IR Thermography
description
Transcript of IR Thermography
10/31/2009
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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)