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).
Foam cored sandwich composite structures are increasingly used in the marine sectors for its low density characteristics . Considerable attempts have been made by the researchers in developing and characterizing the sandwich composites with newer materials for core and face skin. A special class of foam called syntactic foam (a composite prepared by uniform mixing of hollow micro-spherical particles and resin matrix) is one of the challenging foam core material having high compression strength, low density, low moisture absorption, high specific properties and buoyant nature, when compared with other core materials used in marine structural applications. Presence of porosity in the closed-cell form is responsible for higher compression strength of syntactic foams compared to the open cell structured polymeric foams. As a result of closed porosity, elasticity of these materials is less, and they are classified in the category of rigid foams. Other additional advantages of using syntactic foam are low moisture absorption and increased thermal stability when compared to other polymeric foams. A combination of these properties makes them suitable for the structural purpose, especially in marine applications. Syntactic foam based sandwich composites are presently being used in wide variety of marine and naval engineering applications . However, the mechanical properties (tensile and compressive) of syntactic foam mainly depend on the volume fraction and wall thickness of microsphere. Higher the volume fraction of microsphere, lower will be the mechanical properties. Hence, the use of syntactic foam alone as core material may not fulfil all the technical needs of a sandwich composite. In order to benefit from the perspective of syntactic foam, over past three decades, the work on syntactic foam based composites has extended from its two-phase compositions (hollow microsphere + matrix) [16-19] to three and four phase compositions [20-24]. The three or four phase compositions of syntactic foam can be achieved by using hybridization technique. This technique involves the incorporations of nano-particles, chopped fibers, solid particles etc in two phase syntactic foam.
In the present work, an attempt is made to enhance the mechanical performance of syntactic foam core sandwich composite, by filling syntactic foam in resin impregnated paper honeycomb (RIPH) structure. A review of the literature that was carried out gave no related significant publication on mechanical and material properties of paper honeycomb core filled with syntactic foam used in the manufacturing of sandwich composite structure. Therefore, with the escalating use of syntactic foam and honeycomb core in marine sector , it is worthwhile to determine the influence of paper honeycomb core structure and its cell size on the mechanical behaviour and properties of syntactic foam based sandwich composites. This assumes the significance of the present investigation.
The manufacturing process is one of the most important steps in the development of sandwich composite materials. Structural parts, rather than generic material form, are fabricated with relatively simple tooling. Various manufacturing methods suitable for the sandwich composite are available. They include hand lay up or wet lay up, warm press molding, vacuum bag molding etc.
This process is economically most suited to producing low quantities of very large glass reinforced plastic composite mouldings such as boat hulls and building panels, and is highly labour intensive. This is one of the oldest methods that involve laying the dry reinforcement (most often a fabric or a mat) into the mold and applying the resin. The wet composite is rolled by hand to evenly distribute the resin and thereby removing the air pockets. Another layer of reinforcement is laid on top, after which more catalyzed resin is poured, brushed, or sprayed over the reinforcement. This sequence is repeated until the desired thickness is reached. The layered structure is then allowed to harden (cure).
A matched metal mold plates are clamped firmly and heated to the required temperature. The prepreg or reinforcement material is placed in the tool which has a cavity in the shape of the composite core required. The tool is rapidly closed and the cured. Finally, the top mold plate is opened and the composite core removed. In order to aid removal of the composite core from the mold plates, release agents are applied to the surface of the mold plates.
Vacuum bag molding is a single sided tooling process where the preforms (lay up of resin impregnated glass fabric + composite core + lay up of resin impregnated glass fabric) are placed into the tool and vacuum bagged in conjunction with vacuum distribution lines. Vacuum bag moulding is a modification of hand lay-up, in which the lay-up is completed and placed at the top of the mold plate. The mold plate along with lay-up are wrapped using a bag made of flexible plastic film and all edges are sealed. The bag is then evacuated, so that the pressure eliminates voids in the laminate, forcing excess air and resin from the mould. This vacuum consolidation method produces high- quality mouldings, with complete exclusion of air bubbles and improvement to the inner surface of the moulding, which is not in contact with the mould.
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