COMPOSITES

1.05    COMPOSITES

(From Notes on SURFACE FINISHES http//www.gautamshah.in)

DIFFERENTIATING COMPOSITES and STRUCTURAL COMPOSITIONS 

Natural and manufactured raw materials are invariably compounds, made of many materials. Many such compounded materials are in the form of Composites. The composite materials come into being, by putting together natural and manufactured materials in such a special way that the strength and other qualities are different from the constituents, individually and cumulatively.
 
The term -different, is considered here as an improved quality, because man-made composites are designed and created towards specific performance requirements only.

Natural materials, manufactured materials and composites, all are further shaped, re-formed and geometrically integrated to create components as well as Structural Compositions.
 
A composite is a designed material entity with potential utility, but has no  operational functionality. A component, on the other hand  is a configuration of many materials into a utilitarian product. Component manufacturing employs  processes that are many times similar to a composite formation. As a matter of fact for component manufacturing, the ‘composite formation’ and the ‘component creation’ both occur simultaneously. Structural compositions (trusses, bridges, buildings) are geometric-configuration of materials, often assisted by components (nuts, rivets, pins, bearings, etc.). Structural compositions use composites to form the constituent elements. 
 

DEFINITIONS of COMPOSITES
  
1     Consisting of two or more physically distinct and conceptually  separable or visually identifiable materials.


2     Products that can be made by mixing separate materials so the dispersion of one material in the other can be done in a controlled way to achieve optimum properties.

3    Products with properties that are superior and possibly unique in some specific respects compared to the properties of their individual components.


CLASSES of COMPOSITES

Natural composites                  Wood, Bamboo
                                              Bone, Muscle and other tissues

Macro composites                    Galvanized steel
(engineering products)               Reinforced cement concrete (beams, etc.)
                                              Helicopter blades
                                              Skis, Tennis rackets

Microscopic composites             Metallic alloys
                                              Toughened plastic (impact polystyrene, ABS)
                                              Sheet moulding compounds
                                              Reinforced plastics

Nano composites                     Electronics circuits, diodes, transistors
 
Natural composites are easy to identify, such as: wood, bamboo, bones, muscles, etc. First man-made composites were related to the bronze, as man tried to fix natural stones and ceramic pieces by hammering into the bronze. Layered wood composites have been used by Egyptians. Mud bricks reinforced with hay, hair, and rice husk have been used. Cow-dung is also reinforced with granular sand particles for wall plaster. Gypsum (Plaster of Paris) has been applied on a lattice of jute, papyrus and such other fibres.

Macro, Micro and Nano Composites: Composites can be categorized in terms of the size of constituent particulate matter. Ingredients of macro composites can be distinguished by naked eye, whereas one may need an electron microscope to understand the constituents of micro composites and nano composites. Nano composites are created by introducing nano-particulate, which drastically add to the electrical, thermal, and mechanical properties of the original material.


CATEGORIZATION of COMPOSITES on the basis of STRENGTHENING MECHANISMS 

Composite materials can be distinguished into three categories based on the strengthening mechanism. These categories are: Dispersion strengthened, Particle reinforced and Fibre reinforced.

●    Dispersion Strengthened Composites have a fine distribution of secondary particles (filler) in the matrix of the material. These particles impede the mechanisms that allow a material to deform. Many Metal-matrix composites would fall into the dispersion strengthened composites’ category.

●    Particle reinforced composites have a large volume of particles dispersed in the matrix.  The load is shared by the particles and the matrix. Most commercial ceramics and many filled polymers are particle-reinforced composites.

●    Fibre-reinforced composites have fibre as the main load-bearing component. Fibreglass and carbon fibre composites are examples of fibre-reinforced composites.


MATRIX FILLER and INTERFACE 

The constituents of a composite are ordinarily classified as Matrix and Filler. It is the nature of relationship between the filler and matrix, or the Interface that defines the composite. Fillers  serve to resist stresses, mainly tension, and the matrix serves to resist the shear, and all materials present including any aggregates, serve to resist the compression.

Matrix and filler both can be of THREE types: Metals, Ceramics and Polymers. This set provides nine possible combinations.

Composite materials' combinations: Possibilities of combinations and type-examples.

    Matrix    +    Filler                   = Composite Type-Examples
 
●    Metal matrix composites MMC
 
    Metal        +    Metal              Aluminium-Tin are non miscible metals, 
                                               yet can be alloyed as a composite

    Metal        +    Ceramic          Electrical semi conductors, Carbide cutting 
                                              tool tips, Scissors, knives

    Metal        +    Polymer          Not feasible, Metals become soft at veryhigh
                                              temperature -unsuitable for polymer filler   

●    Ceramic matrix composites CMC
 
    Ceramic    +    Ceramic          Carbon-carbon composites

    Ceramic    +     Metal            Metal sprayed optic glass fibre cables 

    Ceramic    +    Polymer          Not feasible, Ceramics require high 
                                              temperature for formation -unsuitable 
                                              for polymer filler

●    Polymer matrix composites PMC
 
    Polymer    +    Polymer          Polyester or rayon fibre reinforced plastics
    
    Polymer    +    Metal             Grinding and polishing abrasives

    Polymer    +    Ceramic          Fibreglass, Fibre reinforced plastic FRP 
                                              Asphalt roads, imitation granite, 
                                              cultured marble sinks and counter tops


INTERFACE of MATRIX and FILLER 
 
In a composite material the Filler in the form of particles, fibres and sheets, is expected to take up the stresses in unison with the matrix because of the  strong interface provided by the later. Composite materials with weak interfaces have low strength and stiffness, but high resistance to fracture, On the other hand materials with strong interfaces have high strength and stiffness but are brittle. The bonding between the matrix and the filler is dependent on the atomic arrangement and chemical properties of filler and on the molecular conformation and chemical constitution of the matrix. A crack that starts in a monolithic  material generally continues to propagate until that material fails, whereas the filler-matrix combination reduces the potential for a complete fracture.

Bonding at the interface: In a simple system the bonding is due to adhesion between filler and the matrix. Adhesion can be attributed to following five main mechanisms:

1    Adsorption and wetting: When two electrically neutral surfaces are brought sufficiently close, there is a physical attraction. Most solids have surfaces that are rarely perfectly in level and without any contamination. So a wetting agent that substantially covers every hill and valley displaces all air and overcome effects of contamination, is required.

2    Inter-diffusion: It is possible to form a bond between two polymer surfaces by the diffusion of the polymer molecules on one surface into a molecular network of the other surface. The bond strength will depend on the amount of molecular entanglement and the number of molecules involved. Inter-diffusion may be promoted by the presence of solvents and plasticizing agents.

3    Electrostatic attraction: When one surface carries a net positive charge and the other surface, a net negative charge, electrostatic attraction occurs (as in acid+base reaction). Electrostatic attraction does not play any major role in contribution of bond strength, but has importance on how things initially begin to get mixed.

4    Chemical bonding: It is formed between a chemical group of filler and a chemical group of a matrix. The bond formation or breakage usually involves thermal activity.

5    Mechanical adhesion: Some bonding occurs by the mechanical interlocking of two surfaces (e.g. fibre shape-section).

Shocks, impact, loadings or repeated cyclic stresses can cause the Individual fibres to separate from the matrix, e.g. a fibre pull-out. In case of laminated or layered construction there could be a separation at the interface between two layers, a condition known as de-lamination.




MATRICES

A matrix is an environment or material within which something develops. A matrix surrounds a Filler material while creating a bond with it. A matrix thus creates a network within which the filler components are supported by maintaining or reinforcing their intended positions. For a matrix to be affective, it  must at some stage have a lower phase than the filler material. The lower phase may occur before or while the filler material is being formed or introduced. The matrix material may turn to a higher phase by evaporation of the solvent, removal of the heat or pressure, and polymerization or action of a catalyst. Polymer matrices are most common, followed by metals and ceramics. However, paper pulp, mud, wax, etc. are some matrix materials that do not fit into any of the above-mentioned categories. Ceramic matrix composites though difficult to form, show greatest promise in material sciences.

Portland cement, Gypsum plaster, mud (clay), and Bitumens are widely used matrix materials. Polymer matrix materials are thermosetting resins such as polymers, poly-amides, epoxies, or thermoplastic resins such as polycarbonate or polysulphones. Typically a polymer matrix composite of Epoxy and carbon fibres is of two thirds the weight of aluminium, and two and a half times as stiff.

