SAMPE Journal - January/February 2017 - 27

Feature Article

Figure 8. CLS geometry (all dimensions in mm).

length. Analytical closed form
solutions26 also indicate that the
fracture toughness is independent
of the crack length. Thus, the CLS
specimen is also applicable for
investigations of crack-stopping
design features.
A zigzag shaped release film is
used to introduce the crack in the
adhesive at the edge from strap
to lap. Thereby, crack initiation
is believed to be more even and
thus predictable. Furthermore, the
zigzag shaped release film allows
an estimation of the crack growth
threshold load level as discussed in27.
A rather long lap section is chosen to
avoid clamping effects for about 100
mm crack length.
For the hybrid bondline concept
the co-cured strips of PVDF are 10
mm in width. The two inserted strips
though are 7 mm in width. The PVDF
position within the sample is shown
in Figure 9.
For the pinning approach, an array
of five pins in two rows is used for
each sample. The pitch between
elements and the overall arrangement

within the CLS sample is given in
Figure 9. A dens arrangement of pins
is chosen to obtain full crack stop
within the bonded joint. However,
a minimum pitch of more than three
times the element's diameter is still
met.
Materials and Manufacturing
All
test
specimens
are
manufactured using HexPly 8552/
IM715
unidirectional
prepreg
material. The composite plates consist
of 16 plies [0/+45/90/-45/0/+45/90/45]s leading to a plate thickness
of 2 mm. The 0° surface plies in
longitudinal direction are chosen to
minimize undesired delamination
failure close to the bondline while
testing. A release film between the
plate's surface and the steel tooling
ensures a constant overall surface
finish. Curing of the prepreg system
(180°C, 7 bar autoclave cycle) is done
in accordance with the data sheet15.
After cleaning with isopropanol,
atmospheric plasma is used for
surface treatment of bonding surfaces
(Plasmatreat generator FG5001). The

process parameters were obtained
in previous studies for optimized
surface activation.
Loctite EA 9695 0.05 PSF K19,
as common high strength film
adhesive for aerospace application,
is used in this study. As with the
prepreg material, curing is done in
an autoclave cycle (180°C, 3.5 bar)
to ensure high quality bonding.
Subsequently, all plates are trimmed
and cut to the final specimen
dimensions as specified in Figure 8.
Fatigue Test Campaign
All tests are carried out with
a constant amplitude at 8 Hz
sinusoidal with a load ratio R of
0.1 (tensile-tensile loading) using a
servo-hydraulic test machine. The
test is terminated either after 106
load cycles or after the crack length
equals 90 mm. Thus, the maximum
testing time per specimen amounts
to approximately 35 hours.
Fatigue testing of the pristine CLS
samples is thoroughly discussed and
evaluated in27. As a consequence
of this preliminary study, a

Figure 9. Position of welded thermoplast (left) and pinning array within CLS sample (right); dimensions in mm.
SAMPE Journal, Volume 53, No. 1, January/February 2017

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