FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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PULSED PILATOMETRIC INSPECTION OF UNBONDED LAYERS
Thermographie techniques are particularly convenient for the inspection of
high emissivity surfaces such as graphite-epoxy laminates or anodized
coatings. However, the thermographie signal is normally lower by an order of
magnitude when inspecting metallic surfaces such as Al-epoxy honeycomb panels
such as the one shown in fig. 7. This is a consequence of the low emissivity
of bare aluminium surfaces, which is typically of the order of 0.1. On the
other hand, thermography measures an indirect property of the material i.e.
thermal resistance of the layer-to-layer interface, rather than the property
which is of real interest i.e. the adhesive bond at the interface. A lack of
adhesion between the aluminium skin and the honeycomb core in a laminate such
as the one shown in fig. 7 would be quite difficult to detect thermographically,
because the thermal conductivity of the air-filled honeycomb core
is lower by several orders of magnitude than the thermal conductivity of the
aluminium skin.
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The holographic interferometry approach • » , on the other hand, provides
a direct bond evaluation by mechanically lifting the bonded layer between the
two hologram exposures. Although either vibration, pressure or heat can be
applied to lift the unbonded layer, thermal stressing is the most convenient
technique for a fast and non-contact inspection of laminates ~ . With this
technique, the structure is holographically inspected before and after surface
heating by a lamp or a hot-air gun. unbonded areas are revealed by the
appearance of localized fringe patterns if the therraoelastic bending forces
produce a- normal displacement of the unbonded layer of more than half a
wavelength.
In order to take advantage of the mechanical inspection capability of
holography while avoiding its sensitivity to ambient vibration and stray
light, a thermoelastic interferometric technique has been developed at our
Institute 21 .
The basic principle is shown in fig. 8. The layer to be inspected is heated
by a pulsed and focused laser beam. The pulse duration, typically 1 ms or
less, is comparable to the thermal propagation time across the layer
thickness. Consequently, a transient therraoelastic moment causes the layer to
lift, if unbonded, while its position is monitored by a sensitive focused
interferometer. Because of the short duration of each pulse, large surfaces
can be scanned rapidly. Apart from eliminating fringe-counting and heat
uniformity problems, this approach has the following advantages with respect
to holography:
1. Every point being inspected during a very short period of time,
low-frequency ambient vibration noise can be avoided. Operation on the
industrial floor Is thus possible with our approach. High-power pulsed
lasers have been used in the past to avoid hologram blurring during the
exposure time, but ambient vibrations during the heating period between
the two exposures cannot be avoided with such a method. Moreover, the
colllmated interferometer beam can be spatially filtered to avoid stray
light, so that our system can operate under conditions of normal ambient
illumination.