Determination of fatigue curve parameters at cyclic strain limited loading according to the mechanical characteristics of power energy structural materials

Over 280 structural materials used in the Russian nuclear power energy industry were tested at Kaunas University of Technology, as commissioned by St. Petersburg Central Research Institute of Structural Materials in the period of 1970–2000. Alloyed structural steels, stainless steels and metals of their welded joints with different types of thermal treatment were under research in the conditions of symmetric low cycle tension-compression (N ≤ (1–2) × 104) at room and elevated (200–350 °C) temperatures. During these experiments the characteristics of monotonic tension, cyclic stress–strain curves and low cycle fatigue curves, which are expressed by linear dependence of the total strain ɛ on the number of cycles N in co-ordinates log ɛ − log N, parameters C1exp and m1exp were determined. The range of cycle strain ɛ when this low cycle fatigue curve may be used is determined. The definition of the approached values of low cycle fatigue curves parameters C1cal and m1cal at room temperature by the mechanical characteristics of alloyed structural steels and their weld metals, stainless steels and their weld metals is analyzed in this work. The new relationships ln C1exp − m1exp among low cycle fatigue curves parameters, obtained by analyzing experimental data of 22–48 materials in each group, are determined. These relationships were used for determination of dependencies of low cycle parameters C1cal and m1cal by the mechanical characteristics of analyzed groups of materials. From theoretically and experimentally determined equations by using calculated values C1cal and m1cal dependencies for revision of Coffin–Manson fatigue curves parameters C and m were proposed. These parameters for the cyclically hardening or softening materials are calculated by estimating the equivalent value of the cycle plastic strain range. The proposed analytical dependencies make it possible to more exactly calculate the number of cycles N ≤ N0 (N0 = 106) before the fatigue crack initiation for the structural materials of analyzed groups under cyclic strain limited loading by using mechanical characteristics of materials.