58 research outputs found
Etude expérimentale et modélisation des effets de l’hydrogène sur les propriétés mécaniques et le comportement en fatigue d’un acier à haute limite d’élasticité
No full textThe sensitivity to hydrogen embrittlement of S690QL steels is studied by local approach to fracture. Fatigue tests in air and in saline solution under cathodic protection are performed on micro-notched specimens. Crack is monitored by direct current potential drop method. The mechanical response of the material at the notch-tip is simulated by finite elements calculations.Fatigue mechanisms are investigated by a decomposition method of the flow stress to extract internal stress and the thermal and athermal components of the effective stress. Hydrogen effects on these stresses are also studied.Hydrogen trapping in the microstructure is investigated using electrochemical permeation tests and thermal desorption spectrometry. A phenomenological model is developed thanks to experimental results. This model is able to simulate the local hydrogen concentration in samples with complex geometry.Our results show that crack initiation is highly dependent on the plastic strain accumulation. For fatigue tests on micro-notched specimens, hydrogen trapping in dislocations elastic field is the predominant phenomena in the embrittlement mechanism. For smooth specimens under low cycle fatigue testing, hydrogen trapping in dislocations cells walls is significant and crack initiation is probably localized in this microstructural defect. Both components of the effective stress are also influenced by hydrogen.La sensibilité à l’hydrogène d’un acier de type S690QL a été étudiée par une approche locale de la rupture. Des essais de fatigue ont été réalisés à l’air et en milieu salin sous protection cathodique sur des éprouvettes micro-entaillées. La fissure a été contrôlée en cours d’essai par une méthode de suivi électrique. La réponse mécanique du matériau en avant de l’entaille a été simulée par calculs par éléments finis.Les mécanismes de fatigue ont été étudiés par une méthode permettant d’isoler une contrainte interne et deux composantes de la contrainte effective, activée et non-activée thermiquement. L’effet de l’hydrogène sur ces contraintes a également été étudié.Pour comprendre le phénomène de piégeage de l’hydrogène dans la microstructure de cet acier, des tests de perméation électrochimique et de désorption thermique ont été réalisés. Les paramètres expérimentaux ainsi déterminés sont utilisés dans un modèle phénoménologique de la diffusion. Il a été développé pour simuler la répartition locale de l’hydrogène sur des géométries complexes.Les résultats montrent une forte dépendance de l’amorçage de la fissure avec l’accumulation de déformation plastique. Pour les essais de fatigue réalisés sur les éprouvettes entaillées, il ressort que le piégeage dans le champ élastique des dislocations est le phénomène prépondérant dans le mécanisme de fragilisation. Dans le cas d’éprouvettes lisses sollicitées en fatigue oligocyclique, le piégeage dans les murs de cellules de dislocations est majoritaire et l’amorçage de la fissure est certainement localisé sur ces défauts. L’hydrogène affecte également les deux composantes, thermique et athermique, de la contrainte effective
Experimental and numerical investigations of hydrogen effects on mechanical properties and fatigue behavior of high strength steel
No full textLa sensibilité à l’hydrogène d’un acier de type S690QL a été étudiée par une approche locale de la rupture. Des essais de fatigue ont été réalisés à l’air et en milieu salin sous protection cathodique sur des éprouvettes micro-entaillées. La fissure a été contrôlée en cours d’essai par une méthode de suivi électrique. La réponse mécanique du matériau en avant de l’entaille a été simulée par calculs par éléments finis.Les mécanismes de fatigue ont été étudiés par une méthode permettant d’isoler une contrainte interne et deux composantes de la contrainte effective, activée et non-activée thermiquement. L’effet de l’hydrogène sur ces contraintes a également été étudié.Pour comprendre le phénomène de piégeage de l’hydrogène dans la microstructure de cet acier, des tests de perméation électrochimique et de désorption thermique ont été réalisés. Les paramètres expérimentaux ainsi déterminés sont utilisés dans un modèle phénoménologique de la diffusion. Il a été développé pour simuler la répartition locale de l’hydrogène sur des géométries complexes.Les résultats montrent une forte dépendance de l’amorçage de la fissure avec l’accumulation de déformation plastique. Pour les essais de fatigue réalisés sur les éprouvettes entaillées, il ressort que le piégeage dans le champ élastique des dislocations est le phénomène prépondérant dans le mécanisme de fragilisation. Dans le cas