FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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Ultrasonic and radiographie methods are extensively used for the
nondestructive inspection of laminate structures » . Commercially available
instruments based on such methods have reached an advanced stage of
development, and are now capable to detect most types of bonding defects.
Research is nevertheless quite active for the development of alternate
techniques which would lead to faster and less cumbersome devices, requiring
no coupling media or physical contact with the inspected structure, and which
can be applied where access is difficult and possible from one side only.
Moreover, none of the presently used techniques is satisfactory for the
detection of lack-of-adhesion defects between the layer and the adhesive,
i.e. areas where the adherents are in physical contact with each other, but
with a zero bond strength > .
A research program has thus been established at the Industrial Materials
Research Institute to explore unconventional inspection techniques for
stratified materials. Laser-holographic and therr.ographic techniques are
attractive because they are non-contact and provide in a relatively short time
a full image of the inspected surface. However, after nearly 20 years of
development efforts such techniques have found few applications out of the
research laboratory. Part of the reasons for such an inertia are technical:
in particular, thermography is surface-emissivity dependent and provides
little quantitative Information, while holography is very sensitive to ambient
vibrations and to background light, usually requiring that the inspected
object be mounted on a cumbersome vibration-isolated table in a ' dark
environment. Such requirements make it difficult to apply such techniques to
bulky structures on-the-field. A research effort is thus under way at the
Industrial Materials Research Institute to develop alternative optical
techniques which, while maintaining the non-contact and scanning rapidity
advantages, may provide more quantitative information and show a potential for
on-the-field applicability.
Two optothermal approaches under evaluation at our Institute are described in
this paper. The first is a pulsed thermographie technique which has been
applied to a quantitative thermal analysis of graphite-epoxy laminates. The
second Is a novel thermoeleatic technique using a localized heating source and
focused Interferometer to inspect the transient thermomechanical behaviour of
the unbonded layer. A description of such methods and of the experimental
results as well as a discussion of the relative merits and fields of
application of each of these approaches Is presented.
PULSED PHOTOTHERMAL INSPECTION OF LAYERED MATERIALS
The effectiveness and convenience of thermographie techniques for the
detection of delamlnattons and other subsurface defects has been demonstrated
by a number of authors " . Usually, the surface of the layered structure is
heated by a heat lamp or a hot-air gun and the temperature distribution during
or after heating Is observed with a thermographie camera. The presence of a
delamination is revealed by the appearance of a hot spot because of the
thermal-barrier effect of the unbonded interface. More detailed analyses have
been performed recently » » in order to better quantify the retrieved
information. Part of the research effort at our Institute is directed toward
the assessment of the thermographie method and the expansion of the amount of
Information that can he obtained from the detected signal. In particular, the
possibility of a thermal analysis of the Inspected materials by properly