For metal matrices most commonly used metals are aluminum, titanium, magnesium, and copper. Composites with metal matrices generally have metal or ceramic as filler materials. Aluminum reinforced with fibres of the ceramic silicon carbide is a classic example of a metal matrix with ceramic filler. The composite material combines the strength and stiffness of a silicon carbide with the ductility of aluminum. Metal to metal composites consist of two immiscible metals (metals that do not form alloys), such as magnesium and titanium. Such metal-metal composites with bronze matrixes have been in use since Bronze Age to create many useful materials.



FILLERS

Fillers are inevitable constituents of composites. Fillers, besides providing the reinforcement, also impart special properties to the new material. Fillers have many forms, such as fine particulate, staple fibre (whiskers or short fibres), filaments (long or continuous fibres), unwoven (felt) and woven fabrics, knit textiles, aggregates, and sheets. Filler materials are natural (wood, plant, hair), minerals (asbestos, sand, stones, powders), and man-made (polymers, metals, ceramics).

Straw, hair, coir, hemp, jute, papyrus, rice-husk etc., have been mixed with clay to form bricks. Sand, ash, and mineral dust were added to mud to reduce the plasticity for plaster work. Wood planks were glued together to form block board or plywood like construction in 15th C BC.
 
Man-made materials include: Fibreglass, quartz, Kevlar, Dyneema or carbon fibre, graphite, carbon-graphite, silicon carbide, titanium carbide, aluminium oxide, boron, coated boron, boron carbide, alumina, alumina-silica, niobium-titanium, niobium-tin, etc.

Fillers Particles (of 10 to 250 μm in diameter) help block the movement of dislocations in the composites and provide distinctive strength properties. Staple fibres used as fillers have high length to a diameter ratio and are generally in their random orientations. Whereas filaments are used for high performance structural applications and are prearranged (for a particular structural use) before introduction of matrix, or in certain cases a fixing compound. Depending on the load conditions the reinforcement is random, unidirectional (align ed in a single direction), or multi-directional (oriented in two or three directions). Continuous fibres are more efficient at resisting loads than are short ones, but it is more difficult to fabricate complex shapes from materials containing continuous fibres than from short-fibre or particle-reinforced materials.

Particles (fillers) of one material are dispersed in another material (matrix) in many different ways. Particles are mixed in a liquid phase of the matrix and allowed to harden to a solid phase, the particles are allowed to grow in the matrix or particles are pressed into the matrix and inter-diffusion is encouraged by mechanical working or other energy input. Particulate fillers in ceramic matrices enhance characteristics such as electrical conductivity, thermal conductivity, thermal expansion, and hardness. Particles of Alumina, Silicon carbide and Boron nitride embedded in a polymer matrix formed abrasives are used for grinding and polishing stone floors, tools etc. Carbon black (as powder) added to vulcanised rubber provides hardness and toughness for automobile tyres. The rubber is further reinforced with metal, rayon, polyester and other threads as continuous fibre filler.

High-performance ceramic composites are strengthened with filaments that are bundled into yarns. Each yarn, strand or tow may contain thousands of filaments, each of which with a diameter of approximately 10 micrometers (0.01 millimetres).

Often components are formed that are strong in all directions, by creating a three-dimensional lattice of filler component. The filler component itself could be a composite material.

Fillers affect the quality of a composite. Fillers are usually combined with ductile matrix materials, such as metals and polymers, to make them stiffer. Fillers are added to brittle-matrix materials like ceramics to increase toughness. The length-to diameter ratio of the fibre, the strength of the bond between the fibre and the matrix, and the amounts of fibre are variables that affect the mechanical properties. It is important to have a high length-to-diameter aspect ratio so that the applied load is effectively transferred from the matrix to the fibre.

A variety of reinforcements can be used, including particles, whiskers (very fine single crystals), discontinuous (short) fibres, continuous fibres, and textiles perform (made by braiding, weaving, or knitting fibres together in specified designs).

Glass: Glass is the most common and inexpensive fibre and is usually use for the reinforcement of polymer matrices. Glass has a high tensile strength and fairly low density (2.5 g/cc).

Carbon-graphite: In advance composites, carbon fibres are the material of choice. Carbon is a very light element, with a density of about 2.3 g/cc and its stiffness is considerable higher than glass. Carbon fibres can have up to 3 times the stiffness of steel and up to 15 times the strength of construction steel. The graphitic structure is preferred to the diamond-like crystalline forms for making carbon fibre because the graphitic structure is made of densely packed hexagonal layers, stacked in a lamellar style. This structure results in mechanical and thermal properties are highly anisotropic and this gives component designers the ability to control the strength and stiffness of components by varying the orientation of the fibre.

Polymers: A variety of polymer materials are used as filler material for composites. The strong covalent bonds of polymers offer tailor-made properties in the form of bristles, whiskers, staple fibres, filaments, yarns or tows, spun yarns, threads, ropes, unwoven and woven fabrics, knitted compositions. Nylons, polyesters, rayon, acrylic, Kevlar and many other fibres are used for composite formation.

Ceramics: Ceramic fibres made from materials such as Alumina and Silicon carbides are used in very high temperature applications, and also where environmental attacks are severe. Tungsten-boron filaments,  Ceramics have poor properties in tension and shear, so most applications as reinforcement are in the particulate form.

Metallic: Metallic fibres have high strengths but since their density is very high they are of little use in weight critical applications. Drawing very thin metallic fibers (less than 100 microns) is also very expensive.


●    MMC -Metal Matrix Composites:
 
Metal matrix composites have either Metal or Ceramic as the filler material. Polymer fillers are nominally not feasible, because at processing temperature of metal matrix material, most polymers cannot survive. Majority of MMCs are aluminum matrix composites, but a growing number of composites are produced with the matrix of superalloys, titanium, copper, magnesium, or iron. Lightest metal-matrix composite is heavier than any other polymer or polymer matrix composite and  are comparatively complex to process.

Metal matrix composites are as old as the Bronze age. However, work on  modern MMC began in the late 1950s. A copper-clad 316 stainless steel shell was manufactured by electroforming, possessing an outer skin of nickel and a reflective platinum final surface, for aerospace industry in USA. Today, metal cladding (a layered metal composite) is very common, as seen in coins, cooking ware, armour, and other items. Metal-matrix composites are used for the space shuttles, commercial airliners, electronic substrates, bicycles, auto-mobiles, golf clubs, and a variety of other applications. Super-alloy composites reinforced with tungsten alloy fibres are used for components of jet turbine engines that operate at temperatures above 1000 °C. Under sea cables for communication and cables for shipping and elevators are invariably of composite materials.

Compared to monolithic metals, MMCs have higher strength-to-density ratios, higher strength and stiffness, better wear resistance, fatigue resistance and elevated temperature properties, a lower creep rate and coefficients of thermal expansion. MMCs have higher cost, involve difficult to use technologies but provide wonderful super alloys and super-conductive materials.

Carbide drill bits are a metal matrix composite. Tungsten carbide powder is mixed with cobalt powder, and then pressed and sintered together, the tungsten carbide retains its identity. The resulting material has a soft cobalt matrix with tough tungsten carbide particles inside.


Metal Matrix composites in comparison to other materials: Compared to monolithic metals and polymer matrix composites, MMCs  are high cost systems with relatively newer and immature technology. However, MMCs have better temperature capability, superior fire resistance, higher electrical and thermal conductivities.

MMC reinforcements are metal wires, and ceramic materials like particulate, whiskers, filaments, fibres. Key continuous fibres include boron, graphite (carbon), alumina, alumina-silica, boron carbide and silicon carbide.

o    Aluminium Matrix composites have the advantage of low cost over most other MMCs. Aluminium matrix composites have excellent thermal conductivity, high shear strength, excellent abrasion resistance, high-temperature operation, non flame-ability, minimal attack by fuels and solvents, and the ability to be formed and treated on conventional equipment. Aluminum MMCs are produced by casting, powder metallurgy, in situ development of reinforcements, and foil-and-fibre pressing techniques. Aluminium matrix composites are used in brake rotors, pistons, and other automotive components, golf clubs, bicycles, machinery components, electronic substrates, extruded angles and channels, and many structural and electronic applications.

o    Copper Matrix composites have metal wires and filaments of tungsten, beryllium, titanium, and molybdenum for reinforcement. Ductile superconductors are fabricated with a matrix of copper and superconducting filaments of niobium-titanium and niobium-tin. Copper matrices are also fortified with fine tungsten wires. Copper matrix reinforced with tungsten particles or aluminum oxide particles, is used in heat sinks and electronic packaging. Copper-Graphite composites can be designed to have very specific qualities such as high temperatures in air, excellent mechanical characteristics, as well as high electrical and thermal conductivity. Copper matrix materials are easier to process compared to titanium. Copper matrix composites have lower density compared with steel.
o    Titanium Matrix composites are new age composites. Titanium reinforced with silicon carbide fibres is under development as skin material for the National Aerospace Plane. Stainless steels, tool steels, and Inconel are among the matrix materials reinforced with titanium carbide particles.

