FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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2. Holography requires displacements of some wavelengths, while laser
interferometers are sensitive to much smaller displacements > .
Heterodyne holographic techniques can be used to detect small
deformations , but this requires the use of complex double-wavelength
exposure techniques demanding an accurate control of the alignment'and of
the ambient conditions while still requiring point-by-point scanning.
3. In the holographic approach, a broad heat source is used to heat a large
area of the bonded structure. The generated thermoelastic stress
distribution inside the unbonded layer is of the type shown in fig. 9a.
A large longitudinal stress is produced in a direction parallel to the
surface, but the bending component which makes the.-unbonded area to lift
outward is quite small, unless the layer is initially slightly convex.
Bending of the unbonded layer appears to be more related to differential
thermal expansion between the layer and the substrate and to longitudinal
thermal gradients produced by the thermal barrier effect of the unbonded
interface » . Such lifting mechanisms are weak and unreliable. On
the other hand, the transient strain configuration under localized pulsed
heating shown in fig. 9b is much more effective in producing a bending
moment, as was verified using both analytical and finite-element
thermoelastic modelling.
The advantages of conventional holographic techniques over interferometry are
that a full picture is obtained without scanning and that scanning surfaces
can be inspected. However, focused interferometers are now commercially
available which can probe perfectly rough surfaces at any angle of incidence
and at operating distances of several meters . As to the scanning speed, a
rate of the order of 100 cm /s could be obtained with 1 ms pulses and a 3 mm
resolution; such a speed compares favorably to the exposure, heating and
development time required with the conventional holographic approach.
The interferometric technique was experimentally tested on different
adhesively-bonded samples. Some preliminary results were obtained using a
Cu-Be layer, 125 um thick, epoxy-bonded to a massive plexiglass plate. Some
unbonded areas at the metal-plexiglass interface could be easily localized
visually through the plexiglass. A nearly 1 J, 1 ms pulse from a Nd:YAG laser
was used to heat the layer surface to some degrees C above ambient. Fig. 1Ü
shows two curves obtained by scanning the two concentric laser beams over two
delaminated areas of different dimensions. Curve (a) was obtained over a 1
cm-diameter unhond with a 6 mm-diameter heating YAG beam, while curve (b) was
obtained over a 3 mm-diameter delamination with a 0.7 mm-diameter heating
beam. The unbonds are clearly visible, and the signal level of some \m is
much higher than the ultimate sensitivity of the interferometer.
The same interferometric system was also used to inspect an Al-epoxy laminate
of the kind shown in fig. 7. Some artificial bonding defects were produced by
inserting some Teflon foils between the aliminium skin and the epoxy or
between the epoxy and ths honeycomb core. The skin thickness was of 300 ym,
and the honeycomb cells had a size of 3 mm. Fig. 11 shows some curves
obtained by scanning the skin surface over a 2 cm-diameter unbonded area. The
heating-beam diameter was of 1 mm, 3 mm and 6 mm, respectively for curves (a),
(b) and (c). All of the three curves show a clear evidence of the unbonded
area, which is situated between the 5 mm and the 25 mm positions on the
abscissa. A nearly constant background signal is also obtained on the