Properties > Stress, Strain, Strong & Tough
Although many people can define precisely what a force is by quoting Newton’s 2nd Law of motion, F=ma, we need to think about it a bit more to understand it. F=ma means that force is equal to the mass of an object multiplied by its acceleration, and this is its technical meaning. So let’s step back and put this in simpler terms. If you pick something up and hold it in your hand, what you are feeling is the force that object is exerting on you (and that you are exerting on it, Newtons 3rd Law). The object has mass, and gravity is acting as the acceleration. If we then were transported to the moon, that object would feel lighter because the acceleration or gravity has reduced. So when we think about force imagine how heavy something would feel, like when we described the forces in the fan section as up to 100 tonnes.
The difference in force between picking up an object and an engine component is the acceleration. Although we think gravity is strong, in real terms as a force it is pretty weak. Imagine yourself on a roller coaster, the force you are feeling when you speed up, slow down and turn corners comes from your mass and the acceleration of the ride rather than from gravity. Newton’s 1st Law of motion says that if you are travelling in a straight line at a constant speed with no effort (cruising along), to deviate from that straight line or go around a corner, you need to apply a force. This is called a centripetal force and is what you feel when you go around a corner really fast, and its also what a rotating object feels. Now up scaling that idea to components in a jet engine that rotate very, very fast, they generate a lot of centripetal acceleration, and therefore experience very large forces.
(For those who were playing Newton’s Laws of Motion bingo, that’s all 3!)
When an object is strong, it means it can withstand a lot of force (or can handle a lot of weight). The Yield Strength is how much force a material can handle and still react elastically. The Ultimate Tensile Strength is the maximum force the material can handle.
So how much force and object can handle is obviously pretty important but we need a bit more knowledge to make important decisions. So this is where the idea of stress comes in. Stress is the force a material experiences divided by the area over which its acting. If you think about it, stress describes force per size. If I am designing a component, stress lets me figure out how big that component has to be to handle the force acting on it.
When an object experiences a force or stress, it stretches. Strain is how much difference there is in length between the original component/material and what it is in when you apply things like forces and temperature.
When a material is described as tough, we are talking about the ability of a material to absorb energy by using plastic mechanisms without fracturing. Think about your car, if something hits it with only a little bit of energy then the metal will bend when impacted and then return to its original shape leaving no dent (elastic deformation). If something with a lot of energy runs into your car then you end up with a dent (plastic deformation). If something with heaps of energy runs into your car then it experiences failure or fracture. Tough is a way to measure the exact amount of energy you need to go from a dent to a fracture, aka from plastic deformation to failure. It is a combination of strength and ductility. In terms of a person, toughness is the point at which the energy of a punch goes from bruising to breaking.