Properties > Creep

In engineering we call something elastic if when we stretch it, it returns to its original shape.  Now if a stress is below something called the yield point, it acts elastically, like when you stretch a rubber band and it returns to its original size.  Usually we don’t think of elastic behaviour as time dependent in that no matter how long we apply the force for, we still expect the component to return to its original shape. Once we heat a material up over about 40% of its melting temperature, this changes.  If we apply a stress below the yield point and then remove it very quickly, the material will act elastically.  If instead we apply a stress/force then hold it there, the material will start to permanently stretch so when we let it go it does not return to its original shape.  If you think about it, this is what happens a lot in a jet engine.  The hot air passes through the engine imparting stresses and temperature on the components.  The thing is that in a jet engine the components are precisely designed and there isn’t much room for them to stretch before problems start to occur.

So what is happening in our material?  Well just like fatigue even though we are in the elastic region, damage is being done inside the material.  A material is made up of atoms and these are bonded to each other.  When we add enough kinetic energy (stress) and thermal energy (temperature), the atoms can start to move about.  When we have a low energy combination of stress and temperature the atoms in a material undergo a process called diffusion.  This is where the atoms jump about into existing holes.  If these holes are  along the grain boundaries then this is called Coble creep.  Because the grain boundaries are a chaotic region where two grains have joined to each other, the atoms therefore only need a little bit of energy to jump from hole to hole.  If they jump from hole to hole inside the grain or through the crystal lattice this takes a different amount of energy.   This is called Nabarro-Herring creep and you can see how it works in the animation below.

Now once we get more stress and temperature, the atoms have more energy and so they move about differently.  This is called dislocations.  Dislocation is when instead of jumping into holes already present, the stress and temperature cause holes that move about, join together and eventually cause a component to fail.  There are different ways these holes travel about inside the material depending on how much energy is available.  Check out how dislocations form and some ways they move about in a material.

Once we get near to breaking, we start to see holes on a larger scale.  Grain boundary sliding is where the grain start to move or rotate as the bonds at the edge that join them to their neighbour are chaotic and therefore not as strong as the bonds within the grain.

If you watch the above video you can see that big holes start to appear at the edges of the grains and what we get is the start of failure.  If you have a look at the picture below, this is exactly what has happened and we can see something called a triple point crack.  This happens where three grains meet and the holes start to join together.  From here this crack grows and the material breaks.