Abstract: Experimental study is performed on the probability- based cyclic constitution models for LZ50 axle steel. It is verified that the random cyclic constitutions, like the wide recognized random cyclic strain-life relations, are intrinsic fatigue phenomena of engineering materials. Taking into account the integrated effects on the probabilistic assessment of the scatter-ing regularity and the sampling size, probability-based constitu-tion models with arbitrary survival probability and arbitrary confidence are constructed, respectively, by the Ram-berg-Osgood equation and its modified form. Availability and feasibility of the models are indicated by analysis of the present test data.

Key words: LZ50 axle steel, Modified Ramberg-Osgood equation, Random cyclic constitutions