Low cycle fatigue and ratcheting behavior of 35CrMo structural steel at elevated temperature

Low cycle fatigue and ratcheting deformation of 35CrMo steel under uniaxial stress-controlled loading were investigated at 500 °C. The effects of stress rates for 0.125 MPa/s, 0.5 MPa/s, 2.5 MPa/s, 10 MPa/s, 25 MPa/s and 40 MPa/s, and peak stresses σmax for 200 MPa, 300 MPa, 400 MPa and 500 MPa, were discussed in detail. Results indicated that the ratcheting strain curve versus cyclic number is similar to traditional static creep curve with three stages and the ratcheting stress rate is lower than the static creep rate with the same peak stress. Shakedown behavior occurs when σ̇ is greater than or equal to 2.5 MPa/s for σmax = 200 MPa, but ratcheting happens for the other loads tested according to the method proposed by the Committee on Three Dimensional Finite Element Stress Evaluation (C-TDF). Moreover, the viscous stress component σv increases significantly with the decrease of stress rate, which represents the ratcheting deformation of 35CrMo steel is obviously time-dependent under high temperature. Furthermore, stress rate has little effect on fatigue life when the stress rate is less than 2.5 MPa/s, while the fatigue life increases exponentially with the increase of stress rate when the stress rate is greater than 2.5 MPa/s at σmax = 500 MPa. It is of interest that the maximum fracture strains for various stress rates remain approximately constant, and the tested average value is 11.6%.