Active Thermography Maierhofer

InnoTesting 2015 Active Thermography

Application of active thermography in production

processesChristiane Maierhofer, Mathias Ziegler, Rainer Krankenhagen,

Philipp Myrach, Florian JonietzBundesanstalt für Materialforschung und –prüfung

FB 8.7

Energy Infrastructure Environment Material


Content

• Principle of active thermography• Actual research for applications in production

processes: theory and experiment� Planar excitation: Comparison of flash and lockin

thermography

� Light weight constructions

� Join connections

� Local excitation: crack characterisation

� Large structures: Windmill and building structures

• Outlook: Standardisation

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InnoTesting 2015 Active Thermography 3

Principle of active thermography

Flash thermography of an airplane side rudder

• Reflection configuration using 4 flashes from the front side

• InSb IR camera, 640x512 pixel• 100 Hz frame rate• Subtraction of zero image• Measurement duration: 10 s

ca. 300x350 mm2

C. Maierhofer et al, Composites Part B57, 2014, 35-46

Reflection configuration


Heating sources

• Flash lamp• Halogen lamp• Infrared radiator• LED-Array• Laser• Hot air• Ultrasonic sonotrode• Induction

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Theory

5

Non-stationary heat conduction process in anisotropic solids is described by the proportionality of heat flux and temperature gradient using the heat equation (diffusion equation):

T: temperature: position vector

t: time

Heat equation

Solution of parabolic DE:• Spatial boundary conditions

(temperature or heat flux) have to be set

• Temporal starting conditions, i.e. temperature distribution at t = 0, have to be set

• Strong attenuation

Comparison to solution of hyperbolic DE (wave equation):• Spatial boundary conditions have to be

set• Two temporal starting conditions:

temperature distribution at t = 0 and first derivative to time

• Little attenuation

λ: therm. conductivityρ: densityc: spec. heat capacity

[ ] )(),()(),(

)()( rqtrTrt

trTrcr p +∇⋅∇=

∂∂ λρ

rq: introduced heat

per volume: nabla operator∇


Theory of flash excitation

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Solution for Dirac pulse (flash excitation)

t

qtTe

t

qtzT tz

πεπεα == − ),0(,),( 42

1D, for semi infinite isotropic bodies

effusivity

diffusivity

cρλε =

max. traceable penetration depth Lmax depends on heat density , thermal sensitivity of IR camera (NETD) and spec. heat capacity (n=2 bis 3)NETDTnc

qL

∆⋅= 1

max ρ

Penetration depth L depends on time t and on diffusivity αtL απ=

Depth resolution ∆L depends on time resolution and thus on frame rate of the IR camerat

tLL ∆=∆

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cρλα =


Testing of steel and copper

Comparison of penetration depths at similar ρρρρ and c(thermograms after subtraction of zero image)

copper

t = 0.11 ms t = 0.2 ms t = 0.47 ms

t = 0.5 ms t = 1.1 ms t = 3.2 ms

steel

Similar penetration depth of about 8 mm

FBH with 12 mm Ø and remaining wall thickness of 2, 4, 6 und 8 mm

Depth resolution in steel is 10x better as in cupper

TNS Project Flash Thermography >> Draft standard on flash thermography in DIN AA 062-08-27 AA Visual and thermographic testing

NETDTncq

L∆⋅

= 1max ρ

Cu steel


3D Reconstruction: Inverse Solution

thermogram reconstruction

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back sidegeometry

z

front sidetemperature field

T(x, y, t)

inverse problemF-1(T) = z

?

direct problemF with F(z) = T

Regina Richter, DissertationTU Braunschweig, Dez. 2013

DFG-Project for reconstruction ofdamage, together with ZIBVh 8590, 2014-2017

pipe with corrosion

Wall thickness: 3.6 mm

Pre knowledge: Material parameters are known


Objective: Determination of diameter of spot welds in reflection configuration without additional coating

M. Ziegler, IGF project Diatherm 17686 N / 1Project partner: BAM 9.3 Prof. Rethmeier

Visualisation of heat diffusion by projection of a laser line along the sample edge:P = 50 W v = 10 m/s t = 50 ms

Laser

Adhesive joint

Optimum

Diameter of spot welds

Adhesive joint

D = 2.33 mm

Optimum

D = 5.23 mm

Spatter limit

D = 6.12 mm

Full automation pf spot welding in automotive industrywww.kuka-systems.com

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Frame rate: 40 Hz

Intensity devolution at mid point

Division

Result: Good correction of inhomogeneous emissivity, radial symmetry can be recognized

� Data analysis is possible (e.g. full width at half maximum)

Diameter of spot welds

Thermogram at time of max. intensity , strong emissivity variations � no reasonable data analysis

Thermogram at the end of the sequence shows only emissivity differences (due to scratches, discoloration, pollution etc.)

