This paper presents an elastoplastic framework for mechanical-electrochemical damage in metals for simulating corrosion fatigue. The proposed numerical approach combines classical rate-independent isotropic von Mises elastoplasticity with an electrochemical kinetics model to simulate anodic dissolution-driven corrosion. The model's capabilities are demonstrated through benchmark tests and experiments. A hollow specimen was tested in a water environment, incorporating a membrane electrode for corrosion potential measurement and potential drop for crack initiation detection. The formulation accurately reproduces key features of corrosion fatigue, including diverse pit morphologies, time-dependent corrosion kinetics and the formation of multiple crack initiation sites, consistent with experiments.

Fatigue and corrosion are two dominant mechanisms responsible for the failure of structural materials. Fatigue results from the progressive accumulation of damage under cyclic mechanical loading and is estimated to cause over 90 % of all mechanical service failures. Corrosion, caused by chemical or electrochemical reactions with the environment, leads to material loss and structural degradation. In Sweden, the annual cost of corrosion is estimated to be approximately 4 % of the national GDP. When these two phenomena interact, as in corrosion fatigue, the degradation process accelerates significantly. This is particularly critical for engineering components operating in harsh environments such as offshore structures, aerospace systems and industrial atmospheres.

In the context of the global transition toward a fossil-free society, the continued safe operation of existing nuclear reactors is essential to meet rising energy demands. Extending the service life of light water reactors (LWRs), without compromising human safety, is widely recognized as a critical step in this transition. This highlights the need for accurate predictive models that capture the complex interaction between mechanical fatigue and electrochemical corrosion of 304L stainless steel in LWR water environments.

A comprehensive mechanical characterization of 304L stainless steel was performed in air at both room temperature and 300 °C. This included tensile tests, fatigue tests, fatigue crack growth tests and cyclic stress-strain experiments in both the rolling and transverse directions. The results revealed isotropic behavior and reduced mechanical performance at elevated temperature. The cyclic plasticity response was modeled using a von Misesradial return-mapping algorithm incorporating Voce isotropic and Armstrong-Frederick kinematic hardening laws, demonstrating good agreement with experimental hysteresis loops. The mechanical characterization provided the foundation for the corrosion fatigue investigation.

To investigate mechanical-electrochemical degradation, strain-controlled corrosion fatigue tests were performed on hollow specimens in simulated boiling water reactor (BWR) water. The direct current potential drop (DCPD) technique was used in-situ to monitor crack initiation while corrosion potential was measured using a reference electrode. The results showed a pronounced reduction in fatigue life compared to air, especially at low strain amplitudes. Fractographic and microstructural analyses of fracture surfaces revealed that crack initiation preferentially occurred along shear planes within grains favorably oriented for slip, in conjunction with localized oxide film rupture and repassivation. These findings support an anodic slip dissolution mechanism and a clear relationship between strain amplitude and surface crack density was observed.

Based on the experimental insights, a crystal plasticity-based constitutive model was developed to simulate the coupled mechanical-electrochemical processes leading to corrosion fatigue crack initiation. Implemented in Abaqus through a UMAT, the model combines a phenomenological crystal plasticity framework with a corrosion damage law based on Gutman’s theory of mechanoelectrochemical interactions. The model parameters were calibrated using the experimental data and simulations were performed on a representative section of the hollow specimen. The model successfully captured key experimental features, including the localization of damage in favorably oriented grains. The predicted cycles to crack initiation agreed well with experimental results across multiple strain amplitudes.

The combined experimental program and modeling framework in this thesis enhance the predictive capabilities for corrosion fatigue crack initiation in nuclear environments. The work contributes to the long-term operation of nuclear reactor components and lays the groundwork for future research incorporating more complex environmental interactions.

This work investigates the interaction between two competing mechanisms on the fatigue life of 304L stainless steel, martensitic transformation and viscoplastic relaxation, as well as the potential fatigue life enhancement of a single hold time applied prior to cyclic loading. At 300 °C, a tensile load hold time of 15 h applied prior to alternating cyclic loading resulted in an increase in mean fatigue life, exceeding 20 % in the studied low cycle fatigue regime. The observed enhancement is primarily attributed to viscoplastic effects during the hold time, which reduces the maximum stress and fatigue crack growth rate in cyclic loading. At room temperature, the opposite effect was observed. A strain-induced martensitic transformation resulted in a secondary cyclic hardening and a brittle final softening phase. The transformation was enhanced by the hold time, which led to increased brittleness and therefore reduced fatigue life. However, viscoplastic relaxation attenuated the detrimental effect of martensite, as was observed by a 15 % decrease in maximum stress. This study not only demonstrates the positive impact of an extended hold time at elevated temperature on the low cycle fatigue behavior but also analyzes underlying competing mechanisms at room temperature through an in-depth experimental investigation.

