Download Influence of environmental parameters on impulse thermography

Oral Presentation
Application of quantitative impulse-thermography for structural
evaluation in civil engineering – comparison of experimental results
and numerical simulations
Arne Brink, Christiane Maierhofer, Mathias Röllig, Herbert Wiggenhauser
Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, D-12205 Berlin, Germany
phone: +49-30-8104-4234, fax: +49-30-8104-1447, e-mail address: [email protected]
Keywords
impulse thermography, numerical simulation, non-destructive testing, concrete, civil
engineering
Abstract
Impulse-thermography is an active approach for a quantitative thermal scanning of the surface
of various structures and elements [1]. It is well known for material testing in other industry
branches and within the scope of a national (DFG, organisation for academic research in
Germany) founded project it is used to investigate its possible applications as a nondestructive testing method in civil engineering [2]. The technique is intended for the detection
of near-surface inhomogeneities, normally defects, in typical structural elements and for the
determination of their geometrical parameters.
Typical testing problems are voids and honeycombing in concrete elements. Therefor a test
specimen was built having a size of 1.5 x 1.5 x 0.5 m³ containing voids with different sizes
(10 x 10 x 10 cm³ and 20 x 20 x 10 cm³) and concrete covers. Measurements were performed
by heating the surface of the specimen with a heating unit of three infrared radiators with
varied heating times from 5 to 60 min. After switching off the heat source the cooling-down
process was monitored with an infrared camera (Inframetrics SC1000) and thermal images
were recorded with a rate of 2 Hz for a duration of 120 min. Figure 1 shows a thermogram of
the surface of the test specimen which was taken after 15 min heating and after a cooling
down time of 8.1 min.
Figure 1.
Thermogram of a concrete specimen with voids recorded 8.1 min after switching off the heat
source. The heating time was a 15 min.
The main approach in analysing the thermal data was to interpret the function of surface
temperature versus cooling time (figure 2) for selected areas with and without
inhomogeneities (figure 1). As shown in figure 2 these transient curves were compared and
temperature difference curves were calculated.
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3
T max = 3 .6 1 K
35
Reference (1)
Defect (2)
Difference
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2
1
25
20
0
tmax = 486 s
-1000
1000
0
2000
3000
4000
5000
6000
7000
Temperature difference in K
Temperature in °C
40
8000
Time in s
Figure 2.
Temperature versus time curves for selected areas (figure 1) and the respected difference curve
with a maximum temperature difference at a distinct time.
Besides, numerical calculations based on the finite differences method were carried out for
voids with different concrete cover (see simulated difference curves in figure 3) for further
data interpretation and for solving the inverse problem. The results of the measurements and
the comparison with these simulations will be presented.
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Defect depth 1 cm
Defect depth 2 cm
Defect depth 3 cm
Defect depth 4 cm
Defect depth 5 cm
Defect depth 6 cm
Defect depth 7 cm
Defect depth 8 cm
Defect depth 9 cm
Defect depth 10 cm
Temperature difference in K
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10
9
8
7
6
5
4
3
2
1
0
-1
0
600
1200
1800
2400
3000
3600
4200
Time in s
Figure 3.
Simulated temperature difference curves with different defect depths for 15 min heating time.
Additionally, the influence of environmental conditions and measurement parameters like
environmental temperature, different amounts of heating power and temperature losses at the
surface were investigated.
Acknowledgements
The presented work was supported by the Deutsche Forschungsgemeinschaft within the
research project “Investigation of structure and moisture content in the surface near region of
building structures with impulse thermography“ (WI 1785/1-1).
References
[1]
Maldague, X. P. V: Nondestructive evaluation of materials by infrared thermography,
1st Title, London, Springer-Verlag, 1993.
[2]
Vavilov, V., Kauppinen, T. and Grinzato, E.: Thermal characterisation of defects in
building envelopes using long square pulse and slow thermal wave techniques,
Research Non-Destructive Evaluation 9, 1997, pp. 181-200