Testing > Creep Testing
To do creep testing we need to apply a stress/force and a temperature. Now we have two ways of doing this. We can apply a constant force known as constant load creep testing. This means we hang a weight off the back of a machine and the force on the specimen does not change no matter how long the specimen stretches. This is the most common type of creep testing and the mathematics behind how it works is shown below.
The other type of creep testing is called constant stress creep testing. As a sample stretches along its length, the cross section becomes narrower. To keep the stress constant, it means that as the sample stretches we need the amount of load applied to reduce in order to keep the force per cross sectional area constant. To do this we need the level at the top of the machine to change as the sample stretches. Having a specially shaped cam, as the sample stretches the lever ratio reduces meaning the load reduces and we achieve constant stress. The following video shows how this happens.