SAMPE Journal - January/February 2017 - 25
Feature Article
Figure 4. Micro-section of welded area within bonded joint.
spread, resulting in a contact area
development. The high temperatures
at the interphase lead to interdiffusion of polymer chains, a
process referred as healing17. Due to
the homogenous polymer network,
welded joints do not rely on adhesion
at the material interphase.
For the thermoplastic phase a
polyvinylidiene fluoride (PVDF) film
material18 of 0.125 mm thickness is
chosen due to its favorable properties.
Besides
its
ductile
behavior
(elongation at break of more than
50%), the thermoplast has a melting
temperature below the adherend's
curing temperature of about 167°C.
The low melting temperature
allows co-curing with the composite
material and simultaneous welding
of the thermoplast and curing of the
epoxy adhesive in the epoxy's curing
cycle. PVDF's static tensile strength
is only 13% below the strength of the
chosen epoxy adhesive. Thus, the
thermoplast is capable of carrying a
large share of loads in case a crack
grows close to the crack-stopping
thermoplastic zone.
The material properties mentioned
above were obtained by uni-axial
tests in a preliminary study. The test
was also carried out for the chosen
epoxy adhesive system (Loctite EA
9695 0.05 PSF K, see19) for the sake of
comparison. A summary of the test
results for both adhesive systems
(PVDF and epoxy film) is given in
Table 1.
More details on manufacturing
of the hybrid bondline concept and
preliminary static test results are
summarized in20.
Pinning Approach
Working Principle
A second concept focuses on the
implementation of small diameter
pinning elements as reinforcement
entities
within
the
bondline.
Table 1. Averaged results of uni-axial tensile tests for both materials.
Elements of small diameter such
as pins or spikes are a promising
approach as a new technology for
composite parts. In contrast to wellexplored z pinning approaches21,
pins shall here not be implemented
in the uncured composite and are
instead incorporated into the bonded
joint after curing. However, the
working principle is assumed to be
somewhat similar to conventional z
pins. Thus, the elements shall bridge
cracks and consequently suppress
crack propagation. Evidence is given
in the literature that z pins reduce
mode I (peel) stresses at the crack tip
and thereby delay crack extension22.
One might argue that state of
the art rivets could also be used as
crack stoppers. However, replacing
conventional fasteners by small
diameter elements has certain
benefits. Former studies showed
that decreasing the element diameter
increases the overall joint strength
for static load cases, especially for
elements with less than 2 mm in
diameter23. Furthermore, replacing
a certain number of conventional
fasteners (e.g. rivets) by a greater
number of smaller elements of the
same overall shear plane increases
the overall available load bearing
area. This relationship can be proven
easily by following formulas. The
maximum force Fmax transmitted by
the elements is given by:
[3]
Here, S is the bolt's shear strength,
d the bolt diameter, and i the number
of bolts. The maximum bearing
pressure pmax is directly related to
the diameter if we assume a constant
overall shear area.
[4]
In Equation 4, t represents the
overall material thickness. It is,
therefore, concluded that small
diameter elements improve the
bearing strength.
Replacing conventional bolts by
small diameter elements requires a
way of axial fixation. Amongst other
options (e.g. staple-like elements,
see24), solid rivets made out of casehardened steel are used here as
depicted in Figure 5.
Figure 5. Rivet elements (I), rivet head forming (II) by use of a toggle lever press, and
microsection (III) of pinned composite parts.
SAMPE Journal, Volume 53, No. 1, January/February 2017
25
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
SAMPE Journal - January/February 2017 - 4
SAMPE Journal - January/February 2017 - 5
SAMPE Journal - January/February 2017 - 6
SAMPE Journal - January/February 2017 - 7
SAMPE Journal - January/February 2017 - 8
SAMPE Journal - January/February 2017 - 9
SAMPE Journal - January/February 2017 - 10
SAMPE Journal - January/February 2017 - 11
SAMPE Journal - January/February 2017 - 12
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SAMPE Journal - January/February 2017 - 16
SAMPE Journal - January/February 2017 - 17
SAMPE Journal - January/February 2017 - 18
SAMPE Journal - January/February 2017 - 19
SAMPE Journal - January/February 2017 - 20
SAMPE Journal - January/February 2017 - 21
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SAMPE Journal - January/February 2017 - 25
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