Strain-induced crystallization around a crack tip in natural rubber under dynamic load

The local degree of crystallinity around a crack tip in natural rubber under dynamic load was measured by time-resolved scanning wide-angle X-ray diffraction (WAXD). Using a high-flux synchrotron microbeam, WAXD patterns with less than 20 ms exposure time were acquired while the notched rubber sample was subjected to cyclic dynamic stretching at a frequency of 1 Hz, similar to the loading conditions in tear fatigue experiments. By scanning the continuously cycling sample, a complete crystallinity map at any given strain phase angle was obtained. The crystallinity at the crack tip is considerably reduced compared to static crack tip scans. Further investigations revealed the underlying structural reasons for the well-known relation between R-ratio and crack growth resistance. By performing static crack tip scans on increasingly stretched rubber samples, the mechanisms behind crack deflection and branching were studied.     

Entwicklung allgemeiner Entwurfsgrundsätze für Hängernetze von Netzwerkbogenbrücken

Um die statischen Vorteile von Netzwerkbogenbrücken effizient nutzen zu können, ist die Anordnung der Hänger von großer Bedeutung. Der Tragwerksentwurf wird deshalb wesentlich von der Wahl des Hängernetzes bestimmt. Hängerneigung, Hängeranzahl und die Lage der Hängeranschlusspunkte beeinflussen die Spannungszustände sowohl in Bogen und Versteifungsträger als auch in den Hängern in erheblichem Maße. So wird zum Beispiel die Größe der Biegemomente in Bogen und Untergurt durch die Anzahl der Hänger und die damit verbundenen Abstände der Anschlüsse bestimmt. Des Weiteren ist die Hängerneigung ein entscheidender Faktor für die Anzahl der Ausfälle der druckschlaffen Hänger oder die Größe der Spannungsamplituden der Hänger. Nicht minder wichtig ist jedoch auch das ästhetische Erscheinungsbild des Tragwerkes, welches erheblich durch die Art und Weise der Hängeranordnung geprägt wird. Um das effizienteste Hängernetz zu ermitteln, werden fünf mögliche Hängernetze mit variierenden Hängerneigungsparametern, Stützweiten und Bogenhöhen (f/l‐Verhältnisse) hinsichtlich gezielt ausgewählter statischer Kriterien untersucht und bewertet. Daraus resultierend werden Empfehlungen formuliert, die dem Ingenieur die Wahl eines für entsprechende Rahmenbedingungen geeigneten Hängernetzes erleichtern. Abschließend erfolgt ein Vergleich zwischen einer Brücke mit einem nach den erhaltenen Erkenntnissen konstruierten Hängernetz und einem Tragwerk, bei dem die Hänger ähnlich zu bereits ausgeführten Netzwerkbogenbrücken angeordnet sind. Development of general design principles for the hanger arrangements of network arch bridges. To use the static advantages of network arch bridges efficiently, the arrangement of the hangers is very important. The stress conditions in the arch and lower chord are strongly affected by the slope of the hangers, the number of the hangers and the position of the hanger connection points. For example, the size of the bending moments in the arch and lower chord depends on the number of hangers and the associated distances between the connection points. Furthermore, the slope of the hangers is a decisive factor for the number of relaxed hangers and the size of the stress amplitudes. Not less important is the aesthetic view of the structure, which is influenced by the kind of hanger arrangement. To optimize the hanger‐network, different kinds of hanger arrangement methods have been investigated, considering different hanger slopes for each hanger. Five different and possible hanger arrangements with varying parameters (slope of the hangers, number of the hangers, span of the bridge, arch rise) have been analysed in respect to systematic selected static criterions. On the basis of these investigations, recommendations are given for engineering applications how to choose an optimized hanger arrangement according to different geometrical bridge parameters. Finally, a comparison between a network arch, based on the results of the analyses, and a structure with a hanger slope similar to existing network arch bridges is shown to clarify the advantages of the developed hanger arrangement.     

