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|Title: ||Development of improved active thermographic techniques for nondestructive inspection of building and civil engineering structures|
|Other Titles: ||Hong wai re neng tu xiang de kai fa ji gai shan bing ying yong yu jian zhu ji tu mu jie gou gong cheng de wu sun jian ce de ji shu ji ping gu|
|Authors: ||Chen, Yunshen (陳昀生)|
|Department: ||Department of Manufacturing Engineering and Engineering Management|
|Degree: ||Doctor of Philosophy|
|Issue Date: ||2009|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Infrared testing.|
Buildings -- Testing -- Thermographic methods.
|Notes: ||CityU Call Number: TA417.5 .C45 2009|
xxi, 184 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2009.
Includes bibliographical references (leaves 175-182)
|Abstract: ||Recently infrared thermography has received increasing acceptance for nondestructive
testing applications. It has the advantages of being full-field, non-contact and yielding
quasi real-time results. Basically, thermography measures surface radiation. When
surface emissions are known, surface temperature distribution can be determined. A
major industrial application of active thermography is nondestructive inspection.
During inspection, a thermal excitation is applied to the test object and a series of
thermal images (images of the object’s surface radiation) is recorded at equal timeintervals.
The presence of a material defect will interfere with the heat conduction and
cause a localized temperature distribution anomaly to appear on the surface of the object
being tested. A subsurface flaw is revealed by the presence of an anomalous
temperature rise on its surface. This phenomenon is the cause of a transient flow of heat
which is partially interfered by the flaws. The flaw shape and size can be determined by
the boundary of the localized anomalous temperature distribution. Through the
consideration of the time parameter and from reference calculations, its depth-location
can also be estimated.
Although active thermography is applicable to any material, it can yield false results on
objects having an anomalous surface emissivity distribution. This is commonly found in
building materials. This problem can be alleviated by a novel algorithm developed by
the research described in this dissertation. The reduction of the influence of this
emission is based on the idea of self-referencing. Subsequent to the thermal excitation, a
series of thermal images (typically 320x240 pixels image) is sequentially digitized with
time-intervals and is stored in the computer memory. The sequential data of each pixel
represents the thermal history at the pixel point. The data is first reconstructed according
to thermal signal reconstruction (TSR) which is based on the one-dimensional diffusion
theory, therefore the temporal hardware noise can be removed. Then, the emissivity
reduction algorithm is applied to process the sequential data through self-referencing.
The self-referencing reconstruction process successfully diminishes the influence of
emissivity, since the parameters of emissivity (temperature, wavelength, and direction)
are the same at each pixel within the inspection time. Therefore, the capability of flaw
detection in this active thermographic technique can be greatly enhanced by this selfreferencing
algorithm. The validity of the method is experimentally verified and
demonstrated by applying the method to objects of non-uniform surface emissivity, such
as concrete and composite materials.
Another main contribution of this thesis is the development of an electro-thermographic
technique. The technique of electro-thermography is a combination of electromagnetic
induction excitation and thermographic inspection techniques. This method is superior
to the traditional active thermography technique using the irradiative excitation method.
The electro-thermographic technique diminishes the influence of emissivity during
excitation since it can directly interact with the target material. During the inspection,
the eddy current is induced directly on the surface of the conducting material only. Flaw
detection is based on the anomalous “hot-spots” generated by the eddy current. The
eddy current density is affected by the induction magnetic flux density correlated with
the object’s geometry. The penetration depth of the magnetic field can be controlled by
the driving frequency. Therefore, surface, micro-surface or subsurface flaws can be
identified by this technique which was developed during the research described in this
thesis. For instance, electro-thermographic technique has been successfully employed in
several building and civil engineering applications due to its electromagnetic features, for example in the detection of reinforcement steel bars, fiber reinforcement bonding
integrity inspection, and the flaw inspection for steel rail and pressure vessels. Unlike
the traditional covermeter, ground penetration radar, ultrasonic or acoustic emission
measurement which requires contact and point-by-point measurement, the proposed
technique is full-field and non-contact. It has the advantages of both fast inspection
speed and high reliability.
It is believed that the self-referencing algorithm for reducing emissivity influence and
the development of electro-thermographic technique reported in this thesis will
significantly advance and facilitate the application of thermography in nondestructive
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b2375026|
|Appears in Collections:||MEEM - Doctor of Philosophy |
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