Investigation On Extra-Low Cycle Fatigue Fracture Design For Middle Carbon Steel Under Axial Loading

Under the subsidy of the talents project fund of Gansu Province and academic step & characteristic study of Lanzhou University of Technology, the extra-low cycle fatigue (ELCF) fracture design for middle carbon steel (MCS) under axial loading was investigated in this paper, the available parameters to the application of crack theory in engineering was obtained.On the basis of the existing achievements of fracture mechanics and crack technology, the stress, strain and displacement field of plain crack and notch were analyzed, and the stress intensity factor (SIF) and fracture criterion of the crack and notch were discussed. The SIF of different form of notch was calculated, and the results of stress, strain and displacement shown that taken the V-shape notch as ELCF sensitive stress field is feasible.According to the characteristics of ELCF fracture design, some experiments were planned and performed on the SHIMADZU hydraulic-electric servopulser fatigue testing machine.The data that obtained in the experiment was studied with evaluated method of fatigue life and consuming of distortion energy. Several problems, such as the relations of the cycle times of fracture on the effects of depth, tip radius of notch, loading frequency, loading range and the suitable parameters of fracture design for middle-carbon steel in extra-low axial fatigue loading were discussed through the experiments and the shape of fatigued fracture face. The experimental results shown that the tensile-compression fatigue loading mode is feasible to extra-low cycle fatigue fracture design for middle carbon steel, and it settles for low energy consuming, fracture surface with high quality, low cycle times, and high efficiency. The appropriate parameters are: loading frequency 3⁵Hz, tip radius of notch, r=(0.2⁰.3) mm, notch angle α=60°, notch depth t=(0.14-0.17)D.The results of fatigue characteristics reveal that cyclic hardening occurs when MCS is under axial ELCF loading. Based on the analytical comparison of calculation of fatigue notch factor K_f , the following formula adaptive to ELCF was put forward: K_f=0.986 + (0.692-0.018K_t)(K_t-1). It is proved that modified SWT method using K_fσ_maxε_a as research parameter is suitable to predict ELCF life under axial loading.