SAMPE Journal - January/February 2017 - 29
Feature Article
Table 3. Summary of static tensile residual strength for the different configurations.
pristine reference samples that were
tested as part of the reference test
campaign27.
The hybrid bondline samples
achieve a fracture load of 41.45 kN
which is 55% above the reference
strength. However, larger scatter is
observed since the standard deviation
is determined to be 11.3%. Fracture
for all hybrid bondline samples
occurs due to stock break failure of
the strap laminate at the transition to
the welded thermoplastic area.
The pinning samples achieve
fracture strength of 27.9 kN with a
standard deviation of 4.7% which is
4% above the reference results. As for
the hybrid bondline concept, failure
occurs by breakage of the strap close
to the elements.
Discussion
It can be concluded that the CLS
test is a suitable configuration for
fatigue crack growth studies of
bonded joints, since it represents
a realistic shear dominant loading
case and allows stable and linear
crack propagation as proven by the
reference samples.
Both crack stopping concepts
worked in the desired way as growing
cracks are efficiently stopped for 106
cycles. The welded area within the
hybrid bondline concept influences
the fatigue behavior of the epoxy
adhesive. Crack growth appears
faster close to the welded area.
This is believed to happen due to a
slightly thicker bondline close to the
thermoplastic area. Usage of slightly
thinner thermoplastic films might
reduce this effect. However, the
influence is only detected rather close
to the crack stopper. For a distance
more than 5 mm, no deviations in
comparison to the reference samples
could be found.
The residual strength of the hybrid
bondline samples is much higher
than the static strength of reference
samples. A better load distribution
compared to an unmodified joint is
expected. The position of the ductile
material at the (new) overlap edge,
i.e. at the crack tip, correlates to the
design principle of mixed-adhesive
joints. Accordingly, the adhesive's
shear modulus G should be inversely
proportional to the shear stress
distribution (see Figure 2). However,
from a technological point of view,
it is hardly feasible to smoothly
modify the adhesive's stiffness. Thus,
different adhesives with different
properties are combined. High
stiffness adhesive is placed in the
lightly loaded trough in the center of
an overlap joint whereas adhesive of
lower stiffness is placed at the locus
of peak stresses (in the particular
case at the crack tip). By doing so, a
larger share of loads is transferred in
the middle of the overlap and stress
peaks at the edges are moderated
which increases the strength of the
joint.
The pinning elements also worked
well as crack stoppers since the crack
growth is fully stopped for all samples
for 106 load cycles. It is assumed, that
a larger share of loads is transferred
through the pins which reduce stress
concentrations at the crack tip within
the adhesive below the critical value
for crack growth. Certainly, more
investigations would be needed for
a thorough understanding of the
working mechanism. Comparable
investigations (e.g.11) with rivet
elements as crack stopper state that
suppression of peel, i.e. mode 1 load,
within the bond plays a significant
role for crack retardation in bonded
and bolted joints.
Residual strength of pinned
specimens is only slightly above
reference samples. An abrupt
stock-break failure between the pin
elements indicates that the pitch
between elements is too small for the
chosen configuration. Hence, design
optimization would be needed
for further strength enhancement.
However, no strength degradation
is found due to presence of the pin
elements.
Conclusions
In this study, two novel crackstopping
design
features
for
bonded
composite
joints
are
introduced to address the concerns
of the certification authorities for
Figure 11. Fracture pattern for reference sample (left), hybrid bondline sample
(middle) and pinning sample (right) after testing.
SAMPE Journal, Volume 53, No. 1, January/February 2017
29
Table of Contents for the Digital Edition of SAMPE Journal - January/February 2017
Contents
SAMPE Journal - January/February 2017 - Cover1
SAMPE Journal - January/February 2017 - Cover2
SAMPE Journal - January/February 2017 - Contents
SAMPE Journal - January/February 2017 - 2
SAMPE Journal - January/February 2017 - 3
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SAMPE Journal - January/February 2017 - 7
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