JEC COMPOSITES MAGAZINE - Issue #97 - Special JEC Americas - May/June 2015 - 62

health monitoring
Table 1: Number of cracks counted on a section passing along the axis of the OFS for a growing number of cycles

Sample

0 cycle

1000000 cycles

1001000 cycles

2000000 cycles

4000000 cycles

layer has four 0° plies and the optical fibre is embedded in
the centre. The side contours are curved due to the limited
detector screen area, while the upper and lower edges are the
sample's top and bottom surface. One can observe a richer
resin film at each 0/90 interface layer: a darker colour is the
result of a lower X-ray attenuation, meaning a lower density,
which corresponds to a resinricher area.
The enlargement of the sensor surroundings (Figure 3 bottom) already highlights damage around the optical fibre of
Sample 8, which has an 80-μm fibre with a 190-μm thicker
coating. A transverse crack starts from the sensor's coating towards the interface with the nearest 90° layers. Samples 1 and
5 show no crack, resulting in a better coating/composite, and
coating/cladding, adhesion. A point of interest ensues from
the plies redistribution around the sensor creating resinrich
areas sideways and, at the same time, compacting the reinforcing fibres above and below the sensor. This may results in
higher local fibre volume fractions. The resin-rich areas appear

more marked for the smaller sensor, while the distortion of
the plies around the fibre is less important.
Damage evolution
After the initial scan, the laminates were tested for fatigue and
scanned at defined intervals in order to investigate the overall
damage evolution through the thickness. This was achieved by
analysing the 3D rendered model of the scanned volume. As
an example, Figure 4 depicts the number of cracks detected in
Sample 1 after 1,000,000 cycles. Because of the cross-ply layup
and the loading conditions, the cracks were expected to be
initiated in the 90° layers perpendicular to the direction of the
applied force [5].
To quantitatively evaluate damage through the thickness of
the sample, the number of cracks arising in a cross-section
along the optical fibre axis was tracked [2], [6]. For each sample, the number of cracks was compared at different number
of cycles. Table 1 summarizes the overall number of transverse
cracks counted.
An increase in the number of cracks in the 90° layers during
fatigue testing is clearly noticeable. Besides this, however,
no evolution of damage in the central 0° layers (interlaminar
cracks) was yet found.

Conclusion and outlook

Fig. 4: Transverse cracks (confined in white boxes) evolving in the 90° plies due to
fatigue cycling; the optical fibre is highlighted in red in the centre of the laminate

62 jec composites magazine / No97 May - June 2015

Due to the high curing temperature (180°C) of the M55JM18 carbon fibre prepreg, the thermal expansion coefficient
difference between the hosting and the host materials was
high enough to induce cracking in the area surrounding the
fibre optic in Sample 8. In fact, in this case, the larger coating
thickness, and its overall dimensions, distorted the composite
architecture significantly more.
However, it is not yet clear whether the crack was initiated at
the cladding/coating or at the coating/composite interface.
When considering a smaller OFS diameter (80-μm fibre with
thinner coating and 60-μm fibre), the arising thermal stresses
were not sufficiently high to induce any damage. However,
when analysing the 0° plies surrounding the OFS during
fatigue cycling, no crack initiation was detectable after 4 million cycles. Also for Sample 8, which was already showing a
crack, no further damage evolution was noticeable. Analysing
the data contained in Table 2, the samples with a thicker fibre

JEC COMPOSITES MAGAZINE - Issue #97 - Special JEC Americas - May/June 2015

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #97 - Special JEC Americas - May/June 2015