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Tribologie des céramiques
(2013)
Les ceramiques modernes sont differentes des ceramiques traditionnelles (carrelage, materiaux de construction, porcelaine); on les distingue par des adjectifs tels que « techniques », « industrielles » ou « thermomecaniques ». Les ceramiques industrielles sont caracterisees par leur caractere refractaire marque et de grandes duretes. Leur mediocre tenacite explique leur sensibilite ä la presence de defauts, d'oü leur reputation d'etre peu resistantes et cassantes.
Les ceramiques techniques sont utilisees pour:
• leurs proprietes electriques ou dielectriques;
• leurs proprietes magnetiques et de supraconducteurs ;
• leurs proprietes optiques;
• leurs proprietes chimiques;
• leurs proprietes biologiques;
• leurs proprietes nucleaires.
En outre, eiles sont employees pour leurs proprietes mecaniques, thermomecaniques et tribologiques, ainsi que pour leur tenue face ä la corrosion. Nous allons nous concentrer sur leur comportement tribologique.
Für die bisher der winterlichen Witterung schutzlos ausgelieferten acht Figurengruppen auf der Schlossbrücke in Berlin wurde innerhalb des vom Landesdenkmalamt Berlin und der Senatsverwaltung für Stadtentwicklung initiierten Forschungsprojektes geprüft, ob und welcher Typ einer Einhausung in der Lage ist, die schädlichen Umwelteinflüsse zu reduzieren. Um den hohen konservatorischen, bauphysikalischen, bautechnischen, künstlerischen und stadträumlichen Anforderungen gerecht zu werden und mögliche Lösungen erstmalig umfassend auf ihre Effektivität zu untersuchen, war die Zusammenarbeit von Denkmalpflegern, Restauratoren, Architekten, Ingenieuren, Geologen, Materialwissenschaftlern und Bauphysikern notwendig. Das durch die Deutsche Bundesstiftung Umwelt (DBU) geförderte Projekt zielte darauf ab, in einem Modellvorhaben mit seinen Partnern, der Technischen Universität Dresden (Institut für Bauklimatik), der Universität Göttingen (Abt. Strukturgeologie und Geodynamik), der Bundesanstalt für Materialforschung und -prüfung (Fachgruppe 7.1, Baustoffe) und unter Beteiligung der Universität der Künste (Fachbereich Konstruktives Entwerfen und Tragwerkslehre) in Berlin, eine innovative und nachhaltige Wintereinhausungskonstruktion auf die besondere Situation der Marmorskulpturen der Schlossbrücke anzupassen und ihre Wirksamkeit zu überprüfen.
Due to their extreme high hydrogen contents, high molecular weight (HMW-) and ultra-high molecular weight (UHMW-) polyethylene (PE) are a comprehensible choice as neutron radiation shielding material in casks for storage and transport of radioactive materials. But as a direct consequence of inserting radioactive material in such casks, gamma radiation occurs. Hence, the impact of gamma radiation on the molecular structure of polyethylene has to be taken into consideration. Consequently, PE has to withstand any type of gamma radiation induced degradation affecting safety relevant aspects in order to be applicable for long term neutron radiation shielding purposes during the whole storage period (in Germany, for instance, up to 40 years). The scope of our investigation comprises an estimation of the impact of gamma radiation and temperature on the molecular and supra molecular structure of the two types of PE used as neutron radiation shielding cask components. A further point which is worth exploring is to what extent these changes are detectable by conventional analysis methods. Therefore, thermoanalytical measurements were performed such as differential scanning calorimetry (DSC), thermo mechanical analysis (TMA), dynamic mechanical analysis (DMA), and thermo gravimetric analysis (TGA). Additionally optical and weighing methods were applied. With those methods it is possible to detect structural changes in polyethylene induced by exposure to gamma radiation. The observed amounts of changes of the irradiated material are not safety relevant for the application of polyethylene as neutron radiation shielding material; moreover, some properties actually improve via irradiation.
As shown recently, the quantification of damage in historic masonry structures is possible by using active thermography. In this paper, a case study is presented concerning systematic studies of the determination of damage size and prognosis of damage increase inside a sandstone column by using different approaches of active thermography. Various heating sources as well as impulse and periodic heating have been compared. Reproducible investigations in regular time intervals for structural monitoring are possible.
Active thermography is sensitive to inhomogeneities at and below the surface of objects investigated. Thus, it should be useful for detecting plaster delaminations on concrete. In this paper, the results of field and laboratory investigations into plaster-covered concrete were compared. For evaluating the bonding state of the plaster it is not sufficient to study only the thermal contrasts at the surface of the investigated objects. The experimental results suggest that the overall thermal behaviour has to be considered.
Pulse thermography is a non-destructive testing method based on infrared imaging of transient thermal patterns. Heating the surface of the structure under test for a short period of time generates a non-stationary temperature distribution and thus a thermal contrast between the defect and the sound material. In modern NDT, a quantitative characterization of hidden imperfections in materials is desired. In particular, defect depth and shape are of interest. The reconstruction of the defect from thermography data is a nonlinear inverse problem, and ill-posed. We propose an algorithm for the identification of subsurface defects based on the travel time of the reflected thermal pulse. Our work extends results by Lugin and Netzelmann, taking lateral thermal flows directly into account while retrieving the defect depth. This requires significantly less computational work. Quantitative information about the defect shape and depth is obtained. Application of our method to both thermography data generated by a finite element simulation and experimental heating of PVC test specimens with different defects yields good reconstruction of the actual defects.
Cina
(2013)
Biolubrifiants
(2013)