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Validation at CFRP test specimen

Comparison of flash thermography and CT, specimen R 1

3rd layer1st layer

T R

x = 1.2 mmx = 0.48 mm

Set-up:120x120x2 mm3

5 layers 0°/90°CFRP fabric6k carbon fibre bundleEpoxy matrixDefects during production:Reinjected areaAdditional fibre bundleWooden stick

TT

CT

Cooperation with BAM 8.5 and Benteler SGLEC project Thermobot (www.thermobot.eu)

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2 cm∆x = 81 µm

∆x = 240 µm


Comparison of flash thermography and CT, specimen R 3/4

3rd layer

x = 1.28 mm

Set-up:2 plates glued with 4 beadsDefects:Broken fibre bundleBeads:1: release agent2: without sand blasting3: o.k.4: wrong mixture

TT

CT

R

Phase image from mid

x = 2.1 mm

T3

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1

3

2

1

C. Maierhofer et al., Composites Part B, 64, 2014, 175-186

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2 cm

Validation at CFRP test specimen


Theory of lockin excitation

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Solution for periodic heating(lockin excitation, thermal waves)

Penetration depth (or thermal diffusion length ) of thermal wave: • decreases with increasing frequency• Increases with increasing diffusivity

Phase velocity of thermal wave: • Increases with increasing frequency and

with increasing diffusivity

>> thermal waves have a high dispersion and high at tenuation>> broadening of heat impulses (multi frequency)

)4//(/0),( πω −−−− ⋅= µztiµz eeTtzT

ωα2=µ

ωαω 2=⋅= µv

1D, for semi infinite isotropic bodies

effusivity

diffusivity

cρλε =

cρλα =


Comparison of flash/lockin thermography

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time

A

ϕ

FT(fA)

time

FT(f)A

ϕ

Flash thermography Lockin thermography

• Thermograms at raising or max. contrast• Pulse-Phase-Thermography (PPT)• Thermal signal reconstruction (TSR)

• Online FFT or 4 point method at excitation frequency

• Offline FFT at excitation frequency

f


Comparison: 4.4 J impact damage in CFRP

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Frequency in Hz

1 Hz 0.1 Hz 0.05 Hz0.45 mm 0.63 mm 2 mm

Depth in mm

• Phase images at 1 Hz are similar• Phase images at lower frequency have less contrast for lockin

excitation• Possible reason: Material separation due to thermal expansion

is larger for flash excitation>> Comparison to laser speckle interferometry/shearography

C. Maierhofer, ECNDT 2014, Keynote, www.ndt.netCFRP samples from ZFL Haldensleben, Prof. Dr. J. Häberle, ZIM Kooperationsnetzwerk


Comparison: Cu-Shunts am LHC, CERN

2 mm Cu sheet

3 mm Cu sheet

Flash

Lockin1 Hz20 P.

Flash

Lockin1 Hz20 P.

www.cern.ch

Cu-Shunts are supporting the connection of two cable heads

Customer: CERN, C. ScheuerleinC. Maierhofer et al, NDT&E International, 52, 2012, 103-111

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New excitation sources: LED -Array

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Stand atControl 2014

LEDcontrol Computer

IR camera

Photo-diode

Sample

M. Ziegler, MNPQ project, 2013-2014

Advantages of LED excitation source

� Can be modulated up to 1 kHz

� Narrow spectrum, no overlap with IR camera


Current: Pores in aluminium

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Al pressure casting: rear axle carrier of the chassis of BMW i3

http://www.springerprofessional.de/, Hengst, BMW

Cooperation with Bergakademie TU Freiberg

Thermogram after flash

Phase image at 4.6 Hz

Section of a crankcase, Length150 mm, thickness 2 mm

KCT fromBAM 8.5

Lockin excitation at 8 HzLaser widening

U. Richter, C. Maierhofer, R. Mischke, M. Röllig, Int. Foundry Research, 2015, accepted


Flash thermography Lockin thermography

Advantages:• Very short measurement time• Multi spectral

>> all penetration depth until Lmax

Disadvantages:• Strong thermal strain• less SNR especially at higher

penetration depths

Advantages:• Less thermal strain• Higher SNR at larger penetration depth• Higher frequencies (LED, LASER)• Depth selective

Disadvantages:• Much long measurement times• Each measurement is only optimized for one

penetration depth

time

A

ϕ

FT(fA)

time

FT(f)A

ϕ

f

Comparison of flash/lockin thermography


Cracks and local excitation

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Characterisation of vertical cracks by heating with a laser spot

