Fatigue Testing

Testing > Fatigue Testing

Fatigue testing is about applying a cyclic stress whilst holding a constant temperature.  When we do this we need to define the R-ratio which is the minimum stress divided by the maximum stress.  An R=1 is a static load where the maximum and minimum stress are equal.  R=0 means that the minimum stress is zero, so basically we put a force on the specimen then completely remove it.  An R=-1 means that we are applying tensile and compressive stresses i.e. we are stretching it using a certain amount of force, then we squash it using the same amount of force.

A fatigue test applies a temperature, cyclic stress then measures the growth of the resulting crack.  The problem with measuring a crack is that the length of the crack on the surface does not always represent how big that crack is inside the component.  Knowing how big the crack is very important as it tells us how broken a component is.  To measure the size of a crack we measure a potential drop or voltage across the test piece. The bigger the crack, the less material there is for electrons to travel across.  This gives us the area of the crack at any specific stress cycle.

Now when we test something the goal is about being able to predict and quantify its behaviour.  With crack growth we use the Paris-Erdogan law which relates the change in size of the crack per cycle to ΔK.  This ΔK is essentially a combination of the changing stress, a geometry factor (depending on the specimen type), and the crack length.  From the Paris-Erdogan law we use the values of C and m to tell us how much bigger a crack will get depending on the stress, size of the crack, and shape of the specimen/component.  The following animation shows how we get these values of C and m.