●    PMC -Polymer-Matrix Composites:
 
These are most widely used types of composite materials. PMC consists of a polymer matrix of thermosetting and thermoplastic materials with nearly all types of fillers. Most popular fillers are wood, glass and carbon fibres.

o    Thermosetting Materials for Polymer Matrix: Thermosetting polymer molecules develop interlinks during the curing process which usually occurs at room temperature, but can have temperature input to achieve the optimum results. Shrinkage during curing and thermal contraction on cooling, can lead to inbuilt stresses in the composite material. Thermoset materials have a shelf life problem, often requiring freezing to retard the changes that continuously occur in the resin. Thermoset materials are recyclable so are considered to be environment friendly. Epoxies and polyester resins cover a broad range of thermoplastic resins for composites.

Thermosetting composites are manufactured through many different processes. In all these processes wetting of fibre with the resin material is an important aspect. Fibres in the form of a mat are impregnated with resin, tows or bundles of fibres are wetted and laid to shape a component, Paper-covered sheets or narrow stripes (pre-preg) of resin impregnated fibres are partially cured, and pressed to form the final shape of the component, or resin and fibres, are mixed and co-extruded, or hot pressed.

o    Thermoplastic Materials for Polymer Matrix: Thermoplastics can be repeatedly melted and solidified for reprocessing. Thermoplastics as a result are considered recyclable and preferred from the environment point of view. Manufacturing technologies for thermoplastics are not as advanced as those for thermoset.

Thermoplastics derive their strength and stiffness from the inherent properties of the monomer units and very high molecular weight. In amorphous thermoplastics there is high concentration of molecular entanglements, which acts like cross links, and in crystalline materials there is high degree of molecular order and alignment. In amorphous materials heating reduces entanglement and the material turn from solid to viscous liquid. With crystalline materials heating melts the crystalline phase, to give an amorphous viscous liquid.

Thermoplastics are structurally very sensitive to temperature conditions, as under constant load conditions these materials show an increase in strain with time, i.e. materials creep under loads. This means that in composite material with thermoplastics a redistribution of the load between the resin (matrix) and the fibres occur during the deformation.

Thermoplastics are more expensive, and they generally do not resist heat as well as thermoset, however, some thermoplastics with higher melting temperatures are available. Three common thermoplastics matrix polymers are polypropylene, nylon and polycarbonate.

●    CMC -Ceramic Matrix Composites:
 
Ceramic matrix composites have evolved from needs that certain components must maintain their structural integrity at very high temperatures, yet remain operative. Temperatures in boilers, chimneys, exhausts, heat sinks, combustion engines, furnaces, etc. are such that metal materials become soft, whereas polymer materials get degraded. Here traditional ceramic materials work well with their superior heat resistance, low abrasive and non corrosive properties, but brittleness is a major drawback. Ceramic Matrix Composites reinforced with ceramic or metal fibres provide better structural properties against not only traditional ceramics but also metals and alloys. Ceramics by themselves are often not conductive of heat, electricity, etc. but this can be altered by suitable addition of a filler material. The desirable characteristics of CMCs include: high-temperature stability, high thermal-shock resistance, high hardness, high corrosion resistance, light weight, non-magnetic and non-conductive properties.

Unlike polymers and metals, which are processed by techniques that involve melting (or softening) followed by solidification, constituents of high-temperature ceramics cannot be melted. They are generally produced by some variation of sintering, a technique that renders a combination of materials into a coherent mass by heating to high temperatures without complete melting. In some instances a polymer-filler mixture, on pyrolysis, is converted to a ceramic matrix (consisting of silicon carbide, boron carbide, boron nitride, and a silicon oxy-carbide -SiOC glass), without reacting with the carbon fibre. Where continuous fibres or textile weaves (as opposed to short fibres or whiskers) are involved, sintering is preceded by impregnating the assembly of fibres with a slurry of ceramic particles dispersed in a liquid.

Technologies for CMC are still new compared to PMC, but technological achievements are very remarkable. Major barriers to wide use of ceramic matrix composites are: lack of detailed specifications, information, problems of fixing other materials to ceramic matrix components, in-service repair methodology, high cost.

Ceramic matrices can be categorized as either oxides or non oxides and in some cases may contain residual metal after processing. Common oxide matrices include alumina, silica, mullite, barium aluminosilicate, lithium aluminosilicate and calcium aluminosilicate. Common non oxide ceramics include, SiC, Si3N4, boron carbide, and AlN. Oxide matrices are considered environmentally stable yet non oxide ceramics, with their superior structural properties, hardness, and corrosion resistance are considered commercially better.

Ceramic reinforcements in the form of discontinuous fibre include whiskers, platelets, and particulate having compositions of Si3N4, SiC, AlN, titanium di boride, boron carbide, and boron nitride are commonly used. Of these, SiC is most prominent because of its stability with a broad range of ceramic oxide and non oxide matrices. Discontinuous oxide ceramic reinforcements are less prevalent because of their incompatibility with many common ceramic matrices.

Some of the more common continuous reinforcements of ceramics include: glass, mullite, alumina, carbon, and SiC. Of these, SiC-fibres are commonly used because of their high strength, stiffness and thermal stability.

Common trade names for silicon carbide fibre include Nicalon™, Hi-Nicalon™, SCS, Sylramic™, and Tyranno. For applications where temperature is lower (< 1100°C) or exposure times limited, mullite fibres are used because of their lower cost. Nextel is a common trade name for both mullite and alumina fibre. Continuous ceramic fibres are favoured because of their ability to provide pseudo-ductile characteristics to otherwise brittle ceramic materials.



ADVANCED COMPOSITES 

‘Advanced-composites’ is a misleading term. In case of composites it could mean different concepts. It could mean composites requiring: high tuned multiple processing, composites of uncommon materials, or composites with very specific material properties.

Advanced composites had come of age in the early 1960s with the development of high-modulus whiskers and filaments. While whiskers were easily made, their composites were of poor quality. But the 60 million modulus strength boron filaments with reinforcing epoxy, were very successful and were used in fighter aircraft and, later, in golf-club shafts, fly-rods, and tennis rackets.

But it was the improvement of graphite fibre that led to the major jump in mechanical advantages for military and sports applications. A sprinkling of boron now turned up but, just like the early gasoline with boron, its contribution was minor; still, it sold. The large modulus differences for fibre and matrix were also accompanied by large differences in expansion coefficients and consequent residual thermal stresses.

In the First category, as an example, one can include, Carbon-carbon composites which are closely related to Ceramic Matrix Composites, but differ in the methods by which they are produced. Carbon-carbon composites consist of semi-crystalline carbon fibres embedded in a matrix of amorphous carbon. The composite begins as a PMC, with semi-crystalline carbon fibres impregnated with a polymeric phenolic resin. The resin-soaked system is heated in an inert atmosphere to pyrolyze, or char the polymer to a carbon residue. The composite is re-impregnated with a polymer, and the pyrolysis is repeated. Continued repetition of this impregnation/pyrolysis process yields a structure with least voids. Carbon-carbon composites retain their strength at 2500 C and are used in the nose cones of re-entry vehicles. However, because they are vulnerable to oxidation at such high temperatures, they must be protected by a thin layer of ceramic.

In the Second group, as an example, one can include composites of piezoelectric materials which generate an electrical current when they are bent, conversely, when an electrical current is passed through these materials, they stiffen. This property can be used to suppress vibration, the electrical current generated during vibration could be detected, amplified, and sent back, causing the material to stiffen and stop vibrating. Other examples are efforts to develop smart or responsive material systems, that mimic a living organism by reacting to stimuli generates a desired response.
 
In the third group, for example, materials are needed with a near-zero coefficient of thermal expansion can be included. These materials have to be thermally stable and should not expand and contract when exposed to extreme changes in temperature.

MANUFACTURING METHODS for COMPOSITES 

Composites to be extra efficient are engineered to form or shape of the component. This involves strategically placing the reinforcements while manipulating the matrix properties to achieve a perfect combination. These fabrication methods are commonly named moulding, casting, injection and extrusion processes. Other processes include curing (through catalysts, radiation, exothermic reactions and heat), heat treatments like baking, melting, sintering, forging, pressing and rolling.

