Histoires inspirantes de femmes scientifiques africainesUncategorized
PRODUCTION OF ARTIFICIAL COLOSTRUM TO REDUCE CALF MORTALITY AND INCREASE THEIR PERFORMANCEUncategorized- Inspiring stories from African women scientistsUncategorized
Addressing gender stereotypes in the classroomGeneral
Gender assumptions that challenge a quality education for girls in UgandaEducation
Strengthening Gender Responsive Pedagogy for STEM in UgandaEducation
RESPECT FOR WOMEN IS PARAMOUNTNetworking
English
Arabic
French
German
Portuguese
Spanish
Swahili
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
Brittleness
Brittle materials bend very little then suddenly crack without any warning examples of brittle materials;-
– Glass
– Chalk
Characteristics of bitterness
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.
Strain
This is the extension produced per unit length
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
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.
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
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.
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.
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.
When BC is removed points B and C move closer together showing that girder BC is a strut.
When girder BE is removed points B and E move further apart showing that the girder is a tie.
In the hanging structure below a heavy rod DE B supported by a nail at A
Identify struts and ties i.e. the structure below;-
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.
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.
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.
When BC is removed points B and C move closer together showing that girder BC is a strut.
When girder BE is removed points B and E move further apart showing that the girder is a tie.
In the hanging structure below a heavy rod DE is supported by a nail at A
APPLICATION OF SRUTS AND TIES
The common designs are triangular is shape and have diagonal girders arranged in such a way that tension and compression and simultaneously resisted.
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.
This occurs in a farm of granite/ mumble /sand stone etc.
Made by mixing clay and water the mixture is molded into suitable shapes and fired at high temperature,
It is a mixture of sand and cement made into a paste by adding water and its used for binding breaks.
Is made by mixing cement sand and gravel [small stones] the concrete used in building in made by mixing;-
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
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.
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.
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.
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.
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.
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.
ASSIGNMENT : MECHANICAL PROPERTIES OF MATERIALS assignment MARKS : 10 DURATION : 1 week, 3 days