Feature 2 | MARINE COMPOSITES
Use of carbon fibre in high-speed passenger ferries
High oil prices over recent years has seen the use of lighter, more sophisticated designs of commercial vessels and technology that until now have only been used in the aircraſt or military industry. Te use of carbon fibre is a typical example of this writes Måns C Håkansson.
technology and design principles are well known. But, just switching the fibre to carbon will in many cases result in non-optimised design due to the material’s different set of properties. An example of this is carbon fibre’s significantly higher Young’s modulus, but only moderately higher strength that will result in more strength problems rather than stiffness problems. Another example is that quality control is more difficult in carbon laminates because they are not transparent, as glass-fibre laminates. Carbon fibre has been used for at least
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30 years in the military and aerospace industry and as in the case for glass fibre the material properties, production technology and design principles are well known. Using these materials and methods for ship design and construction will certainly result in a far too expensive product. To successfully use carbon fibre and
other light-weight, high-cost materials for commercial vessels demands extensive knowledge and long experience of all steps in design, production and marketing. Composites have many advantages in
ship building but also some disadvantages and other issues that must be taken into consideration. Low weight, freedom of design and low maintenance cost are the most important advantages of composite materials in ship structures. High material cost, acceptance among operators and special engineering skills needed when designing and building are a few factors that limit the use of these materials. It is important to an in-depth knowledge of all aspects of the composite materials to be able to create a design optimised for
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lass fibre has been used for boats for more than 50 years and material properties, production
a specific application. Tis includes not only the mechanical properties but also production methods, long-term stability, environmental impact, material supply, cost and so on.
Material properties Carbon fibre is lighter, stiffer, stronger and much more expensive than glass fibre. Carbon fibre is also lighter and stronger than steel but not as stiff. Te properties of carbon-fibre laminates, and other fibre laminates as well, can be controlled by laying the fibres in a certain direction. Tis will improve the properties in that direction but also decrease the properties in the other directions. A laminate with the same properties in all directions is
Young’s Modulus (GPa)
Carbon fibre laminate
Glass-fibre laminate
Steel Aluminium 45 (120) 17 (35) 210 70
called quasi-isotropic and a vacuum infused carbon-fibre laminate with stitched multi-axial reinforcements will usually have a stiffness of about 45GPa. If all the fibres are laid in one direction the stiffness will be about 120GPa in the fibre direction, but as low as 6-8GPa perpendicular to the fibres. Te stiffness of a fibre laminate will also vary with the fibre fraction in the way that a hand laid-up laminate will have lower stiffness that a vacuum infused one. A quasi-isotropic hand laid-up laminate have about 32GPa Young’s modulus compared to 45GPa for the vacuum-infused carbon- fibre laminate. Comparing fibre laminates to metals,
which are isotropic, is therefore not always straight forward and designing with fibre
Strength (MPA)
500
(1500) 300
(600) 360
200 Table 1. Basic material properties typical for ship applications.
Tensile Stiffness to weight ratio
CFRP GRP Steel Aluminium 115 (307)
33 (70) 100 96
Strength to weight ratio
747
363 100 160
Bending stiffness to weight ratio
322
184 100 200
Table 2. Weight effectiveness of the materials with steel as reference. (CFRP = carbon-fibre reinforced plastic).
Ship & Boat International July/August 2010
Density (kg/m3
1450 1800 7800 2700 Cost ) (c/kg) 20 2 1 5
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