Active Thermography for Nondestructive Testing presented by Vetorix Engineering S.r.l.

1. Active Thermography for
Nondestructive Testing
presented by
Vetorix Engineering S.r.l.
1 www.vetorixgroup.com

2. Increased Acceptance of Thermographic NDI
2 www.vetorixgroup.com
Used for NDI of metals,
composites, polymers
and ceramics.
Applications
• Delamination
• Adhesive disbond
• Impact damage
• Corrosion
• Water entrapment
• Thickness measurement
• Porosity
• TBC coating measurement

3. Published Sept 2007
3 www.vetorixgroup.com

4. 4 www.vetorixgroup.com
Examples
Delamination in CFRP
Voids in CFRP
Water inclusion in honeycomb
Hybrid junction

5. NDT Requires Active Thermography
5 www.vetorixgroup.com
IR Camera
Excitation
energy (input)
Emitted
IR energy
Sample
Internal
flaw
Excite sample
View thermal
response via IR
surface
temperature
Some stimulus must be applied to the sample in order
to obtain subsurface information from the IR camera.

6. Active Thermography Basics
6 www.vetorixgroup.com
Light
Hot air
Direct
contact
Heat (or cool) the sample
surface and observe the
response with an IR camera.
IR camera
possible
heat
sources

7. Typical Thermography Results
cooler warmer cooler warmer
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Delaminations obstruct the
flow of heat and cause the
surface to appear warmer.
Trapped water absorbs
incident heat and causes the
surface to appear cooler.

8. Excitation / Detection Schemes
Pulse Step Modulated
response
excitation
response
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temperature
excitation
response
time
temperature
excitation
time
temperature
time
Analyze cooling Analyze heating or cooling Analyze both
The fundamental mechanism for detection of subsurface structure is the
same regardless of how the heat is applied or when it is detected.

9. Pulse Response is Easiest to Interpret
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excitation
response
Surface temperature
falls as 1/√t for an
infinitely thick sample
Tsurf
time
ln(Tsurf)
response
ln (time)
Log-log plot of temperature vs. time
is a straight line with slope -0.5

10. Flash Thermography: Basic Technology
• Most widely implemented
• Fastest
• Non-contact
• Most repeatable
• Provides quantitative measurement
10 www.vetorixgroup.com
Flashlamp
IR camera
defect
PC
IR Image

11. Pulsed Thermography Basics
Heat conduction
11 www.vetorixgroup.com
Probe Internal
Structure
Measure
Surface ΔT
Lamps
IR camera
Excite Surface
Layer
Light
IR radiation
to PC
Process
PC
Blackbody
radiation

12. Cool air IR radiation
12 www.vetorixgroup.com
Heat Transfer Basics
Conduction: Diffusion into
material from higher energy
state (hotter) to lower
(cooler).
Convection: Heat is carried
away from surface by
moving particles (e.g. air
current).
Radiation: Heat is carried
away from surface by
electromagnetic radiation
(light, IR)
How does a sample heated
at one face cool?
Conduction provides the most information about internal structure.
Convection and radiation depend solely on surface properties of the sample.

13. 1
defect
T
13 www.vetorixgroup.com
Defect Model
surface
3
2
Heat from the inbound pulse is blocked by the defect.
Trapped heat energy will diffuse out through the sides of the “trap”.
Area above insulating defect appears hotter than defect-free areas.
Temperature profile above defect is not flat
1
2
3
4
4

14. Log Plot of Temperature vs. Time
6.5
6
5.5
5
4.5
4
3.5
Back
wall
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5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
0 1 2 3 4 5 6
time (sec)
T
bkgd
defect
3
0 1 2 3 4 5 6
ln(time)
ln(T)
bkgd
defect
In the log plot, the defect-free background follows a straight
line until heat flow is obstructed by the back wall of the sample.

15. Break time
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Interpreting the Log Plot
ln (time)
ln (temperature)
normal diffusion
(no defect)
Back wall or
defect slope = - 0.5
The log-log plot is a powerful diagnostic tool that allows analysis without
comparison to a reference or other pixels.

16. diffusion
blocked by
back wall
16 www.vetorixgroup.com
Log Plot: Ideal Slab
ln(t*) ln(t)
ln(Tinit) 1-d diffusion
slope = -0.5
ln(T – T0)
ideal
Thermally isolated: assume
1-d conduction is the only
cooling mechanism.

17. 17 www.vetorixgroup.com
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-4.5 -3.5 -2.5 -1.5 -0.5 0.5 1.5 2.5 3.5
ln(time)
ln(T- T0)
25 mm Plate
1.2 mm FBH
3.3 mm FBH
5.3 mm FBH
Flaw Depth and the Log Plot
Steel Plate with Flat Bottom Holes

18. Case studies
• Water inclusion in honeycomb sandwich panel,
18 www.vetorixgroup.com
rivet junction – helicopter tail
• Impact damage–fusolage
• Flaws – wing leading edge