Each matrix system requires specific manufacturing method. Polymer matrices, need no or low heat input, followed by metal matrices. Ceramic matrices need very high temperatures for composite formation.

The most common form of material used for the fabrication of composite structures is the pre-impregnated tape, or Pre-Preg. There are two types commercial pre-preg materials, Tapes (of 75 ml width) and Broad goods (of several metres widths) intended for hand lay-up and large sheet applications. To make a composite pre-preg with surface treated fibres are passed through a resin bath and laid on to a component form or sheet form. For thermosetting polymers, the resin coated structure of tape or broad goods are autoclaved or baked. Thermoplastic systems do not require heat input, so prove to be less costly and easier to handle.

Pultrusion is a continuous process where fibres and the resin are pushed through a heated die for manufacturing long shapes. Resin transfer moulding is generally limited to low viscosity materials. Fibres are laid in a mould and resin is injected in the closed mould.

To make a low temperature super conductor, Niobium-tin alloys (Nb3 Sn), are found to be brittle. However, Copper and Niobium nominally immiscible, when melted together provides a ductile material. The new material has Niobium solidified within the copper into structures called dendrites. When drawn into a thin wire, the Niobium dendrites form small filaments within the copper. After these stage the wire is passed through molten tin. The tin combines with the copper to provide a wind able wire of super-conducting properties.


SHORT COMINGS of COMPOSITES 

Although composite materials have certain advantages over conventional materials, composites also have some disadvantages. For example, PMC and other composite materials tend to be highly anisotropic, that is, their strength, stiffness, and other engineering properties are different depending on the orientation of the filler material. Joining composite components is difficult. Advanced composites have not only high manufacturing costs but require very exact quality controls. The composite material must result into a definite component, which makes the processes very much labour-intensive, and not easily amenable to automation.

Further reading:

An Introduction to Composite Materials: Hull Derek / Cambridge University Press
Selection and use of Engineering Materials : Charles, Crane, Furness / Butterworth-Heinemann
Engineering Materials : Budinski, Budinski / Prentice Hall
Engineering Materials and their Application : Flinn, Trojan

PRIVACY and INTIMACY

Behaviour in space is conditioned by two personal factors Privacy and Intimacy. Such behaviour develops from the primary concern for survival. Survival is a defensive action where one tries to create a protective layer, yet it is an offensive activity where people form groups and make themselves more accessible to others.

PRIVACY
Privacy is achieved through number of means and conditions. It is basically achieved by obscuring own self, or by isolating from things and people. One can diffuse own self by merging with background or become less perceptible. Isolation is achieved by barricading. A person or group achieves insulation through physiological posturing. By posturing one can adjust the body exposure, control the communication, command the expression, contain or extend the reach of the body and re-calibrate the sensorial perception. Privacy  can help overcome many inhibitions through mental isolation. Psychological motivation helps one to ignore some side effects of privacy. 

Privacy provides the isolation whereas degree of intimacy determines the interference by others. Both are important means for individualization or branding of unique personality. In a space, primarily one tries to anchor to a spot where privacy and intimacy are controllable. And whenever these are compromised one may try to distance from others, adjust the posture and reorient. People inherently discern their relationship with others in terms of distances or spaces between them. In other words certain quality of distancing from others is required for individualization.

One requires many different types of privacy: Physical privacy -against someone making a close approach (touch or near approximations), Visual privacy -to limit others’ view of oneself, Audio privacy -insulation against being overheard and interference from background noise, and Olfactory privacy -that limits to reveal own physiological state or experiencing someone else’s such a state through hormones-odours. Other privacy parameters include the body temperature, breathing rate, heart beats, pulse rate, vibrations of the body, sweating and perspiration.

VISUAL PRIVACY
Visual privacy addresses the ability to limit others’ view of oneself. Inherent in human behaviour is the tendency to avoid situations in which one can be watched without being aware of who is watching. Visual privacy can be achieved through the use of furnishings, partitions or walls. In a private space or an office, people will often orient their desk in order to visually control the doorway and achieve a visually private space on one side of the desk. Similarly, people prefer to sit with a protected back, controlling the area they cannot see directly. In restaurants, the first seats to be filled are usually those along the walls. In outdoor spaces, people tend to sit against or beside objects such as trees and bushes rather than in the open.

AUDIO PRIVACY
Audio privacy in an interior space results from effective treatment of the acoustic environment as an interrelationship of many components: ceiling, partitions, furniture, equipment, and floor. A complete acoustic system will generally provide adequate speech privacy. Speech privacy is achieved when there is sufficient acoustic shielding to allow conversation to be unheard beyond the participants of the conversation. A high quality of speech privacy will significantly affect the level of communication, social interaction, and productivity. An appropriate relationship between background noise and that produced within the activity space is conducive to speech privacy. However complete insulation of a space, such as a study room, cuts-off the background noise, leading to loneliness or alienation. Hospital wards are hard finished due to issues of bacterial infections. The wards during daytime have high presence background noise that suppresses or subdues the noise from within the space. However, past midnight, in absence of background noise, the noise from inside the room becomes unbearable.

SOCIAL PRIVACY
Crowding occurs when personal space and territoriality mechanisms function ineffectively, resulting in an excess of undesired external social contact. Social privacy is often equated to ‘crowding’ in a space. It is reflected in degrees of accessibility a person or group offers to others. Crowding can have different meaning depending upon the situation. Crowding may be tolerated if it is temporary and for a definite purpose like for fun. Crowding or means both heightened accessibility or reduced interaction  depending on the need for expression, communication, physiological requirements and sense of belonging for group behaviour mechanisms (common purposes, beliefs). The scale of a room it's size relative to the occupants also influences conversational distance. As room scale diminishes, people tend to sit closer together. Likewise, increased noise levels and distractions drive people to sit closer together.

INTIMACY
Intimacy is a feeling of closeness or affinity between a person and another, or an object. If privacy is personal as well as group-based requirement, intimacy is an intra personal affair. Intimacy is also a biological need. Intimacy is considered a product of distance, and it relies on compatibility, sexual needs, glandular secretions, social acceptability, etc.  It is an attitude, mental conditioning or mental posture. Intimacy could be one-way feeling that is without reciprocal feeling. One can be intimate to another person or group of persons without the apparent need for privacy. So intimacy is not always a function of physical proximity. One can feel close to a person who is long dead -an illusory presence or through notional links (clothes, odours, recorded sounds, etc.). Distancing is also a matter of time, like remembrances.

Beyond security a person or group seeks privacy to flourish in an environment. It instills a sense of intimacy, a one to one relationship. An intimate relationship is with a person but an intimate space is one where occupant and object have intense relevance to each other. A private abode is an own world. An intimate situation is safe, predictable and reassuring.  Intimacy is like a domain where everything is under an exclusive command. Privacy  allows a person to develop a distinctive identity. In case of groups intimacy increases the affinity, unity, communication and expression.

Privacy and intimacy are factors of environment for conducting certain tasks. In appropriate environments it is easy to control ingress, distraction and unwanted participation by others. These are achieved in several ways: by placing required types of barriers, by physical and metaphorical declarations of the territorial markings, by suitable space planning and style of space occupation. Group behaviour is defined by the nature of privacy and degree of intimacy that is available.
 
Meaning of intimacy varies from relationship to relationship, and within a given relationship. Intimacy has more to do with rituals of connection. Intimacy is both the ability and the choice to be close, loving, and vulnerable. Intimacy requires identity development. Intimacy can have two main forms: emotional intimacy and physical intimacy. There could be other forms of empathy like cultural, intellectual, spiritual, social that are akin to intimacy in some conditions. Strategic relationship developed to take advantage of anyone could be very close but it is a make-believe intimacy.


DISTANCING FOR PRIVACY AND INTIMACY
Privacy is achieved by distancing from the threatening, disturbing or overwhelming elements. Privacy and intimacy both are determined by the distance one creates and perceives from other beings and objects. The distance from others is perceived in terms of space, intervening objects, physical reach and perceptibility. Distance is required to conduct tasks and for communication and expression. Distance is governed by environmental factors, social requirements, psychological make-up, time duration and the relevance of objects and people. Edward T. Hall defines four distinct distances at which interpersonal transactions normally take place: Intimate, Personal, Social and Public. Sociological determinants such as group dynamics and communication also affect personal interactions within an environment.

Interpersonal distance refers the range of distance between two or more people required to be maintained for best meaningful communication. The interpersonal distance may vary with culture.

