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NDE using IR Thermography

C.V.KrishnamurthyOctober 30, 2009

What is IR Thermography ?

• Collecting radiation of heat in the infrared

band of the Electromagnetic Spectrum

• Quantifying the measured radiation and

assessing the Temperature

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Infrared Spectrum

IR Absorption Characteristics

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Matter Radiates Heat

Gases/Liquids/Solids at T > 0 Radiate Heat

Thermal and Heat Transfer Characteristics

of Materials

• Thermal Conductivity (k )– capability to transfer heat across a domain

having a steady temperature gradient; higher the value, faster the approach to equilibrium; it is high for metals and low for porous materials

• Heat Capacity (Cp)– ability to store heat (also referred to as thermal

capacitance); structures with low thermal capacitance reach equilibrium sooner when placed in a cooler environment

• Thermal Diffusivity ( = k/ Cp )– relates more to transient heat flow; widely

used in NDE for flaw detection

• Convection

– requires medium; involves physical movement

of molecules; depends on flow parameters;

depends linearly on thermal gradient

• Conduction– requires medium; involves vibrational

characteristics of molecules; depends linearly

on thermal gradient

• Radiation– No medium required; involves

electromagnetic wave propagation

characteristics; depends on the fourth power of

the absolute temperature of the object

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Thermal Properties of Common Materials

C

kCke

Blackbody Radiation Characteristics

1

12/2

3

TkhffBec

hfe

Tkhf B

2

2 Tke B

f

(Planck Radiation Law)

(Rayleigh-Jeans Approximation)

2 2

1 1

, ,

1 2 2 14

,

0

( ) ( )

( , ) (0, ) (0, )

( )

B B

B

L T d L T d

e e eTL T d

For

Wien’s displacement law

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Black and Gray Bodies

Radiometry

)()1( 0 aTL )()1( 0 aTL

)(0 oTL )(0 oTL

)()1( 0 atmTL

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Radiation Transfer

Reflected (Wr TB4)

Transmitted (Wt TA4)

Emitted (We Te4)

Source A Source B

Wr+Wt+We = Specimen Radiosity

Specimen

Temperature Te

reflectivity

transmissivity

emissivity

Radiation Transfer Characteristics

Transmitted Energy = Eo

Source A

Specular

Reflection = Eo

Smooth Surface

Diffuse

Reflections

Rough Surface

: absorptivity

: reflectivity

: transmittivity

+ + = 1 (from the law of energy

conservation)

Absorbed Energy = Eo

Kirchoff’s law: ratio of radiation intensities for two surfaces is equal to the ratio of their absorptivities.

Implies = (i.e., 1 - )

When transmittivity is low (for opaque bodies), 1 -

Thermal equilibrium is achieved when a body is emitting radiation at the same rate that it absorbs it from the surroundings

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Surface Features and Emissivitysmooth slightly rough moderately

roughvery rough

Spectral Emissivities of Common Materials

Material T (C) Wavelength Range

( m)

Emissivity

( )

Concrete (dry) 36 2 – 5 0.95

Plastic (acrylic) 36 2 – 5 0.94

Wood (polished) 36 2 – 5 0.86

Water 0 8 – 14 0.98

Ice 0 8 - 14 0.97

Snow 0 8 - 14 0.8

Rubber 0 8 - 14 0.95

Lampblack 0 8 - 14 0.96

Steel - oxidized 0 8 - 14 0.88

Steel – rolled freshly 0 8 - 14 0.24

Steel – nickel plated 0 8 - 14 0.11

Sources: AGEMA Infrared Systems, Inc. and Linear Laboratories, Inc.

