Fatigue test result.
According to Mareike Hahn of the Institut for Product Engineering at the University of Duisburg-Essen, fatigue tests show that metal materials do not endure an infinite number of repeated loads even if the stress amplitude remains below the yield strength at a macroscopically elastic load cycle.
The sustainable load depends on the time and respectively the number of load cycles. To create a Wöhler curve, approximately 10 samples of a material with uniform surface conditions are tested at a constant average stress level and different stress amplitudes until failure. The determined pair of values δa and NA (number of cycles until failure) are entered into a double-logarithmic diagram. The compensating curve is the Wöhler curve (figure 1). Please note that the Wöhler curve is only determined by means of tests at controlled stress levels. δa on the y-axis refers to the endured stress level and N on the x-axis to the number of cycles.