermography of a composite delamination as a result of water infiltration

Crack damage in a composite

http://www.youtube.com/watch?v=‐LYXWdam5Pchttp://www.youtube.com/watch?v=UgNJ5hvUETU

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Radiography ‐ principle

• Sample or structure is radiated with electromagnetic radiation with short wavelengths

• Typically Rontgen or X‐rays are used (<1nm f>3 1017)• Typically, Rontgen or X‐rays are used (<1nm, f>3.10 )

• Either changes in thickness or changes in material can be seen, due to different absorption of the radiation

• Volumetric errors are easily detectable, plane‐shaped errors depend on the angle of radiation, so a complementary NDT is sometimes necessary


• Often, a penetrating agent is applied, which has a high absorption for the used radiation, thus obtaining high contrast for the flaws in the structure. Also, matrix cracks can be made visible this way.

Radiography ‐ principleTensile test on carbon epoxy

[±25°/90°]s

At 95% uts At failure

Fatigue test of a carbon epoxy[0°/90°/±45°]s

Matrixcracks and delaminationsaround the hole (due to stress concentrations


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Micro‐tomography ‐ principle

• Aka CT‐scanning• Uses X‐rays to create a large number of cross sections of a 3D‐objectj

• These images can then be recombined with specific software to a virtual 3D‐model

• Again based on the difference in X‐ray absorption of the different constituents of the 3D‐object

• Virtual model can then be investigated:– Highlight materials with the same absorption


g g p– Remove parts– Convert model into an array of segments or cross‐sections, along a pre‐defined path

– …


• Spatial resolution of the scan depends on the maximum width of the specimen, di id d b h b f

sample

divided by the number of pixels on one detector row(typically 1000)

• e.g. cylinder with 2cm diameter yields 20 micron resolutions

• Sample preparation is notnecessary (only cutting to the

X‐ray source

Rotating fixtureX‐ray detector


desired dimensions)

http://www.youtube.com/watch?v=bAaME5yjY2s

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Micro‐tomography ‐ exampleshttp://www.ugct.ugent.be/examples.php

Air bubbles inside a piece of ice

Granite

3D‐view of a guppy


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Micro‐tomography – composite examples

Void content

Going through the thickness of a

balanced glass‐epoxy

Epoxy matrixAnd cladding


Glass fibres

Micro‐tomography – composite examplesCarbon‐PPS with optical fiber Carbon‐PPS, failed in bending

Carbon‐PPS with optical fiberVisualizing the optical fibre


Remark: for carbon reinforced polymers, the difference in x‐ray attenuation for the fibres and the polymer is small (both carbon based). Hence, it is more difficult to obtain a nice contrast

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Optical fibre with bow‐tie gratingg g



Carbon‐epoxy with optical fibre


Crack is present due to curing

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Acoustic emission ‐ principle

• At loads far below ultimate strength, already micro‐cracks will occur

E h k lt i l l h f t t t• Each crack results in a local change of stress state, thus producing a short elastic vibration an acoustic emission

• Larger cracks produce an audible signal, micro‐cracks can only be heard using specific sensors

• Pre existing cracks cannot be determined


• Pre‐existing cracks cannot be determined

• This is an online monitoring method

• Interesting technique to monitor fatigue


• Acoustic emission of a matrix crack differs from that of a fibre failure, but is it detectable?

• Sound has to travel through the material to theSound has to travel through the material, to the sensor– Damping of the sound– Dispersion of the sound– Reflection of the sound– Properties of the used sensor have influence on the signal being picked up


signal being picked up– Other noise on the structure (e.g. friction)

• Can determine damage growth, but it is very difficult to distinguish the failure mechanism

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• To determine the origin of the sound, multiple sensors are necessary.

