Light Weight Structures for Maritime Applications - Design, Fabrication, Characterisation, Testing & Simulation

Abstract

Single layer laminates for hulls: Laminated composite hull panels are widely used in high speed light craft (HSLC), small pleasure craft and sailing yachts. The superior stiffness of the high-modulus fibres has resulted in an increase in use of these materials for fast marine vessels from lengths ranging a few meters long to vessels in excess of 50 m. Properties such as light weight, corrosion resistance, low maintenance costs, enhanced properties, and low magnetic characteristics have been successfully applied to marine vessels of all kinds. Composite vessel hull sizing presently relies on linear (small deflection) theory and many organisations worldwide are constantly searching for new methods to establish new standards to satisfy the testing of the rapid advance in composite materials and sandwich fabrication techniques. Classification societies rely on rules solely on maximum lateral deflection related to the panel span. However, it is known that even at low load levels composite panels exhibit a non-linear response to loading. Furthermore, although the process of introducing a core material to produce a sandwich material is used by the top of the range boat/ship international manufacturers, no testing procedures for the material process has been undertaken on a national level. During the fabrication process and general boat building, the ratio of the fibre to resin depends upon the fabrication process, the type of resin used, the method of application of the fibres and upon the skill of the person who is involved in the fabrication process. A high volume of fibre to resin produces a composite with a superior mechanical property when compared with a lower volume ratio. However, there are limits of this ratio since all the fibres need to be encapsulated in the resin. The fabrication process and techniques also determine the amount of air inclusions and imperfections, which must be reduced to a minimum, in the resulting panels. Single skin composite laminate panels may lack stiffness due to their relatively low thickness, and when dealing with long ships, hull flexibility must be considered, since composite vessels have relatively low stiffness when compared to similar vessels of aluminium or steel construction, and fatigue damages in the hull may result. In small craft design and the typical boat building locally, the boat builders, traditionally introduce stiffeners of wood encapsulate in additional fibre and resin, producing additional frames for strength, thereby adding to the weight of the boat and increasing the construction complexity.

Sandwich construction for vessel fabrication: A sandwich construction consisting of two high strength composite laminate skins are separated by a core material. The core material is light weight, with favourable properties of shear strength and stiffness. The panel which although may increase in thickness, does not suffer a weight penalty, and offers additional strength support, which acts as an I-beam shear web, as the global sandwich panels bend and flex during the in-service loading, such as hydrostatic loads, bottom slamming loads, and impact load on the side shell.

Hybrid Marine Structures: Marine structures encompass a wide range of applications, where such structures are fabricated using various methods and materials and are subjected to the harsh marine working environmental conditions. The work focuses on composite lightweight structures. The materials selected are dependent on the purpose of the structure and are to be of the higher specification marine grade. The materials to be considered are traditional metallic materials, combined with non-metallic materials such as glass, carbon (higher strength) and Aramid & Kevlar (abrasion and ballistic penetration resistance) fibres encapsulated in polyester and/or epoxy resin forming a laminate structure and renewable fibres such as kemp; additionally closed cell foam core layers or equivalent are also introduced, providing structural shear stiffness, structural rigidity and an energy absorption mechanism. The design of a structure needs to follow proper design rules, optimising on the material selection based on weight and strength, to combine with a few or many other constituents (as the application requires) to produce a structure fit for its purpose. Marine structures sizes vary greatly, adopting the correct method of fabrication is crucial to achieve correct bonding of the many constituent layers. Such methods need to be reviewed and a proper understanding of the current trends needs to be studied. The design stage requires the knowledge of the resulting hybrid lightweight structure to be determined using various scientific analytical theories, ranging from traditional strength of material, composite laminate, sandwich theories and mechanics of material fundamentals. Material characterisation through experimental techniques is essential to confirm the designed hybrid structure properties, in so doing that validation of the design principles adopted are verified and can be used predictively in other various loading or damage scenarios. Testing the hybrid material, designed specifically for various applications, is required to show that the structural integrity conforms to the expected trends. Various loading and damage, even up to complete catastrophic failure, is envisaged. Characterisation and testing must comply with current BS and ASTM standards. FEA methods of simulation, using the validated characterised properties of the hybrid lightweight marine structure, using explicit solution techniques, is also to be introduced.

Spin offs from the work that is proposed and to be undertaken can also be adopted for sports protection as well as for ballistic testing and protection.

Design: The local small craft motor yacht industry has gained popularity over many decades, with the cost of such vessels within the reach of many individuals. Therefore, the understanding of the underlying principles and their performance characteristics has led to the on-going research within this field of small, medium and large vessel design, single or multihulled. Such craft need to comply with both local and international maritime classification societies and standards to gain approval and certification for manufacture and purchase of such craft. This research considers the hull construction and scantlings from standards of the determination of the design pressures and design stresses, together with classification society rules and Malta Transport regulations. Consideration during such work is of the build material such as wood, aluminium, steel, fibre reinforced plastic, sandwich, method of construction, structural arrangements and details, rudders and propulsion (self and wind assisted) devices, intact and damage stability