Research underway at the Port Melbourne-based Maritime Division of the Defence Science and Technology Organisation (DSTO) is paving the way for new applications for thick advanced composite structures.
Dr Asintha Nanayakkara and Dr Nigel St John are members of a team focused on developing skills and knowledge of thick composite structures, covering the spectrum from material selection, design and structural analysis, manufacture and testing, through to damage and fatigue prediction techniques.
While there has been a lot of research conducted on the manufacture, design and testing of thin composite components there is less knowledge of the behaviour of thick composites, says Dr Nanayakkara.
“A better understanding will lead to broader applications. Our focus is on thick curved composite structures for applications involving hydrodynamic loading such as rudders and control surfaces (hydrofoils),” she says.
“Thick composites are very complex, and have different properties to thinner composites due to their three-dimensional structure, thus their performance characteristics are different and, under water, the forces from shock are unique.”
Starting in early 2013, at the well-equipped DSTO research laboratories, work began to design and manufacture four versions of the 1.5m long hydrofoil in various combinations of core, outer ply materials and using out-of-autoclave manufacturing methods. The final model was laidup using a glass sourced from local manufacturer, Colan, for the core plies and a large tow carbon fabric from GURIT for the outer plies. The resin was infused using a closed mould RTM process.
The thickness of the hydrofoil tapers from 100mm to 50mm. Curing the thick composite using RTM presented the first of several challenges for the project team.
“One of the issues with the manufacture of thick composite structures is controlling exotherm in the resin because you get too much heat from the resin,” says Dr St John. “So we did it in two halves, manufacturing each at 50mm thickness to give a combined thickness of 100mm in the final structure.”
Drawing on the DSTO’s advanced tools in modelling and test validation and the resources of partners at the UNSW Australia (University of New South Wales) and the University of Tasmania’s Australian Maritime College work on smaller scale foils, the large foil has been put through rigorous fatigue testing, with the results now being validated using Finite Element Analysis.
The results will inform specification for thick composite structures for future Australian Navy ships and submarines as well as identification and/or development of techniques such as acoustic emission, for the early identification of delamination and compressive, tensile and fatigue failure.
As well as the work on rudders and foils, the DSTO is testing composites in frigates and looking at robust composite sonar dome structures.
Like the Boeing Dreamliner, the team hopes to see future Navy vessels built with increasing proportions of advanced composite components as research proves the benefits and provides solutions to barriers.
“One of the issues for advanced composites is to get people to take account of the life-cycle costs,” says Dr St John. Using more composite components would reduce life-cycle costs due to the reduced weight; the ability to make complex shapes (less machining); and the anti-corrosion properties; and tailored mechanical behaviour in hydrodynamic control surfaces and rudders, increasing efficiency of the vessels’ movement through water.
DSTO Maritime Division