Jajnabalkya Guhathakurta

Zusammenfassung

Abstract Fiber orientation is of paramount importance for the design of fiber-reinforced concrete (FRC) structural elements, because it markedly influences the postcracking properties of such material. For this reason, structural codes introduce orientation factors which aim to correlate the real mechanical properties of the structural element with the ones determined from standard beams. Although the need of considering fiber orientation in design codes is commonly accepted, the orientation factors are still based on a limited number of research studies, raising the need to better determine fiber orientation to improve the current standards and support the design process of FRC elements. In this research, a steel fiber-reinforced concrete (SFRC) with a hybrid system of macro and microfibers is steered into a broad range of fiber orientations and cast into standard beams. Besides measuring the mechanical performance of these SFRC beams, three different methods for assessing fiber orientation are employed, namely electromagnetic induction, image analysis, and micro-computed tomography. The comparison between the outcomes of the different methods provides detailed information about the accuracy and suitability of each method, considering the corresponding domain of applicability at structural level. Finally, a critical review of the most common 2D and 3D orientation parameters found in literature is performed, and the equations are adapted to account for the hybrid system of fibers.

Zusammenfassung

Abstract Graphene and its related materials are increasingly applied in fiber-reinforced polymers to tailor their material properties for high performance composites. Yet, the use of graphene derivatives that underwent a plasma functionalization process is not well understood. This study uses 1.50 wt.\% of unfunctionalized (rGO) or plasma-treated reduced graphene oxide with amine-functionalities (frGO) in an epoxy matrix to prepare unidirectional pre-impregnated carbon fibers. The material characteristics of the graphene-modified carbon fiber-reinforced polymers (g-CFRP) are compared to the neat carbon fiber-reinforced polymer (CFRP). Both g-CFRP configurations exhibit a higher void content than the neat CFRP. No significant difference between the CFRP and the g-CFRPs is observed with respect to the Young's modulus, glass transition temperature, storage modulus, specific heat capacity, thermal conductivity at room temperature, and in-plane electrical conductivity. Yet, a reduction in ultimate tensile strength of up to −13\% is noted. In addition, the apparent interlaminar shear strength and transverse electrical conductivity are increased by up to +12\% for the rGO-CFRP and +52\% for the frGO-CFRP. This knowledge will support the selection of additives for fiber-reinforced polymers for, for example, lightning strike protection in aircrafts, sensory materials, electromagnetic interference shielding or heat transfer elements.

Zusammenfassung

An aqueous fabrication method is investigated for a composite reinforced with chitosan and flax fibers. The composite is characterized structurally, mechanically and chemically. A strong influence of molecular weight (MW) is identified on the composite properties. A strong fiber-matrix interface, which is associated with porosity and effective fiber impregnation, is achieved by applying low molecular weight (LMW) solution followed by casting using LMW or medium molecular weight (MMW) solution. Porosity is analyzed using μ-CT analysis. Increasing porosity with increasing molecular weight results in a decline of the tensile and flexural properties of the composites. The chitosan-flax composites have a low density compared to synthetic and natural fiber composites, which is a competitive advantage as a replacement material for particle board or plyboard in suspended ceilings, furniture compartments, sports or leisure equipment. A multiscale simulation is carried out to compute the directional effective elastic properties and predicts a potential 21% improvement of the tensile modulus if the process is optimized. This work shows the potential of chitosan-flax composites as a sustainable green material with an aqueous fabrication procedure and useful mechanical properties.

Zusammenfassung

For investigations of concentration profiles around bubbles in millichannels as Taylor flow and in bubble columns two in situ real-time process analysis systems were developed. The first system uses laser Raman spectroscopy combined with real-time digital holography. The Raman part of the system is based on a custom pulsed high-energy laser. With this process analysis system, it is possible to measure concentrations of many chemical compounds selectively, with a spatial resolution in the micrometer range during a 10 µs laser pulse. Due to the two combined principles, the determination of the position of a measured local concentration relative to the gas bubble is possible and has been demonstrated. The second real-time process analysis system is especially suited for colored chemical reactions. The system is based on real-time UV/VIS 2D tomography such that with time the third dimension of a 3D concentration profile in the bubble wake can be determined. It consists of fast line sensors illuminated by a laser light sheet. This light sheet originates from a laser spot moving around one third of the bubble column repeatedly. This system is applicable to freely ascending single bubbles as well as bubbly flows in bubble columns.

Zusammenfassung

Due to throughput requirements above 1 gigapixel/sec for the real-time compression of modern image and video data streams, parallelism for encoding and decoding is inevitable. To achieve parallel decoding, a well-established technique is to insert markers into the variable length code (VLC) stream. By locating markers, it is then possible to extract the sub-streams that are, in turn, decoded in parallel. The use of markers adversely affects compression especially when a high degree of parallelism is required. In this paper, we propose an architecture of a marker-free parallel decoding approach of VLC streams. Instead of multiple local entropy decoders, the proposed architecture is based on using a single parallel entropy decoder in conjunction with a novel format to construct the VLC stream. The approach runs at high clock rates supporting parallelism to a high number of decoders. A synthesized clock frequency well above 110 MHz is achieved for up to 20 decoders on a medium-sized FPGA.