The literature survey reveals the importance of determining the durability of materials before fatigue crack initiation. For this purpose, various criteria are used, including the most popular ones, featuring the microstructural characteristics of metal materials. However, incorporating these characteristics in the defining dependencies leads to complications associated with using specific values that are difficult to determine or find in the reference literature. In this regard, it is proposed to use the French line (FL) and the calculation-experimental method for its construction, considering the polycrystal's characteristic microstructural unit. The author has previously proposed such a method, substantiated by the experimental literature data on the FL accelerated construction. When applying this method to the new data on the experimental determination of the FL performed by the conventional, deviations in the calculated and experimental durabilities were obtained. The regularities of these deviations were established, their cause was identified, and a method for their elimination was proposed in this study. For this purpose, the concept of a variable endurance limit in the cyclic loading process was substantiated and accepted. The obtained dependencies were subdivided into two stages of a smooth specimen's fatigue damage: (i) from the beginning of loading to FL, and (ii) from FL to complete failure.
