A composite material is a macroscopic mixture of two or more distinct constituent materials, all of which are present in reasonable proportions and have different properties so that the composites property is noticeably different from that of each of the constituents. Out of two distinct constituents, one being reinforcement phase which is usually stiffer, stronger and discontinuous, and other being matrix phase which is less stiff, continuous phase and is required to bind the reinforcement materials together. Further, in composite material system, reinforcement phase should not dissolve in and/or react with matrix phase.
The homogeneity of the composite material is solely depends on the uniform distribution of reinforcement phase in matrix phase, however, non-uniform distribution leads to heterogeneity. Homogeneous composite material yields consistence best properties when compared to heterogeneous composites. In addition to this, isotropic and anisotropic nature of the composites is significantly affected by the type, shape, size and orientation of reinforcement phase in matrix phase .
Composites used in medium load bearing structures make use of reinforcement materials in the form of short fibers, particles. On the other hand, continuous fibers are used as reinforcement materials for high load bearing structures, because of its high strength and stiffness. The matrix phase provides protection for the sensitive fibers, bonding, support and distributes the load uniformly throughout the reinforcement phase of the composite structure. There exists one more phase in between the reinforcement phase and matrix phase called an interphase. This phase does have significant influence on the strength, stiffness and failure mechanism of the composite structures.
Composite materials apart from aerospace applications; are being widely used in marine industry. Some of the applications of composites in marine sector include boats, ships, submersibles, ship hull structures such as keel, frame, plating, decks, bulkheads offshore structures etc. Marine engineering structures are mainly designed to carry static loads due to its own self weight and/or dynamic loads such as environmental loads due to wind or waves. Performance of marine structure depends on several parameters such as geometry of the structure, type of materials, mechanical properties (strength and stiffness) and physical properties (density), environmental resistance, damage tolerance capability etc. Materials used in marine applications demand high strength-to-weight ratio and high stiffness-to-weight ratio , high impact strength, good corrosion resistance, buckling resistance, excellent flammability, better damping characteristics, low thermal and acoustics conductivity etc. Sandwich construction is one such technique, if properly designed, can satisfy these requirements.
A sandwich construction essentially consists of two thin face sheets (face skins) separated by a lightweight, thicker core which occupies about 80% of the overall volume of sandwich construction. The function of the core is to stabilize the facings against buckling and define the flexural stiffness, out-of-plane shear and compressive behaviour. The role of the skin is to provide required flexural and in-plane shear stiffness. Sandwich composites are mainly used for beams and panels in marine engineering structures. Sandwich composites in flatwise orientation are commonly used as structural panels such as roof, floor and decks, where the face sheets (face skins) carries the flexural load and the core carries the shear load. In beams, sandwich structural components are used in the edgewise orientation for higher stiffness.
The core in the sandwich composite may be honeycomb (made from aluminium, papers etc) [2-6], polymeric foams (polyurethane, polypropylene, polyvinylchloride, polyol-isocyanate, poly ethylene terephthalate etc) [7-11], metallic foams , foam filled honeycomb [13, 14] etc. Skin in sandwich composite may be made from metals (aluminium, titanium, copper-nickle etc) and composites (glass polymeric, carbon polymeric, aramid polymeric etc).
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