This fact suggests that crystals are made of small particles called atoms or molecules arranged in orderly way in plates. Metals consist of tiny crystals.

Tensile, shear and compression force.
(i) Compression force
Compression is when the force acts as in the diagram below. This results in the particles to be pressed more closely together. So the length of the material decreases but the thickness of the material increase.

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(ii) Tensile force
Tensile force is when the force acts as in the diagram below. This result in the particle of the material to be pulled further apart from one another. So tensile forces increase the length of the material but its thickness decreases.

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Differences between tensile and compression force.

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(iii) Shear force
Shear force is the force needed to fracture the material in a direction parallel to the applied force in that one section (or layer) of the material slides over its neighbour.
A shear is produced when two equal but opposite forces are applied to a body. The effect depends on the turning effect or movement of the force.

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Important materials used for construction.

(a) Metals:

Large varieties of metals are available from which different alloys or combinations of these metals are made into various shapes. Metal can be rolled, pressed, and drawn, and are usually strong, rigid and elastic.

Some of the common metals are; copper, Zinc, Lead, Tin, Nickel, Chromium. e.t.c

(b) Alloys:
Alloys are made by mixing one metal with one or more other metals and in some cases non metals.

Steel alloys. Steel is an alloy of Iron and carbon. Iron is alloyed with a variety of the other materials like:-

Examples of steel

Mild steel (Iron and carbon) used in making cars, ships etc.

Stainless steel has high corrosion resistance due to its composition of chromium and nickel. It is used in making knives, watch casing etc.
Lead and sulphur steel. It is used in the making of screws because it is easy to cut.
Duralium is an alloy of aluminium and is used in the making of aircrafts because of its lightness and strength.

Nickel-Chromium alloys
i) Have good resistance to corrosion
ii) The electrical conductivity is independent of temperature
iii) Have a high melting point.
For these properties, nickel-chromium alloys are useful for making elements of electrical heaters.

Invar: is a nickel-iron alloy with low expansivity. It can be used to make accurate measuring tape and parts of watches.
Brass: Is copper-zinc alloy? It is ductile and with high tensile strength. It is used in stamping, pressing or drawing. It is used in the making plumbing fittings.
Bronze: Is an alloy of copper and tin is harder and stronger than brass. It is useful in ornamental work.

(c) Stony materials
Bricks: Are made by moulding a mixture of clay and water and beating the mixture strongly.
Concrete: A concrete is a stony material which is a mixture of cement, sand gravel and water. This is left to harden in desired form.

Properties of concrete which makes it a suitable building material
i) It is resistant to weather
ii) It is resistant to compression
iii) It is very durable
iv) It is resistant to fire

Concrete can be primarily subjected to compression like column and arches because it compression strength is high. However, concrete is relatively brittle material whose tensile strength is small compared to its compressional strength. This makes concrete unsuitable for use in structure membranes which are subjected to tension like tie rods, beam.

In order to overcome the limitation of low tensile strength, steel (with high tensile strength) is interlocked and completely surrounded by hardened concrete mass to form integral part of the membranes called reinforced concrete.

Reinforced concrete is a combination of steel rods, Cement, sand, gravel and water.
Concrete is reinforced by interlocking and surrounding the steel rods with the hardened concrete mass.

Advantages of reinforced concrete.
i) It has high compressive strength
ii) It has high tensile strength
iii) It has much greater ductility
iv) It is tough
v) It is weather and fire resistant

However, the disadvantage of concrete is its volume instability caused by shrinkage of concrete, which results in cracks.
The cracks can be filled with mixture of special tar, sand, cement and water.

Cement Mortar: Cement mortar is composed of sand, cement mixed with water and left to harden.

Reinforcement: sisal-fibre, bamboo stripes, wood strands are also used in reinforcing concrete and cement mortars.

The reinforcing improves on tensive strength and weather resistance of the materials.
Glass: Glass can be melted and formed into various shapes.

Advantages of glass which makes it useful as construction material.
i) It is transparent
ii) Its surface quite harder
iii) Very few chemicals react with glass.
iv) Can be melted and formed into various desired shapes.

Safety glass: Is used motor vehicle windscreen safely glass is made by heating plate glass cooling the two surfaces in a stream of air.

These contract and compress the glass in the middle resulting a very strong glass which when hit hard enough breaks into small fragment that are less dangerous than large pieces.

Wood: Wood is a poor conductor of both heat and electricity.
The hardness and strength of wood varies from one sample to another.
Thin sheets of wood are glued together to form a laminate (plywood) which is stronger than solid wood of the same thickness.

Provide room for expansion and contraction.

The Notch effect:
Cracks and fractures:
A notch is a cut or weak point in the surface of a material.

When a notch is made in the reinforcing material, the fibres, stripes and strands in the length of concrete or mortar are broken down.
This result in such materials to fail to withstand compression or tensile force.

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Glass tubes are easily broken after notch is made on the side.
A notch, crack or scratch on the surface of brittle material like concrete and glass, spreads more readily under tensile force than under compression.

Reducing notch effect:

(a) For concrete and cement mortar;
Notch effect can be reduced by; pre-stressed concrete containing steel rods that are in tension because they were stretched while the concrete was poured on them.
This is advantageous in that as well as resisting tension forces they keep the concrete in concrete in compression even if the whole structure is not.

(b) For glass:
Notches can be removed from glass
i) By making the surface of glass as smooth as possible. So glass usually making smooth to reduce the breaking due to notches.
ii) For safety glass used in motor vehicle screen is made by heating plate glass and cooling the two surfaces in a stream of air where they contract and compress the glass in the middle. This is called thermal toughening.
iii) By reinforcing glass with transparent polythene

(c) For wood;
Thin sheet of wood are glued together to form a laminated structure which is able to resist notches more than solids because for solid structures, the crack or a notch goes right through while in a laminated structure it may be stopped by one of the layers.