Search

• A computational strategy based on gradient damage is developed for low-cycle fatigue. • Mixed-hardening model is combined with elasto-plastic gradient damage. • Developed framework captures stress-strain hysteresis better than Chaboche model. • Low-cycle fatigue crack growth is accurately simulated by the developed framework. • The framework avoids computational burden of conformal meshing and remeshing. This work presents a novel computational strategy based on gradient enhanced damage and cyclic yielding with nonlinear mixed hardening behaviour for simulating low-cycle fatigue of ductile materials. A new yield function is introduced which accommodates the cyclically evolving state variables during mixed hardening and the damage induced during the application of repetitive loads. The resulting constitutive relations are derived in an incremental form. The non-local strains facilitate the computation of incremental damage in the material. A finite element scheme is developed from the governing physics through full coupling between damage and cyclic-elastoplastic deformations. In order to obtain an accurate computational response, a stress-return mapping scheme is formulated for the damage-dependent yield function. The developed numerical strategy is investigated for specimens made of various ductile materials exhibiting elasto-plastic behaviour under low-cycle fatigue loads. The stress-strain hysteresis and cyclic softening computed by the present numerical framework agree well with experimental data available in literature. The low-cycle fatigue crack-growth results are also accurately captured for various fatigue loading conditions. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights • Heterogeneous coarsening of nanolath structure is characterized under the low plastic strain level condition. • Two regions in CSS curve are classified based on the cyclic softening behavior and the homogeneity of microstructural coarsening. • A dislocation-based viscoplastic constitutive model is developed by introducing an intergranular back-stress related to the heterogeneous coarsening. • The macroscopic mechanical behaviors and microstructural evolution of a nanolath strengthened steel at high/low plastic strain levels are reasonably reproduced. Abstract Cyclic softening behavior at elevated temperature of nanolath strengthened steel was rationalized under different strain amplitudes. Results indicated that the evolution of internal stress components and the coarsening of subgrains are significantly depending on the plastic strain level. This paper proposes a dislocation-based viscoplastic constitutive model to represent the cyclic deformation response especially at low plastic strain level. The heterogeneous coarsening of subgrains is explicitly formulated via employing different evolution laws for multiple dislocation populations. An additional back-stress hardening component is introduced to account for the retarded cyclic softening. The cyclic deformation response and microstructural evolution of a nanolath strengthened steel at high/low plastic strain levels are perfectly reproduced. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • Comparative study on cyclic softening evolutionary characters in different subzones. • Comparison of fatigue strength of different subzones in CrNiMoV welded joint. • Study the ratcheting behavior of welded joints under different stress ratios. • Comparison of cyclic and fatigue features for joints under different control mode. • Explanation of the failure location shifting behavior in the welded specimen. Abstract The cyclic plasticity and the low cycle fatigue failure behavior of the weld metal (WM) and base metal (BM) of the CrNiMoV steel welded joint under the strain and stress-control modes were investigated respectively. Significant cyclic softening was observed for both the WM and BM under the low cycle fatigue tests with the two control modes. Besides, obvious ratcheting happened in the WM and BM under the stress-controlled cyclic loading conditions. It is shown that both the WM and BM exhibited lower fatigue strength at the stress control mode than that at the strain control mode due to the influence of tension-compression asymmetry. Meanwhile, the WM showed larger cyclic softening rate, lower ratchetting deformation and fatigue strength than the BM under the same loading levels. The failure location of the WM specimens shifted from BM region (nearby the heat affected zone) to the center of WM with the increasing of strain amplitude under the strain-controlled tests, which can be explained with the similar maximum equivalent plastic strain amplitude location shifting behavior observed from the corresponding finite element simulations. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Ferritic-martensitic (F/M) steels are a common choice for high-pressure, high-temperature boilers and steam piping in power plants, but cyclic softening without saturation is a major degradation mechanism of F/M steels. This degradation mechanism is more pronounced and complex under creep-fatigue loadings. For reliable, but not overly conservative design a better understanding of this phenomenon is necessary. The intent of this work is to characterize and model the softening and lifetime behavior under creep-fatigue conditions at high temperature. Therefore, low-cycle fatigue (LCF) tests with hold time were performed at 550   °C. A distinct dependence of the softening behavior on hold time was found. Additionally, detrimental effect of compressive hold time on lifetime was also observed in this study. All effects of hold time were more pronounced for small strain amplitudes. Based on these experimental observations, a unified viscoplastic deformation and damage model is further developed to describe the high-temperature cyclic softening and lifetime behavior of F/M steels under creep-fatigue loading. An optimized parameter set for the extended model is determined based on experimental results. Good agreement of experimental results and model descriptions is achieved both under pure fatigue as well as under creep-fatigue conditions. The detrimental effect of compressive hold times on lifetime of ferritic-martensitic steels is well represented. [ABSTRACT FROM AUTHOR]