Metalwork - Materials - Properties
If you read the page at the start of the Materials section you will remember that we asked why are different materials chosen to do different things. Here in this section you will see that materials can have many different Properties, and the combination of all these Properties make a material suitable for one job or another. Look at a car for a second, why are the windows made from glass ? An easy question.... or is it ? What else could we have instead of glass... clear plastic, wire mash, or why have anything at all ? When designers are creating a product they have to take all options into consideration and then make the best decision. Why there is a glass window in a car is not a difficult problem, but imagine you had to design a kitchen knife as a metalwork project, (still an easy problem compared to designing a fighter jet!). What material would you use for the handle and why ? Wood, metal or plastic ? Perhaps you choose to have a plastic handle, now you have to decide which type of plastic. And then there is the blade to consider. If you understand the Properties that materials can have you will be able to make informed decisions when designing a product. This knowledge will help you save money, time and a lot of hard work. I have one question for you that has always puzzled me... you know that little black box in an airplane that never gets destroyed.... well why don't they make the whole plane out of the same material ?!!
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.
You can test the strength of materials in your metalwork room. Get a Snips and some strips of different materials, preferably about 1mm in thickness. Cut the different materials and by the amount of force you need to exert on the Snips you'll be able to tell which has the most Strength.
Hardness is the ability of a material to withstand scratching or penetration. Glass is an example of a reasonably hard material. If you try to scratch it with a Scriber you will find that it takes more effort on your part than if you scratched a piece of plastic. The Hardness of metals can be changed by the use of heat. This is called Heat Treatment, strangely enough. If you sharpen a Scriber or a Punch you should Heat Treat it in order to bring back its Hardness, otherwise it will go blunt again quickly.
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 slightly more accurate way of comparing the Hardness of 2 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. In the diagram on the right you should be able to tell which material is harder, and below you can see the results. This simple method is the basis of methods Engineers use to determine the Hardness of different materials. Engineers use machines like the Rockwell Indenter, or the 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. This way accurate readings can be taken of the Hardness of different materials.
Further down you can read about Toughness which is a different property from Hardness. In the English language 'hard' and 'tough' mean pretty much the same thing, but in Engineering they are very different and it is important to know the difference. Glass is Hard but it is not Tough. Did you know that the same Element makes up the lead in your pencil and the diamond that you would find in a ring ? If you want to know how click here.
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. Would you have any idea why heat might make a material more Ductile ? If you have not already read about The States of Matter then now would be a good time to give it a glance. By applying enough heat you can change Matter 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. In more accurate terms the heat is breaking the Chemical Bonds in the material and allowing them to move about more freely. It is important to remember that a Ductile material can be stretched a long way before it breaks or fractures. It also retains the shape you stretch it to. An elastic band is not Ductile... can you think why ?....the elastic band returns to its original shape if you let it go. This is another property called Elasticity. Chewing gum on the other hand is Ductile. If you don't find this too disgusting, take a piece of chewing gum, chew it for a while, and then stretch it. You will notice that it 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. Can you spot one other physical property that chewing gum has that is not in our property list ? It's sticky. Don't forget to put the chewing gum in the bin when you are finished experimenting !
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... more about that later. If you look at a modern glass window you will notice nothing unusual about it, however if you could come back in 2 or 3 hundred years the glass wouldn't look the same. You would notice that the glass looked like it had small waves in it. Prove it I hear you say... well if you know of any very old building near where you live go to it, and have a look at the glass. You will see what I described. In old glass the waves are due to 2 reasons, one is that the way in which glass was manufactured years ago was not as good as it is today and the second reason is Creep. Creep increases as the temperature of the material increases and so areas in industry where materials are under a constant load or force and the temperature is high there is a greater chance that creep will occur. Take the example of turbine blades in a steam plant. The steam raises the temperature of the material from which the blades are made and because of Centrifugal Force the blades are creeping away from the center of the turbine. As turbines are housed in tunnels you can imagine, that if the Creep in the blades was allowed to continue there would be a lot of damage when the blades stretch to the edge of the tunnel. Would you like to see Creep in effect yourself ? If so follow the following instructions and the diagrams below should help a little. Get a strip of Lead and straighten it as beast you can. Clamp one end securely and hang a weight, (the larger the better), from the other end. Measure the length of the strip. Now leave the experiment alone for 5 to 7 days and then come back and measure the length of the strip again. You will notice that the length of the strip of lead will have increased and this is all due to Creep. If this worries you then you'll be glad to know that Lead Creeps at room temperature, whereas most other metals don't start to Creep until much higher temperatures are reached or the force has been applied for a much longer time.
Malleability is the ability of a material to be stretched or shaped in all directions without breaking or fracturing. Again copper, gold and aluminium have good Malleability. Generally all metals become more Malleable as their temperature is increased and this allows Engineers to press or roll metals into quite intricate shapes. Some products that are manufactured as a result of the material being Malleable are girders, sheet metal and car panels.
If you ever get involved in beating copper sheet into a bowl in the metalwork room then you will be making use of the Malleable property of copper. You will also notice that beating metals is a difficult skill to master but the results can be very pleasing on the eye.
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. If you remember when we were talking about Hardness we said that it is important to know the difference between Hardness and Toughness, and we used the example of glass, which is Hard but definitely not Tough. You should know that 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 good example of this is High Speed Steel. This material is Harder than mild steel but it is not as Tough. You could ask you teacher to demonstrate this with a Hammer, and do ask your teacher as this is a slightly dangerous experiment. Toughness in a material is important in many areas. 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 some, or possible impact it is important to have a material that can withstand the force of the impact. One of the best materials for withstanding sudden impacts is actually the polymer known as Kevlar®. You can compare the differences in Toughness of different materials yourself. Get a Hammer and pieces, that are the same shape, of different materials. Hold the pieces of material firmly in a vice and hit them with the Hammer. You will be able to tell which are the Tougher materials by the effort you have to put into the blow or the number of blows necessary to break or fracture the material.
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, and you know what happened then.
Elasticity is the ability of a material to return to its original shape after it has been stretched. A regular elastic band is a very 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 let go of one end, (usually to the detrement of one of your fingers!), the elastic band returns to its original shape. However all materials do have an Elastic limit. It is reached when the material is stretched so far that it will not return to its original shape, and sometimes the material will even 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 and if you continue on to do Engineering you will find out more about this. 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, potentially with catastrophic consequences, due to the Elastic limit being exceeded.
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 pack 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.
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 beacause 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 are put to, such as electrical wiring, pots and pans, and electronics. Water also has very good Conductivity of heat and electricity, which makes it good for 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 and 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.
You can test different materials to compare their Conductivity in a number of ways. Here is one which you may have done in your Science class. Get a metal box with a number of holes in the top and some water in it. Now get rods of a number of different materials. Put some candel wax on the tops of the rods and press a ball bearing into the wax. Now put the ends of the rods not covered in wax into the holes in the metal box. Make sure all of the rods are in the water. Use a Bunsen Burner under the box to heat the water. After a while you will notice that the wax is starting to melt, and the ball bearings will start to fall off the rods some of the rods. The rods which lose their ball bearings first have the best Conductivity. The rods which don't lose their ball bearings have a low Conductivity and are Insulators.