Low cycle fatigue behavior of a modified 9Cr-1Mo ferritic-martensitic steel in lead-bismuth eutectic at 350 degrees C - Effects of oxygen concentration in the liquid metal and strain rate

The synergistic effects of oxygen concentration and strain rate on the low cycle fatigue behavior of a 9Cr–1Mo steel have been investigated in lead–bismuth eutectic (LBE) at 350 C. The results showed substantial fatigue life reduction due to liquid metal embrittlement (LME), compared to vacuum. Nevertheless, no evident life degradation was observed under a combination of low strain amplitude, high oxygen concentration in LBE and slow strain rate. This is because fatigue crack initiation was delayed by surface oxide films, rather than because LME disappeared. Fatigue crack propagation remained unaffected by oxygen concentration. A new LME mechanism is proposed.status: publishe

Temperature dependence of liquid metal embrittlement susceptibility of a modified 9Cr-1Mo steel under low cycle fatigue in lead-bismuth eutectic at 160-450 °C

Low cycle fatigue properties of a 9Cre1Mo ferritic-martensitic steel (T91) have been tested in a low oxygen concentration (LOC) lead-bismuth eutectic (LBE) environment and in vacuum at 160e450 C. The results show a clear fatigue endurance “trough” in LOC LBE, while no such a strong temperature dependence of the fatigue endurance is observed when the steel is tested in vacuum. The fractographic observations by means of scanning electron microscopy (SEM) show that ductile microdimples are prevalent on the fracture surfaces of the specimens tested in vacuum, whereas the fracture surfaces produced in LOC LBE at all the temperatures are characterized by quasi-cleavage. Interestingly, using electron backscatter diffraction (EBSD), martensitic laths close to the fatigue crack walls or to the fracture surfaces of the specimens tested in vacuum are found to have transformed into very fine equiaxed subgrains. Nevertheless, such microstructural modifications do not happen to the specimens tested in LOC LBE at 160e450 C. These interesting microstructural distinctions indicate that liquid metal embrittlement (LME) is able to occur throughout the fatigue crack propagation phase in the full range of the temperatures investigated, i.e. LME is not very sensitive to temperature during the fatigue crack propagation.status: publishe

Effect of liquid metal embrittlement on low cycle fatigue properties and fatigue crack propagation behavior of a modified 9Cr-1Mo ferritic-martensitic steel in an oxygen-controlled lead-bismuth eutectic environment at 350 °C

The low cycle fatigue properties of a modified 9Cr–1Mo ferritic-martensitic steel (T91) have been tested in stagnant liquid lead-bismuth eutectic (LBE) with oxygen concentrations ranging from 1.16 x 10¯⁶ to 6.0 x 10¯¹⁰ wt% at 350 °C. The effect of liquid metal embrittlement (LME) on fatigue endurance, fatigue crack propagation modes and secondary cracking has been studied. The results showed that the fatigue lives of T91 steel in a low oxygen concentration LBE were drastically reduced compared to those in vacuum due to the presence of LME. The microstructural observations on the fatigue crack propagation modes revealed that fatigue cracks in LBE mainly propagate across prior-austenite grain boundaries and then cut through martensitic lath boundaries, simultaneously leaving a few plastic flow traces and characteristic brittle features. Intergranular and interlath cracking occurred occasionally and their occurrence depended on the orientation of the boundaries relative to the stress axis. The complexity of the LME-induced fracture features can be attributed to a mixture of the multiple failure modes. No obvious plastic shear strain localization was present around the crack tips when LME occurred. However, using a high resolution electron backscatter diffraction (EBSD) technique, highly localized plastic shear strain was observed in the vicinity of the crack tips in vacuum, manifested by the presence of very fine subgrains along the crack walls. A qualitative mechanism was proposed to account for the LME phenomenon in the T91/LBE system. In addition, the secondary cracking at fatigue striations was different in the presence of LBE compared to vacuum. This phenomenon was elucidated by taking into account the influence of the LME on the fatigue crack propagation rate.status: publishe

Multiscale investigation of quasi-brittle fracture characteristics in a 9Cr-1Mo ferritic-martensitic steel embrittled by liquid lead-bismuth under low cycle fatigue

Liquid metal embrittlement (LME) induced quasi-brittle fracture characteristics of a 9Cr–1Mo ferritic–martensitic steel (T91) after fatigue cracking in lead–bismuth eutectic (LBE) have been investigated at various length scales. The results show that the LME fracture morphology is primarily characterized by quasi-brittle translath flat regions partially covered by nanodimples, shallow secondary cracks propagating along the martensitic lath boundaries as well as tear ridges covered by micro dimples. These diverse LME fracture features likely indicate a LME mechanism involving multiple physical processes,such as weakening induced interatomic decohesion at the crack tip and plastic shearing induced nano/micro voiding in the plastic zone.status: publishe

L’exercice de français au primaire et au collège

Dans la structuration ordinaire du travail à l’école, l’exercice est central, si l’on entend par là une démarche didactique fondée sur des supports divers (souvent textuels) qui porte sur un savoir ou un savoir-faire identifié et sont l’objet d’activités précises, matérielles et cognitives, des élèves. Qu’il concerne la découverte, l’approfondissement ou l’évaluation de contenus d’enseignement, l’exercice semble inhérent à la pratique didactique, ce qui en fait un objet récurrent des discours ordinaires sur l’école. Il existe pourtant relativement peu de travaux sur la question dans le champ de la didactique du français, sans doute du fait d’une tradition critique de la didactique à l’égard de l’exercice, mais aussi car d’autres termes, dans les discours didactiques, lui ont été substitués. Pourtant, l’absence d’une appréhension théorique systématique de l’exercice n’empêche pas que ce dernier résiste et perdure dans les pratiques, tant dans les représentations et les discours des différents acteurs de l’école que dans les manuels et dans les classes. Ce numéro de Repères veut contribuer à mieux cerner ce qu’on entend par exercice dans la classe de français, au primaire et au collège. Plus précisément, il s’agit d’identifier quelle peut être l’approche didactique de l’exercice, qui ne saurait être traité indépendamment des contenus en jeu. In the classwork, as it is ordinary structured, we know how important are the exercises, namely the didactic approach based on various mediums (which are often textual), that involves an identified knowledge or know-how and can result in precise, material and cognitive activities of the pupils. Whether it concerns the discovery, the deepening or the evaluation of teaching contents, the exercises seem inherent in didactic practice, and make them a recurrent object of ordinary discourses about school. However, there are not a lot of studies about that topic in the field of didactics of French tongue, probably because of a critical tradition of didactics with regard to the exercises, but also because other terms have been prefered in the didactic studies. But the absence of a systematic theoretical understanding of the exercises does not prevent that they resist and persist in the practices, as well in the representations and the discourse of the various actors of education as in the textbooks and in the classrooms. This issue of Repères aims a better understanding of what is meant by exercice in the French tongue classroom, in primary and lower secondary education. More precisely, the matter is to identify what can be the didactic approach to the exercise, which can not be treated independently of the involved contents