Abstract
The results of analysis of fatigue damage development in the X10CrMoVNb9-1 (P91) steel specimens subjected to various cyclic loading combinations are presented. The study concerning a quantitative assessment of damage is based on analysis of the fatigue hysteresis evolution in subsequent loading cycles. According to the elaborated algorithms, damage parameters were determined on the basis of two indicators, i.e., average strain and strain amplitude variations. Each of them was responsible for a single damage development mechanism. The average strain characterized ratcheting effect, which is usually related to local generation of plastic strain surrounding microstructural elements. The second damage mechanism called cyclic plasticity is connected with micro slips preceding slip bands formation. The paper also discusses the results of metallographic and fractographic microscopic (SEM) observations of the tested specimens in order to indicate microstructural aspects of fatigue damage development mechanisms.

Key words
fatigue test, steels, fracture, scanning electron microscopy, microstructure

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