COMPOSITE MATERIALS

composite

It happened that the first composite materials in the history of mankind were used in construction. In ancient Greece (5th century BC) marble columns were reinforced with metal bars, and St Basil’s Cathedral in Moscow since 1555 AD stands on stone slabs fastened with iron. It is interesting that the Frenchman J. Motier, who received a patent for reinforced concrete in 1867, unwittingly followed in the footsteps of the ancients. Lightweight and durable racing yachts these days are made of another composite – polymer reinforced with fibreglass. This is a classic example of a modern approach to composites.

This is because technologists have learned how to pull the finest threads of many substances. Today they can pull the finest threads of many substances, even from materials like basalt. They are used to reinforce concrete or ceramics. With this kind of fibre they get a material stronger than steel, which doesn’t lose its strength up to 10000 Celsius. It is used for the manufacture of gears, moulds, dyes, parts and assemblies of engines for automotive and space industry.

Another example of making composite materials from traditional materials is glass. From one cubic centimetre of glass you can get the thinnest thread of as long as 450 kilometres. At the same time, the properties of glass dramatically change. It loses its fragility completely and bends easily.

Composite materials are artificially created materials that consist of two or more components that differ in composition and are separated by a boundary, and which have new properties that have been designed in advance.

  1. The major elements

In most composites (with the exception of layered ones), components can be divided into a matrix (or a binder) and reinforcing elements (or fillers) included in it. In constructional composites, reinforcing elements usually provide the necessary mechanical characteristics of the material (strength, stiffness, etc.), and the matrix ensures that the reinforcing elements work together and protect them from mechanical damage and aggressive chemical environment.

A component that is continuous throughout the entire volume of a composite material is called a matrix. The component is discontinuous, divided in the volume of the composite material, called the reinforcing material or filler.

Matrices in composite materials can be used the following:  metals and their alloys, organic and inorganic polymers, ceramics, carbon and other materials. The properties of the matrix determine the technological parameters and its operational properties: density, strength, operating temperature, resistance to fatigue failure, and the effects of corrosive media.

The reinforcing components are distributed in the matrix volume. They, as a rule, have high strength, rigidity and modulus of elasticity and by these indicators considerably exceed the matrix.

  1. Classification

Composite materials are classified according to the geometry of the reinforcing material and its location in the matrix. The geometry of the filler composite materials is divided into three groups:

First group has zero-dimensional fillers whose dimensions have the same size. The second group has one-dimensional fillers but with one of the sizes significantly exceeding the other two. The third group has two-dimensional fillers whose two sizes significantly exceed the third.

According to the layout of the fillers, three groups of composite materials are distinguished:

First group: With a uniaxial (linear) arrangement of the filler in the form of fibres, filaments, threadlike crystals in the matrix parallel to each other;

Second group: With a biaxial (planar) arrangement of the reinforcing filler, mats of threadlike crystals, and foil in the matrix in parallel planes;

Third group: With a three-axis (volumetric) arrangement of the reinforcing filler and the absence of a preferential direction in its location.

In composite materials with zero-dimensional filler, the metal matrix is the most common. Metal-based compositions are strengthened by uniformly distributed particles of various shapes and sizes. Such materials are characterized by isotropic properties.

In composite materials with one-dimensional fillers, reinforcements are one-dimensional elements in the form of threadlike crystals, fibres, wires, which are held together by a matrix into a single monolith.

  1. The main production technology of composites

There are many technologies for the production of composite materials with different fillers, but in this article we would like to stop at one-dimensional fillers, mainly fibres of organic and inorganic origin, such as glassfibres, basalt fibres, carbon fibres, aramid fibres, and others.

3.1 Manual manufacture/Handmade composites

This is the easiest and most common method of producing parts from composite materials.

The reinforcing material – mat, fabric, yarn, roving – is cut into measured pieces, and if necessary, cut off according to a template. Filler impregnates with binder (different resins) and puts into the form with the required number of layers to achieve the calculated thickness of the product.

The mould is then placed in a heat unit (using a term curable resin), where the curing process is performed. After opening, the product is removed from the mould, trimming other finishing operations are performed. This method is advisable to use in small-scale making of products of complex shape, to which there are low requirements for strength.

1 – the mould; 2 – separation layer; 3 – outer layer; 4 – filler;            5 – hand roller; 6 – resin

Very often, this method is used for the manufacture of boats and small vessels.

3.2  Manufacture by dusting

The chopped fibre and resin are put in an open form. Fibreglass roving passes through a chopper device and is blown into the resin stream.

The polymer composition impregnates the glassfibre, and the combined stream sprayed by the operator in the mould according to a predetermined pattern. After applying resin with fibreglass into the mould, the formed layer is rolled by hand to remove air, the fibres are compacted to obtain a smooth surface. The technology of hardening and trimming the edges is similar to that used in the moulding of manual forming.

