Carbon Fiber Reinforced Plastic—Material for Strong Lightweights
Low weight, high tensile strength and
rigidity as well as low thermal expansion—with these properties, carbon
fiber reinforced plastics (CFRP) are being applied in more and more
industries. Effective new processes for simulation and production have
also made an economic series production of CFRP components possible.
Carbon fiber reinforced plastics (CFRP) are up to 70 percent lighter than steel; compared to aluminum, a weight saving of up to 40 percent is possible. But, lightweight is only one forte of this compound material; its other benefits are its very high tensile strength and rigidity, resistance to corrosion, as well as its low thermal expansion. With these properties, CFRP has picked up points in the aerospace industry for about two decades already. The share of components made from this lightweight material is increasing steadily—in the Airbus A380, more than 20 percent is made from this material, and in the new Boeing 787 Dreamliner, it is 50 percent.
Lightweight Components for Vehicle
Production
The reduction of carbon dioxide emissions,
saving of resources and increase in the
economic performance are all driving forces in
the automotive industry for the development of
new processes for the series production of
CFRP components. The latest example in this
field is the CFRP roof of the new BMW M3
coupé, which is produced in a highly automated
production line at the BMW plant in Landshut
(Germany). The roof consists of several layers
of a CFRP batt, which are first pre-shaped in
dry condition, and then saturated in resin in the
so-called RTM process. For standardized and
economical series production in significantly
higher numbers, the motor-vehicle manufacturer
developed innovative tools and process
techniques. Apart from a reduction in weight of
about 11 pounds, the light material originating
from the aerospace industry permitted the
center of gravity of the new sports car to be
lowered even further, thus positively influencing
the vehicle dynamics.
At present, all motor-vehicle manufacturers are dealing with the subject of lightweight construction. Thus, the producers are in the process of developing structural and cassis components made of CFRP, such as the B pillar and the crash box, shock absorbers and axles. Significantly larger components are required for the development of a 40-ton towing vehicle made of a carbon fiber reinforced compound construction which started in autumn of last year. Primarily, the articulated lorry consists of the lightweight material in the area of the chassis, the driver’s cab as well as a number of mounting parts, and thus, will weigh between 5.6 and 6 tons less (towing vehicle and three-axle semi-trailer). On one hand, this reduction in weight permits a considerable saving of CO2 emissions, and on the other hand it is available for additional payload and thus contributes to the payback of the costs which will increase by about 20 percent.
Use in Machine and Device
Construction
Aerospace, vehicle construction as well as
racing and sports equipment belong to the
“accustomed” fields of application for
CFRPs, but the application possibilities of
this lightweight material have not been
exhausted by a long way yet. By reducing the
mass of the components to be accelerated,
such as skids or spindles in tool machines and
handling devices, for example, the dynamics
and productivity can be increased, and at the
same time, the wear is reduced as well. This
also applies for all oscillating moving
machine parts, such as levers and skids in
printing, textile and packaging machines. For
fast running rollers, CFRP components
permit higher speeds through the reduction of
the centrifugal forces. Axles and shafts made
of carbon fiber reinforced plastics reduce the
load on bearing, reduce thermally induced
deformations, and thus increase the service
life of machines and devices. In robot
construction and automation, CFRP components
are beneficial for movement and
positioning structures. Apart from excellent
damping behavior, they ensure faster and
more precise movement, and consume less
energy in the process due to the low weight.
The Direction of Fiber is Decisive
Carbon fiber reinforced plastics are
available for the construction of CFRP
components in different variations, such as
short or long fibers, continuous fibers, fiber
hoses as well as batts, fabrics and mesh. In
the so-called uni-directional batts, for
example, the fibers are arranged in parallel,
whereas fabrics consist of warp and weft,
which cross and sling around each other.
Multi-axial batts consist of several randomly
arranged batt layers which are connected to
each other. The versatility of material permits
the precise setup of the components for the
load to be expected or present. The significant
characteristic feature is that the high
tensile strength and rigidity is achieved only
in longitudinal direction of the fibers—
comparable to the human bone structure
which is designed according to the direction
of load. The design of a CFRP component, in
which the material can demonstrate its
material properties in an ideal way is thus
always characterized by a certain organic
flow, which means there are soft radii and noclear-
cut dividing lines. The material is not
suited for sharp edges and corners as they
will lead to enormous peak stress. In order to
influence certain properties, such as the crash
behavior of CFRP structures, the material can
be combined with glass-fiber reinforced
plastics. The fiber orientation for components
and structures can be optimized by means of
the Finite Element Method (FEM).
Effectively Analyzing and Optimizing
Fiber Composite Structures
Software tools are available also for an
effective analysis of fiber composite
structures. They can render information from
the simulation process even before the first
prototype is available for test purposes. The
influence of the microscopic behavior can be
simulated, for example, on the overall
behavior of the fiber composite materials, the
consideration of discrete or layered reinforcement
as well as the fiber orientation, modeling
of layer failures and delamination in fiber
composite materials, the targeted use of
anisotropy of the fiber matrix material,
sandwich structures and stiffened panels
made of fiber composite materials, modeling
of progressive damage, the consideration of
various failure criteria, the simulation of
impact behavior of fiber composite structures,
stability examinations and post-buckling
behavior as well as optimization options.
Economic Production Methods
Up to now, the most frequent manufacturing
process for lightweight components
has been the prepreg method. Fabric pre-impregnated
with resin is placed into a mold,
which is covered up air-tight and evacuated.
The curing of the semi-finished fiber product
is effected in an autoclave at a pressure of up
to 10 bar and a temperature of up to 356°F.
This process takes between three and six
hours. Apart from long cycle times, extensive
manual work and the accruing costs stand in
the way of economical series production
For this reason, the last few years have seen the development of new processing technologies which permit a more cost-effective production, including, for example, the RTM (resin transfer molding) and the VARI (vacuum assisted resin infusion) process. These two processes use so-called preforms (dry fiber structures), into which the resin is injected. This permits the manufacture of component parts with complex contours as well as the use of different types of fiber, such as fabric, multi-axial butts and mesh for a component. This also permits a reinforced functional integration, through which components can be produced without screwing or welding individual components. Another advantage of the preforms is their improved drapability.
In the RTM process, the resin is injected under pressure into a closed mold. In the VARI method, the fibers in the mold are covered with a special distribution medium. The mold is then evacuated, through which the resin is drawn into the mold under atmospheric pressure. On the one hand, the medium permits equal distribution of the resin across the entire surface of the component, and on the other hand it has a distinctly higher permeability than semi-finished fiber products. By applying injection methods, CFRP components can be produced in shorter cycle times. In addition, compared to prepregs, they have a distinctly better surface quality. In order to further reduce process times, a further progressing automation of production as well as a faster and more efficient curing of the plastic resins by microwave radiation, for example, will contribute to the development of innovative manufacturing technologies.
Doris Schulz has worked as a freelance journalist for more than 15 years. Her specialty is the field of technical topics, especially lightweight materials and surface treatment.