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Advantages of high

• It follows from Stefan-Boltzmann’s law that surfaces with a high emit a higher intensity radiation at a given T, thereby providing a larger signal (high S/N ratio)

• High surfaces are, by definition, poor reflectors. Low surfaces tend to reflect radiation from other sources. Detected signal unrelated to the object (low S/N ratio)

• High surfaces also absorb more radiant energy. Radiant sources can therefore be effective in inducing a thermal gradient – a favourable feature in testing

Coating Substrate

Thermography - Techniques

• Passive

• Active

– Pulsed (pulse heating with observation during the cooling phase – time domain)

– Step heating (long pulse with observation during continuous heating – time-resolved infrared radiometry)

– Lock-In (continuous sinusoidal heating with observation while heating – frequency domain)

– Pulsed Phase (mix of pulsed and lock-in methods using Fourier transforms)

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Infrared Thermography - Features

• Non-contact

• Sensor collects radiated heat flux over a narrow wavelength interval

• Sensors can be a point sensing type, line scanners or focal plane arrays (2D)

• Detection can be either pyroelectric (signal is proportional to absorbed thermal energy) or photonic (photoconductive or photoelectric - signal is proportional to the number of IR photons collected)

• Real-time information of radiating object – can generate images from which anomalies can be identified and assessed

• Radiation temperature is evaluated from the received/collected energy/photons

Application Areas

• Electrical Systems

• Building Envelopes and Structures

• Mechanical Systems

• Petrochemical Applications

• Electronic Equipment

• Environmental Applications

• Automotive Applications

• Aerospace Applications

• Medical / Veterinary Applications

• Pulp and Paper

• Steam Turbine and Hydroelectric Generators

Pipe thinning

Moisture Ingress

Heat leaks

Overheating

Material

characterisation

Online Process

Monitoring

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IR Imaging - I

Example:

(IFOV) = 1 mrad; d = 1m; D 1 mm

HFOV =30 ; VFOV = 20 leads to inspection area of

0.53 m 0.36 m at d = 1m

IR Imaging - II

SNR

TNETD

FtMTF

VIFOVHIFOVNETDMRTD

eyeO

O

)(

SNR

TNETD

eye integration time frame rate

e.g., MRTD 0.05 C at 25 C

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Modulation Transfer Function (MTF)

oM

MMTF

LL

LLM ;

minmax

minmax

IR Detectors

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Passive Thermography

Infrared Thermography has been proven to find, quantify, and document problems caused by:

Electrical Mechanical Building

• Faulty components • Shaft misalignment • Moisture infiltration of roof

• Poor connections • Worn bushings & bearings • Air infiltration or exfiltration

• Corrosion • Improper tension of belts & pulleys • Areas of potential mold growth

• Contamination • Over or under lubrication • Air circulation and distribution

• Load imbalances • Gear box anomalies • Leaky or clogged pipes

• and much more • Excess friction • and much more

• and much more

Examples of Passive IR Thermography

Electrical Inspections

The anomaly, or hot spot, indicates a probable problem

with the disconnect.

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Examples of Passive IR Thermography

Leak in radiant heating system under a concrete pad.

Subsurface Leaks

Examples of Passive IR Thermography

Mechanical Inspections

Infrared Image: Bearing

Over Lubrication

Digital Image

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Moisture ingress in building walls

Effects of Environment on T Measurement

• Solar radiation– Masks the inspection strongly; periods of quick heating (day) /

cooling (night) preferred

• Cloud cover– Absorbs and scatters; slows heat transfer; little or no cloud cover

preferred

• Wind – Aids heat transfer; < 6.7 m/s (15 mi/hr) preferred

• Moisture (also humidity)– Moisture on the ground masks the true features; humidity affects

inspection as water vapour absorbs strongly

• Ambient temperature– Need to factor temporal fluctuations

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Thermography - Techniques

• Passive

Active

– Pulsed (pulse heating with observation during the cooling phase – time domain)

– Step heating (long pulse with observation during continuous heating – time-resolved infrared radiometry)

– Lock-In (continuous sinusoidal heating with observation while heating – frequency domain)

– Pulsed Phase (mix of pulsed and lock-in methods using Fourier transforms)

