Properties of Materials 1. Strength • Strength is the ability of a material to withstand the forces of compression, tension and shear. • The Strength property of a material is very important in areas such as building construction, bridges and aircraft. • In the diagrams below the forces are represented by the arrows. Above we can see a Above you can see a crane Above we can see a pillar under lifting a load. plank of wood with 3 Compression. The crane is pulling up and the forces acting on it. load is pulling down, therefore If the column is not the cable is in Tension. The forces are not in constructed out of the line with each other and right materials then it For a crane, the cable is made so if the material is not would collapse. of a number of steel cables that strong enough it will are wound around each other. Shear. Concrete columns are This increases the strength of often reinforced by the cable. A scissors cutting paper steel cable or rods. Materials that do not have is Shearing the paper. enough strength would stretch and break in this situation. 2. Ductility • Ductility is the ability of a material to be stretched out by a force into thin wire. • Materials such as copper, aluminium and gold have excellent Ductility. • Steel also has excellent Ductility when it is heated. • By applying enough heat you can change materials from one state to another. So if you apply a small amount of heat to a Solid you are slightly changing the material towards a Liquid state. • It is important to remember that a Ductile material can be stretched a long way before it breaks or fractures and it retains the shape you stretch it to. • Take a piece of chewing gum, chew it for a while, and then stretch it. The chewing gum stretches for a good distance before it will break. • If it didn't break and let one end go, the chewing gum doesn't return to its original shape. This is Ductility. In the diagram below you can see a simple diagram of how a metal is drawn into a thin wire. 3. Creep • Creep is the stretching of a material due to a constant force over a period of time. • This is an unusual property because it is difficult to see, however you can conduct an experiment in the metalwork room to demonstrate Creep Creep Experiment The Lead strip is a fixed After 5 to 7 days the Lead strip length at the start of the is longer than it was at the start experiment. of the experiment. 4. Hardness • Hardness is the ability of a material to withstand scratching or penetration. • Glass is an example of a hard material. • If you try to scratch it with a Scriber you will find that it takes more effort on your part than scratching plastic. A simple way of testing for the hardness of materials is to use a scriber to scratch the surface. The more force you have to use the harder the material is. This is only a method of testing the Hardness of a material relative to another material, and it is not very accurate. • A more accurate way of comparing the Hardness of two materials is to use a Bench Vice and a hardened steel ball. • As the Bench Vice is tightened the hardened steel ball will be pushed further into the softer material. • This simple method is the way Engineers use to find the Hardness of different materials. • Engineers use machines like the Rockwell or Brinell Indenter for this purpose. • These machines press a hardened ball or point into a material using a limited force, and then measure the depth of the indentation The harder material showing The softer material showing where the hardened steel ball where the hardened steel ball penetrated less. penetrated more. 5. Malleability • Malleability is the ability of a material to be stretched or shaped in all directions without breaking or fracturing. • Copper, gold and aluminium have good Malleability. • Generally all metals become more Malleable as their temperature is increased and this allows them to be pressed or rolled into quite intricate shapes. • Some products that are manufactured as a result of the material being Malleable are girders, sheet metal and car panels. A girder made from hot rolled steel A panel from a truck which was to increase the steels Malleability. bent into shape from sheet steel. 6. Elacticity • Elasticity is the ability of a material to return to its original shape after it has been stretched. • A regular elastic band is a good example of the Elastic property of a material. • An Elastic band is made from rubber which has good Elasticity. • You can stretch it a long way and when you it go, the elastic band returns to its original shape. • All materials do, however, have an Elastic limit. • This is reached when the material is stretched so far that it will not return to its original shape, and sometimes the material will break or fracture. • Metals have a reasonably good level of Elasticity, which you might find surprising. Special tests can be conducted on materials to see how far they can be stretched before the Elastic limit is reached, • When constructing buildings and structures, it is important to know the Elasticity of the materials being used especially if the forces or loads on the structure are changing. • If the wrong material is being used then eventually the structure will fail, often with catastrophic consequences, due to the Elastic limit being exceeded. 7. Toughness • Toughness is the ability of a material to withstand impact. • The larger the impact needed to fracture or break a piece of a material determines how tough the material is. Differences with Hardness & Toughness • Example 1: Glass which is Hard but definitely not Tough. • Example 2: Steel is a very tough material and it can be alloyed with other materials in order to make it Tougher still. • However by making a material Tougher you also reduce its Hardness. • A good example of this is High Speed Steel. This material is Harder than mild steel but it is not as Tough. • We could demonstrate this with a Hammer, but safety precautions must be observed as this is a dangerous experiment. • Toughness in a material is important. • Think of a car, the body needs to be Tough in order to protect the driver and passengers. So anywhere there is likely to be impact we need a material that can withstand the impact. • One of the best materials for withstanding sudden impacts is actually the polymer known as ‘Kevlar®’. 8. Brittleness • A Brittle material can be easily broken or fractured by an impact. • Brittleness is the opposite of Toughness, so you should be able to tell, at this stage, that glass is Brittle. • It is very important to know if a material is Brittle in case you use it in a situation where an impact may occur. • Materials become more Brittle as temperatures decrease, which is one of the reasons that the Titanic sank, • The Steel hull of the Titanic lost its Toughness and became more Brittle in the cold waters of the North Atlantic and when the ship hit the iceberg the hull was no longer able to withstand the impact. 9.Conductivity • Conductivity is the ability of a material to allow heat or electricity to flow through it. • Metals in general make very good Conductors and this is because of their Chemical Bonding. • Copper, Aluminium and Gold are very good Conductors of heat and electricity, and you can see this in many of the uses that these materials • Water also has very are put to, such as good Conductivity of electrical wiring, pots heat and electricity, and pans, and which makes it good for electronics. cooking with, but very dangerous near electrical equipment. You should never put electrical equipment in water, and never swim during a thunder storm. • Materials which have good Conductivity, like metals, are called Conductors , • Materials which have very poor Conductivity, like plastics, are called Insulators. • You will often see Conductors and Insulators working together. Take the example of a normal plug. • The pins are made from Brass to conduct the electricity, but the casing is made from plastic, to save you from an electric shock. • The wires from the plug to the electric appliance are also covered in plastic as protection. Plasticity • Plasticity is the ability of a material to be stretched or formed into another shape and then hold that shape, without breaking or fracturing. • This property is very different from Elasticity, as the material does not return to its original shape. By heating metals you can increase their Plasticity, and this goes back to breaking some of the Chemical Bonds that hold the Molecules of a Solid together. •You will have seen a horse-shoe at some stage in your life. The horse-shoe was manufactured by a blacksmith using the Plastic property of the metal. •The blacksmith heats the metal to increase its Plasticity and then deforms the metal by hammering it into the horse-shoe shape. •When the metal cools the horse-shoe retains its distinctive shape. If the metal had broken or fractured the Plasticity of the metal would have been exceeded. •This could have been prevented by heating the metal more.
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