JEC COMPOSITES MAGAZINE - Issue #85 - November/December 2013 - 49

lated arm. The weight saved by the carbon fibre parts is just a
small component of the virtuous cycle induced for the complete
machine, which needs a smaller counterweight, smaller hydraulic cylinders to open the arm and, as a result, a smaller truck.
This is the cornerstone of the project, which also makes it feasible from the economic point of view. Actually, the higher cost
of composite booms is partially balanced by the reduced truck
size, which makes it possible to keep the overall cost acceptable,
offering the following advantages to the final customer:
- Weight reduction: -25%
- Greater working length: +20%
- Better accessibility to city centres: no oversize load regulations
- Reduced fuel consumption
Fig. 2: Metallic bushing epoxy-glued in the carbon fibre arm

Redesigning
From a technical point of view, the component involves two
main difficulties. The first one is a structural concern due to the
need to increase the overall section and thickness to obtain the
same stiffness as the steel benchmark, because a composite layer
has a lower Young's modulus than a steel layer. In fact, composites have optimum specific mechanical properties, but in absolute terms it is very hard to reach the stiffness of steel with the
same design. As a reference, normal PAN-based carbon fibres
combined with an epoxy resin have a stiffness of about 120 GPa
for a unidirectional layer, in the fibre direction, and 60 GPa with
a fabric layer. Common steel, instead, has a Young's modulus of
207 GPa, but its density is 5 times higher.
This is the main reason why simple "metal replacement" is
generally not a very efficient approach, even if composites have
huge potential in this type of application.
Thanks to the expertise of RIBA Composites in carbon-fibrereinforced materials and composite technology, CIFA could
redesign the section of its booms to fulfil all the technical
specifications. The finite element method was used to control
strength and stiffness and a comprehensive testing campaign
was conducted on specimens to correlate the right material properties with calculation models. The boom is mainly
subjected to bending due to its own weight and to concrete
flowing inside the pipes, so the best geometry to optimize the
strength-to-weight ratio was found to be a hollow rectangular
section, with unidirectional fibres oriented along the isostatic
lines (stress trajectories).
Developing the new design required not only an evaluation
of the main loads carried by the boom, but also a study of the
joints: pinned connections are required for the rotation of the
articulated arm. This was a great concern because of the reduced
bearing capability of composites in comparison to metals. To
avoid damaging the epoxy matrix in the pin area, the design
choice was to bond metallic bushings after producing the part.
Figure 2 shows a detail of a metallic bushing, which is bonded

Fig. 3: Boom extension

onto the carbon part using a high-performance epoxy adhesive,
whose thickness is controlled by microspheres to guarantee
the repeatability of the technological operation. According to
RIBA's experience, bonding parameters are the real key factors
in a good joint. Achieving perfect reliability of the adhesive
thickness is paramount to achieve efficient polymerization and
maximum shear strength of the connection. Moreover, +/- 45°
fibres were chosen to reduce the stress intensification factor
around the holes in the laminate.
To sum up, the following structural specifications were taken
into consideration during the design phase:
- Static loads of the weight to be carried
(concrete and structure's own weight)
- Loads due to the joint forces generated during the boom
movement, as shown in Figure 3
The above considerations, together with the necessary damage tolerance in case of misuse by operators (accidental crash
against objects in the building site), resulted in the decision to
use a solid laminate with up to 30 mm wall thickness.

Curing simulation
The second concern of the project, directly linked to the first

No85 November - December 2013 /

jec composites magazine 49

JEC COMPOSITES MAGAZINE - Issue #85 - November/December 2013

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