| Picture 4.2 Illustrations of stress and strain. | | Stress has units of newtons per square metre, N m^{-2}, or pascals, Pa. It is often expressed in: megapascals [1 MPa = 10^{6} Pa] or gigapascals [1 GPa = 10^{9} Pa]. | | | | Stress allows us to get a fair comparison of the effects of a force on different samples of a material. A tensile force will stretch and, possibly, break the sample. However, the force needed to break a sample will vary - depending on the cross sectional area of the sample. If the cross sectional area is bigger, the breaking force will be bigger. However, the breaking *stress* will always be the same because the stress is the force per unit area. In picture 4.2, sample A breaks with a force of 60 kN. Sample B breaks with a force of 120 kN because it has twice the area (you can think of it as being two pieces of sample A next to each other, with each one needing a force of 60 KN to break it). Although the force is bigger for sample B, the stress is the same for both samples – it is 60 kN cm^{2}. So breaking **stress** is a more useful measurement than breaking **force** because it is constant for a given material. It allows us to fairly compare the strengths of different materials. | |

| Picture 4.3 A special heavy duty machine puts a huge force on test samples of steel bars to break them. | | | Tensile strength | | A tensile test is used to find out what happens when a material such as steel is stretched. A steel bar is placed in a device that pulls one end away from the other fixed end. The **tensile strength** is the maximum stress that the bar can withstand before breaking. | Stretching | | So far, we have looked at only the *breaking* stress of a material. But what about the stretching effect of a force. We would expect that different samples of the same material would stretch in the same way when we apply the same stress to them. However, in order to compare the amount they stretch, we need to think of more than simply their extension. This is **strain**. On the next page we will look at strain and stiffness. | |