Experimental and numerical characterization of anisotropic damage evolution of forged Al6061-T6 alloy

AbstractAluminum alloy 6061-T6 (Al-Mg-Si) has been selected as the material of the vessel for the construction of Jules-Horowitz material testing reactor. Fracture mechanism of this alloy has been investigated using mechanical testing of smooth and notched tensile specimens loaded in different directions. A strong anisotropic fracture behavior has been observed. Microstructural studies using tomography and image analysis have shown a presence of anisotropic distributed coarse precipitates which is the key microstructural feature affecting the damage evolution. These observations were complemented by investigations on fractured tensile samples. A damage scenario of anisotropic growth and coalescence of voids is proposed to explain the fracture behavior associated with the distribution of precipitates. A GTN (Gurson-Tvergaard-Needleman) damage model is used to simulate this scenario and to predict damage evolution

Understanding the nature of variations in postgraduate learners’ willingness to communicate in English

Willingness to communicate in a second language (L2 WTC) is a learner’s volitional participation in oral communication using L2. Previous research has expended considerable attention to the stable, trait-like disposition of learners’ L2 WTC, while less focus has been accorded to the complex nature of variations in L2 WTC on multiple timescales. Using dynamic systems theory, the present article examines the complex nature of variations on three timescales: during conversation, between classes and over time. The data were obtained from six postgraduate students through structured classroom observations, learners’ diaries, stimulated recalls and biographic questionnaires. The findings show that while variations in L2 WTC within and between the classes were influenced by situational variables, such as interlocutors, topic and perceived opportunity, variations over 14 classes were strongly affected by enduring factors, such as personality and trait-level motivation. Introverted learners showed dependency on their interlocutors to provide them with opportunities for L2 use. Conversely, extraverted learners proactively exploited opportunities to use L2 regardless of the nature of topic of discussion and the behaviour of interlocutors. The study also discusses a number of implications for language teaching and teacher education

Pre-study of the hydraulic works for the Seine-Scheldt-East Project

peer reviewe

Étude de l'influence des paramètres d'extrusion sur la morphologie du polypropylène isotactique par spectroscopie Raman

L'étude porte sur la caractérisation microstructurale d'un matériau polymère par la spectroscopie Raman. Les objectifs du travail sont de justifier l'utilisation de la spectroscopie Raman comme une technique fiable pour la caractérisation des matériaux polymères et de mieux comprendre la formation des structures hétérogènes de type cœur/peau issues des conditions de mise en forme (extrusion). Nous nous sommes particulièrement focalisés sur les paramètres opératoires suivants : la nature et la température du fluide de refroidissement et l'étirage mécanique de la matière en sortie de filière

Volume Variation Process of High-Density Polyethylene During Tensile and Creep Tests

Samples of high-density polyethylene have been subjected to tensile tests and creep experiments by means of a video-controlled testing system (VidéoTraction ©). The evolution of specific volume in this semi-crystalline polymer is determined versus true strain. In the elastic stage, we measure a hydrostatic expansion and then, in the plastic stage, we observe a competition between a compaction effect and a dilatation phenomenon. Although compaction is probably overestimated in the present testing technique, it represents a pertinent mechanism that is ascribed to the orientation of the amorphous chains during stretching. This phenomenon is characterized by X-ray diffraction measurements that show a reduction of average distance between amorphous chains. Dilatation process is explained by the diminution of crystallinity and by the formation, growth and coalescence of crazes inside and between spherulites. Electron microscopy reveals these defects. The competition between compaction and dilatation, controlled by the mobility of the amorphous phase, depends on temperature and time