Properties > Fatigue

Fatigue resistance is a very important material property in aerospace design.  Engineers define fatigue as the process of repeatedly applying a load/force (below the breaking force) to a component.  This repeated loading weakens the material as each time a force is put on the component, a little bit of damage is done.  This damage leads to small cracks, and eventually crack propagation i.e. things breaking apart.  Now in a jet engine we have lots of components spinning very fast at high temperatures and stresses.  These stresses are not constant and rotating components often experience vibrations, therefore we need to know how well a material can handle these conditions.  To judge how well a material will perform in a jet engine, there are three types of fatigue we investigate at the ISM.

 

High Cycle Fatigue or HCF

HCF is where we repeatedly apply stresses/forces in the elastic region.  Because we are in the elastic region, we apply these forces in a really fast cycle hence it is called high cycle fatigue.  The force we apply stretches the material and when we remove it, the material returns to its original shape.  Although this seems like that force has done nothing to our material, it actually causes a little bit of damage each time.  This damage builds up and eventually the material crack and breaks.  Now this type of fatigue happens lots in jet engines as the flow of air past blades and stators creates air currents and therefore forces that bend the components.

 

Low Cycle Fatigue or LCF

In this type of fatigue we apply more force to the point where the material undergoes plastic behaviour.  Basically the force we apply permanently deforms the component.  This is done in a slower cycle as plastic behaviour is not instantaneous.   This type of fatigue also happens in jet engines as a blade has features like notches, damage and holes that mean stress concentrates in that area.  This means part of the blade may experience higher stress/force than the rest of the blade and so we need to know how it will cope.

 

Thermo-Mechanical Fatigue or TMF

In HCF and LCF stress is applied cyclically but the temperature is held constant.  In TMF both the temperature and stress is applied in cycles.  This more closely represents what happens in a jet engine whilst it is in service.  The problem is that by having both changing stress and temperature we have a far more complicated situation and so figuring out how and why it breaks become a lot more difficult.