According to Edward T. Hall there are four interpersonal distances: 

Intimate distance is used in intimate relation for embracing, kissing, touching or whispering and ranges from 0 to 450mm.

Personal distance is useful for interaction between good friends and family members and ranges 450mm to 1200mm.

Social distance is for interaction between social acquaintances and its range is from 1200mm to 3.5mt 

Public distance is mainly for public speaking and is above 3.5mt.

Distance also means the size of intervening space or affective extent of space.
Some Space Distance definitions are:

Intimate Area: There is a very Intimate area close to the body, within which it is possible to have physical touch, non verbal communication and emotional interactions. To gain such an intimate position one needs to be familiar with the other person or coerce. Even in the intimate space close to the body, the nature and level of intimacy is affected by the attitudes of the persons involved.
In case of objects such as tools and support systems are intimate. Here due to the intimate relationship one senses the texture, temperature, moisture, vibrations, energy, etc. One has to be very sure that by using these no harm will occur.

Personal area: There is an area immediate to the body that marks the Personal area. It is a zone of regulated and selective participation. This is an area where one can reach out through projection (expression), channels of communication, physically (through body limbs) or stretch out with gadgets (walking stick, stethoscope, etc.). Here the intimacy is regulated but it is not a private affair, unless some screening barriers  are used to achieve the privacy. One can dwell in a culture or state formed of metaphysical elements (beliefs, customs, etc.), to achieve the same.
In case of objects that are personal are not shared with others or its use is restricted. Others also sense such ownership or exclusive rights. The strong association to a person is imprinted on the object’s form or position. Chief guests chair, head chair at the end of a dining table are such personal entities.
 
Nearness Zone: There is a zone of varying nearness. The nearness is defined by the position and duration of the eye contact, sound pitch and language call, olfactory sensation and body heat perception. The zone  is used for sensing the time and space of happenings. The time (duration) and space (distance), are used alternatively to compensate the other. This is a zone of participation without personal involvement. It is also called a non-committal area.
As per the protocol, leaders of the two nations never share a seat, but rather occupy separate seats distanced with a small table or flower vase.  Similarly deputies accompanying their leaders, are made to sit at some distance, from where they get a sense of participation but have no chance of intervention. On public platforms one intentionally uses lower sound pitch to draw attention. On very large dining table one can effectively hold conversation with members sitting on the two sides, but not across the table. In gatherings one uses differing sound pitch to reach desired distance.

Reach Zones: These are within the domain that one occupies. Though these are not within an anthropometric ambit, but one can if required ‘reach’ out. Such zones are marked by a predominantly assigned task, such as dining area, entrance, cooking, study, etc. Zones like prayer area are marked by the associated sanctimony. In reach zones intimacy or privacy of personal nature is not available, yet one can announce it through metaphoric presentations. Reach zones usually have one consistent environment. Such zones are unitary spaces and so there is a sense of belonging, safety, security and personalized identity. 

Neighbourhood areas: These are spaces beyond the current domain, like the street, neighbourhood, and such public areas. Here the ‘privacy’ is afforded by the familiarity of the surroundings and security of nearness. These areas have intimacy of the distance such as within the  visible range, reach of shout or call, and the odour. These are part of the ‘home’ setting. Such public areas foster non-intimate group behaviour. Mothers allow children to play in these spaces, as these are under surveillance.

Domain edge zones: These have personal relevance so far as there is a link active in time and space. It is a place for a sporadic encounter, however effects of which could persist as a remembrance or experience. Frequent occurrences belie a consistency or permanency. It functions like a networked zone (web) where something relevant is always available. So no matter where a person arrives, it begins to manifest familiarity or is pregnant with possibilities.

TERRITORIALITY AS A DECLARATION OF PRIVACY
A person or group carves own space by different territorial markings. The territorial markings such as of physical, metaphysical, metaphorical nature present spaces of  many different characteristics. Territoriality is a means of achieving a desired level of privacy. It involves creating and maintaining an exclusive control over a space by an individual or group. This control implies privileges and may involve aggressive actions in its defence. For the individual, territorial control provides security and identity, and is communicated through personalization and definition of the space. For the group, territorial control, is the cause of their being an entity, and is expressed through the common behaviour.

Hargie & Dickson identify 4 spatial territories:

Primary territory: This refers to an area associated with someone who has exclusive use of it. For example, a house that others cannot enter without the owner’s permission.
 
Secondary territory: This is like ‘social’ sharing territory.  There is no right to occupancy, but people may still feel some degree of ownership (belonging) of a particular space. For example, someone may sit in the same seat on a train every day and feel aggrieved if someone else sits there.

Public territory: This are public spaces that are available to all, but only for a set period with conditions. Such as a parking space or a seat in a library. Although people have only a limited claim over that space, they often want to exceed that claim, to show their interest in it. For example, it was found that people take longer to leave a parking space when someone is waiting to take that space.

Interaction territory: This is space created by others when people are interacting. For example, when a group is talking to each other on a footpath, others will walk around the group rather than disturb it.

SPATIAL MANIFESTATIONS OF INTIMACY
The need to be alone or be part of a group arises from factors like personal (psychological and physiological), intra-personal (communication, exchanges) and group behaviour mechanisms (sense of belonging, sharing, participation, confirmation).

For an individual the space that permits one to ‘distance’ from others is a private space. However, for a group, a space that allows easy communication and recognition (without any aids or extra strain) are intimate spaces.

Enclosing space forms (corners, cones, concaves) force individuals to be closely spaced and be intimate enough to form a group.

Crowded spaces force intimacy of coexistence. The coexistence may force an individual to mentally barricade own-self or open-up.

Some environmental and other effects are highly focussed (illuminated spot, under the fan area, sunny patch, breezy path)  and force a group to share the same zone and so breed intimacy.

Eye contact could be both intimate and non-intimate. A direct eye contact allows better nonverbal communication (empathy), but a direct eye contact also subdues the other person and thus is less-intimate. Persons meeting  in one to one setting are more intimate, than their encounter in public presence or a gaze.

Audio intimacy occurs over a chat on telephone or mobile, web chatting, videoconferencing. These are forms of being audio intimate.

Virtual proximity Intimacy is also achieved by entering into some one’s private domain like home, a bedroom, toilet, study area. Homes and work places of great persons provide illusion of physical intimacy.

A person projects privacy and intimacy in various proportions, whereas the space facilitates various levels of physical closeness, isolation and insulation. The mental needs ultimately define the degree of involvement. The culture defines the acceptable or inappropriate types of intimacies. A crowded stair, elevator, metro carriage do not project an intimate atmosphere. A hand shake or hug nominally has no sexual meaning in many cultures. In some cultures privacy achieved by a veil is considered retardant of intimacy. Intimacy could be a display or an expression with physical touch but with no apparent mental feelings.

Two persons or members of a group can talk in whispers and give out an impression of intimacy in spite of the apparent distance between them. Conversely talk-discussions in high pitch could be used to present bonhomie and thereby a close-knit entity. Politicians and celebrities talk in whispers to state things that need to be made public and talk loudly things that need not be public, both ways they draw the attention. A public orator changes the pitch from normal to very low or high to draw the attention of the audience and thereby register a point.

SPATIAL SEPARATION AND RELATIONS
Hall (1959) has stipulated that spatial separation also serves an expansive function. He  made a study of the spatial relations that seem appropriate to various kinds of interactions. They vary with intimacy, and depend on the possibility of eye contact. They vary with the culture.

One can easily distinguish strangers from friends in an airport lounge. Strangers will keep a distance taking alternate seats wherever possible. Friends tend to form clots, and families even pile one on the top of another. Total strangers will comfortably seat themselves only inches apart if the seats are back to back, but friends and the members of the family never arrange themselves in this way. Eye contact invites interaction and so is sought to the degree that intimacy already exists. (Hall E. T. 1959 The silent language).

Hall argues that there are spatial zones appropriate to various types of interactions. Distances through virtual communication technology mediated  interactions are likely to be different. Hall has also shown the cultural variations that South American needs much closer distance for impersonal information than a North American desire or is accustomed to.

