JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 43

when relaxation of the patch occurs and gauge C did not exhibit
strain.
In the unpatched specimen, linear strains are consistent with
the load vs. displacement curve up to 76 kN. After this point,
plasticity is represented by a non-linear strain increment. In the
patched specimen, the linear region rises up to around 240 kN
with strains much higher than unpatched specimens. The effect
of the patch holding the crack is reported by gauges and FBGs
that show similar values at the yield point (∼400me) and from
here strains in patched specimen are increasing around crack
tips until patch failure (∼1000me). The use of a bonded patch in
the cracked structure results in an increased load capacity in the
safety region of the structure.

Fig. 4: Load vs. displacement curves of patched and unpatched specimens
Table 1: Yield point and maximum load of tested specimens

Yield (kN)

Max. load (kN)

Unpatched

76.5 ± 1.5

134.0 ± 3.0

Patched – Sand blasting

242.5 ± 4.6

Figure 6 also shows a comparative graph of the strain values
measured with conventional strain gauges and fibre optic
sensors. The same behaviour found between both measuring
systems shows that the new system proposed for crack monitoring runs properly. Some minor differences between the gauges
and FBGs close to the debonding loads could be explained by a
non-symmetric crack opening.

258.5 ± 3.5

Regarding strain measurements, Figure 5 shows the values
obtained by gauges placed in the locations shown in Figure 3
during tensile tests. In positions A and B, strains increase when
the load increases but, on the contrary, the gauge located on
the top of the patch (gauge C) shows a completely different
behaviour. When the load increases, strain decreases; in other
words, a compression force is exerted on the patch. This
explains the effectiveness of the patch in slowing down crack
growth because the adhesive layer transfers force from the crack
tip to the composite, causing a similar bending effect on the
global specimen. In preceding loads of the maximum value,
from 160 kN, the strain increases until final debonded failure

Fig. 6. Strain values measured in patched specimen with conventional
gauges and fibre optic strain sensors

In conclusion, the use of composite patches to repair vessels
significantly increases the load capacity of the damaged structure and the use of fibre optic strain sensors is suitable for monitoring the repair status during service because they can detect
debonding and crack openings. n
More information:
www.co-patch.com
Contact:
erodriguez@aimen.es

Fig. 5: Strain gauge values of patched and unpatched specimens

Acknowledgements
The research leading to these results has received funding
from the European Union’s Seventh Framework Programme
(FP7/2007-1013) under grant agreement No. 233969.

No78 January - February 2013 /

jec composites magazine 43

http://www.co-patch.com

JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013