JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 42

repair
Composite patch repairs do not involve hot works in any way
and, therefore, existing deadweight loading or proximity to
explosive environments have no particular consequences.
The Co-Patch project (www.co-patch.com), recently launched
by a large consortium including shipbuilders and a number of
research bodies from eight European nations, aims to establish
this repair technique in the naval sector to demonstrate that
composite patch repairs can be environmentally stable, so that
they can be used as long-term repair measures. The main technical challenge is to design and produce a strong and durable
adhesive joint between the steel and composite.
The joint studied in the present work was a carbon fibre laminate patched over artificially-cracked metal laboratory specimens. The use of fibre Bragg grating (FBG) optic strain sensors
was studied as a method for in-situ monitoring of the crack
propagation rate as well as debonding failures in the repaired
metallic plates.

mize the specimen-fixtures assembly tolerances in a universal
testing Hoytom machine with a 600 kN maximum load capacity
(Figure 2). The loading was applied in the form of linearly-increasing tensile displacement at a rate of 0.5 mm/s.
Force versus total displacement curves were recorded during
each test. Strains at various locations of the specimens were
monitored with FBG strain fibre optic sensors and strain gauges
(as conventional reference) in order to detect patch debonding
and steel plasticity in locations close to the crack tip. Three
strain gauges (Kyowa, 5-mm long) were bonded, one (named
GC) on the top of the patch in the centre of the crack and the
other two (named GA and GB) in the back side at 100 mm of
the crack tip. Two strain FBGs (Fibersensing, 5 mm grating)
were also bonded on the back side at 100 mm from the crack
tip but on the opposite side (FA and FB) for comparison with
strain gauge values. The specimen geometries, crack positions,
and strain gauges and FBGs positions are shown in Figure 3.

Materials and experimental procedure
Grade-A steel was used to manufacture the test specimens
(yield strength = 315 MPa). The composite laminated patches
were manufactured by hand lay-up with unidirectional carbon
fibre and vinylester resin (tensile modulus[0] = 74 GPa). The
dimensions of the steel plates were 200 mm x 550 mm x 5
mm and artificial cracks 140-mm long and 0.3-mm wide were
manufactured by electrical discharge machining (EDM). The
composite patch was 15-mm thick, 200-mm long and 200-mm
wide. The total patch length was 300 mm, since it additionally
includes the tapered edges of the patch to avoid high debonding
stresses at the
patch edges. The
surface of the
steel was prepared
using isopropanol
to remove rust,
grease and oil
before laminating
the composite
patches. Then, the
test specimens
were abrasively
treated by sand
blasting to obtain
SA2.5 and roughness values of
Ra=10.6 m (ISO
4288).
Tensile static
tests were carried
out with a 25 kN
preloading in
Fig. 2: Specimen on the hydraulic testing machine
order to mini-

Fig. 3: Specimen dimensions and artificial crack positions

Results
Figure 4 represents the global displacement versus applied
tensile load curves and Table 1 summarizes the failure values
of both types of specimens. The unpatched specimen exhibits a
predictable behaviour, with an initial linear elastic deformation
to 76.5 kN, followed by entrance into plasticity to a maximum
load value of 134 kN and finally crack opening occurs decreasing the load. However, the linear elastic response region of the
patched specimen (242.5 kN) is much larger than the corresponding one of the unpatched specimen. This can be explained
by the fact that, due to the presence of the patch, the steel substrate is subject to lower stresses at the same applied load, so that
it behaves elastically up to higher loads. In the loading sequence,
bonding failure between the patch and the steel plate occurs
at 258.5 kN, resulting in a sudden drop of the load carried by
the specimen since the patch is no longer carrying load and the
plasticity of the steel occurs at this point. This result shows the
effectiveness of the patch in increasing the load capacity of a
cracked structure.

42 jec composites magazine / No78 January - February 2013

http://www.co-patch.com

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

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