• By using the time difference for the ‘same’ signal between each sensor, the location can be estimated. (signals change as function of the travelled distance)


400m3 Ammonia pressure vessel

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Acoustic emission – signal processing

• Determining ‘signal’ from noise by setting a ‘discrimination level’ (=certain threshold)D t i i ‘ ti t’ AE• Determining ‘acoustic event’ AE

For damage type characterization: h


Examining each AE• Rise time (short vs. long)• Duration• Amplitude• …

Acoustic emission – signal processing

• Accumulating all the AE (counting)

• Accumulating the entire signalt

• Activity can be monitored (constant noise vs. spikes)

2

1

2t

t

E I dt


Fatigue test• Constant increase during

cycling• At specific points, jumps

occur, signifying larger events.

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Acoustic emission – example

• Quasi‐static test till failure on a carbon reinforced PPS [(0°,90°)]4s(CETEX)(CETEX)

• Different representations


Ultrasonic inspection ‐ principle

• Uses ultrasound (f>20kHz, usually between 1 and 15 MHz)

• Is mechanical wave (not electromechanical)travel speed depends on the materialdepends on the material

• Different for longitudinal and transverse waves

• Each material has an acoustic impedance

• When a wave hits a boundary between two materials, the difference in impedance determines whether a wave is transmitted or reflected (thus allowing to detect foreign bj i )


objects or air)Longitudinal wave Transverse (shear) wave

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• Because of the very low acoustic impedance of air, a coupling medium is necessary, hi h i ll t

Sent pulse Registered echo’s

which is usually water (impedance is 3700 times larger then air).

• An ultrasonic (known) pulse is sent by a transmitter

• The signal is received by the receiver, which can be underneath the sample

water transmitter


underneath the sample (transmitted signal) or above the sample (reflected signal). In this case, the transmitter also serves as receiver.

receiver


• Since both reflected and transmitted contain valuable (and different) information, it i i t ti t i b th

Transmitter/receiver

is interesting to acquire both signals.

• To do so with only one transmitter‐receiver, the signal is reflected at the bottom of the water container

• As the speed of sound is know in water ±1500m/s (and

specimen

mirror

The received signal contains:• Reflection on the top of the specimen• Reflection on delaminations


in water, ±1500m/s,(and preferably also in the material), all signals can be determined by using the corresponding time frame

Reflection on delaminations• Reflection on the bottom of the specimen• Transmitted signal, reflected on the mirror.

This signal is reduced twice, since it travels two times through the specimen

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• Since the speed is know, the time interval for the top and bottom reflection can be determined.– When there is no bottom reflection, this means that the entire pulse has

been absorbed usually voids or very large delaminationsbeen absorbed usually voids or very large delaminations

• Anything in between represents damage (always lower amplitude)

• Small cavities (voids) will not reflect, but absorb the signal– These area’s can be determined from the transmitted part

• The representation of the entire signal (or its amplitude) as function of time is called ‘A‐scan’


Ultrasonic inspection ‐ representation

• A‐scan gives the information for one point (xi,yi) on the surface (=one passing through

A‐scan(=one passing through the thickness)

• Usually, the entire surface is scanned (trajectory)

• C‐scan plots the (desired) information of the A‐scan along the trajectory on the top planeB d D l t th

zy

x

(xi,yi)

B

C‐scan

Scanning trajectory


• B‐ and D‐scan plot the information of the A‐scan on a through‐the‐thickness cross section

• For composites, usually the C‐scan is considered

B‐scan

D‐scan

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Ultrasonic inspection ‐ representation


Ultrasonic inspection ‐ actual setup


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Ultrasonic inspection ‐ a pulse

• Usually, one specific frequency is triggered.• The image below illustrates an actual pulse (left) with its frequency

content (right)


Ultrasonic inspection ‐ C‐scan Steel fabric reinforced epoxy – subjected to impact loading


• Virgin sample.• Blue on the left: clamping of the specimen

• Blue on top and bottom: damage resulting of the start of the water jet cutting

• Pattern: related to the weave pattern and the material

• Low energy impact• Impact damage in the centre is clearly visible

• High energy impact• Impact damage in the centre is clearly visible and obviously larger than for low impact

