Active Infrared Thermography in Non-destructive Testing

Transcription

1 Active Infrared Thermography in Non-destructive Testing M. Hain, J. Bartl, V. Jacko Institute of Measurement Science Slovak Academy of Sciences, Bratislava, Slovak Republic

2 Outline of the presentation Motivation of the research Basic physical principle of infrared thermography Passive / Active infrared thermography Numerical modelling of heat propagation in nonhomogenious environment Finite elements method Description of the experiment Results Aplications Conclusions Measurement 2009, Smolenice 2

3 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 Measurement 2009, Smolenice 3

4 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 unity of time to a half-space is described by Planck s law of radiation M 2 hc ( T) 5 2 e 1 hc kt 1 T M (T) Measurement 2009, Smolenice 4

5 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 Measurement 2009, Smolenice 5

6 The principle and basic set-up of pulse active infrared thermography system 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 Measurement 2009, Smolenice 6

7 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 Measurement 2009, Smolenice 7

8 Heat propagation in material The thermal energy propagation within the material can be described by Fourier s partial differential equation. C p T t.( k. T ) Q 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 ), C p is specific heat capacity at constant pressure (J.kg -1.K -1 ), k is thermal conductivity (W.m -1.K -1 ) Measurement 2009, Smolenice 8

9 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 Measurement 2009, Smolenice 9

10 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 Measurement 2009, Smolenice 10

11 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 n.( k. T ). 4 4 T T h( T T) s a where T is temperature of the object, T s is temperature of the surrounding (background), T a 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 ) Measurement 2009, Smolenice 11

12 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/m Measurement 2009, Smolenice 12

13 Thermal contrast maximization To find the optimal time for observation, we will evaluate thermal contrast defined as: Td t Ts t CT t T t T 0 where s s T d (t) is surface temperature of defect area, T s (0) is surface temp. of sound area before heating pulse is applied, T s (t) is surface temperature of sound area at the time t Analysis of the thermal contrast shows a local extreme at the time 360s after start of the heating pulse. thermal contrast between defect and sound area Measurement 2009, Smolenice 13

14 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 m; it corresponds to the spatial resolution 1 mm on the object s surface Measurement 2009, Smolenice 14

15 Experiment Object under test Front Rear Material: concrete, gypsum Measurement 2009, Smolenice 15

16 Experimental results 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. Thermogram of the object under test with subsurface defect in the middle Temperature difference between defect and sound area is around 2.5 K, as it was predicted by the theory and FEM simulations Measurement 2009, Smolenice 16

17 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 Measurement 2009, Smolenice 17

18 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 Measurement 2009, Smolenice 18

19 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 Measurement 2009, Smolenice 19

20 Thank you for your attention! Measurement 2009, Smolenice 20