FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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A micrograph of the laminate is shown in fig. 2. As can be seen from fig. 2a,
Che upper layer was partially unbonded because of interface stress during the
mechanical manipulation. The laminate was inspected by the experimental
apparatus shown in ftg. 3. A CO2-TEA laser with volumetric ratio equal to
0.05 provided 60mJ, 50 ps pulses with pulse shape optimized to' obtain
efficient surface heating without overheating .
The single-transversal-mode beam had a gaussian distribution with 1/e 2 radius
of 4 mm. The surface temperature was monitored by an InSb infrared detector
pointed in the center of the irradiated area. Such a detector has a spectral
sensitivity range from 2 to 5.5 pm, so that it is insensitive to the 10,6 pm
reflected intensity of the CO2 laser.
The experimental results obtained with such a system are shown in fig. 4.
Curve (a) corresponds to a well-bonded region, while curves (b) and (c) were
obtained on regions where the upper layer was partially unbonded. The
discontinuity in the slope of curves (b) and (c) after approximately 10 ms
corresponds to the arrival of the thermal front on the thermally resistive
air—filled unbonded interface. The thermal propagation time of 10 ms over a
125 vim thickness corresponds to the expected thermal properties of the
graphite-epoxy material.
While the slope of curve (a) is nearly equal to -1/2, as for a uniform sample,
curve (c) has a significantly different slope, even before 10 ms. This may be
interpreted by taking into account the heterogeneous nature of this material.
As can be seen in fig. 2b, the fibers are randomly distributed in the matrix,
some fibers being in contact with each other while other fibers are separated
by a significant thickness of the resin. As the thermal conductivity of
epoxy, 4 . 10~ W/cm °C, is much smaller than the conductivity of graphite
fibers 17 ' 18 , 0.5 w/cm °C, the fiber distribution may significantly affect
the transverse thermal properties of the composite.
Figure 5 shows the results of the fiber distribution in the composite matrix.
In the three-dimensional model used to obtain such curves, 25 ym x 25 urn x 1
cm graphite elements are embedded in an epoxy matrix with volume ratio of
0.5. A contact thermal resistance equivalent to 2.5 urn of epoxy is assumed
between adjacent graphite elements. Curves (a) and (b) in Figure 5 correspond
to a well-bonded and delaminated layer, respectively, with a uniformly
distributed graphite-epoxy mesh, as shown in Figure 6a. Curve (c) in Figure 5
corresponds to a delaminated layer with a random matrix obtained by a
random-number generator, Figure 6b. We can see that curve (c) has a more
gradual slope, similar to the experimental curve (c) of Figure 4. These
results suggest that photothermal techniques may provide information not only
on the presence of sub-surface delaminations, but also on the fiber content
and distribution in fiber-resin composite materials.
As a conclusion to this first section, it can be said that pulsed photothermal
techniques are an attractive unconventional approach for the inspection of
coated or layered materials. Their rapidity and non-contact nature makes such
techniques convenient to use for scanning the surface of large structures.
Moreover, the possibility to use such techniques for an on-the-field thermal
evaluation of layered materials may open new opportunities in the NDE field.