Feature 2 |MARINE COMPOSITES & CONSTRUCTION MATERIALS
Table 1. Comparison of conventional and natural composite performance (ILSS – interlaminar shear strength)
microscope images of (b) unaged and (c) aged in water glass/epoxy specimens show the clean, adhesively failed, fibres in the aged specimens, compared to the more ideal cohesive failure in the unaged specimens in (b), reflecting the degraded fibre/resin bond in the aged specimens.
Research at the
University of Southampton’s Faculty of Engineering and the Environment has therefore concentrated on multiscale approaches to build confidence in the performance, manufacturing and long term durability of natural composites in the marine environment. One of the main issues with natural
materials has been the prediction of the mechanical properties. Conventional boatbuilding composite materials can exhibit wide variation in performance off-design due to production processes; for example, cloth saturation and volume fractions, fibre and cloth architecture, void content and cure profiling. Further variation can occur for natural composites due to the climate under which they are grown, meaning an individual fibre may vary its properties along its length, or that fibres are grown in different climates, meaning one source of cloth made from flax, for example, can be different from another. It has therefore been important to investigate these materials and their properties at different scales from bundles through cloths to laminates. The analysis at the University of Southampton shows that whilst these materials do vary more at
Panel Glass/Epoxy
Dynamic properties Static and dynamic experimental testing carried out at the University of Southampton has shown that whilst absolute properties of natural composites cannot yet compete with conventional composite materials, their specific properties are impressive. For example, whilst natural fibre composites do not have quite the same mechanical performance as glass fibre composites, they are significantly lighter. Weight for weight therefore, natural fibre composites compete well with glass fibre composites. Table 1 gives examples of the mechanical properties of different conventional composite and natural composite layups. Whilst the specific tensile properties
of flax fibre and epoxy resin can be seen to compete well with glass fibre and epoxy, one area of note for a structural material is in the low flexural stiffness of the natural fibre composite which is almost half, in terms of the specific property, of the conventional glass fibre and epoxy composite. However, this low material property
is countered in structural design by the fact that the natural fibres are much bulkier than the equivalent weight glass
Acceleration Peak Main Frequency 4.0 g
11.4 Hz
aACC 0.09
Flax/Epoxy 44 2.9 g 10.8 Hz 0.13
, for a conventional fibre composite and a natural fibre composite
Table 2. Acceleration peak, main frequency and viscous damping factor, αACC
Ship & Boat International May/June 2013
lower levels, the overall properties at laminate level have a similar variation to standard composites. This research also supports the argument that modifications to the standard rule of mixtures must be made to calculate natural composite properties.
fibres: in the case of a hull or deck panel, for example, the thickness is around twice that of the equivalent weight glass/epoxy panel and since the flexural rigidity is governed heavily by panel thickness, panel rigidity is actually significantly superior with the natural composite. Dynamic properties are also crucial
for boat design and natural composites are proving to have an advantage over conventional fibre composites in greater acoustic and vibration damping. Table 2 shows the results of peak accelerations, panel vibration frequency and viscous damping calculations from experimental tests conducted at the University of Southampton on a typical bottom hull section (500mm from keel to chine, 735mm section width, constant deadrise of 25degs, 64 kg section mass) dropped into water from a height of 750mm. Higher viscous damping, αACC
,
demonstrates how quickly the evolved strain in the panels decays with time. Te flax/epoxy composite panel is about 40% more effective in damping the vibrations than the equivalent weight conventional composite panel and the peak accelerations experienced by the natural composite panel in the drop test are less than 75% of those experienced in the conventional glass composite drop test.
High strain rate Two other natural composites listed in Table 1
have been the subject of
research at Southampton. These are conventional fibres with low density
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