Very close          75 to 150mm           Soft whisper, top secret
Close                 200 to 300mm         Audible whispers, very confidential
Near                  300 to 500mm         Soft voice, confidential
Neutral               500 to 900mm        Soft voice, low volume, personal subject matter
Neutral              1300 to 1500mm     Full voice, impersonal information
Public               1700 to 2500mm      Slightly over-loud, information for others to hear
Across room       2500 to 6000mm     Loud voice, talking to a group
Hailing privately 6000 to 7500 mm    Indoors, Loud voice departures
Hailing public     30.00 mts              Outdoors, Loud voice shouting, departures and calls

Impersonal discussion, takes place at 1200 to 1500 mm; cross the inner boundary of this zone and one’s interlocutor will retreat; cross the outer boundary and he will advance or subtly change the manner in which he behaves adopting the pattern appropriate to the new distance. One can move from impersonal discussion to personal matter by reducing the distance, or to a non personal information exchange by increasing the distance. TV anchors do the distance trick on their show. For an intimate question the anchor pushes forward own body (Larry King of CNN ), but as soon as the question sinks in with the guest, the anchor withdraws not just to the nominal position but little further backward. These distancing movements allow the guest to deliver the answer more objectively and the camera frames the guest alone for such a ‘heroic effort’. However, a host may intimidate the guest by doing exactly opposite of this.

DOORS: Myths and Legends

Doors are as ancient as the human abode and so have become part of our conscious and subconscious being. Throughout history and across cultures doors, doorways, portals, gates and thresholds have been potent objects and symbols of superstition, rites and rituals, psychological change, transcendental and religious experience. Doors occur metaphorically in our expressions and recur in our dreams.

Etymologically Doors have been known as: dor (middle English), duru (old English) dur (proto-Germanic), dwer-dwor (proto-Indo-European), Gothic (daúr), Danish dør, Tür (German), turi (old High German), dyr (Icelandic), dyrr (old Norse), foris (Latin), thýra (modern Greek, thura (ancient Greek), dar (Persian), and dver (Russian), Kiwad (Urdu= door shutter).

Doors are synonymous with many objects and expressions such as entrance, gate, gateway, passage, portal, access, adit, admission, admittance, ingress and way. Doors are also refereed to in terms of: indoor -inside a built or enclosed space, outdoor -outside in the open, next door -neighbouring property or territory, at the door -outside or inside the door but on the verge of crossing it, waiting for permission or opportunity to leave or enter, by the door -passing by, at one's door -as compliance or submission, for being responsible, door to door -going or made by going to each house in a neighbourhood, out of doors -deposition, thrown out, removed, through the door -through an appropriate channel, to show the door -thrown out, asked to leave, open-door policies -barriers less, closed doors -private, protected, doors of opportunity, door to success -change of position for success, doorways to the future -future reached through breach or opening.

Doors have been part of our folklore and legends, often with diverse meanings. Doors have been dealt in their interior as well as exterior expressions. The interior expressions relate to way of life, virtues, good manners, exemplary behaviour, restraints, and exterior manifests as supernatural, unpredictable, dilemmas. Doors are equated with other building elements like walls, windows etc. to juxtapose the physical character.

I'm talking to the door, but I want the walls to hear me. The ear is only a door. Walls have ears, paper sliding doors have eyes.

When God shuts one door, He opens another. At death's door a man will beg for the fever. The doorstep weeps for forty days whenever a girl is born. Misfortune only comes in when the door is open.

If you want to keep camels, have a large door. A doorstep is the highest of all mountains. The best kind of closed door is the one you can leave unlocked.

Every dog is a great barker at the door of his own house. Earth is like a prison: we all go in through the same door, but we stay in different cells. Insects do not nest in a busy door-hinge.

He who wants to tell the truth will always stand before closed doors. Luck stops at the door and inquires whether prudence is within. He who is outside the door has already a good part of his journey behind him.
Many open a door to shut a window.
When a door opens not to your knock, consider your reputation.

The door, physically and symbolically, involves a change of state. At mundane level, a door means control over illumination, intrusion, acoustical disturbance, visual engagement, social interference and movement of air and pollution and thermal emission. At symbolic level a door offers hope, new life or fresh beginning, isolation from the familiar, ventures into unknown, initiation into mysteries, fear and expanded communications. At spiritual level it provides an encounter with the supernatural, a communion and unification with the creator. As Christ said, 'I am the door,' and 'no one comes to the Father but through Me.'

A door leads one out of a space to another space and a door perceptible somewhere. The door here is a bridgehead over the connecting passage. The door also takes one out to exploration of an illusive opportunity or intangible entity. The passage, in the first case is narrow with dangers of falling off it, whereas in the second case there are no options except backtracking to the reality of home.

MAKING OF A DOOR:
A door comes into being by three phenomenal elements: a Top, Bottom and the Sides. Japanese gate Torii is essentially composed of the eaves, the omnipresent top. A single stroke did not suffice, so two or three strokes emphasize the horizontal at top (as in Sanchi gate). An Indian door is epitomised by the Laxman Rekha. A territorial mark on the ground that defines whether one is included or excluded from the macrocosm. Here the threshold exists in spite there being no door. The Egyptian temple entrance consists of tall sides formed by a pair of columns, pylons or obelisks. The lintel or head is architecturally less significant, just incidental. A formal door requires all three to be present, and concurrently.

ENTRY - EXIT:
During Aagaman -while entering one must either go in or stay out. And during Nirgaman -while exiting one must either get out or stay in. A door is a point of uncertainty. Ones action must be very certain, and one must never stay put in the door itself. One must be very cautious on crossing a door threshold, as trouble lurks in both directions. All activities in the door space or frame, such as standing, seating, eating, drinking or doing anything else is a taboo. One must not look backward during entry or exit, or the willpower and physical strength will dissipate. The homeward (return) journey must be accomplished without an intermediate stop or engagements. On Aagaman, before entering the house door, one must purify own self (disinfect) with appropriate Vedic ritual, like bathing, washing the feet or at least sprinkling of holy water over the head. The Nirgaman, if for a journey to an unknown destination or for an uncertain purpose must be conducted with the permission of elders, masters or gurus, and goddess of the family (Kuldevi).

Mystically, an open door represents good fortune, a new opening in life, or a desire to open up the feelings. A revolving door means a monotonous period ahead and a trap door predicts shocking news, a door knob means unexpected good luck, hinges bring family problems. A locked door shows missed opportunities, denial of opportunities, or can represent ‘need to close the door over the past’. A door opening outward may show that one needs to be more accessible to others. However, an inward opening door may represent the desire for inner exploration and self-discovery. A front door is a normal entrance and a back door a nominal one. A house with one door is a preferred abode. Evil spirits enter the house from a back door.

THRESHOLD:
A Grih-Pravesh, -a ceremony of entering a home, by crossing over a threshold -Udumbara with right foot forward. It is conducted while entering into a new home, when a new member joins the family like a bride, new born baby, groom, a disciple or an apprentice, and when someone returns after some lapse of time such as recovery from an illness, achievement or trip. Grih Pravesh ceremonies have many forms but it is mainly performed at the door. Most common elements are: a Kumbha either filled with rice or water, a cocoanut and Arati.

The threshold is an element that is to be crossed without touching it. If one stumbles over a threshold by accident the entry or exit must be postponed. The western tradition of lifting the bride by the husband over the threshold reflects similar attitude. A threshold is not always considered an untouchable element, people do touch it with their head to pay obeisance to the place or its owner or touch it with right hand to take a few dust particles and place them over the forehead. The dust near the thresholds or doors of temples, prostitutes’ houses, and very rich persons’ mansions are considered auspicious, and are also used for construction of the Havan kund (ceremonial and sacrificial fire place).

‘The tradition to carry a bride over the threshold of the married couple's home is a very old one and followed in many societies of the world. The belief is nearly same everywhere, that a bride must not trip over a threshold. Some believed demons haunted doorways and could cause the bride to trip. Others felt that threshold was ominous of evil, and one must never step on to it, but rather cross over it. But people do touch the threshold in reverence. Some believe that one must open a door at midnight to allow evil spirits to depart, and that the first person to open the door on Christmas morning will have good luck. It's bad luck, some believe, to leave a house by a different door than the one used to enter the house or to eat in front of a door’. Ogden Nash has stated that ‘only a door is what a dog is perpetually on the wrong side of’, and for some, the dame luck also stays on the wrong side, at least most of the time.

‘A woman who had just given birth was forbidden to tread on another family's threshold, for it was believed that the woman would become a threshold cleaner in her next life. In Liaoning province of North-East China, people still follow the tradition during Dragon Boat Festival (the 5th day of the 5th lunar month) of sitting on the threshold of the door to eat an egg before sunset. After finishing the egg, they throw all the shells outside the house. Such an act is believed to clear the house of disease and bad luck in the following year’.
Houses confirming to Vastu have a properly oriented entrance door, so the place for home-entrance ceremony is also well set. In grih pravesh ceremony the home owner enters the house by touching the right shoulder to the right jamb, and by crossing the threshold with right foot first, but without touching it.

