“Thermal (IR Thermography) in Non-Destructive Testing”

Technical Seminar Report

Subject: DAMAGE ASSESSMENT THROUGH NDT & EVALUATION

Submitted by

RANGREJ SOMNATH PRAKASH

(Reg. No. : 140954002)

II Semester M.Tech (Advanced Thermal Power & Energy Systems)

DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING

MANIPAL INSTITUTE OF TECHNOLOGY

(A Constituent Institution of Manipal University)

MANIPAL-576104, Karnataka, INDIA

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1. INTRODUCTION

Infrared thermography is one of the non-destructive thermal methods which is becoming ever more

popular in nondestructive testing of materials and structures since it is completely noncontact and may be

faster than many other techniques that are being used. Thermal methods generally consist of the thermal

stimulation of the object (under examination) and monitoring of its surface temperature variation during the

transient heating or cooling phase. The analysis of heating and cooling processes during and after warming

up with an internal or external heat source is well established technique for the characterization of

composites and metallic materials.

In Civil Engineering, the application of infrared thermography is not limited to passive

investigations of the quality of thermal insulation of building envelopes. Defects like voids in concrete or

masonry, delaminations at interfaces of composites which have different density, heat capacity and/or heat

conductivity in comparison to the bulk material can be localized and characterized. Infrared thermography,

due to its non-contact character that allows for quick 2D surface mapping, represents a powerful tool for

non-destructive evaluation (NDE) of materials and structures. Notwithstanding this, Infrared thermography

is still not completely exploited.

In contrast to the conventional use where natural temperature gradients are utilized,

the NDT applications take an active approach. A heat pulse is applied and the surface

temperature is monitored and analyzed. Typically, the temperature distribution at the

surface at the time of maximum contrast is used for the detection of any defects. The

most important condition for infrared thermography to provide useful results is that a

temperature difference or thermal contrast ΔT, exists between the feature of interest,

e.g. people on a scene or an internal flaw on a specimen; and its surroundings. A

second condition is to have the appropriate thermal imaging equipment to produce

thermal images or thermograms. It is necessary to count with an experienced

thermographer to interpret thermographic results.

2. CLASSIFICATIONS OF IR THERMOGRAPHY• Passive mode of Thermal NDT requires only using IR cameras.

• Active mode of Thermal NDT involves additional thermal stimulation of objects under test. Several

types of heaters (coolers) are used in combination with IR cameras and computer stations and

Control units.

1. Pulsed thermography

2. Lock-in thermography

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2.1 PULSED THERMOGRAPHYIn pulsed thermography (PT) the specimen surface is submitted to a short heat pulse using a high

power source such as photographic flashes, see Figure1. The duration of the pulse may vary from a few

milliseconds (~2-15 ms) to several seconds depending on the thermo physical properties of both, the

specimen and the flaw. After the thermal front comes into contact with the specimen’s surface, it travels

from the surface through the specimen. As time elapses, defective zones will appear at higher or lower

temperature with respect to non defective zones on the surface, depending on the thermal properties of both

the material and the defect. The temperature evolution on the surface is then monitored in transitory regime

using an infrared camera. A synchronization unit is needed to control the time between the launch of the

thermal pulse and the recording with the infrared camera.

The one-dimensional solution of the Fourier equation for a Dirac delta function in a semi infinite

isotropic solid is given by:

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2.2 LOCK-IN THERMOGRAPHYIn lock-in thermography (LT), the specimen’s surface is periodically illuminated

by one or several modulated heating sources, e.g. halogen lamps, to inject thermal

waves into the specimen. The periodic wave propagates by radiation through the air

until it reaches the specimen surface where heat is produced and propagates through

the material. Internal defects, acting as barriers for heat propagation, produce

changes in amplitude and phase delay of the response signal at the surface. Figure 2

depicts an LT experiment. The lamps send periodic waves (e.g. sinusoids) at a given

modulation frequency ω, for at least one cycle, ideally until a steady state is

achieved. Different techniques have been developed to extract the amplitude and

phase information. Fourier analysis is the preferred processing technique since it

provides single images, ampligrams or phasegrams (the weighted average of all the

images in a sequence).

The Fourier’s law one-dimensional solution for a periodic thermal wave

propagating through a semi-infinite homogeneous material may be expressed as:

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3. LITERATURE REVIEWTitle: “The Role of Infrared Thermography in Nondestructive testing of Civil Engineering Structures”

Author: Bojan, Faculty of Civil Engineering, University Of Zagreb

To investigate the detectability of voids in concrete, two concrete test specimens were built as demonstrated

in Figure 1, having a size of 1.8 x 2.0 x 0.25 m. Before concreting, voids, simulated by polystyrene cuboids

with different sizes were positioned by polyamide threads in the wooden formwork.

a), b), c) Figure 1 Concrete test specimen including polystyrene cuboids with different sizes at different

depths, a) photo before concreting, b) and c) schemes of the specimens

Thermal imaging was performed according to ASTM in the summer period, between the 18.00 and 22.00

hours with the periodic imaging every hour. During the day both specimens were exposed to direct

insolation while the shades moved over the specimens when the sun was setting. The day was sunny, and

there was no rain for at least a week before the thermal imaging.

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4. RESULTS

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By using active infrared thermography and appropriate post processing

techniques, detection of near-surface inhomogeneities and common subsurface

defects in typical structural elements is possible. The quantitative determination of

their geometrical parameters and defect depth is the main objective for the practical

problems like:

locating and quantifying voids and honeycombing in concrete

locating delaminations of plaster at concrete and masonry

locating delaminations and voids behind tiles on concrete embedded in mortar

Assessment of bonding of carbon fiber reinforced laminates glued on concrete

identifying poorly grouted ducts.

5. OTHER APPLICATIONS Aerospace

Process Industry

Medicine

Condition Monitoring

Electronic Inspection

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6. CHALLENGES

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CONCLUSION

• Many common electrical & Mechanical problems begin as an increase in temperature, a thermal

image lets you easily detect them without interrupting operations.

• We can improve equipment maintenance, reliability and safety.

• Ultimately can save money.

8. REFERENCES

• [1] Nondestructive Handbook, Infrared and Thermal Testing, Volume 3, X. Maldague technical ed.,

P. O. Moore ed., 3rd edition, Columbus, Ohio, ASNT Press, 2001, 718 p.

• [2] Maldague, X.: „Theory and practice of infrared technology for non-destructive testing“, John

Wiley and Sons, New York, 2001.

• [3] Wiggenhauser, H.: „Active IR-applications in civil engineering“, Infrared Physics & Technology

43, pp. 233–238, 2002.

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• [4] Stimolo M.: “Praktische Anwendung der Thermografie im Straßenbau und fur

Abdichtungssysteme”, DGZfP - Berichtsband 77, Thermografie-Kolloquium, Stuttgart, 2001 [in

German].

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