JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 46

bonding

Fig. 3: Failure patterns in differently pre-treated CFRP specimens

rements are fully met. Parameters producing surfaces such as
shown in Figure 1b result in a failure within the top matrix resin
layer (CSF – cohesive substrate failure). If the laser energy is
too high, delamination occurs below the top fibre layer due to
too strong heating in the material. The failure patterns can be
directly correlated with the achievable lap shear strengths. A
summary of the maximum shear stresses depending on the laser
parameters and materials is shown in Figure 4.

Fig. 5: Results of the pull shear tests on abraded and laser-treated aged
specimens using the 120°C matrix system

mens curing at 180°C only local delamination of individual
fibres occurs and the average crack propagation energy of the
abraded references is achieved. The results of the mechanical
tests after accelerated ageing (2000 hours, 70°C, 90% relative
humidity) are summa-rized in Figure 5.
The results clearly show that, due to ageing, the strength
decreases by about 35% in both abraded specimens and lasertreated ones. This loss of strength is more or less the same, so it
can be concluded that the laser pre-treatment does not have any
negative influence on the ageing behaviour of the entire system.

Conclusions

Fig. 4: Lap shear strength of CFRP adhesive joints with a matrix curing at 120°C
and 180°C

First of all, a small delamination can be observed in the failure
areas, even in the case of manual polishing pre-treatment. With
too low processing intensity (P13), the top matrix layer is not
completely removed, but thermal damage occurs resulting in a
failure within the top matrix layer in the case of low forces.
When choosing favourable laser parameters (P39) and using
the matrix system with the higher curing temperature, the
failure occurs completely cohesively in the adhesive, so that the
strength of the adhesive can be fully utilized. Thus the requirements for the pre-treatment procedure are met. The slightly
higher forces in the 120°C matrix system compared to the
180°C curing matrix system are presumably a result of the lesser
thickness of the adherents (~ 1.8 mm as against ~ 2.1 mm).
In laser treatment with parameter 39 or with parameter 12,
complete delamination occurs, yet the strengths of the abraded
references are still achieved.
Even when Mode I loads were applied, the stronger delamination tendency of the matrix curing at 120°C could be shown.
The specimens made of this material delaminate across the
entire surface after the laser treatment, whereas in speci-

The results achieved in the MoLaB project show that pretreatment of fibre-reinforced components with near infra-red
solid-state laser systems is possible. With both tested materials,
the reference strengths of abraded specimens can be achieved
and the procedural disadvantages of manual polishing can be
overcome by using laser technology. In particular, when choosing the matrix resin system curing at 180°C, the full potential
of the adhesive joint can thus be used. Tests of woven materials
and materials with strongly differing matrix resin thicknesses
show that the application is problematic in this area, as inhomogeneous ablation can occur or matrix resins can remain on
the surfaces. If damages or a crash occur, structural repairs of
the CFRP have to be carried out. In this case, adhesive bonding
of CFRP is an interesting solution. In this context, a “safe” pretreatment is required, allowing subsequent reproduci-ble and
reliable adhesive bonding of the components with the highest
possible bond strengths. n
More information:
fabian.fischer@tu-braunschweig.de
www.ifs.tu-braunschweig.de
Acknowledgments
The MoLaB project is funded by the Federal Ministry of Economics and Technology, based on a decision of the German Parliament. The authors expressly
thank the AiF for their encouragement and administrative project sup-port,
and Clean-Lasersysteme GmbH, Germany.

46 jec composites magazine / No78 January - February 2013

http://www.ifs.tu-braunschweig.de

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

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