1 – rowing; 2 – resin; 3 – chopper device;  4 – resin; 5 – finished layer; 6 – roller;  7 – mould

This method is used for the manufacture of various large containers or tanks without increased requirements for strength.

3.3  Pultrusion

The method of pultrusion of composite materials was first developed in 1948. However, it was intensively developed starting in the 1960s, when the demand for profile products from polymer composite increased.

The reinforcing fibre is pulled at a given speed through a resin bath and a preforming zone, where the fibre bundle is shaped into the desired profile with pre-compaction.

Then fibre enters the metal die that defines the exact shape of the product and removes excess resin. For curing of the resin, sometimes a heated die or a continuous furnace is used. Here, the final moulding of the profile takes place and its shape is fixed, after which the product is fed into the cutting device by the pulling device. The profiles thus obtained exactly correspond to the required section-size and do not need further machining.

  1. Direct rowing; 2. Impregnation bath; 3. Die; 4.Furnace; 5. Pulling mechanism; 6. Cutting device; 7. Product

In this way, various profiles, rods and pipes are obtained.

3.4  Manufacture by winding

Fibre-winding is a relatively simple process in which the reinforcing material in the form of continuous roving (tow) or yarn (yarn) is wound onto a rotating mandrel. Special mechanisms, control the winding angle and the location of the reinforcement material. It can be wrapped around the mandrel in the form of adjacent strips to each other or on some kind of repeating pattern to complete coverage of the surface of the mandrel. Sequential layers are applied until the desired thickness is obtained. The winding angle can vary from very small-longitudinal to large-circumferential, that is close to 90° relative to the axis of the mandrel.

Usually, the curing takes place under the temperature without excessive pressure, and the final stage of the process is the removal of the product from the mandrel.

The method is actively used for the manufacture of high-pressure cylinders and pipes.

3.5  Manufacture by compression

For compression moulding of large-sized products, hard or soft forms of negative and positive types are used. The surfaces of the mould are covered with a layer of anti-adhesive release agent. The blank obtained by the method of preforming is placed in a mould. The blank is a package of reinforcing material pre-impregnated with a binder. The so-called “prepreg” materials with a pre-applied thermosetting binder are often used. Under the pressure of the punch and the matrix, the workpiece acquires a given shape and then heats up, which triggers the polymerization mechanism. Thus, products with high mechanical and precision parameters are obtained.

This method is used to create thin and durable products with increased requirements.

3.6  RTM

Traditionally, RTM (Resin Transfer Moulding) technology involves injecting resin into a hermetically sealed form that contains reinforcing material. The injection of the resin occurs under low pressure (or a vacuum), the pressing of the punch and mould takes place with the help of a vacuum.

The mould and the punch are covered with the non-adhesive composition. Then the necessary package of reinforcing material applied, and resin gets injected. After completion of the injection, mould remains in a closed position until the resin is completely cured. RTM enables the manufacture of complex parts with a single operation, thus achieving optimal impregnation characteristics, while improving the rigidity and at the same time the lightness of the composite material.

In this way, the hull of high-speed boats and blades of the wind turbines are made.

This article has indicated only the basic methods of obtaining products. However, over the entire period of composite materials technology, many subspecies and varieties of each technology have appeared.

  1. Applications of composite materials

The use of new composite materials is an important factor in solving such fundamental economic problems as limited natural resources, lack of strategic materials, maintaining the pace of economic development and productivity growth, maintaining competitiveness in the global market. They can be applied in many areas of human activity.

In ordinary life, composite materials are used in consumer goods. For example, there are fishing rods made of fibreglass and carbon, boats, tyres, etc, made of fibreglass. In the sports world, some sports equipment is made of fibreglass: bicycles, skiing sticks and skis, snowboards, hockey sticks and skates, kayaks, canoes and paddles, body parts for racing cars and motorcycles, helmets, etc.

 In the medical world, there is considerable use of composite materials such as dental fillings, where the filler is glass dust.

 

In engineering, such as mechanical engineering, composite materials are widely used to create protective coatings on friction surfaces, as well as to manufacture various parts of internal combustion engines (pistons, connecting rods, cylinders) and housings.

Armament and military equipment. Due to its characteristics (strength and lightness) composites are used to produce various types of armor: body armor, armor for military equipment.

Aviation and cosmonautics. Since the 1960s, there has been an urgent need for the manufacture of durable, lightweight, and wear-resistant structures in aviation and aerospace. Composite materials are used for the manufacture of primary structures of aircraft, artificial satellites, heat insulating coatings of space shuttles. Horizontal and vertical stabilizers, rudders, tail elements, side members, propeller blades, wing trim, are made of them. Modern airliners are made up of 70% of composite materials.

In construction and infrastructure, there is concrete reinforcing, decorations, bridges, pedestrian and communications crossings, ladders, gratings, protective systems, fences, electric and light poles and much more.

In the next part, we will stop on the application in construction, describe physical and mechanical properties, and the major pros and cons of composite materials in today’s world.

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