JEC COMPOSITES MAGAZINE - Issue #85 - November/December 2013 - 50

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Strict quality control

Main features
Carbon composite booms for concrete pumping trucks are produced by
one-shot autoclave curing, with a wall thickness up to 30 mm and a
length of 14 m.
Prepregs and autoclave technology guarantee a reliable and efficient
laminate with high strength and damage tolerance, allowing a 25%
weight reduction in comparison to steel. The working length of the
machine has been increased by 20% with the possibility to reduce
the truck size, achieving good economic balance for the project.

Particular reference has to be made to the quality control
activity that is necessary to ensure production reliability. The
raw materials are monitored at a specific statistical rate by
differential scanning calorimetry (DSC) to verify the chemical
compliance of the matrix and the B-stage status of the incoming prepregs. Moreover, a machining cut-away part undergoes
visual inspection and DSC in different areas throughout
the whole thickness to have a direct feeling of the laminate's
debulking and to verify the glass transition temperature
(Tg). It should not be forgotten that this kind of application
is intended for use worldwide: it should work under the sun
of Arabic countries or in the wet and cold weather of Nordic
nations. Having a high Tg and, in particular, an onset above
110°C is absolutely mandatory, and may be a concern, considering the thickness of the laminate. The DSC check also allows
verifying the curing rate through the thickness. The areas directly in contact with the mould are the most critical because
they could remain colder during the autoclave cycle; this is the
reason why it is useful to check and monitor them.

one, or better, caused by it, is the technological difficulty
in guaranteeing the polymerization of such an amount of
composite material just by one-shot autoclave curing. Epoxy
resin, in fact, undergoes a very strong exothermic reaction
during cross-linking and the thicker the material, the higher
the temperature peak.
Therefore, thermal simulations were performed during the
project development stage, in parallel to the structural analysis, in order to ensure proper temperature homogeneity in
the mould and in the laminate. The resin was cured based on
the autocatalytic model first proposed by Horie and further
modified by Kamal and Sourour according to the following
equation:
dα/dt = k(T)αm(1-α)n
where ɑ is the curing degree, m + n is the total reaction order
and
k(T)=Ae(-Ea/RT)
is the Arrhenius function used to take into account temperature dependence (A is the pre-exponential factor, R is the gas
constant and Ea is the activation energy).
This theoretical model made it possible to forecast the resin's
curing behaviour and to optimize the temperature cycle to
be used, in order to avoid high peaks inside the laminate. In
particular, some dwell steps were suggested to allow correct
and homogeneous heating of the mould and to prevent material degradation in the central part of the laminate wall. The
overall curing cycle, including the cooling phase, lasts about
18 hours.
It is worth mentioning the importance of a suitable cooling
rate to avoid distortions in the laminate and, above all, to allow correct stress release inside the material. Composites are
inherently stressed after curing due to the different coefficients
of thermal expansion between the resin and fibres. This condition is particularly disturbing in components which have to
be machined and need to fulfil dimensional and geometrical
tolerances. In the present application, the need to install the
hydraulic cylinders for motion and the articulated systems
requires some mechanical operation and is paramount to control thermal distortions.

After curing, each boom is also tested by ultrasonic inspection
(100% control over the whole length) to check the porosity
level and the absence of delamination. This kind of verification
is done by a qualified operator directly at RIBA and provides
feedback about the thickness in the most critical sections. In
fact, it is very important to keep under control the areas where
the vacuum bag could have some difficulties in debulking the
laminate, near the corners for example. Effective debulking
is generally the primary condition necessary to avoid crack
propagation during laminate loading.
After solving all the technical aspects and verifying the integrity and stability of the component directly in the field, CIFA
and RIBA Composites could start mass production. Today,
the quality of the composite booms is very consistent and reliability is one of the most important results obtained.

Commercial production
Following the prototype phase, the boom has now reached the
market at full production rate, with major investments in tooling and equipment. The current production rate is approximately 100 booms per year. The Top Carbon joint venture was
established in Faenza, Italy, last February to increase the production rate up to 150 parts a year. The maximum dimension
currently under production is 14 m, made in a single autoclave
curing step, for a total weight of up to 1000 kg including approximately 650 kg of carbon fibre. n

50 jec composites magazine / No85 November - December 2013

More information:
www.ribacomposites.it
www.cifa.com

http://www.ribacomposites.it http://www.cifa.com

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

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