With the development of ultrasupercritical power generation technology, creep strength of high-temperature materials should be considered for safety evaluation and engineering design. However, long-time creep testing should be conducted by traditional creep assessment methods. This paper established a high-efficient prediction method for steady creep strain rate and creep strength based on short-term relaxation tests. Equivalent stress relaxation time and equivalent stress relaxation rate were defined according to stress relaxation characteristics and the Maxwell equation. An accelerated creep prediction approach from short-term stress relaxation tests was proposed by defining the equivalent relaxation rate as the creep rate during the steady stage. Stress relaxation and creep tests using high-temperature material 1Cr10NiMoW2VNbN steel were performed to validate the proposed model. Results showed that the experimental data are in good agreement with those predicted solutions. This indicates that short-term stress relaxation tests can be used to predict long-term creep behavior conveniently and reliably, and the proposed method is suitable for creep strength design and creep life prediction of 9–12%Cr steel used in ultrasupercritical unit at 600 °C.

Cyclic uniaxial stress-controlled ratcheting behaviors of 35CrMo structural steel were investigated at 500 °C. The effects of stressing rates and stress amplitudes were discussed in detail. Results present that the accumulated ratcheting strain increases with the increment of stress amplitude, while decreases with the reduction of stress rate at elevated temperature. Moreover, the ratcheting evolution curves with increasing number of cycles are similar to traditional static creep curves, and can be mainly divided into three stages. Especially, the ratcheting strain rate during the steady stage can be easily obtained to estimate the ratcheting or shakedown behavior. Furthermore, the steady ratcheting strain rate significantly decreases with increasing the stressing rate when the stressing rate is relatively slow. However, when the applied stressing rate is greater than a critical value, such as ≥ 2·5 MPa s⁻¹ for = 200 MPa and ≥ 25 MPa s⁻¹ for σ_max = 500 MPa, the effect of stressing rate has little effect on the cyclic ratcheting strain rate.

A new prediction model of stress relaxation from creep data in terms of average creep rate was established. An incremental calculation procedure has been further established to obtain stress relaxation data from creep data. To validate the effectiveness of the proposed conversion models, the predicted results were compared with those obtained by the previous continuum damage mechanics model as well as the stress relaxation experimental data using 1Cr10NiMoW2VNbN steel. Results showed that the creep–stress relaxation conversion model based on the average creep rate is better than that based on the continuum damage mechanics model because the former avoids the dispersion effect of fracture strain data, although the predicted results from the two models are both well in agreement with the experiment data.

Analytical ratchet limits of cylindrical pipelines subjected to several typical cyclic nonproportional loading combinations, such as internal pressure, tension, compression, moment, torsion, and thermal loading in practical engineering, were deduced theoretically according to the noncyclic method. The effects of the axial compression stress and radius ratio on the ratchet limit of structures were investigated intensively. Results revealed that the ratchet limit reduces significantly when axial compression stress is applied, while it clearly increases with the increment of radius ratio. Moreover, several Bree-like diagrams under the corresponding loading conditions of this paper were obtained, which can be used to conveniently estimate the ratchet limit of cylindrical pressure vessels and piping for engineering design.