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This study reports the fatigue behaviour of 2.25Cr1MoV steel under low cycle fatigue (LCF) loading and creep-fatigue interaction (CFI) loading at 355, 455 and 555 °C. Various hold durations up to 600 s were introduced in the CFI tests at the peak/valley strain under strain or stress control. In LCF tests, the steel exhibited remarkable strengthening at 455 °C, which can be ascribed to the effect of dynamic strain aging. In CFI tests, tensile holds were found more damaging than compressive holds but considerably less harmful than the combined tensile-compressive holds. A modified plastic strain energy approach based on the damage mechanisms was proposed to predict fatigue life under LCF and CFI conditions. The predictions obtained compared very favourably with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: Tensile properties and cyclic stress–strain response of Z2CND18.12N austenitic stainless steel were investigated at the strain rate of 1×10−3/s in the temperature range between 293K and 723K. Low cycle fatigue tests were also carried out at the strain rate of 6×10−3/s at 293K and 623K. SEM and TEM analyses were performed on the fatigue specimens. Tensile strength and ductility were found to reduce drastically with the temperature increasing from 293K to 423K but almost remained on the same level in the range between 523K and 723K. Serrations occurred during the stress approaching the ultimate tensile strength at 623K and 723K. The cyclic stress responses at temperatures ranging from 293K to 623K were characterized by a rapid initial hardening to the maximum stress, followed by gradual softening, whereas at 723K continuous cyclic hardening was present. The maximum cyclic hardening ratio, defined as the maximum peak stress divided by the initial peak stress, increased with increasing temperature in the present temperature range. Phenomenological friction and back stresses were derived from an analysis of hysteresis loop shapes using the Cottrell scheme. The results indicated that the increase of back stress was mainly responsible for the cyclic hardening. Fatigue life decreased with increasing strain amplitude at both 293K and 623K. Transgranular fracture failure mode was observed at both temperatures. TEM observations revealed that the dislocation structure changed from the cellular structure at 293K to the planar slip band at 623K. Dynamic strain aging has been believed to play a significant role in tensile properties, cyclic deformation and fatigue behavior of the material. [Copyright &y& Elsevier]

Abstract: Monotonic tension, isothermal/anisothermal fully reversed strain cycling and zero-to-tension cyclic tests were conducted within the temperature domain from room temperature to 823K to investigate the mechanical behavior of Z2CND18.12N austenitic stainless steel under various uniaxial loading conditions. Interesting results were observed from these tests, including obvious rate-dependence at room temperature but lack of rate-dependence at elevated temperatures with the occurrence of serrated flow stress in tensile tests, more cyclic hardening at higher temperature in strain cycling tests, and tendency to reach shakedown condition at elevated temperatures in zero-to-tension cyclic tests. Dynamic strain aging (DSA) effect was presumably believed to contribute to these characteristics of the material. A thermo-viscoplastic constitutive model was proposed to describe the mechanical behavior of the material under uniaxial loading conditions at small strains. Kinematic hardening rule with two components of back stress and isotropic hardening rule incorporating DSA effect are the novel features of the proposed model. The simulated and predicted results show reasonable agreement with the experimental data. [Copyright &y& Elsevier]

Abstract: The nonlinear viscoelastic–plastic responses of proton exchange membrane (PEM) under immersed condition at 30 and 70 °C are experimentally studied. For the description of the viscoelastic–plastic behavior of the material, Schapery''s nonlinear viscoelastic model is used in combination with a nonlinear viscoplastic constitutive law. PEM''s creep–relaxation behavior is predicted by the model with a constant duration at various stresses and a fixed stress level with different durations. The prediction results are found to agree quite well with experimental values at 30 °C. A step-stress input profile is conducted to validate the model and the result shows reasonable agreement with the experimental data indicating that the proposed model provides the capability to describe the characteristic of membranes in liquid water. However, this model cannot describe the creep behavior at 70 °C precisely mainly due to the distinct behavior of the membrane at the temperature. Thus, a modified model is developed and prediction values of the modified model at 30 and 70 °C are both found to be in good accordance with experimental results. [Copyright &y& Elsevier]

Abstract: Experimental results of monotonic uniaxial tensile tests at different strain rates and the reversed strain cycling test showed the characteristics of rate-dependence and cyclic hardening of Z2CND18.12N austenitic stainless steel at room temperature, respectively. Based on the Ohno–Wang kinematic hardening rule, a visco-plastic constitutive model incorporated with isotropic hardening was developed to describe the uniaxial ratcheting behavior of Z2CND18.12N steel under various stress-controlled loading conditions. Predicted results of the developed model agreed better with experimental results when the ratcheting strain level became higher, but the developed model overestimated the ratcheting deformation in other cases. A modified model was proposed to improve the prediction accuracy. In the modified model, the parameter mi of the Ohno–Wang kinematic hardening rule was developed to evolve with the accumulated plastic strain. Simulation results of the modified model proved much better agreement with experiments. [Copyright &y& Elsevier]

Abstract: In this study, a series of uniaxial tensile, strain cycling and uniaxial ratcheting tests were conducted at room temperature on Zircaloy-4 (Zr-4) tubes used as nuclear fuel cladding in Pressurized Water Reactors (PWRs) for the purpose to investigate the uniaxial ratcheting behavior of Zr-4 and the factors which may influence it. The experimental results show that at room temperature this material features cyclic softening remarkably within the strain range of 1.6%, and former cycling under larger strain amplitude cannot retard cyclic softening of later cycling under lower strain amplitude. Uniaxial ratcheting strain accumulatesin the direction of mean stress, and the ratcheting stain level is larger under tensile mean stress than that under compressive mean stress. Uniaxial ratchetingstrain level increases with the increase of mean stress and stress amplitude, and decreases with the increase of loading rate. The sequence of loading rate appears to have no effects on the final ratcheting strain accumulation. Loading history has great influence on the uniaxial ratcheting behavior. Lower stress level after loading history with higher stress level leads to the shakedown of ratcheting. Higher loading rate after loading history with lower loading rate brings down the ratcheting strain rate. Uniaxial ratcheting behavior is sensitive to compressive pre-strain, and the decay rate of the ratcheting strain rate is slowed down by pre-compression. [Copyright &y& Elsevier]