Nicolas Larrosa

Corrosion-fatigue: a review of damage tolerance models

ABSTRACT The synergistic combination of mechanical fatigue stresses and environmental agents acting together can be more detrimental than that of the summation of the contributions of each mechanism acting separately. Major attempts to understand the contribution of the different agents (microstructure, chemical composition of environment, temperature, loading conditions, etc.) have been reported in the literature. Nevertheless, current knowledge is insufficient to address life estimation with a sound physical basis from the initiation of localised corrosion (such as pitting) to the estimation of crack propagation. Major simplifications and assumptions have been required in the development of life prediction methodologies. This paper reviews recent efforts made by the different interested parties, in both academia and industry, in the development of corrosion fatigue (CF) lifetime prediction procedures. The paper mainly focuses on the methodologies proposed in the literature for oil and gas, nuclear, energy generation and aerospace applications, dealing with pitting CF damage in aluminium alloys, carbon and stainless steels. The transition of a pit into a small crack (SC) and its growth is influenced by the interaction of the pit stress/strain concentration and the local environmental conditions, making the modelling of this stage of the utmost complexity. A major trend in the models reviewed in this paper is to simplify the analysis by assuming the pit (a volumetric defect) as a sharp crack, decouple the CF problem and account for the mechanical and environmental contributions separately. These procedures heavily rely on fitting experimental data and exhibit low generality in terms of application to varying system conditions. There is a clear opportunity in this field to develop mechanistically based methodologies, considering the inherent dependence of the damage mechanism on the interaction of environmental, metallurgical and mechanical features, allowing more realistic lifetime estimates and defect tolerance arguments, where pit-to-crack transition and SC initiation stages pose a significant challenge. Abbreviation: ASME: American Society of Mechanical Engineers; API: American Petroleum Institute; BP: British Petroleum; BS: British Standards; BWR: Boiling Water Reactor; CF: Corrosion fatigue; DNV: Det Norske Veritas; FCGR: Fatigue crack growth rate; FCI: Fatigue crack initiation; FCP: Fatigue crack propagation; FFS: Fitness for service; HA: Hydrogen assisted; HRR: Hutchinson, Rice and Rosengren stress fields; LC: Long crack; LEFM: Linear Elastic Fracture Mechanics; S-N: Stress vs. number of cycles

Ductile fracture modelling and J-Q fracture mechanics: a constraint based fracture assessment approach

The reduced state of stress triaxiality observed in shallow cracked components allows an increased capacity to resist crack propagation compared to that observed in deeply cracked specimens. This may be regarded as a higher fracture toughness value which allows a reduction in the inherent conservatism when assessing components in low constraint conditions. This study uses a two-parameter fracture mechanics approach (JQ) to quantify the level of constraint in a component (e.g. a pipe with a surface crack) and in fracture test specimens, i.e. single edge tension [SE(T]) and compact tension [C(T)] specimens, of varying constraint level. The level of constraint of the component is matched to a specific test specimen and therefore the ability of the structure to resist fracture is given by the fracture toughness of the test specimen with a similar J-Q response. Fracture toughness values for different specimens have been obtained from tearing resistance curves (J-R curves) constructed by means of a virtual testing framework. The proposed engineering approach shows that the combination of a local approach and two-parameter fracture mechanics can be used as a platform to perform more accurate fracture assessments of defects in structures with reduced constraint conditions. KEYWORDS. Crack tip constraint; Ductile fracture modelling; J-Q fracture mechanics.

Assessing fatigue endurance limit of pitted specimens by means of an integrated fracture mechanics approach

From an operator/engineering perspective, the correct assessment of the severity of corrosion defects (e.g., pits) can have enormous economic, social and environmental benefits; therefore the development of a generally applicable and simple to apply procedure for fatigue assessment of key components is recognised as a valuable tool, seeking to reduce the current overly conservative procedures whilst maintaining structural integrity. The critical condition for a crack emanating from a pit (pit-to-crack transition) to start to propagate is analysed in this paper. The pitcrack configuration is re-characterized into that of a hemispherical crack of length equal to the pit depth, and this assumption is analysed by detailed 3D FEA. A propagation threshold approach is used to estimate the fatigue resistance from intrinsic material properties. The proposed approach is validated by comparison with experimental results available in the open literature.Copyright

Characterization of the effect of notch bluntness on hydrogen embrittlement and fracture behavior using FE analyses

This paper introduces a method to characterize the effect of notch bluntness on hydrogen embrittlement for high strength structural steel, FeE 690T, C(T) specimens. Hydrogen concentration depending on notch radius is assessed via finite element (FE) hydrogen diffusion analysis already developed and validated by the authors. Reduction in fracture toughness, KIC or JIC, due to hydrogen embrittlement is evaluated by means of a coupled hydrogen diffusion-ductile damage analysis. The ductile damage simulation used in this study is based on the model known as ‘stress-modified fracture strain model’. Tensile properties and fracture strains are modified according to the level of hydrogen concentration in the simulation and its effect on the fracture behavior of the specimen is simulated for different notch radii.Copyright