Composite Materials Research Progress

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The advantages are numerous:
W. Van Paepegem, I. De Baere, E. Lamkanfi et al.
Figure 2. Measurement principle of the optical fibre sensor.
• the measurement is absolute and does not drift in time,
• fibre optic sensors are rugged passive components resulting in a high lifetime
(>20 years) and are insensitive to electromagnetic interference,
• the fibre Bragg grating forms an intrinsic part of the optical fibre and has very
small dimensions which makes it very suitable for embedding in composite plates,
• many fibre Bragg gratings can be multiplexed employing only one optical line so
more sensing points can be read out at the same time.
Doyle et al. [9] experimented on the use of fibre optic sensors for tracking the cure
reaction of a fibre reinforced epoxy, with success. They also successfully demonstrated the
feasibility of these sensors for monitoring the stiffness reduction due to fatigue damage, for
thermosetting matrix.
De Baere et al. [10,11] have shown that the optical fibre sensors also survive the
production process for carbon fabric thermoplastics (both autoclave and compression
moulding) and that the correspondence between the axial strain measurements from the
extensometer and the optical fibre sensor were identical in tension-tension fatigue tests (see
Figure 3). That means that the adhesion of the embedded optical fibre sensor to the
surrounding thermoplastic material is very good.
The accumulation of permanent strain is another important phenomenon to monitor.
Especially in composites with large residual stresses built up during manufacturing, the relief
of thermal stresses due to fatigue cracking can result in accumulation of permanent strain.
There again, optical fibre sensors are very sensitive sensors to measure these permanent
strains. Figure 4 shows the stress-strain curves of intermediate static tensile tests during a
tension-tension fatigue test on a carbon thermoplastic. The accumulation of permanent strain
can be clearly seen.