Joachim Schlichting, Dissertation, TU Berlin, 2012Adolf-Martens-Award 2013

Static laser spotDetermination of crack depth and crack angle is possible

Moved laser spotFast detection of shallow and

narrow cracks is possible

time


Test parameters:• Scan speed: 12 mm/s

• Trace distance: 0.5 mm

• Scan time: 40 s• Power density: ~0.7 kW/cm² (9 W, 1.3 mm Spot Ø)

• Camera resol.: 39 µm/pixel

Calibration block 1 (aka MTU Nr. 3) Ø 50 mm:• Reference for MT (fluorescent) EN ISO 9934-2 Annex B • 90MnCrV8 steel, hardened, browned• Stress corrosion cracks and grinding cracks:

width: 0.1-5 µm, depth: 2 µm-1 mm, approved by microscopy

Reference specimen for magnetic particle testing

Sub-µm cracks are recognized! Below diffraction limit (~5 µm)

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Natural cracks

P. Myrach, M. Ziegler, C. Maierhofer, M. Kreutzbruck, AIP conference proceedings, QNDE, 1581, 2014, 1624-1630

INS Project Laser-Thermography M. Ziegler


Testing of large scaled structures

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Objective:Testing and

inspection withoutscaffolding and

access aids

Building facades

Funded by

Rotor blades of windmills

VIP-Project IKARUS, BMBF R. Krankenhagen


Testing of rotor blades

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Cross section of a rotor blade

leading edge

trailing edge

beam

foamFull GFRP

Adhesive defects

Which structures and which defects and inhomogeneities can be detected with active thermography?

18 to 22 °C

Project IKARUS (Funding program VIP of BMBF, FKZ 03V0135, 2011-2015):Infrared-Kameratechnology for non-touching Analysis of Rotor blades under Offshore conditions


10 p.m.

~ 180 cm

~ 20

0 cm• Segment of a rotor blade with natural and

artificial defects and inhomogeneities• Comparison of measurements and numerical

simulations under different environmental conditions (solar radiation, air temperature, background radiation)

Testing of a rotor blade segment

103 cm 57 cm

Measurement Simulation

Worzewski et al, DGZfP Jahrestagung 2014

Testing of rotor blades

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Detection of voids during solar heating

Thermogram after 3 h sun radiationTmax = 48 °C

Plaster scratches at Magdeburger Dom

From: maps.google.deFrom: maps.google.deFrom: maps.google.deFrom: maps.google.de

• Plaster scratches from 13th century• Voids and delaminations are getting warmer due to

solar heating• Heating as well as cooling down (e. g. due to

shadowing) enable further structural investigations >> active thermography

Steel engraving of central picture: Otto I. with his two wives Adelhaid and Editha

BBR project 3D-Mapping, Cooperation with with FhG IFF and IDK, local restorers

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Alternate heating with sun and clouds

11:00 12:00 13:00 14:00 15:00 16:00

0,0

0,5

1,0

Nor

mal

isie

rt

Zeit in Stunden

Temperaturentwicklung Fassade Globalstrahlung Hukseflux

Testing of building facades

BBR project ERBE, ZukunftBau, 2013-2015

• Culture house in Cobbelsdorf, Coswig• Mural painting Industrialized agriculture of

Erich Enge 1970/71• Beam construction• Sun radiation

from 150 W/m2 to850 W/m2 on 9.9.2014

• Time: 10:45 to 15:50 o’clock

2nd phase image of the sequence

10th phase image of the sequence

steel beams

cracks

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Standardisation

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DIN 54192Active thermography

Draft standardflash thermography

Structure of draft standardlockin thermography

DIN NA 062-08-27 AA Visual and thermographic testing

CEN TC 138/WG 11 Infrared and thermographic testing

prEN 16714-1Part 1 Gener-al principles

prEN 16714-2Part 2 Equip-ment

prEN 16714-3Part 3 Terms and definitions

Active thermography

EMRP VITCEA: Validated inspection methodsfor composites in energy applications

Pre Normative EMPIR-Proposal of EURAMET Focus Group on Standardization of Thermal Imagers (from 2016): Calibration of infrared cameras, crack detection


Thermography Team

Thank you!

Mathias ZieglerPhilipp MyrachFlorian JonietzMarco LuchtTamara WorzewskiManoucher DoroshtnasirHenrik SteinfurthMathias Röllig

Rainer KrankenhagenNick RothbartSven AugustinErik ThielMarc KreutzbruckRegina RichterJoachim SchlichtingMercedes Reischel

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Active Thermography Maierhofer

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