Pure Geometrical Size Effect in fatigue tests with constant stress amplitude and in programme tests

The investigation considers the geometrical size effect for heat‐treated 37 Cr 4 steel specimens in the case of one‐level and multi‐level fatigue tests. It was shown that the ratio Notch Factor/SCF of geometrically similar specimens increases asymptotically with the notch radius, tending to a value less than 1. The formula of Stieler for the ratio SCF/Notch Factor do not describe correctly the decrease of the fatigue limit with specimen diameter. A more exact description of the size effect is obtained with the aid of a two‐parameter formula, which takes into account the support effect of the cyclic strain hardening and of the stress gradient. An effect of specimen size on fatigue life was also investigated in the case of multi‐level tests. It seems that this effect is associated with a smaller diameter interval than the effect of size on the fatigue limit. This is connected with the higher plastification of the notched area in programme tests with maximum stress much higher than the fatigue limit. The programme fatigue life and the damage sum were considerably larger for the smaller (d = 8.9 mm) than for the larger (d = 22.26 mm) specimens, but further increase of the specimen diameter seems to have a slight effect.