Strong materials

Materials can be tested for strength. In this picture there are two rods of the same size but made from different materials. Both rods have the same length hanging over the edge of the table. A weight is added to the end of each rod. If the rod is made of a strong material it stays straight, but if it is made of a weak material it bends. In this picture you can see that only a small weight was added to the rod on the right and it bent, while a large weight was added to the rod on the left and it stayed straight. The rod on the left is made of a strong material and the one on the right is made from a weaker material.


How does pulling, pushing and tearing affect materials?
Pulling makes a material stretch. If the material is strong it may not stretch or it may stretch then go back to its original length when you stop pulling. A weak material may snap when it is stretched.
Pushing squashes or crushes the material. If the material is strong it will not change shape but if it is weak it will go flat or crush.
If a material is pulled or pushed in opposite directions, it may tear. A strong material may not tear but a weak material may tear easily.

Why is stone a useful building material?
Stone (rock) has been used for making buildings for thousands of years because it is strong in a special way. It does not go flat or crumble when it is pushed or squashed. This is important because the stones at the bottom of a wall support all the stones above them. If the stones at the bottom squashed or crushed under this weight the wall would fall down.

What makes stones strong?
If you look at a piece of rock with a magnifying glass you may see that it is made of tiny particles stuck together. You may even see some small crystals. Rocks, such as sandstone are made from sand grains which have become stuck together by a natural cement. Others, such as granite, have crystals interlocking like pieces in a jigsaw puzzle. This helps to give granite its strength.

What makes bricks strong?
Stone is a natural material but is a manufactured material. Brick is made from clay. Clay is made from very tiny particles which move past one another quite easily. This is why clay is soft and can be moulded to shape. This is not a useful property if you want a strong materials, however. A brick is clay with its properties changed. To change the properties, the clay is moulded into a brick shape then heated strongly in a special kind of oven called a kiln. Changes take place in the clay that create tiny drops of glass between the particles. When the brick cools the glassy substance sticks the particles together and makes the brick strong.

What is concrete?
Some materials are combinations of other materials. A material which is made from several other materials is called a composite. Mixing materials gives them special properties. One common combination makes concrete.
Concrete is a mixture of cement, water and small stones called gravel. The largest part of the mixture is the gravel. After the concrete has been mixed it is left to set.
During this process changes take place in the cement and it forms crystals between the pieces of gravel. The crystals hold the gravel together and make it strong. Concrete has an advantage over bricks and stone. It can be poured into a mould and it will set into almost any shape without heating. Concrete is not a new material. It was invented by the Romans who used it build town walls as early as the third century BC.
Can concrete be made even stronger?
Yes it can. Very strong concrete is made by having steel bars running through it. This type of concrete is called reinforced concrete. It is used to make bridges and large buildings that are strong, cheap and reasonably lightweight.

Are other materials mixed together to make them stronger?
Yes. One of the most common is called glass-reinforced plastic (fibre glass). It is made from long thin threads of glass called glass fibres and plastic. The glass fibres are in the form of a matting and the plastic (called resin) is poured onto it. The plastic spreads between the fibres and then goes hard (it sets) to make a very strong yet lightweight material. It is used for making such things as canoes, the bodies of some cars and crash helmets.