EAST ENTRANCE:
The rising sun and the East face entrance have a very intimate relationship in many cultures across the world. The spiritual relationship of Eastern light falling on to an East facing deity (sited in the western section of the building) or place of worship is a well-established fact.

It was universal early practice for the great door to be 'the Gate of Sunrise.' This door of enormous size was properly the sole opening to the temple, serving as much for light as to enter by, it was thrown open at dawn. Dr Hayes Ward in the American Journal of Archaeology (Vol. 3), shows some dozen Babylonian seals, with intaglios of the Sun-god passing through the double-valved gate of the East and beginning to climb the mountain of the sky. The gate has two guardian figures.

It was usual to cover the Eastern door with shining metal for it to glow in the morning sun rays. Sun rays were also received on the metal-clad shiny inclined top faces of the tall obelisks, shining metal finials over tall masts and over furling flags and festoons. The Sun is also metaphorically represented by soaring eagles, circular objects like a wheel, red disc or the winged globe. Images of soaring Eagles and wheels (Chakra) were part of door heads. The moving Dharma-Chakra of the Buddhist temples and stupas atop a Stambha -pillar is perhaps reminiscent of this.

Babylon temples had ‘gate of glory as brilliant as the sun’. In Syria it was the same; at the temple of Mabog (Hierapolis) the doors were gilded, as also was the entire sanctuary, walls and ceiling. ‘Two immense columns, one hundred and eighty feet high, flanked the door, inside which, on the left, was placed the throne of the sun’.

The preference for East for the main door changed when in pre Gothic period cathedrals began to have Western entrances, so that deities could be backlit with stained glass in the morning sunlight.

Egyptian’s temples and tombs had two openings: the Gate of the East, and the Gate of the West. Through the former the sun enters in the morning to pass out from the other in the evening, and there from pursue its journey way back by the dark path of the under world.

FALSE DOORS:
In the tombs of early dynasties a false door was the focus of the offering chamber that was adjacent to the tomb. It was here the family members placed their offerings for the deceased on a slab in front of the door. False doors were most typically placed on the west wall of the offering room. It was a threshold between the world of the living and the dead. It was a point through which the spirit of the deceased could transit. In some instances, there were two false doors affixed to the west wall, with the left one serving the tomb owner while the right door was meant for his wife.

In Egyptian tombs of earlier period, the false doors were indicated like the door of an ordinary house, low, small and narrow but not pierced through. These doors were not copies of real doors, bur rather a combination of an offering niche and a stela with an offering table. The doors were engraved on a single piece of wood, alabaster or stone panel, or drawn on the walls. They often had one, two or even three pairs of jambs leading to a central niche, so arranged to convey the illusion of depth and a series of frames, a foyer, or a passageway. The niche within the false door had carved statues and the side panels had inscriptions naming the deceased along with the titles, and a series of standardized offering formulas. These texts extol the virtues of the deceased and express positive wishes for the afterlife.

GUARDIANS OF THE DOOR:
A door as an entrance needs protection so that evil spirits are warded off it, and as an exit point good fortunes or luck does not escape out of it. Doors have protective charms. The most common charm for the door is the horse shoe. Other objects include olive branches, statuettes of gods, angels and saints. (For more refer to Chapter: 4.3 Openings systems: Treatments).

Portals, doors and gates had inevitable protectors or guardians in the form of real or horrible humans, beasts and monsters. These figures 'fawn on all who enter, but rend all who would pass there again (after death)’. The creatures were such as: winged genii in the form of bulls, scorpions, human-headed lions -the sphinx, lions, Dwarpal, Yaksha. Other forms like Christ, Michael, Gabriel, archangels, Ganesha, Hanuman, signs of the zodiac and sculptures of the months. In India, China, Siam, Japan, the gates are protected by Dwarpal, and presence of the Kshetrapal (the guardian of the local territory) was also necessary.

China has many legendary door guardians. The guardians are brave warriors. The pictures of door gods are hung in pairs, facing each other, it is considered bad luck to place the figures back-to-back.

There are two types of door gods: martial door gods and literary door gods. Martial door gods are usually generals depicted in life-size proportions, wearing full battle armour and wielding weapons, loyal men, great fighters. Commonly seen door gods of this type include ‘Shen Tu and Yu Lu’, ‘Qin Qiong and Weichi Gong’, ‘Zhong Kui’, ‘Guan U’ and ‘Guan Sheng’. Whoever the door gods may be, the common denominator of all front gate door gods is their trustworthiness, strength and loyalty, bolstered by a fierce martial countenance and impressive weaponry. In the past, each Chinese household hosted numerous gods, such as the stove god, door gods, the property god, the wellness god, etc. To keep ghosts and monsters at bay, prints of such door gods were pasted on front doors.

Emperor Tang Taizong (599 : 649 AD) was beset by demons howling outside of his bedroom at night. Qin Shubao and Hu Jingde, two generals volunteered to guard the door to the emperor's chamber. Thereafter there was no more trouble. In honour of the two brothers' bravery the emperor ordered pictures of the two to be drawn and posted on the palace gates. The two came to be associated as the Guardians of Doors or Door Gods. Qin Shubao has a white face, and Hu Jingde either a red or black one. They protect households from the evil forces outside, as well as marking ‘spaces safe’.

Shentou and Yulei are other immortals who were ordered by the Jade Emperor to guard peach trees which demons were gnawing at. Shentou and Yulei carry a battle axe and a mace, respectively. Zhong kui is not a door god but a mythical exorcist (ghost catchers) whose image is often displayed as the ‘backdoor general’.

‘According to an ancient Chinese text, "The Classic of Mountains and Seas" (shanhaijing?), there was a very large peach tree on Dushuo Mountain (dushuoshan?) whose branches formed an archway through which evil spirits could pass between the spirit world and the earth. The Emperor of Heaven (tiandi ?) was concerned that the evil spirits might harm the people on earth and so he assigned two brothers, Shen Tu and Yu Lei, to guard the passageway. If the evil spirits caused any harm, the two brothers were instructed to tie them up and feed them to the tigers which lived at the base of the mountain’.

A tradition gradually developed to engrave their portraits in peach wood which were then hung on gates and doors for protection from evil influences. Images were also painted on the doors, but only few could afford it. Ordinary people placed a light-coloured broom and a black piece of coal outside their doorways to represent Chin and Yu-Chih, respectively. With the advent of wood block printing in the Sung Dynasty, (960-1297) artistic renderings of deities became easily available. It was during this period that door God charms (taofu?) were replaced by auspicious verses written on red paper (spring couplets -chunlian?). The verses are hung above and at the sides of doors and gates. Traditionally prior to the Chinese New Year houses are cleaned and red trimmings are placed on doorways and windows to scare away the monster Nian as it is afraid of the colour red. Red banners featuring words like ‘longevity’, good luck’, ‘happiness’, etc., are displayed in homes.

Door guardians used for Buddhist temples are different from those at Taoist temples. At Buddhist temples, the most commonly seen door gods are Wei Tuo and Chia Lan, two guardians of the blessed state of enlightenment. The four Buddhist grand lords flank their sides, each holding, either a musical instrument, sword, umbrella or snake. At Taoist temples, such as those dedicated to Matsu, Goddess of the Sea, the most popular characters for painting on the entrances are the Thousand-Mile Eye God and the Wind Ear God. The Chinese temples dedicated to Confucius, are not decorated with door gods, for the Great Sage did not speak of superstition, deities or demons.

Over the years not only many different guardians for doors evolved but many different image versions became available. The door gods protected one from evil influences of demons and endowed luck, wealth, longevity and happiness. Each character is believed to have different powers. For example, images of the infant door god are suitable for the bedrooms of newly-weds, whereas the ox door god is appropriate for cowsheds.

JANUS ROMAN GOD OF DOORS:
In Roman mythology, Janus was the god of gates, doors, doorways, beginnings and endings. Janus also represented the sun and the moon. Janus is always associated with some form of duality. Janus is depicted with two faces -looking in opposite directions.

