active infrared thermography in non-destructive...

Active Infrared Thermography

in Non-destructive Testing

M. Hain, J. Bartl, V. Jacko

Institute of Measurement Science

Slovak Academy of Sciences,

Bratislava, Slovak Republic

e-mail: [email protected]

mailto:[email protected]

Outline of the presentation

• Motivation of the research

• Basic physical principle of infrared thermography

• Passive / Active infrared thermography

• Numerical modelling of heat propagation in non-

homogenious environment

• Finite elements method

• Description of the experiment

• Results

• Aplications

• Conclusions

24. 5. 2009 Measurement 2009, Smolenice 2

Motivation for the research

Development of fast, non-contact and reliable method for

non-destructive revealing of hidden non-homogenities

(defects) inside 3D objects.

For this purpose also the X-ray radiography or CT tomography are very

effective tools with superior resolution but they have several limitations:

1. limited size of objects under test,

2. transmissive methods - require access from both side of the object,

3. can’t be used for in situ testing


source of radiation

detector

source of radiation

detector

transmissive method reflective method

Basic physical principle of infrared

thermography


Bodies with T>0 are emitting thermal radiation

Energy in wavelength band (, +d) emitted from unity of surface per unityof time to a half-space is described by

Planck’s law of radiation

1

12)(

5

2

kT

hc

e

hcTM

T M (T)

Passive / Active infrared thermography


Passive infrared thermography

Thermal radiation emitted from object’s surface is scanned by infrared

camera, and gives information about the surface temperature of the

object in thermal equilibrium.

Active infrared thermography

The object under test is thermally excited - usually irradiated by a source

of infrared radiation. In this case the object is in non-equilibrium state

and the transient temperature field measured by thermographic camera

provides information about the thermo-physical properties, defects and

inhomogenities inside the object.

Active methods:

- pulse methods

- lock-in (modulated) methods

- pulse phase methods

The principle and basic set-up of pulse active

infrared thermography system

24. 5. 2009 6Measurement 2009, Smolenice

In the pulse thermography the

object under test is during a limited

time thermally excited by an

infrared source of radiation.

Some part of the IR radiation

incident on the object’s surface is

absorbed and transformed into a

thermal energy, which propagates

by thermal diffusion from surface

to the inward of the object.

Subsurface defects change

thermal diffusion rate and

therefore they can by detected as

surface areas of different

temperature with respect to normal

areas.

Optimal design of the pulse active

thermographic system


To make an optimal design of the pulse active infrared thermographic

system we have to answer these questions:

1. How long should be the object under test irradiated?

2. How much infrared energy should be absorbed in the object?

3. How long should be the delay between thermal excitation (irradiation)

and sensing of the thermographic image?

4. What should be the minimal thermal sensitivity (NETD) of the

thermographic camera?

To find quantitative answers to these questions we need to create and

solve a model of heat propagation in non-homogeneous environment.

The thermal energy propagation within the material can be described by

Fourier’s partial differential equation

where T is absolute temperature (K),

t is time (s),

Q is supplied thermal energy (J),

is mass density of the material (kg.m-3),

Cp is specific heat capacity at constant pressure (J.kg-1.K-1),

k is thermal conductivity (W.m-1.K-1).

Heat propagation in material


QTkt

TC p

)..(.

Heat propagation in material


In very simple cases it is possible to find analytical solution of the heat

propagation problem.

In the case of complex non-homogeneous 3D objects it is necessary to

use numerical solution of the Fourier’s differential equation.

One of the suitable methods is finite element method – FEM.

Finite Element Method


The basic concept of this method is to divide the area of solution into smaller

parts called finite elements, connected at nodal points (mesh generation).

In each node the physical equilibrium equation is defined, and on the object’s

boundary the boundary conditions are defined.

In this way the physical problem described by partial differential equation

PDE is break down into linear system of equations.

For thermal FEM simulation of active thermography method the Comsol

Multiphysics program was used.

Numerical modelling of heat propagation in

non-homogenious environment


For the heat propagation model the Fourier’s PDE equation was used with the

following assumptions:

1. heat energy is exchanged between object and surrounding only through upper side

of the object

2. upper side of the object is irradiated by the IR radiation

3. the energy equilibrium through the upper side of the body is described by the border

equation

)(.)..( 44TThTTTk as n

where T is temperature of the object,

Ts is temperature of the surrounding (background),

Ta is temperature of the air layer next to object,

ε is emissivity of the object’s surface,

h is heat transfer coefficient (W.m-2.K-1)

Results of FEM modelling


Temperature field of the object with

subsurface defect irradiated by IR radiation –

pseudo-colour presentation of FEM model

results

Time evolution of the temperature along the

surface line of the object with subsurface

defect irradiated by IR radiation – results from

FEM model

Modelling parameters:

material gypsum

heat flux 500W/m2

Thermal contrast maximization


To find the optimal time for observation, we will evaluate thermal

contrast defined as:

0ss

sdT

TtT

tTtTtC

where Td(t) is surface temperature of defect area,

Ts(0) is surface temp. of sound area before heating pulse is applied,

Ts(t) is surface temperature of sound area at the time t

thermal contrast between defect and sound area

Analysis of the thermal contrast

shows a local extreme at the time

360s after start of the heating

pulse.

Experiment


To prove the theory and FEM modeling results, the experimental active

thermographic measurements were done.

Basic set-up of the measuring system consist from thermographic camera

NEC San-ei Thermo Tracer TH7102WX and two IR panel heaters.

The camera is equipped with

FPA micro-bolometric array

320x240; camera’s

instantaneous field of view is

1.6 mrad. Distance camera -

object under test was 0.625

m; it corresponds to the

spatial resolution 1 mm on

the object’s surface.

ExperimentObject under test


Front Rear

Material: concrete, gypsum

Experimental results


Thermogram of the object

under test with subsurface

defect in the middle

Thermogram shows the temperature field of the object’s surface after IR

heating pulse was applied; in the middle of the thermogram a defect area is

clearly identifiable.

Temperature difference between defect and sound area is around 2.5 K, as it

was predicted by the theory and FEM simulations.

Applications of the active infrared

thermography.


- non-destructive testing of building materials

- non-destructive testing of airplanes

- non-destructive testing of cultural artefacts – frescoes

and wall paintings

Application of the active infrared thermography.

non-destructive testing of frescoes


Before thermal excitation

After thermal excitation Frescoes in the church St. Stephans in Žilina

Discussion and conclusions


• The finite element method was successfully used for the modelling of

thermal energy propagation in 3D objects with subsurface defects.

• Simulation results have allowed optimization of the active infrared

thermographic method, optimal setting of heat pulse parameters and

the thermogram observation time.

• In the active thermography experiments special care should be taken if

the emissivity of the surface is non-uniform, or the surface of the object

is non-uniformly irradiated by the IR source. This can introduce

temperature gradients on the object’s surface similar to changes

induced by subsurface defects, and therefore these non-uniformities

should be eliminated.

• Results of FEM simulations were experimentally verified in laboratory

experiments.

• Active pulse thermography method was successfully used at the

investigation of frescos and mural paintings.


Thank you for your attention!

active infrared thermography in non-destructive...

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