SAMPE Journal - January/February 2017 - 24
Feature Article
Looking
at
a
conventional
overlap joint, the qualitative stress
distribution is characterized by
high stress peaks at both ends and a
lightly loaded trough in between. If
we take the representative equation
after Volkersen14 (although some not
negligible simplifications are made)
for an exemplary overlap l of 25 mm,
we obtain the stress distribution
as function of the position x along
the overlap by use of the following
formula:
transition of both materials. Hence,
the impact on the overall bond
strength for an intact joint is expected
to be small and tolerable.
Manufacturing Concept
The joint manufacturing can be
divided into four steps as shown in
Figure 3. First, the thermoplast strips
are put in place before curing of the
adherend parts (Step I). The prepreg
curing cycle is conducted as specified
[1]
In Equation (1), t is the adherend
thickness, σ the tensile stress within
the adherend, Ψ the stiffness
ratio of both adherends, and ρ the
characteristic
bonding
number
which is calculated as follows:
[2]
If we consider two equal composite
adherends (Ψ=1) with a young's
modulus E of 67500 MPa which are
bonded using an epoxy adhesive
of 0.15 mm in thickness (ta) and a
shear modulus Ga of 900 MPa, one
obtains a value for ρ = 7.36. Note,
that the chosen material parameters
match the material properties of the
experimental study presented here.
It becomes obvious in Figure 2
that larger parts of the overlap are
barely loaded. The same qualitative
stress distribution applies for long
overlap
doubler
configurations
(e.g. stringers) which show a rather
long low loaded area in a certain
distance from the overlap edge. Due
to the thermoplast position in areas
of low stresses, only minor stress
concentrations are expected at the
by the material data sheet15. An
ethylene tetrafluoroethylene (ETFE)
release film between the plate's
surface and the steel tooling ensures
a constant overall surface finish. As
a strip of thermoplast is applied to
an uncured prepreg surface and
processed in a prepreg curing cycle,
it is incorporated into the material
during autoclaving. Hence, a plane
surface is created due to applied
pressure (Step II).
Since the thermoplast's melting
point matches the curing temperature
of the CFRP, a strong bond is
realized between the thermoplast
and the composite's matrix system.
A similar approach is described in
Hou's work16 where an amorphous
thermoplastic film as outermost layer
of a thermoset composite is used to
obtain a fusion bonded joint.
The thermoplastic strips are
placed on both adherend surfaces.
Thus, the strips face each other
when both parts are joined. For all
test specimens strips of 10 mm in
width are used for this process step.
In a third step (Figure 3), the actual
bonding is conducted by placing the
epoxy film adhesive on one adherend
with the area of thermoplastic strips
Figure 2. Relative shear stress
in a representative SLS
joint after Volkersen14.
left open. Two additional strips of
thermoplast (width of 7 mm) are put
here. Small adjustment errors during
manufacturing of the strips are
acceptable since the strips that are
already placed within the prepreg
are slightly larger. Therefore, a
proper welding of the inserted strips
shall be ensured.
In the final step IV, the adhesive
is cured and the bond is thus
established. Both, adhesive curing
and thermoplastic welding are
performed
simultaneously.
In
doing so, all 4 thermoplastic strips
are welded through coalescence of
the material. The applied curing
temperature of 180°C and pressure
of 3.5 bar ensures proper welding
at the interface of the thermoplastic
layers. Pressure and temperature
are maintained for 2 hours to allow
complete polymerization of the
epoxy adhesive. A micro-section
of the resulting hybrid bondline is
shown in Figure 4.
In contrast to adhesive bonding,
welding does not need surface
pretreatment of the adherends.
Applied heat causes the thermoplast
to soften, while applied pressure
causes the softened asperities to
Figure 3. Manufacturing steps for hybrid (bonded & welded) joint.
24
SAMPE Journal, Volume 53, No. 1, January/February 2017
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
SAMPE Journal - January/February 2017 - 13
SAMPE Journal - January/February 2017 - 14
SAMPE Journal - January/February 2017 - 15
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
SAMPE Journal - January/February 2017 - 22
SAMPE Journal - January/February 2017 - 23
SAMPE Journal - January/February 2017 - 24
SAMPE Journal - January/February 2017 - 25
SAMPE Journal - January/February 2017 - 26
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SAMPE Journal - January/February 2017 - 28
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SAMPE Journal - January/February 2017 - Cover4
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