Left : this is the output from a Dynamic Mechanical Thermal Analysis (DMTA) on a mould tool laminate. The technique takes a sample of laminate, bends it up and down and measures its stiffness as the temperature of the sample is increased from room temperature up to 150°C. The upper curve demonstrates that the stiffness remains constant until the softening temperature (Tg) of 107°C, at which point it start to decline. This value is important as the Tg of a resin or laminate system is indicative of the state of cure. If a resin has not been cured sufficiently the Tg will be depressed. The second curve is the loss tangent which is also indicative of the Tg but occurs at a slightly increased temperature. The objective is for this curve to exhibit a smooth single peak. When we see multiple peaks this indicates the presence of partially cured material within the sample
Left: this is the log from our oven during the partial cure of a 16m racing boat deck. The 15 traces from temperature sensors show the temperature at different points in the oven over a period of 23 hours. For this particular laminate and mould tool we chose to hold the temperature at 65°C for two hours, then slowly increase to 80°C and hold again for 12 hours. Some of the thermocouples are directly on the laminate, others are in the air above or on the floor beneath the mould. Different zones of the oven are controlled independently to get the flow we want for each part of the deck
At room temperature the resin is a solid
and dry to the touch; at 35° it is a sticky liquid; at 65° it is a liquid with the viscosity of water; at 75° the chemical reaction is starting to take hold, but the resin remains mobile for several hours before it ‘gels’. If we get too much ‘flow’ we will
remove too much resin from the laminate, resulting in areas where the laminate has a large number of very small voids (porosity). Too little flow and we degrade the bond between the plys of materials and trap larger air voids in the laminate. For honeycomb bonding operations the
size of the fillets between skin and core are in part determined by the time the resin is allowed to flow. All raceboat yards will have developed a series of cure schedules for different resin systems and laminate types, in our case these have been perfected over many years by trial and error.
3. De-bulking The carbon fibre that we use here is
pre-impregnated with uncured epoxy resin and stored in a freezer to prevent it curing. At room temperature the resin is workable for between 30 and 50 days, depending on exactly what temperatures it has seen before freezing and during transport. Completing the laminate of a complex
inner or outer skin for a large yacht can take three or four weeks, so some of the material will be close to the end of its workable life out of the freezer before the hull is cooked. As time goes by the tack and flow characteristics of the resin will change, and the tendency to trap air between plys of carbon increases. Experienced laminators know how to
deal with this, partly by altering the method of laying down the materials and partly by small adjustments to the oven temperature. Very small adjustments, that is. Too hot and the resin turns tacky and you trap air. Too cold and you risk humidity and/or tooling shrinkage. Good technique helps, but trapping a small amount of air between plys is
inevitable so we need to de-bulk laminate frequently by applying a vacuum bag to reduce air inclusion to a minimum. De- bulk techniques have evolved significantly and the frequency of de-bulking has also changed. We now recognise that trying to pull air up through a laminate of more than 150g/m2 is not effective, so it has been important to find efficient methods to de-bulk almost every ply of laminate. The extreme care with which modern
raceboats are built bears a closer resemblance to work carried out in an aircraft factory than to most people’s idea of working in a boatyard. By engaging with the latest NDE
techniques we have been able to fine-tune the details of all our build processes, so that with good technique and careful control we can produce excellent carbon composite structures at a fraction of the budget that is typically available in the aerospace industry. Geoff Stock, Fibre Mechanics
www.fibremechanics.com
SEAHORSE 55
q
FIBRE MECHANICS – LYMINGTON UK
APPLIED POLYMER DEVELOPMENTS APD/NEWPORT IOW
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