PHY2: MECHANICAL PROPERTIES OF MATERIALS

This unit is about the mechanical properties of materials.

MECHANICAL PROPERTIES OF MATERIALS

Materials used in construction of the structures like buildings, bridges, dams, tanks etc.

Before these materials are put to use it is important to know whether they will stand the conditions of finished structures and tods they are subjected to.

These conditions are made apparent by the test on their mechanical prosperities. Some of these properties include;-

– Strength

– Stiffness

– Ductility and brightness

Strength

It is the measure of how great an applied force a material can with stand breaking ie a strong material require a large force to break it.

FACTORS AFFECTING THE STRENGTH OF A MATERIAL

The cross section area

The bigger the cross section area of a material the larger the force required to break it.

The nature of the material

It is for example trader to break a steel rod than to break a wood on rod.

Force applied

The strength may depend on how a force is applied on it eg concrete is very strong when pressed but weak when stretched.

Breaking stress

This is the force needed to break a piece of material of a section area  = Force/Cross section area

Stiffness

It is the ability of a material to restrict bending. stiff materials resist force which try to change their shapes or sizes. They require constant large compression force but stretched by little amount when pulled apart.

Ductility

This is the ability of a material to be worked into any shape without breaking. Examples of ductile materials include;- metals like steel, copper, silver, iron etc.

These can be hammered, cut, rolled or stretched into useful shapes.

Characteristics of ductile materials

  • They can be deformed considerably without breaking
  • Crocks are not easily formed and where formed are not easily transmitted through.
  • They can with stand fairly large forces of tension and compression.

Brittleness

Brittle materials bend very little then suddenly crack without any warning examples of brittle materials;-

– Glass

– Chalk

Characteristics of bitterness

  1. They undergo very little deformation before breaking
  2. They can withstand fairly large compression forces but break suddenly when subjected to tension
  • Creaks form easily and are rapidly transmitted through material once formed.

Elasticity

Is the ability of a substance to recover its original shape after distortion i.e. when a force is applied and when a force is removed it returns to its original shape.

The amount of deformation depends on the nature of material and strength of the distorting force on the nature of the material.

STRESS AND STRAIN

Stress

This is the force applied to a material per unit cross section area.

materials

Strain

This is the extension produced per unit length

materials 1

BEAMS

A beam is a piece of materials with uniform cross section whose width and thickness are small compared to the length

Effects of stress on beams

materials 2

If a beam is acted on by two opposing forces acting away from each other its particles are stretched further apart and the beam is said to be in a state of tension.

materials 3

If two forces acted towards each other, particles of the beam are pushed close to each other and it is said to be in a state of compression.

Bending of beams

materials 4

materials 6

 

When a beam is bent one side is compressed and the other is stretched, particles along AB are in tension while those along CD are in compression.

Particles along xy are called the neutral line. They are neither stretched nor compressed. If the bending force along the region of tension and the beam breaks.

STRUTS AND TIES

A strut is a beam or girder under compression. A tie is a beam or girder under tension. A tie can be replaced with a rope or string but a strut cannot be replaced with a rope or string.

Struts and ties are found in structures

Structures used for support are made of wood iron or steel and they are called grinders.

The material out of which girders are made depends on the load to be supported.

Whatever the material, the design and arrangement of the girder is such that they can resist tension and compression forces. It is noticed that certain shapes give support while other collapse easily.

materials 7

Triangular structures provide better support than rectangular ones for the same land.

The rectangular structures can be made stronger and stiffer by fixing more girders diagonally as shown below.

materials 8

Before a structure is built its model is first made to enable engineers to know which girders are struts and which girders are ties. In order to determine which girder is a strut or a tie, each girder is removed one at a time and the effect it causes on the frame work is observed.

If the system buckles or bends on removal of the girder then the girder is a strut, but if the system tends to move further apart, then the girder is a tie.

Girder’s intention can also be tested by replacing them with strings. If the string is pulled tightly then the girders are in tension.

materials 9

When BC is removed points B and C move closer together showing that girder BC is a strut.

materials 10

When girder BE is removed points B and E move further apart showing that the girder is a tie.

materials 11

In the hanging structure below a heavy rod DE B supported by a nail at A

materials 12

Identify struts and ties i.e. the structure below;-

materials 13

Structures used for support are made of wood iron or steel and they are called guiders. The material out of which girders are made depends on the load to be supported. Whatever the material, the design and arrangement of girders are such that they can resist tensional and compression force. It is noted that certain shapes give support while others collapse easily.

materials 14

Triangular structures provide better support than rectangular ones for the same land.

