JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 45

Materials and testing

Fig. 1: Fast removal of the top matrix layer and direct discharge of the
applied energy using a short-pulsed solid-state laser

Federation of Industrial Research Associations (AiF). A near
infrared Q-switched solid-state laser with a wavelength of 1064
nm, a pulse repetition rate of 50 kHz and an average power of
100 W was used for these investigations. The beam is guided by
an optical fibre and focused via an f-theta lens to a spot measuring only a few tens of microns. The beam quickly passes over
the component by means of a galvanometer-driven scanner
with a deflection speed of up to 4000 mm/s. The high-intensity
short pulses cause the fast removal of the top matrix layer and
the direct discharge of the applied energy (see Figure 1). Using
this technique, the heating of the material can be controlled in a
way that excludes any damage.
Due to the scanning of up to 50,000 laser pulses per second
with a pulse duration of just a few nanoseconds, the entire surface is treated. In this process, no dust is produced, thus avoiding any contamination of the environment. As no abrasives
and chemicals are used, the laser procedure also helps protect
the environment.
Due to their small size, the near infrared solid-state lasers used
are also suitable for mobile systems, e.g. for repairs. Compared
to CO2 lasers, they are characterized by a good performance
efficiency as well as low investment and operating costs. An
essential and more or less indispensable advantage of the
mobile and compact laser systems is the beam transmissibility
by means of fibre optic cables. The robust systems produced by
Clean-Lasersysteme can be equipped with fibre optic cables,
due to the loss-free guidance of the laser beam via glass fibres,
and thus the laser beams can be flexibly transmitted. Thus, with
cable lengths of up to 50 m, large and three-dimensional components can be processed by hand or by an automated system
using compact processing optics. CO2 lasers – laser systems
emitting in the 10,600 nm range – do not have the advantage
of a flexible beam guide via an optical fibre. This underlines the
high flexibility of near infrared solid-state lasers against CO2
laser systems.

The project focuses on the pre-treatment of CFRP materials
with an epoxy resin matrix used in aircraft construction. In
addition to their excellent mechanical properties, these materials have a very high reproducibility rate as regards their fibre
volume and the thickness of the top matrix resin layer.
Two prepreg materials with curing temperatures of 120°C and
180°C were chosen for the project and pre-treatment tests were
carried out with two solid-state lasers with a wavelength of
1064 nm. In a first step, the process parameters were assessed
by means of surface analyses in a light and scanning electron
microscope. The generated topography was evaluated by means
of pictures made in a confocal laser microscope. In the next
step, it was attempted to detect delaminations by means of cross
sections. Adhesive joints were then produced at selected parameters with a one-component epoxy foil adhesive and tested
under pull-shear (DIN EN 1465) and Mode I loads. The ageing
behaviour of adhesive joints pre-treated by laser was investigated after storage for 2000 hours at 70°C and a relative humidity
of 90%.

Results
The surface images displayed in Figure 2 clearly show the
strong dependence of the processing result on the chosen laser
parameters.

Fig. 2: Surfaces of CFRP specimens pre-treated using various laser parameters

In the specimen shown in Figure 1a, which was treated with the
parameter P13, the applied energy is not sufficient to remove
the matrix from the top fibre layer. After treatment, there is still
a sealed matrix layer. The light areas, however, indicate local
delaminations between matrix and fibres. The surface shown in
Figure 1b was processed with a higher intensity. It can be clearly
seen that the matrix is peeled off from the surfaces but not
completely removed. Only a treatment with further increased
intensity (parameter P12) achieves the surface shown in Figure
1c, where the matrix is completely removed from the top fibre
layer with no more matrix residues. This is the intended condition for the adhesion laser pre-treatment.
A strong dependence on the laser parameters used can be seen
not only in the surfaces, but also in the failure patterns of the lap
shear specimens (Figure 3).
Untreated surfaces show adhesive failure, as no bonding can
be established between the adhesive and the fibre-reinforced
material. With optimally treated surfaces (parameter P39),
cohesive failure occurs in the adhesive so that the potential
of the joint can be fully utilized and the pre-treatment requi-

No78 January - February 2013 /

jec composites magazine 45

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

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