Transient Thermography - I

Reflection Mode

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Response of a semi-infinite homogenous

medium to a short heat pulse

t

Q

dt

dTc

dx

Tdk

2

2

One-dimensional equation for

Temperature evolution when heated by

an instantaneous pulse of strength Q

c

k

t

x

t

QtxT ;

4exp

)(2),(

2

- density; k – thermal conductivity; c – specific heat

Thermal

diffusivity

Response of a homogenous slab to a

short heat pulse

ckeeebb

bR effusivity;/;

1

112

slab

x = 0 x = l

Deeper

delamination

Back wall

Shallow

delamination

ln (t)

ln (

T)

Semi-infinite

medium (slope –0.5)

Reflectivities of

common materials

Al/Air 1.0

Al/Epoxy Resin 0.95

CFRP/Air 1.0

CFRP/Epoxy Resin 0.3 ( )

0.7 ( )

t

lnR

t

QtxT

n

n22

1

exp21)(2

),0(

Reflection coefficient

accounts for

multiple reflections

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Transient Thermography –

Defect detection with derivatives

Derivatives can be evaluated quickly and without adding noise

Source: Steven Shepard, James Lhota, Bharat Chaudhry, and Yulin Hou, Thermal Wave Imaging, Inc.

Thermographic Signal Reconstruction (TSR)

Transient Thermography - II

Transmission Mode

2 2

21

( , ) 1 2 1 expn

n

Q nT L t t

DCL L

CKlT

QC

t

l

M

;;38.1

21

2

2

w =1.38 at T/TM = 0.5

where,

T : Temperature, L: Thickness, t : Time, Q : Pulse of radiation energy,

D: Density, C: Heat capacity, α : Thermal diffusivity,

Flash method for determining thermal diffusivity,

heat capacity and thermal conductivity of a slab

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Features of commonly used

excitation sources

Stimulus Source Power/Energy Approximations Pulse Duration

Flash Lamps 1kW ~1 – 50 ms

Quartz Bulbs 10kW 1 – 15 s

Ceramic Heater 250W – 5 kW > 60 s

Hot/Cold Water 1 kW > 60 s

Hot/Cold Air 1 kW > 1 s

Pulsed Laser ~ 100 J 1 – 10 ps

Ultrasonic Transducer 2 kW Few ms

from N.P. Avdelidis et al, Progress in Aerospace Sciences vol.40 (2004) pp 143-162

• 6 mm thick soft steel samples, Xenon flash lamps or Quartz heaters

• > 6 mm thick soft steel samples, Quartz heaters

Source: ASNT Handbook (2001)

Transient Thermography - Issues

Typical heat pulse – Spectral content changes with

time !

Initial frames get saturated

Long tail affects many frames

time

amp

litu

de

IR Camera

Integration

time

Frames captured by the IR camera

Video frame rate

(60 Hz)

Need “clean” pulses of

short duration and

without long tails

Non-uniformity of irradiation

Ambient noise can become significant

Need reference or

“non-defect” image

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Pulse Thermography - Honeycomb

Sample

Planar defect sizes of 25 mm 25 mm

and 40 mm 40 mm were considered

Pulse Thermography –

Corrosion under Paint

Paint thickness varies between 0.2 mm to 0.8 mm

Maximum metal plate thickness is about 1.1 mm

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Corrosion under Paint - Results

Th

ick

ness

(m

m)

Scan length (mm)

0.38 0.53

Depth Estimation - Comparison between Different Methods

Ultrasonic

C-Scan (avg.)