d’éprouvettes lisses sollicitées en fatigue oligocyclique, le piégeage dans les murs de cellules de dislocations est majoritaire et l’amorçage de la fissure est certainement localisé sur ces défauts. L’hydrogène affecte également les deux composantes, thermique et athermique, de la contrainte effective.The sensitivity to hydrogen embrittlement of S690QL steels is studied by local approach to fracture. Fatigue tests in air and in saline solution under cathodic protection are performed on micro-notched specimens. Crack is monitored by direct current potential drop method. The mechanical response of the material at the notch-tip is simulated by finite elements calculations.Fatigue mechanisms are investigated by a decomposition method of the flow stress to extract internal stress and the thermal and athermal components of the effective stress. Hydrogen effects on these stresses are also studied.Hydrogen trapping in the microstructure is investigated using electrochemical permeation tests and thermal desorption spectrometry. A phenomenological model is developed thanks to experimental results. This model is able to simulate the local hydrogen concentration in samples with complex geometry.Our results show that crack initiation is highly dependent on the plastic strain accumulation. For fatigue tests on micro-notched specimens, hydrogen trapping in dislocations elastic field is the predominant phenomena in the embrittlement mechanism. For smooth specimens under low cycle fatigue testing, hydrogen trapping in dislocations cells walls is significant and crack initiation is probably localized in this microstructural defect. Both components of the effective stress are also influenced by hydrogen
Microstructural aspects of Ti6Al4V degradation in H2O2-containing phosphate buffered saline
No full textTi6Al4V surfaces were exposed to simulated inflammation conditions in H2O2-containing phosphate buffered saline with and without FeCl3. Scanning electron microscopy analysis revealed significantly different degradation modes for the α and β phases. While the α grains are covered by a ca. 400 nm thick protective nanostructured oxide layer, the attack of the β phase generates a porous microstructure with microscaled cracks and a low polarization resistance. The β phase is postulated to be sensitive to H2O2 reduction products and less able to generate a passive oxide film. The presence of FeCl3 enhances the cathodic activity and the β phase degradation
Quantification of Hydrogen Flux from Atmospheric Corrosion of Steel Using the Scanning Kelvin Probe Technique
No full textThe atmospheric corrosion of high-strength steels can lead to hydrogen absorption directly linked to hydrogen embrittlement or delayed fracture phenomena. A scanning Kelvin probe (SKP) and electrochemical permeation technique (EPT) were applied to correlate the potential of an oxidized surface with the flux of hydrogen across a thin steel membrane. The side of the membrane opposite the corroding or electrochemically charged area was analyzed. The potential drop in the oxide was calibrated in terms of surface hydrogen activity, and SKP can be applied in situ for the mapping of hydrogen distribution in the corroding metal. A very low flux of hydrogen can be characterized and quantified by SKP, which is typically observed under atmospheric corrosion conditions. Therefore, hydrogen localization that drives steel durability under atmospheric corrosion conditions can be evaluated
Experimental and numerical analysis of hydrogen interaction with plastic strain in a high strength steel
No full text8 pagesInternational audienceCyclic loading tests were performed on micro-notched samples of high-strength steel S690QT in air and under cathodic polarisation in a saline solution. These specimens were modelled and their behaviour simulated by finite elements calculations with a combined nonlinear isotropic-kinematic hardening constitutive law. This model can simulate cyclic softening and ratcheting effect of the high-strength steel. Stress and strain fields in the vicinity of the notch-tip were calculated. Results show that a strong dependence of the crack initiation with plastic strain accumulation. Hydrogen assisted cracking mechanism is discussed based on arrangements of dislocations structures
Effects of Cyclic Plastic Strain on Hydrogen Environment Assisted Cracking in High-Strength Steel
No full text8 pagesInternational audienceCyclic loading tests were performed on micro-notched samples of high-strength steel S690QL (standard EN 10137-2) in air and under cathodic polarization in a saline solution. These specimens were modeled and their behavior simulated by finite elements calculations with a combined nonlinear isotropic-kinematic hardening constitutive law. This model can simulate cyclic softening and ratcheting effect of the high-strength steel. Stress and strain fields in the vicinity of the notch-tip were calculated. Results show a strong dependence of the crack initiation with plastic strain accumulation. Hydrogen assisted cracking mechanism is discussed based on arrangements of dislocations structures