The rectangular structures can be made stronger and stiffer by fixing more girders diagonally as shown below.

materials 15

Before a structure is built its model is first made to enable engineers to know which girders are struts and which girders are ties.

In order to determine which girder is a strut or a tie, each girder is removed on e at a time and the effect it causes on the frame work is observed.

If the system buckles or bends on removal of the girder then the girder is a strut, but if the system tends to move further apart, then the girder is a tie.

Girder’s intention can also be tested by replacing them with strings. If the string is pulled tightly then the girders are in tension.

materials 16

When BC is removed points B and C move closer together showing that girder BC is a strut.

materials 17

When girder BE is removed points B and E move further apart showing that the girder is a tie.

materials 18

In the hanging structure below a heavy rod DE is supported by a nail at A

materials 19

APPLICATION OF SRUTS AND TIES

  1. Rood supports

The common designs are triangular is shape and have diagonal girders arranged in such a way that tension and compression and simultaneously resisted.

 

  1. Water tank/ reserve supports

Water reserve/ tanks are raised highly up so that there is enough pressure to drive water from the tank to the houses. They are of large capacitor contain a lot of water when full.

Being heavy, they need strong support. When the tank is full of water, the stand supports are in compression and may buckle.

To prevent the stand from buckling diagonal girders are added. These hold the stands more firmly and make the structure stiffer.

  1. Bridge
  2. Building materials / strong materials
  1. Natural stone

This occurs in a farm of granite/ mumble /sand stone etc.

 

  1. Bricks

Made by mixing clay and water the mixture is molded into suitable shapes and fired at high temperature,

 

  • Mortar

It is a mixture of sand and cement made into a paste by adding water and its used for binding breaks.

 

  1. Concrete

Is made by mixing cement sand and gravel [small stones] the concrete used in building in made by mixing;-

  • 1 Par [by volume of cement
  • Part of sand and 4 parts of gravel.

 

A stronger concrete will have a high concentration of cement.

In a concrete mixture the particles of sand fill up the holes of larger stones while cement sticks all particles together, the right amount of water should be put in a mixture.

CONCRETE IS STRONG IN COMPRESSION BUT WEAK IN TENSION

  • Reinforced concrete

If a slab of concrete is used for a bridge then it is likely to crock when a lorry drives over it.

The undersides of a bridge is put into tension and since the concrete cannot support much tension, it crocks this can be improved by inserting wires or rods of steel through wet concrete.

As concrete dries, it sticks to the steel producing a combination which is strong in both compression and tension.

  • Stone bridges

When concrete is used to support a bridge the slabs are arranged to form a curved outline called an arch. This keeps the stone in compression making the structure stoney and resistant to bending.

Notches

These are cracks along surface of a material. When a supple of material is bided the forces from the load travel through the material setting up a stress.

concrete 1

In the diagram above if the same weight is used it is noticed that in a thinner material, the stress lines are close together. Therefore, the stress is greater in thinner than thicker material, so thinner materials are most likely to break

If a material has a notch, then the stress lines will appear as shown in the diagram below.

concrete 2

The notch causes a stress line to come closer together and thus causing a stress concentration at the tip of the notch i.e. the stress at the top of the notch is very great.

NB.

– If a notch is under tensional forces, it widens and the material eventually breaks

– If a notch is under compression forces, it closes. The weakening effect of tensional forces on notch s is used.

  1. Tearing paper, cloth or plastic
  2. Breaking glass tubes, rods and sugar cane

The large concentration of stress at the notch results in a tendency for cracks to travel through a stressed material and causes damage.

This may be prevented in a number of ways;-Construction of a structure in which all parts are kept compressed. This is made use of in construction of arches, used in bridges, dams and buildings. The strength of a covered surface is utilized in bulbs, drinking glasses, TVs, tubes and dam etc.

Keeping outer parts of the material under compression and inner ones under tension, this is utilized in the making of safety glass for car wind screens. If a notch develops in the outside of the glass, not damage is done. The glass in compression, so the notch is deep enough to reach the inner glass in tension, the glass shutters quickly, breaking into pieces.

  • Keeping the surface of a material, smooth. The surface is made as smooth as possible, so that’s its hard and difficult to scratch.

Reinforcement

This is done with the help of metal rods of fibers, steel rods are reinforce concrete, asbestos fibers are used to reinforce concrete used for roofing purposes fibers of glass and carbon are used reinforce plastics.

Blunting the trip of the notch

A hole is drilled at the tip of the notch to make it bluest and reduce the concentration of stress behind it.

concrete 3

 

 

ASSIGNMENT : MECHANICAL PROPERTIES OF MATERIALS assignment MARKS : 10  DURATION : 1 week, 3 days

 

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