Infra-Red Lock-in Thermography

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Infra-Red Lock-in Thermography
A. Reichold M. Lefebvre 20 June 2000

• Detect features or defects in a material from
  heat wave interference effects on the surface
• Potentially useful for ATLAS Inner Detector, but
  also for the NAPL in general
• A prototype experimental setup was constructed
  and successfully operated
• Principle of operation
• Theory excerpts
• Experimantal setup
• Data acquisition and analysis
• Analysis results
• IR Camera calibration setup
• Prospects

M. Phys project students Alex Ivison and
Steven Mould
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Principle of Operation

• a periodic heat flux is incident on the surface
  of interest
• the surface temperature is measured and local
  variations in the phase and/or amplitude are
  sought

heat
IR camera
measures surface temperature
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Theory excerpts
Consider the simplest case where the object is an
infinite plate of thickness l along x, and with
the steady state boundary conditions
We also assume heat transport via thermal
conduction in the plate, that is no convection
and no radiation. The problem is then, for the
static case,
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Theory excerpts (continued)
We find that the oscillatory part of the solution
is proportional to
where we recognise the forward and backward
damped waves, where
But to study the amplitude and phase of the x0
surface temperature, it is more convenient to
seek a solution of the form
A study of the transient solution reveals a
longest component lifetime to which we can
associate a natural frequency of the object
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Theory excerpts (continued)
At low frequency, the temperature spectrum
follows the heat input. It acquires a p/4 phase
lag at high frequency
The amplitude decrease with frequency, as the
surface temperature falls behind the heat input.
The rate of change of the amplitude is maximum at
the natural frequency
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Experimental Setup
Bakelite object with defects
DAQ control Labview
glass
PC ComputerBoards
HV 30A 60V PowerTen
9 X 200W bulbs for a usable 1500W
IR Camera Jenoptik Varioscan 3011-ST
PC Jenoptik Varioscan software
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Data Acquisition and Analysis

• M. Phys. Project Students Alex Ivison and
  Steven Mould
• DAQ under Labview control
• sinusoidal heat source (frequency and amplitude)
  
• IR camera trigger (number and relative phase)
• T-probes (light array, glass, object)
• Data taken sets of 4 pictures for periods
  between 15 and 500s
• Data analysis extract amplitude and phase from
  each set of 4 pictures

Labview analysis
BMP file
IR Camera
IRBIS
SID file
ASCII file
PC
C analysis
C
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Results
Analysis of pictures of hole 2 with 120s period
Phase picture
Amplitude picture
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Results (continued)
Hole 2, 120s, amplitude
Hole 2, 150s, amplitude
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Results (continued)
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Results (continued)
Compare with the theory for the infinite plates
case with the following nominal quantities for
bakelite
The results are in qualitative agreement clearly
the experimental conditions are not infinite
plates with constant temperature at the back!
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IR Camera calibration setup
Need to obtain the relation between the camera
ADC and the object temperature
An isothermal box was designed and built An inner
1.5cm thick Al box surrounded by 15cm of
isulation material, with one port for the camera
(with shutter) and two ports for heating/cooling
(with plugs). Blackbody suspended in the box, in
the camera field of view. Under construction.
Special paint ordered. T probes on the box inner
face and on the blackbody.
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IR Camera calibration setup (continued)
Views of the isothermal box
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Prospects

• Very promising.
• Next steps...
• try with other materials (eg ATLAS ID carbon
  fibre)
• improve experimental setup
• redesign the light array (more power?)
• build proper support for light array, glass,
  object
• control interference from ambient conditions
  (cooling?)
• allow for higher frequencies
• finite difference analysis?
• Continue C analysis code development (within
  ROOT?)
• fully commission the isothermal box for IR camera
  calibration