Feature 2 | MARINE COMPOSITES
laminates involves one more dimension. Simply switching materials from metals to fibre composites for a specific design is generally a bad idea. In Table 1 is a comparison of a few basic material properties. Te numbers are typical for materials used in ships. In Table 2 the weight effectiveness of the materials are compared with steel as reference.
Sandwich structures A sandwich structure consists of two face laminates and a core that is usually a polymeric foam or balsa wood. The greatest advantage of a sandwich structure is the superior bending stiffness compared to its weight. Tis means that sandwich structures are very effective in taking lateral loads, as sea pressure for example. Weight savings of up 75% compared to steel are possible with sandwich structures with carbon-fibre laminates and polymer foam core.
20m catamaran As an example, some typical figures for a 20m catamaran in carbon fibre are given in Table 3. All structure is sandwich. In the table minimum carbon-fibre weight per m2 demanded by Norway-based classification society Det Norske Veritas (DNV) is also given together with the total weight of the sandwich panel fulfilling the rules.
Limiting factor in the rules From a mechanical point of view the optimal thickness of the face sheets will be less than stipulated in the DNV rules [1] but the rules also take in consideration other aspects such as durability and resistance to penetration. For example, the bottom panel in Table 3 could be made with
Structural part
Hull bottom Hull side
Deck
Watertight bulkhead
Pressure [kPa]
40 15 5
20 Super-structure 10
Typical panel size [m]
1 2 2 2
2.5
Min. carbon-fibre weight [g/m2
]
1600 1100 1100 1100
800 Table 3. Typical sandwich panels for the 20m catamaran. 700g/m2
which would decrease the weight of the sandwich panel from 9.3kg/m2
.
carbon fibre instead of 1600g/m2 to 6.3kg/m2
As in the example above, the limiting
factor is the minimum weight carbon fibre required by the rules, which is also true for most parts of the ship. Tis means that the actual stress in the laminates is low compared to the maximum allowed in a top quality laminate. Te strength demand on the laminates can therefore be relaxed and a wider selection of fibre, reinforcements and resins is available which will have a positive effect on the material cost.
Strength of different carbon fibres Figure 4 shows results of tensile tests on carbon-fibre/vinyl-ester laminates where eight different carbon fibres from six suppliers were tested. All fibres were made into reinforcements of the same types (+/-45° 600g/m2
) by the same
reinforcement supplier with the purpose of eliminating all difference except the
Figure 4. Tensile strength of carbon fibre laminates with different brands of fibres.
fibre itself. Compared with the top quality fibre (no.1) the other fibres had 47-75% of the strength but all fibres had the same stiffness. Te top quality fibre was at the time of testing also the most expensive and the least expensive was about 60% of the cost.
Strength of different reinforcements Figure 5 shows the results of compression test on carbon-fibre/vinyl-ester laminates where 11 different reinforcements from six suppliers were tested. All reinforcements were of the same type (0/90° 450 g/m2
) and
made with the same top quality fibre. Te differences between the reinforcements for the same supplier were in most cases stitch spacing and fibre spreading. Compression strength depends more on the quality of the reinforcement than tensile strength. Results show that the reinforcement with the lowest compression strength had 60% of that of the one with the highest strength. Te differences in cost were in the case small between the suppliers. Te results from these tests show that
it possible to save cost by choosing a fibre with lower strength from a less expensive reinforcement manufacturer. But the ones with the lowest strength will probably not be good enough and being closer to the limits stipulated by the rules will also make the quality control more important.
Weight of reinforcements In order to optimise the structure all laminates must be as close to the minimum weight as possible. Multi-axial fibre reinforcements can be made with area weight of less than 300g/m2
up to several kilos per square metre 36 Ship & Boat International July/August 2010
Weight of sandwich panel per m2
9.3
6.,8 6.0 7.6
7
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