SAMPE Journal - November/December 2021 - 61

frame tubes. We have developed a method to
evaluate the mechanical performance of the bike
lugs and assess the effect of the component design
and fiber layout. To produce a full bicycle frame,
different types of lugs are needed to connect
tubes under different angles. For the mechanical
characterization we have designed a specific test
lug geometry in which one tube is attached in an
angle of 82.5 degrees to another tube. Two types of
test lugs were used: a closed geometry, in which the
vertical tube remains intact at the position of the
horizontal tube, and an open geometry, in which
the vertical tube has a hole at the position of the
horizontal tube, so there is an open connection
between the inside of both tubes. The volume
fraction of carbon content within the printed lugs
is presented in Table 1.
During mechanical testing, a force is applied
on the first tube while the other tube is fixated. For
different materials and geometries, different forcedisplacement
curves were obtained. To compare
these different test lugs, we have determined the
maximum force that can be applied to the test lug
(the strength of the sample) and the displacement
at 1 kN (as an inverse measure of the stiffness of the
sample). The results are displayed in Figure 6.
Compared to the conventionally produced
carbon frame, which is based on a thermoset
carbon composite, the CCF reinforced lugs reach
about 55% of the maximum force and show 1.6 - 2
times the displacement (corresponding to 50 - 65%
of the stiffness). The measured displacement for the
closed geometry CCF reinforced lug is somewhat
lower than that of the open geometry, but overall
no significant different in strength or displacement
was found between the two geometries.
The effect of the fiber reinforcement is most
apparent in the maximum force. CCF reinforced
lugs are more than two times stronger than nonTable
1.
Bike Test lug configuration Fiber volume fraction
CCF reinforced, closed
CCF reinforced, open
CCF + sCF reinforced, open
CCF reinforced tilted, open
15%
14%
27%
12%
reinforced lugs, even though they seem to have the
same stiffness, as indicated by the displacement
at 1 kN. The cause for the latter could be that the
load in the test set-up is applied perpendicular to
the fibers in the test lug. In this way, they do not
contribute strongly to the stiffness of the sample.
Currently, we are investigating the effect of fiber
orientation in the test lug on the mechanical
properties. A first example is shown as the bottom
entry in Figure 6 (tilted). In these samples, the
fibers in the lug are orientated under a different
angle than for the other samples. As can be seen
in the results, these lugs have a lower maximum
force and somewhat higher displacement than the
untilted lugs. However, it should be noted that due
to design constraints the fiber content of the tilted
lugs is lower than the untilted ones (12% for the
tilted compared to 15% for the untilted).
Further optimization of the mechanical
performance of the bicycle lugs is in progress,
both in design of the fiber layout and in improving
strength perpendicular to the fiber direction.
CONCLUSIONS
When designing structural composite products
to be produced by continuous fiber additive
manufacturing, it is important to consider layout
of the reinforcing fibers, relative to the expected
loading conditions. In this paper, we have shown
both experimentally and with theoretical and
numerical models how the mechanical properties
Figure 6. Mechanical performance of different 3D printed test lugs, compared to the reference.
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NOVEMBER/DECEMBER 2021
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SAMPE JOURNAL |
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SAMPE Journal - November/December 2021

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