When variable loads are applied over a mechanical component or structure, cracks may happen even if the level of the loads is under the yield point of the material. This phenomenon is known as fatigue. Getting to know more about it is very important because this phenomenon causes a great number of components to fail. This is also the reason why it is important to to get to know the behaviour of the different materials when they are loaded with alternative forces.
There are two main methods for the calculation of the components durability and, associated to them, there are also two ways to make the characterisation :
S-N curve or Whöler curve: it determines the whole life versus the alternating stress.
E-N curve: it determines the number of cycles before the crack creation versus the alternating strain.