Remark: colour scales are only valid for one scan.Results cannot be compared between scans

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SOURCE

Z

REFLEC

• Ultrasonic Wave• Pulsed (PUPS)• Harmonic (HUPS)l i l i i

Ultrasonic Polar Scan (UPS) ‐ principle

φ

θ

TRANS-MISSION 1

3

X

-TION

COMPOSITE

• Multi‐angle inspection (~1 million incidence angles)

• Single material spot (no trajectory)

• Reflection or transmission• Amplitude recording• Position of the characteristic

contours encrypts the local


Y2

1 contours encrypts the local elasticity tensor

Remark: harmonic waves (which are ‘state of the art’ in a polar scan) are more sensitive to changes in thickness

SOURCE

Z

REFLEC

Representation of the dataRadial axis = incident angle Angular axis = polar angle Color pigment = TRANSMITTED


φ

θ

TRANS-MISSION 1

3

X

-TION

COMPOSITE

amplitude


Y2

1

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Ultrasonic Polar Scan (UPS) ‐ examples

Aluminium Cold‐rolled steel

‘Isotropic’ materials ‐ pulsed


Note the slight elliptic shape of the characteristic contours small degree of anisotropy.

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Carbon‐epoxy [0°]8 Carbon‐epoxy [0°2,90°2]s

‘orthotropic’ materials ‐ pulsed



Carbon‐epoxy [0°2,90°2]s – grayscale(experiment)

Carbon‐epoxy [0°2,90°2]s – colour (simulation)



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Carbon‐epoxy [0°]8 Carbon‐epoxy [25°]8




Carbon‐epoxy [0°2,90°2]sundamaged

Carbon‐epoxy [0°2,90°2]suDelamination between 2nd and 3rd layer

DAMAGED ‘orthotropic’ materials ‐ harmonic


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Carbon‐epoxy [0°2,90°2]suDelamination between 4th and 5th layer

Carbon‐epoxy [0°2,90°2]suDelamination between 2nd and 3rd layer

DAMAGED ‘orthotropic’ materials ‐ harmonic



DAMAGED ‘orthotropic’ materials – pulsedCETEX, fatigued [(+45°,‐45°)]2s specimen


Location A location B location C

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• Ultrasonic wave• Only harmonic

• Multi angle inspection

SOURCE

Z

Ultrasonic Backscattered Polar Scan (UBPS) ‐ principle

• Multi‐angle inspection (~1 million incidence angles)

• Single material spot (no trajectory)

• Back‐reflection• Amplitude recording• High amplitude spots

l

φ

θ

1

3

X

COMPOSITE


encrypts geometrical material characteristics

Y 2

Representation of the dataRadial axis = incident angle Angular axis = polar angle Color pigment = BACKREFLECTED

Ultrasonic Backscattered Polar Scan (UBPS) ‐ principle

SOURCE

Z

amplitude

φ

θ

1

3

X

COMPOSITE


Y 2

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Ultrasonic Backscattered Polar Scan (UBPS) ‐ examples

Inspection of microscopic surface roughness at top or bottom side of structure

Artificial microscopic structure

One‐dimensional structure Multidirectional structurezero‐dimensional structure(=flat plate)


Location of high‐amplitude backscatter spots gives a quantitative measure for the microscopic surface roughness.


Inspection of microscopic surface roughness at top or bottom side of structure

Rolled steel sample. The sample contains an imprint (~1.1micronmeter depth) which was passed by the work rolls during manufacturing.


Location of high‐amplitude backscatter spots gives a quantitative measure for the microscopic surface roughness.

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• Corroded steel sample.

Ch ti lit d


• Chaotic amplitude map

• The global backscatter amplitude level relates to the level of corrosion

• Technique has been proven very useful for evaluating corrosion in

l t

b) Mild corrosiona) virginal sample


an early stage,

• Inaccessible bottom side of the structure can be assessed

c) Moderate corrosion d)Severe corrosion

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