In situ X-ray microtomography characterization of damage in SiCf/SiC minicomposites

The purpose of the present study is to characterize matrix crack propagation and fiber breaking occurrences within SiC/SiC minicomposite in order to validate later on a multiscale damage model at the local scale. An in situ X-ray microtomography tensile test was performed at the European Synchrotron Radiation Facility (ESRF, ID19 beamline) in order to obtain 3-dimensional (3D) images at six successive loading levels. Results reveal a slow and discontinuous propagation of matrix cracks, even after the occurrence of matrix crack saturation. A few fiber failures were also observed. However, radiographs of the whole length (14 mm) of the minicomposites under a load and after the failure were more appropriate to get statistical data about fiber breaking. Thus, observations before the ultimate failure revealed only a few fibers breaking homogeneously along the minicomposite. In addition, an increase in fiber breaking density in the vicinity of the fatal matrix crack was observed after failure. These experimental results are discussed in regards to assumptions used in usual 1-dimensional (1D) models for minicomposites.     

Universitärer Forschungscluster „Sustainable Technologies in Metal Production and Processing” (STMP)

Foundation of a university research cluster — Co-operation along the value creation chain from raw materials to components — Evidence of joint performance offers — Utilization of synergies — Integration of Christian Doppler Laboratories — Flexible arrangement of co-operations and networks     

Crystallization of poly(ethylene terephthalate)/polycarbonate blends. I. Unreinforced systems

The crystallization and transition temperatures of poly(ethylene terephthalate) (PET) in blends with polycarbonate (PC) is considered using thermal analysis. Additives typically used in commercial polyester blends, transesterification inhibitor and antioxidant, are found to enhance the crystallization rate of PET. Differential scanning calorimetry (DSC) reveals two glass transition temperatures in PET/PC blends, consistent with an immiscible blend. Optical microscopy observations are also consistent with an immiscible blend. Small shifts observed in the T_g of each component may be due to interactions between the phases. The degree of crystallinity of PET in PET/PC blends is significantly depressed for high PC contents. Also, in blends with PC content greater than 60 wt %, two distinct crystallization exotherms are observed in dynamic crystallization from the melt. The isothermal crystallization kinetics of PET, PET modified with blend additives, and PET in PET/PC blends have been evaluated using DSC and the data analyzed using the Avrami model. The crystallization of PET in these systems is found to deviate from the Avrami prediction in the later stages of crystallization. Isothermal crystallization data are found to superimpose when plotted as a function of time divided by crystallization half-time. A weighted series Avrami model is found to describe the crystallization of PET and PET/PC blends during all stages of crystallization. © 1996 John Wiley & Sons, Inc.     

Technology Assessment on Renewable Raw Materials: Potentials and Risks of the Use of Starches

Starch as a renewable raw material for chemical/industrial uses has a long tradition. In terms of quantity, starch ranks third behind wood and vegetable oils. In the medium term, this position will be maintained, if not improved. The volume of renewable raw materials used for fuel is even greater and expected to increase in the medium term. By means of a technology assessment on chemical/industrial and energy uses of renewable raw materials, it shall be demonstrated where the specific potentials and risks of starch use can be found. Among others, the following aspects are considered: energy and CO₂ balance sheets, biodegradability and competiveness. Under the headings of „saving of raw materials (degree of self-sufficiency)”︁, „area required (reduction of surpluses)”︁, „employment effects”︁, „contribution to CO₂ reduction”︁ and „need for subsidies”︁, medium-term (up to 2005) potential use of starch in Germany is studied relative to use of renewable raw materials in total.     

CeO2−CoO Phase Diagram

Phase equilibria in the CeO₂−CoO system at temperatures above 1500°C were investigated. The microstructures and the phase compositions of the DTA (differential thermal analysis) samples and the quenched solid pellets were analyzed using SEM (scanning electron microscope), EDX (energy dispersive X-ray), and WDX (wavelength dispersive X-ray). A eutectic reaction was found at 1645 ± 5°C. The eutectic point was calculated to be at 82 ± 1.5 mol% CoO. The eutectic phases were the CeO₂-rich phase (containing <5 mol% CoO) and the CoO-rich phase (containing ∼0.5 mol% CeO₂). At 1580°C, the solubility of CoO in CeO₂ was ∼3 mol%.     

Modeling of the Twin-Roll Casting Process: Transition from Casting to Rolling

During twin-roll casting, an alloy melt is passing the gap between two counter-rotating rolls, where cooling and solidification leads to the continuous formation of a solid strand. In order to describe this process, a two-phase Eulerian–Eulerian volume-averaging model is presented that accounts for (1) transport and growth of spherical grains within a flowing melt, (2) the formation of a coherent solid network above a specific solid fraction and (3) the viscoplastic flow of the solid network with the interstitial melt during casting and compression. For the considered case of an inoculated Al–4wt%Cu alloy, the process conditions are chosen such that two relatively thick viscoplastic semi-solid shells meet between the rolls, and thus, the material is pressed together and squeezed against the casting direction. The squeezed out material consists of segregated melt and some solid that quickly disappears after melting. It is observed during this study that macrosegregation distributions are inherently connected to the mush deformation that is enforced during the hot rolling process.