FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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SOME UNCONVENTIONAL TECHNIQUES FOR THE INSPECTION OF LAYERED
MATERIALS
P. Cietc
National Reiea-tc/i Council o & Canada
Beuche\vMi, Quebec
ABSTRACT
Optothermal methods using a pulsed laser have been developed for the detection
of delaminated areas in layered materials such as Al-epoxy or graphite-epoxy
laminates. The pulsed laser be?m heats the laminate surface to some degrees C
inducing a slight thermoelastic bend of the unbonded layer.
Two techniques have been tested for the detection of delaminatlons. In the
first technique, an infrared detector is used to monitor the thermal decay
rate of the surface after the absorption of the pulse. The slope of the
signal shows a strong variation when the thermal wavefront reaches an
interface such as a delamination or a layer of different thermal properties.
This technique allows to detect subsurface defects and to locate their depth,
or alternatively to evaluate the thermal properties of the layer.
Graphite-epoxy composites were evaluated by this method. The second technique
uses a laser interferometer to measure the thermoelastic bending of the layer,
which is much larger If the layer is unbonded. The displacement of optically
rough surfaces can be measured with great accuracy by using a focused
interferometer. Delaminations on an Al-epoxy honeycomb laminate were detected
by this technique.
These techniques are attractive for the inspection of large and complex
structures because they require no contact with the inspected surface, are
easy to scan and require access from one side only. Moreover, both techniques
exert a lifting thermoelastic moment on the surface which tends to widen the
delamination gap. This facilitates the detection of lack-of-adhesion defects
when the two layers are initially in physical contact.
INTRODUCTION
Laminate structures such as metal or composite lap joints and honeycomb
sandwich structures are increasingly used in the transportation and
construction industries. Their combination of high stiffness and strength
with low weight is particularly appreciated in the aerospace industry. Such
structures are formed by adhesively bonding several layers of metallic or
fiber-reinforced-plastics materials in order to obtain a more uniform stress
transfer and an increased fatigue life. However, these materials require a
careful inspection after being assembled to detect any extended bonding
defect, such as an internal delamination or an unbonded area, which would
severely affect the structural resistance to transverse or shear loading.