Peak contrast slope Pulse Phase

Thermography

Defect #1 0.35 mm 0.38 mm 0.27 mm

Defect #2 0.49 mm 0.53 mm 0.54 mm

Steel: Rules of Thumb

• For < 2.5 mm thick, high carbon content steel, high intensity, short duration pulse ( 4 ms)

• For > 2.5 mm thick steel, lower intensity, longer pulse (> 3s) or stepped mode heating

• 10% material loss detectable (under lab conditions) for < 0.1 inch thick steel plates

• 25% material loss identifiable (under lab conditions) for steel plates 2.5 mm to 25 mm thick

Source: ASNT Handbook, Xavier P.V. Maldague, 2nd Edition (2001)

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Response of a semi-infinite homogenous

medium to step heating

Solution for surface Temperature evolution under step heating with constant flux Fo

per unit time per unit area for t > 0

The logarithmic temperature-time profile of the surface temperature is a straight line

with slope +0.5 for a semi-infinite, defect-free sample.

ln (t)

ln(T

*)

Surface response of a

coating on a

conducting substrate

ln (t)

ln(T

*)

Surface response of a

coating on a

non-conducting substrate

t

Q

t

k

FtxT o2),0(

Response of a layer on a substrate

to step heating

Computed Measured

Zirconia coatings

x = 0 x = l

Source: Spicer et al (1991) in Thermosense XIII, Proc.

SPIE vol.1467: 311-321Source: Oslander et al (1991) in Thermosense XIII,

Proc. SPIE vol.2766: 218-227

t

nlerfc

t

nl

t

lnRtAtxT

ooon

n )exp()(21),0(22

1

Involves the thermal characteristics of the substrate

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Response of a semi-infinite homogenous

medium to periodic excitation

dt

dTc

dx

Tdk

2

2

One-dimensional equation for Temperature evolution

- density; k – thermal conductivity; c – specific heat

surface temperature is prescribed by

Temperature at any interior point as a function of time is given by

)2/();cos(),( xtAetxT x

)cos(),0( tAtxT

Solution represents a temperature wave of wavenumber and wavelength

given by 2 / = where f = /2)/4( f

Propagating speed of these temperature waves is given by - dispersive

Temperature waves at higher frequencies attenuate more rapidly with depth with the

thermal diffusion length given by

Phase of the temperature wave shows a progressive lag which increases with frequency

2

Thermal Wave Characteristics of Materials

)2cos(535

)2cos(),( 2 tT

teTtxT oo

Material

Thermal

Diffusion length

at 0.1 Hz

Thermal

Diffusion length

at 1.0 Hz

Aluminum 108.2 mm 34.2 mm

Titanium 34.2 mm 10.8 mm

CFRP ( ) 7.3 mm 2.3 mm

CFRP ( ) 21.5 mm 6.8 mm

fall in

amplitude

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Lock-in Thermography – Amplitude and

Phase Images

IR Focal Plane Array

Pixel-wise data

retrieval

2

24

2

131 )()()( SSSSxA

24

131

1 tan)(SS

SSx

4

4321 SSSSTavg

using 4 measurements in

one modulation cycle

Amplitude

Phase

Avg. Temperature

Determine

Phase image insensitive to surface

roughness and related issues which

affect amplitude images

Steady-state response assumed

Lock-in Thermography

Experimental arrangement

Transmission Mode Reflection Mode

More than 4 points per cycle required to reduce noise

Acquisition time should be at least one period (eg. f = 0.03 Hz used

to inspect 2 mm thick composite laminates requires an acquisition

time of 2 min)

Phase image provides more depth information than amplitude image

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Example of Lock-in Thermography with

Ultrasonic Excitation

Principle and set-up of lockin thermography with ultrasonic excitation.

Care needs to be taken for avoiding standing wave patterns

The real and imaginary parts of the coupling between the

ultrasonic wave and the defect exist.

Source: Th. Zweschper et al, NDT.net - February 2003, Vol. 8 No.2

Ultrasound Lock-in Thermography –

Illustration

Configuration of the blind hole steel

sample. Metal cylinders were bonded into

the holes with a diameter of 36 mm and

18 mm to serve as ultrasonic absorbers.

Front of sample was painted black.

Phase image and profile of second row of holes. Lock-in

frequency 0.03 Hz. Excitation frequency 19.4 kHz.

