Study of the exfoliation on aluminum specimen using pulsed thermography
Marc Genest
Xavier Maldague (Supervisor)
Problem: Non-destructive evaluation techniques allow the inspection of systems or components without altering their properties. The advantage of a thermography technique is that it allows the inspection of large surfaces and is a non-contact technique. The present study deals with the inspection of exfoliation on aluminum specimen. The goal is to quantify the dimensions and the depth reached by the corrosion. Since aluminum has a weak emissivity, part of the study consists in using several types of coatings to increase the emissivity while ensuring that the latter is uniform.
Motivation: Corrosion causes a deterioration of the metal and consequently of the structures it composes. This deterioration affects the security of people using the corroded structure, leading to repair costs and causing other damage in the structures. By carrying out non-destructive evaluation (NDE) by thermography, one can estimate the characteristics of the corrosion present in the inspected component. This evaluation provides information with respect to when the corrosion will become dangerous for the structure. Moreover, detecting corrosion in its initial stage and monitoring this corrosion, provides a retroaction of the evolution of the corrosion of the specimen with respect to the environment it is used in. This will lead to a better understanding of the phenomenon of corrosion and thus to a better corrosion control and prevention system.
Approach: The use of pulsed thermography (PT) allows the propagation time of the thermal wave to be related to the depth of the default. Moreover, the variation in temperature reached by the zones containing the defaults also depends on the depth of the default. PT thus seems like an interesting approach for the quantification of the depth of the defaults. PT also allows the dimensions of the defaults to be estimated.
Challenges: The difference in emissivity of a specimen can lead to a misinterpretation of data by the operator. When the inspected surface is covered by a uniform emissivity coating, this source of errors is removed. The operator can thus assume that the apparent differences in temperature are due to the defaults in the materials and not due to differences in emissivity. Since exfoliation arises near the surface and aluminum is a very good thermal conductor, most of the information is contained in the first images. The rate of acquisition of the infrared camera must thus be sufficiently rapid to be able to see the evolution of the temperature during the lapse of time which is of interest.
Applications: Corrosion occurs in the air industry, since certain metals, like aluminum, are used in the construction of airplanes. Non-destructive thermography inspection enables a large surface to be inspected without having to come into contact with the surface and without requiring the disassembly of the airplane, as opposed to ultrasound or Foucault current techniques which are the most commonly used methods of inspecting metals such as aluminum.
Calendar: May 2003 - August 2004
Last modification: Sep 28 2007 1:57PM by mgenest


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