Effects of Cyclic Plastic Strain on Hydrogen Environment Assisted Cracking in High-Strength Steel
No full textInternational audienc
Hydrogen entry and distribution in steel : Assessments by different local electrochemical techniques
No full textThe sensitivity of high strength steels (HSS) to hydrogen embrittlement is well established. Cracking is likely to occur even at very low hydrogen content. Hence, it is important to develop new and very sensitive methods (e.g., with high lateral resolution) to study the sources of hydrogen, the distribution of hydrogen in the metal and the mechanisms of hydrogen interaction with microstructural defects and with the surface oxide. This report reviews the application of the localized electrochemical techniques such as Scanning Kelvin Probe (SKP), Scanning Vibration Electrode Technique (SVET), Local Electrochemical Impedance Spectroscopy (LEIS), and Scanning Electrochemical Microscopies (SECM) to study the hydrogenation of austenitic stainless steel, HSS and galvanized HSS under different experimental conditions. For example, SKP can detect 0.01 ppm of atomic hydrogen. This low detection limit is based on the nanoscale interaction of emerged hydrogen with the surface oxide film. Hydrogen diffuses and decreases the Volta potential of the surface at the locations of the emerging due to the reduction of Fe3+ species in the oxide film. It enables real time monitoring of the reduction of oxide film that is related to the failure of the steel passivity. SKP measurements are carried out in air and provide information on the relative hydrogenation due to atmospheric corrosion. In the case of LEIS, hydrogen interacts with surface oxide increasing the capacitance and decreasing the resistance of the film that can be monitored in a water electrolyte. SECM is able to map hydrogen distribution in the electrolyte in the vicinity of the steel surface. Advantages and disadvantages of the different techniques are discussed. The combined effect of mechanical stress and the hydrogen on the steel passivity is also under the scope of this review.</p
Experimental investigations of internal and effective stresses during fatigue loading of high-strength steel
No full textInternational audienceLow cycle fatigue tests are performed on a high strength tempered martensitic steel at different plastic strain amplitudes at room temperature. Internal and effective components of the flow stress are analyzed using Handfield and Dickson's method. The internal stress is affected by the plastic strain amplitude. Conversely, the evolution of the athermal component of the effective stress with the number of cycles is independent of the plastic strain amplitude. The thermal part of the effective stress increases with the plastic strain amplitude, but remains constant with plastic strain accumulation. Microstructural changes in the cyclically deformed material are investigated by means of transmission electronic mycroscopy and X-Ray characterizations. Internal and effective stress evolutions are discussed based on these observation
Scanning Kelvin Probe Investigation of High-Strength Steel Surface after Impact of Hydrogen and Tensile Strain
No full textHydrogen in combination with mechanical stress can lead to rapid degradation of high-strength steels through environmentally assisted cracking mechanisms. The scanning Kelvin probe (SKP) was applied to automotive martensitic steel grade MS1500 in order to detect local reactivity of the surface after hydrogen uptake and tensile deformation. Hydrogen and stress distribution in microstructures can be characterized by SKP indirectly measuring the potential drop in the surface oxide. Thus, the links between electron work function, oxide condition, and subsurface accumulation of hydrogen and stress have to be investigated. It was shown that plastic strain can mechanically break down the oxide film creating active (low potential) locations. Hydrogen effusion from the steel bulk, after cathodic charging in aqueous electrolyte, reduced the surface oxide and also decreased potential. It was shown that surface re-oxidation was delayed as a function of the current density and duration of cathodic hydrogen pre-charging. Thus, potential evolution during exposure in air can characterize the relative amount of subsurface hydrogen. SKP mapping of martensitic microstructure with locally developed residual stress and accumulated hydrogen displayed the lowest potential
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