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Proceedings Paper

T.S.A. and Thermography
Author(s): W. N. Reynolds
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Paper Abstract

The SIRA SPATE equipment for thermal stress analysis is a successful example of the application of modern instrumentation technology to the development and application of classical concepts in physics. The use of Infra Red Optics to monitor small temperature changes over the surface of a solid material has great advantages in many applications as it is rapid, non contacting and non-invasive, and produces signals which can be recorded and analysed in many ways. These advantages have already been exploited to some extent in other applications such as conventional thermography, in which the distribution of heat in a structure is monitored by an I.R. imager using a rotating scanning system, or an I.R. pyroelectric vidicon tube. Such systems are widely employed in monitoring electrical power supplies or chemical plant, but have proved of less relevance to problems of nondestructive testing. In recent years this idea has been combined with that of introducing an alternating or on-off source of heat by means of a chopped laser beam for example, and using recording equipment such as SPATE to monitor the flow heat through a solid. It can be shown that such a source produces a train of critically damped waves of length A where 1/2 A = 2{1T4 where a is the thermal diffusivity of the solid and v is the frequency of chopping and the wave amplitude falls to l/e of its initial value in one wavelength. For waves of frequency 1Hz in copper, A is about 20mm, but normally it is necessary to work at frequencies of 100Hz or more in materials of much lower diffusivity. Consequently this method is chiefly of interest in the examination of thin sheets or coatings up to 1-2mm in thickness. In another development which is more specifically adaptable 'to many problems of NDT, heat is introduced over a chosen area at a high rate for a predetermined time of between lms and about 10s. The subsequent flow through the solid is then monitored by means of an I.R. imager compatible with current TV video standards. This system enables the thermal field to be recorded every 2Oms, and the detailed study of the resulting heat flow has provided effectively a completely new NDT technique. Successful applications so far demonstrated include the detection of disbonds and delaminations in coatings and laminates, voids in castings and cracks in welds. A problem of current interest is the examination of ceramic parts, including unfired materials in which ultrasonics is impracticable. Theoretical models are now being developed to enable optimised systems to be specified for particular cases.

Paper Details

Date Published: 1 April 1987
PDF: 1 pages
Proc. SPIE 0731, Stress Analysis by Thermoelastic Techniques, (1 April 1987); doi: 10.1117/12.937898
Show Author Affiliations
W. N. Reynolds, National NDT Centre (England)

Published in SPIE Proceedings Vol. 0731:
Stress Analysis by Thermoelastic Techniques
B. C. Gasper, Editor(s)

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