Figure 4. Working principle of the micro-rotary fatigue resistance (µRFR) test set-up. (a) A small rectangular beam with a rounded constriction at the tooth-biomaterial interface (diameter, 1.2 mm) is prepared by means of a diamond saw and a lathe. At a fixed distance of 12.5 mm from the interface (l = lever arm), a load (F) is applied, while the tooth part of the specimen is still clamped in the chuck of the lathe. Maximal stress (tension) is induced at the upper part of the interface (•). (b) When the specimen is rotated around its main axis, the outer layer of the interface (•) is subjected to sinusoidal tension-compression cycles (S = stress).