This test is made by putting the specimen through a testing machine and by applying load cycles with a constant strain amplitude, so it is necessary to control the strain during the test. The specimen must be mounted in a way that only suffers uniaxial stress.
The fatigue curves are obtained from the testing of several specimens at several strain levels, considering the elastic and plastic strain, the force and the number of cycles until the crack initiation. The elastic and plastic strain and the specimen life are used to get the strain-life curves.
Only the maximum strain level (peak) and the minimum one (trough) are used. The system must apply load to the specimen in a way in which both, the peak and the trough value, are reached during each cycle. If the system is not able to maintain the test conditions between these limits, the test will be stopped and the data will not be considered.
The strain rate or the frequency must be kept constant during the entire characterisation process. The only exception to this rule is when we want to measure the influence of these parameters on the fatigue behaviour.