Excitation power: 800W.

from Dillenz et al “Lock-in thermography for depth resolved defect characterisation”

at the 15th WCNDT Conference, 2000

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Vibrothermography (Thermosonics)

Sonic energy from the horn should be coupled into the part as

efficiently as possible.

Energy inserted to the part should not be coupled to the fixtures

or mounting hardware.

The part and horn should be mounted rigidly.

The horn should not damage the surface of the part.

Schematic of vibrothermography interaction

Acoustic energy from a horn is injected into a solid sample, causing frictional heating at

the tip, or along the faces of a crack

Issues

Repeatable ?

Irreversible ?

Quantitative models ?

up to about

25 kHz

Pulsed Phase Thermography (PPT) –

Principle

nn

N

k

Nikn

n iekTF ImRe)(1

0

/2

n

nnnnn iA

Re

Imtan;ImRe 122

Pixel-wise data

retrieval

IR Focal Plane Array

N

nfn

Time interval between

thermal imagesNumber of

thermograms in the

sequence

Use of max at each pixel leads to

better image quality

= 16.6 ms fmax = 60 Hz

N = 32 images, fmin = 1.88 Hz

(i,j)T( C)

for each pixel(i,j)

Infrared image sequence

(after thermal pulse)

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Pulsed Phase Thermography - Features

• PPT provides phase images in addition to amplitude images produced by PT (Pulsed technique)

• Unlike in PT, PPT does not require reference or “non-defect” image

• Better than the Lock-in technique since information can be deduced at several frequencies

Plastic plate with a bonded half-sphere

(non-flat specimen)

from: Maldague et al, Can. Soc. Nondestructive Testing Journal, vol. 19 pp 5-10 (1997)

Tomography

Source: “Theory and Practice of Infrared Technology for Nondestructive Testing”, Xavier P.V. Maldague

John Wiley & Sons Inc. (2001)

Thermal tomography of 1.2 mm deep

Teflon insert in carbon-epoxy specimenPrinciple - surface temperature evolution sequence,

after pulse excitation, used to construct a Thermogram

Raw

image

Smoothed

image

Tomogram of

the layer at

0.8 to 1.5 mm

Tomogram of

the layer at

1.8 to 2.0 mm

Tomogram of

the layer at

1.4 to 1.8 mm

CFRP panel specimen 4.25 mm

thick layers (28) with a 10 mm dia

Teflon insert

Timegram

TGMc_max

Timegram

2

264.3 zt

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Scanning Thermal Wave Microscopy

Combines high spatial

resolution microscopy with

thermal wave imaging

(topographic, amplitude and

phase images)

Surface and sub-surface

features can be imaged

Micro- and Nano-scale images

(kHz frequencies used)

Probe-tip – sample interaction

complex due to couplant

(requires standardization)

(Ohmyoung Kwon1, Li Shi and Arun Majumdar, Transactions of the ASME, vol 125 Feb 2003)

Thermal wave imaging of a VLSI via structure: (a) Topography image; (b) Phase lag image at 6.4 kHz; (c) Amplitude image at 6.4 kHz.

Summary

Non-contact

Fast (one sided) surface

inspection

Portability

Large area inspection

(several m2)

Simulations help

Variable Emissivity

Non-uniform heating

(Active mode)

Transitory nature of

signals require fast

recording IR cameras

Deep defects difficult to

detect

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Matter radiates heat, but…

False color images can be misleading!

References

• H.S. Carslaw and J.C. Jaeger, Conduction of Heat in Solids, 2nd Ed. OUP (1959) Reprinted 2004

• Xavier P.V. Maldague, Theory and Practice of Infrared Technology for Nondestructive Testing, John Wiley & Sons Inc., 2001

• Xavier P.V. Maldague, Infrared and Thermal Testing, ASNT Vol. 3, 2001

• Proceedings of SPIE – Thermosense

• Quantitative Infrared Thermography (QIRT) Conferences

• World Conferences on Nondestructive Evaluation

• Reviews of Progress in Quantitative Nondestructive Evaluation (QNDE)