Materials

Structure and properties - Ripping paper

Imagine we're reading a newspaper. We then proceed to take out our magnifying glass and look at the fibres of the paper we're holding. What we would see is long, thin stands lined up in the same direction.

We then proceed to rip the paper in two directions:

    

Firstly, let's think about what would happen when we rip the paper vertically, in the first direction. When we rip the paper this way we will find that it is incredibly easy, and a relatively straight rip can be obtained. That's because all we're doing is separating fibres. Think of the paper as a pack of cards, and each fibre as a single, individual card. When you apply a force that acts to separate the cards, you'll find it incredibly easy. Now we need to think what happens when we tear the paper in the other direction. We should find this a lot harder, because in this case what we actually have to do is break molecules in order to rip the paper, which requires a lot more energy. We therefore seldom see straight rips when we tear paper in this direction. In our deck of cards analogy, what we're doing is trying to separate the cards into two groups by ripping through 52 cards - this is obviously a lot harder than just separating them without having to rip through them.

Materials

Composite materials

If we take a look at the beam below that has has a force exerted on it, we can observe that the two sides of the beam, the inside and the outside, are experiencing different kinds of forces.

Let's say we want to build a beam, and we have two materials that we can build this beam out of. The first is iron. Iron has a very good tensile strength. This means that it can be subjected to a high pulling force, during which the atoms would experience a force pulling them further apart, without deforming. During compression, however, the material does not do so well, and buckles. Compression would occur if the atoms in our beam were "pushed" together, as they are on the top of the beam above.

The other beam that we're looking at is made of concrete. Concrete, unlike iron, has an excellent compression strength. Unfortunately, it does not do so well with tension on the underside, and cracks form in the material. This means that it too, would be unsuitable for a beam. 

        

We have an issue here. Both of our materials would be unsuitable for the manufacture of this beam - they both fail in some respect when subjected to any kind of force. What we do therefore, is form a composite material. This involves taking two materials and combining them to make one new materials, which takes the best aspects of both materials. When iron would fail in compression, concrete takes this force and stops any buckling from occurring. When concrete would be deemed unsuitable on the underside of the beam, iron takes the tension force to stop the new material from cracking.