Janus symbolized change and transitions such as: the progression of past to future, of one condition to another, of one vision to another, and of one universe to another. Janus was worshipped at the beginnings of all events like planting and harvesting, births, marriages, etc. He was representative of the middle ground between barbarity and civilization, rural country and urban cities, and youth and adulthood.
Janus once caused a hot spring to erupt to stop the would-be attackers and forced them to flee. In honour of this, the doors to his temples were kept open during war so that he could easily intervene. The doors and gates were to be kept closed during peace, which rarely happened. But in the time of Augustus it was closed, after he had overthrown Marc Antony, and before that, when Marcus Atilius and Titus Manlius were consuls, it was closed for a short time; then war broke out again at once, and it was opened. Of the several places, Janus had a temple at Rome with double doors, which were called the gates of war.

Janus’s most apparent remnant in modern culture is his namesake, the month of January. The name Genoa, the Italian city is believed to be derivation of Janus. The Latin word for door, ‘ianua,’ itself possibly came from the god Janus.

Cardea is the goddess of thresholds and door-pivots (cardo =door-pivot). She protected little children against the attacks of vampire-witches. She is also a goddess of health like Carna. Cardea obtained the office from Janus in exchange for her personal favours. Her powers were ‘to open what is shut and to shut what is open’.

DOORS IN ROMAN HOUSES:
Roman houses had doors that opened inward. Roman society permitted only rare individuals of high honour to have doors that opened out on to the Street. Plutarch wrote that the Roman people complimented Marcus Valorous, a founder of the Roman Republic, after his triumph with a house built on the Palatine at public expense, but with doors to open outwards as perpetual recognition of his merit as if ‘he might be constantly partaking of public honour’. Caesar was given this plus an additional pediment which Livy mentioned as a decree by the senate for honour and distinction.

For the Romans a door represented the character of the household and was an expression of the owner’s place in society. The front door was always open to a stranger and community. An open street door showed a willingness to serve the community participate fully in political and social life.

‘Pliny the Younger interpreted that an unwillingness to participate with the community was the same as a willingness to engage in a destructive manner against the community. The Roman’s inability, to see behind, the closed doors always caused fear and suspicion, an implication that the paterfamilias had something to hide. A place of concealment was a place of potential revolution as can be seen by the conspirator Catiline in 63 BC. Sallust wrote that Catiline gathered his most trusted friends behind closed doors to attempt overthrowing the Republic’.

The formal entrance to a Roman house was set with certain depth from the street. It was a decorative entryway flanked by half columns or pilasters to create a picture-like frame. Strangers and formal guests were impressed by the passage through the fauces and atrium. The family members, neighbours and other regular visitors however, used a simpler side or back doors set flush with the street. ‘In Roman culture, the front door was always open to a stranger and community but to understand and be treated equal to the family, one had to approach from other means’. Hillier and Hanson state that the side or back-door was left ajar for the working class and to encourage neighbours coming in unannounced. Since the back door typically led to the kitchen, an important room to a house, the entrance carries a high level of presence availability.

JAPANESE DOOR:
For the Japanese ‘the door to happiness opens outward. A door simply imposes itself upon the room when it opens inward. Having the door open inwards has the outside intruding upon the inside’. Feudal schools of etiquette prescribe all kinds of norms for opening a door and coming into a room. Sukisha, well-bred people use the hand, nearest the door to open it a few inches (the length of a forefinger, to be exact) and then switch hands to slide it back the rest of the way. A man is judged by how he opens a door and a woman by how she shuts. This is so because in a room with a group of men, a woman served the food and take a leave. She would be observed closing the door behind her with grace. The balanced and graceful action of folding down one's knees on the floor, moving into a room, keeping at a level equal to others already in the room, were part of larger ceremony. The skills of opening and closing a sliding Japanese doors are part of reishiki, proper form or etiquette.

REVOLVING DOORS:
Revolving doors, when began to be used in American buildings, were marketed with a lot of hype, as ‘doors that always remain closed’. With a clear Victorian attitude they declared as capable of avoiding the ‘noxious effluvia’ and ‘baleful miasmas’. The doors were supposed to save life, by preventing those deadly lung and throat diseases which are sure to overtake the unfortunate salesman, cashier, or clerk whose duty keeps him near the constantly opening front door.

DOOR OPERATORS:
In the early part of Industrial age children hired to work as trappers to operate trap doors in mines and railway yards. They sat in a hole hollowed out for them and held a string which was fastened to the door. When they heard the coal wagons coming, they had to open the door by pulling a string. This job was one of the easiest down the mine but it was very lonely and the place where they sat was usually damp and draughty. This was like lift operator boys who opened-closed the elevator doors, working for long hours, but in fancy places, in late 19th and early 20th C.

DOORS IN LITERATURE:
Doors in literature are used in their various physical constructs, metaphysical effects and metaphorical forms. J. R. R. Tolkien the author of The Hobbit and The Lord of the Rings refers directly or indirectly to a door, window, gate, or other passageway that leads to a change in a character’s physical, metaphysical, or metaphorical state. While not quite as omnipresent in The Silmarillion, the motif is still used extensively throughout the text. The function of transition refers to a passage point that signals some type of change in a character. Barriers are thresholds that represent the dichotomies of safety/danger, us/other, inclusion /exclusion, and control/chaos.

One of the metaphorical meanings of doorways and openings in Tolkien’s work conveys the idea of ‘becoming’. As one passes through the doorway and enters a new stage of development or experience, having gained the ‘key’ necessary to move forward, one ‘becomes’ something more—more capable, more perceptive, more knowledgeable. Although, we often cling to our comfort zone, it is only by venturing outside our experience that we come to know our true selves. In other words, we must cross the thresholds that paradoxically lead us both outward and inward to a deeper understanding of our strengths and weaknesses and recognition of our relationships with the cosmos, just like Tolkien’s fish out of water.

Tolkien’s exploration of openings, and especially doors, gates, and windows exist not only as a motif within his written works, but is something he contemplated outside Middle Earth through his sketches. It is likely that his attraction to such passageways grew out of his daily surrounds as he lived and worked among centuries old architectural icons. Images of tunnels leading to and from openings appear to represent these complex initiatory ideas within Tolkien’s work. Like the ‘Before and Afterwards’ sketches, tunnels, caves, and mines function as extended thresholds between what came before and what new challenges wait on the other side, and as the inner realm of conflict where obstacles are met and overcome.

Much of Tolkien’s dark woodland imagery is also tunnel-like. He juxtaposes the open land with forest. Forests are entrance points and open lands exit points. In most natural environments he inverts that usual association of ‘being inside’ with safety, and ‘being outside’ with danger. More often than not entering a forest is depicted as entering into a dark unknown, as is entering water, caves, and barrows. Coming out into the open light of the fields is to have survived the dark dangers of woods, water, and earth with newly gained knowledge and confidence. For Tolkien passageways as barriers are usually artificial constructs like gates, fences, doors, and bridges and much less frequently a natural element like water or forests. Tolkien has used artificial barriers to symbolically represent the artifice of separation between peoples. (For more Refer to 4.0.3 Openings in Literature)
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Metaphorically, a Revolving door is an instance of the easy movement of individuals from one position or situation to another, and back again. For example, from government-related jobs to lobbying jobs and vice versa, resulting in a conflict of interest for those chosen to represent the public and/or special privileges and benefits to former government officials and personnel. A similar metaphor in the Japanese language is Amakudari, it refers only to former government employees joining companies they were once supervising. This is also used when describing early release of criminals who often end up back in prison after a short time.

The key to a door, permission to enter a door or a domain is given to a person of faith. It is at the door the alienation begins to manifest. To enter a premise, one needs to be consecrated by a person of authority, which means the visitor agrees to abide by the rules that prevail within.

MIHRAB IN MOSQUE:
A mihrab originally was a room for the great prophet to pray in. It was the focus of the wall that was indicative of the directioof Mecca. It later on became a decorative niche in the wall. Mihrabs vary in size, are usually ornately decorated and often designed to give the impression of an arched doorway or a passage to Mecca.

Doors and doorways frequently appear in metaphorical or allegorical situations, literature and the arts, often as a portent of change. Old buildings, castles, haunted houses, lair of villains or mansions of super heroes have secret passages accessed through doors, traps or openings. One enters or exits a building by triggering concealed levers, buttons or even surprising mechanisms. But one enters a supernatural realm, gets transported to another time and space setting, or even gets transformed into a different form by simply crossing a point or invisible threshold. The change is often secretive that is perceptible to few. Bizarre and secret entry ways are used in mythological stories, fiction, television programmes or films, and now video games to denote a change that is out of normal range. In Batman fiction and movies such secret entryways are used. In computer games such points may lead to new complications, new leads or bonuses, termination of the game or could be a false situation. In architecture entry ways are delayed in time and space that is the transition is not simple or immediate.