This study addresses the critical need for a constitutive model to analyze the cyclic plasticity of additively manufactured 316L stainless steel. The anisotropic behavior at both room temperature and 300 °C is investigated experimentally based on cyclic hysteresis loops performed in different orientations with respect to the build direction. A comprehensive constitutive model is proposed, that integrates the Armstrong-Frederick nonlinear kinematic hardening, Voce nonlinear isotropic hardening and Hill's anisotropic yield criterion within a 3D return mapping algorithm. The model was calibrated to specimens in the 0° and 90° orientations and validated with specimens in the 45° orientation. A single set of hardening parameters successfully represented the elastoplastic response for all orientations at room temperature. The algorithm effectively captured the full cyclic hysteresis loops, including historical effects observed in experimental tests. A consistent trend of reduced hardening was observed at elevated temperature, while the 45° specimen orientation consistently exhibited the highest degree of strain hardening. The applicability of the model was demonstrated by computing energy dissipation for stabilized hysteresis loops, which was combined with fatigue tests to propose an energy-based fatigue life prediction model.

This work investigates the influence of a single tensile preload, applied prior to fatigue testing, on the fatigue strength of 316L stainless steel parts manufactured using laser-based powder bed fusion (PBF-LB) with a porosity of up to 4 %. The specimens were produced in both the horizontal and vertical build directions and were optionally preloaded to 85 % and 110 % of the yield strength before conducting the fatigue tests. The results indicate a clear tendency of improved fatigue life and fatigue limit with increasing overload in both cases. The fatigue limits increased by 25.8 % and 24.6 % for the horizontally and vertically built specimens, respectively. Extensive modelling and experiments confirmed that there was no significant alteration in the shape and size of the porosity before and after preloading. Therefore, the observed enhancement in fatigue performance was primarily attributed to the imposed local compressive residual stresses around the defects.

Background: The mechanical characterization of the cyclic elastoplastic response of structural materials at elevated temperatures is crucial for understanding and predicting the fatigue life of components in nuclear reactors. Objective: In this study, a comprehensive mechanical characterization of 304L stainless steel has been performed including metallography, tensile tests, fatigue tests, fatigue crack growth tests and cyclic stress-strain tests. Methods: Isothermal tests were conducted at both room temperature and 300 °C for both the rolling direction and the transverse direction of the hot rolled steel. Mechanical properties were extracted from the uniaxial experiments by fitting relevant material models to the data. The cyclic plasticity behavior has been modelled with a radial return-mapping algorithm that utilizes the Voce nonlinear isotropic hardening model in combination with the Armstrong-Frederick nonlinear kinematic hardening model. The plasticity models are available in commercial FE software and accurately capture the stabilized hysteresis loops, including a substantial Bauschinger effect. Results: The material exhibits near isotropic properties, but its mechanical performance is generally reduced at high temperatures. Specifically, in the rolling direction, the Young’s modulus is reduced by 16 % at 300 °C, the yield strength at 0.2 % plastic strain is lower by 23 %, and the ultimate tensile strength is lower by 30 % compared to room temperature. Fatigue life is also decreased, leading to an accelerated fatigue crack growth rate compared to room temperature. A von Mises radial return mapping algorithm proves to be effective in accurately modelling the cyclic plasticity of the material. The algorithm has also been used to establish a clear correlation between energy dissipation per cycle and cycles to failure, leading to the proposal of an energy-based fatigue life prediction model. Conclusions: The material exhibits reduced mechanical performance at elevated temperatures, with decreased monotonic strength, compared to room temperature. Fatigue life is also compromised, resulting in accelerated fatigue crack growth. The material’s hardening behavior differs at room temperature and elevated temperature, with lower peak stress values observed at higher temperatures. The radial return mapping algorithm can be used to determine the dissipated energy per cycle which together with fatigue testing has been used to propose a low cycle fatigue life prediction model at both temperatures.

A crystal plasticity-based constitutive model is introduced that captures mechanical-electrochemical damage mechanisms in polycrystalline metals, aimed at simulating crack initiation under corrosion fatigue conditions. The finite element-based model integrates classical phenomenological crystal plasticity with an electrochemical kinetics formulation to account for anodic dissolution-driven corrosion. It’s predictive capabilities are evaluated through corrosion fatigue experiments on 304L stainless steel. Hollow cylindrical specimens were tested in a simulated boiling water reactor environment, with a membrane electrode used to monitor corrosion potential and a potential drop technique employed to detect crack initiation. The predicted number of cycles to crack initiation by the model is in good agreement with experimental results. Additionally, it successfully reproduces key characteristics observed in the experiments, including varied corrosion pit morphologies and densities, time-dependent corrosion kinetics and the development of multiple crack initiation sites.

This study investigates the corrosion fatigue behavior of 304L stainless steel under cyclic loading in a simulated boiling water reactor (BWR) water environment. Fully reversed strain-controlled fatigue tests were performed on hollow specimens at various strain amplitudes, revealing a significant reduction in fatigue life, up to an order of magnitude, compared to air environment. The direct current potential drop (DCPD) technique effectively captured crack initiation, while corrosion potential measurements using a copper/copper oxide electrode confirmed strong anodic polarization, consistent with an anodic slip dissolution mechanism. Microstructural characterization through SEM, EBSD and EDS revealed crack initiation along shear planes in favorably oriented grains, localized oxide film rupture and iron oxide repassivation at crack fronts. Fractographic analysis showed increased surface crack density with strain amplitude and evidence of corrosion-assisted crack propagation. Comparisons between solid and hollow specimens indicated similar fatigue performance and internal pressurization had negligible influence on fatigue life. However, copper- and zinc-induced metal embrittlement from brass EDM wire was identified as a potential contributor to premature failure, particularly at lower strain amplitudes.