Effect of sea water and humidity on the tensile and compressive properties of carbon-polyamide 6 laminates

Abstract

Thermoplastic matrix carbon fibre composites offer considerable potential for underwater applications. Various material options exist but there are questions concerning the tension/compressive behaviour and water sensitivity of the less expensive polymers (e.g. polyamides) for these applications. The aim of the current work is to model water diffusion and its effect on the mechanical properties of thick carbon fibre reinforced polyamide-6 composite cylinders immersed in sea water for deep sea applications. To provide the data for such a model, thin specimens (2 mm thick) have been aged under different humidity conditions and tested in tension and compression. As water enters the composite, a significant reduction in the laminate properties is observed. An empirical relationship that links matrix-dominated properties to water content is presented and can be used for modelling purposes.More than 70% of the oceans remain unexplored. With an average depth of over 3800 meters, it is necessary to design exploration devices that are able to withstand high hydrostatic pressures. The lighter these pressure vessels are the more equipment and measuring instruments they can carry, so composites have been used underwater for many years [1]. The use of composites for pressure hulls of underwater vehicles and submarines has been an ongoing research topic for many years, since the early work in the UK by Smith and colleagues in the 1970"s [2]. The use of glass and carbon reinforced thermoset composite materials for deep-sea applications needs a thorough understanding of the behaviour of these materials under hydrostatic pressure.However, the ability to predict the implosion pressures of such materials and under such loadings (bi-axial compression) is not an easy task, as was demonstrated by the results of the World Wide Failure Exercise [3].Applications for thick thermoplastic cylinders for underwater applications are rarer, even though there is an increasing number of thermoplastic matrix polymers available on the market such as polypropylene (PP), polyamide (PA), polyphenylene sulphide (PPS), polyetheretherketone (PEEK), etc. They offer possibilities for forming by local heating, attractive mechanical properties, good environmental resistance and the potential for end of life recycling. More ductile and reparable by melting, they offer a real potential for greatly improved devices. Studies have been conducted in the USA on carbon/PEEK cylinders [4], [5] and by the author [6] and were found to be very promising as they imploded at comparable pressures to those of their carbon/epoxy counterparts. However, it is not clear to what extent the compressive behaviour of thermoplastic composites in the presence of water will limit the operating depth of an underwater pressure vessel, especially for carbon/polyamide 6 (C/PA6) composites. The latter are attractive as they are much less expensive than composites based on high performance matrix polymers such as PEEK.The mechanical behaviour of composite materials has been extensively expl...

Effect of thermoplastic resin transfer molding process and flame surface treatment on mechanical properties of carbon fiber reinforced polyamide 6 composite

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Carbon/polyamide 6 thermoplastic composite cylinders for deep sea applications

“…The mechanical properties used for the finite element calculations are shown in Table 3. Most of the mechanical properties were measured on composite panels manufactured by thermo-compression from the same fibres and matrix, within the project [39].…”

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Experimental Results of Accelerated Long-term Durability Performance of E-glass Fiber Reinforced Polymer Composite Materials

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