Temperature‐Responsive Substrates: Adhesion and Mechanical Properties of PNIPAM Microgel Films and Their Potential Use as Switchable Cell Culture Substrates (Adv. Funct. Mater. 19/2010)

Thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) microgel films are shown to allow controlled detachment of adsorbed cells via temperature stimuli. Cell response occurs on the timescale of several minutes, is reversible, and allows for harvesting of cells in a mild fashion. The fact that microgels are attached non‐covalently allows using them on a broad variety of (charged) surfaces and is a major advantage as compared to approaches relying on covalent attachment of active films. In the following, the microgels’ physico‐chemical parameters in the adsorbed state and their changes upon temperature variation are studied in order to gain a deeper understanding of the involved phenomena. By means of atomic force microscopy (AFM), the water content, mechanical properties, and adhesion forces of the microgel films are studied as a function of temperature. The analysis shows that these properties change drastically when crossing the critical temperature of the polymer film, which is the basis of the fast cell response upon temperature changes. Furthermore, nanoscale mechanical analysis shows that the films posses a nanoscopic gradient in mechanical properties.     

Energieeinsparpotential eines Passivhauses unter Berücksichtigung von Wetterprognosen

Innovative Regelungen in Gebäuden können beachtliche Energieeinsparungen zur Folge haben. Besonderes Augenmerk ist hierbei auf Gebäude gelegt, die in einem gesteigerten Maße von ihrer Umwelt thermisch entkoppelt sind: dazu wird ein nach Passivhausstandard gebautes, real existierendes Bürogebäude modelliert, das Modell mit Monitoring‐Daten validiert und simuliert. Anhand der Simulationen mit der Simulationsumgebung TRNSYS und der Hinzunahme von gemessenen Wetterdaten werden mögliche Energieeinsparpotentiale erkannt und ausgewertet. In dieser Betrachtung, die den Zeitraum eines kompletten Jahres (Zeitraum 2009/2010) beinhaltet, liegt der Fokus auf Energieeinsparungen, die durch intelligentes Heizen und Kühlen des Gebäudes erzielt werden können. Potential for saving energy in a Passivhaus building, taking into account weather forecasts. Innovative controls in buildings can lead to remarkable energy savings. The main focus of this paper is on buildings that are highly thermally insulated from their environment: the model is based on an existing building that was built to Passivhaus standard, and then validated with monitoring data and simulated. By adding actual, measured weather data in the simulations (which were done in the TRNSYS simulation environment) potential energy savings are identified and evaluated. In this analysis, which covers a full year (2009/2010), we concentrate on energy savings in the building’s heating and cooling systems.     

马达设计的技巧

布局约束因素 现今的可调速驱动电路都采用变频器来调整输出电流,以满足三相马达的要求。变频器的形状大小通常会受到应用的限制。在许多情况下,电路板与马达靠得很近,而马达构造的高度也会受限。另外,所用高功率半导体器件的物理性质和所选封装的形状,也要求电路板上有足够的位置空间。     

The relationship between thin film fragmentation and buckle formation: Synchrotron-based in situ studies and two-dimensional stress analysis

Fragmentation and buckling of Ta layers with thicknesses of 50, 100, and 200 nm on polyimide substrates was studied by in situ light microscopy and synchrotron analysis. Buckling indicates the presence of compressive stress, which cannot be explained exclusively by a Poisson ratio mismatch. We extended the classical shear lag model and derived a rigorous analytical solution for the biaxial stress field in a single fragment attached to a uniaxially loaded substrate. The presence of cracks not only gives rise to tensile stress relaxation, but also induces compressive stress in the perpendicular direction, which eventually leads to film buckling. The validity of the model has been confirmed using a synchrotron-based technique for the in situ determination of the biaxial coating stress during uniaxial tensile testing. Taking into account the mean crack distance as a function of the applied strain, the model is utilized to determine the residual stress and fracture toughness of thin films.     

Transport Properties of Poly(lactic acid)/Clay Nanocomposites

Poly(lactic acid) (PLA) is one of the most promising sustainable alternatives to petroleum‐based polymers. One of the main applications is in the packaging industry. Therefore, this article deals with the evaluation of gas transport properties polylactic acid 2003D nanocomposite. Montmorillonite‐based fillers, Cloisite® 10A, 20A, 30B and natural Cloisite® Na⁺, intended to enhance the transport properties, were incorporated into a series of PLA films. The influence of the additive on the transport properties was studied under the conditions of melt processing. Further material morphology was assessed using X‐ray diffraction and transmission electron microscopy. The best results achieved compositions with PLA/Cloisite10A and Cloisite 30B with improvement more than 50%. POLYM. ENG. SCI., 59:2498–2501, 2019. © 2019 Society of Plastics Engineers     

Direktes Herstellungsverfahren von Komposit‐Hohlfasermembranen mit sinusförmiger Geometrie

Composite hollow fiber membranes are fabricated in a one‐step process by superimposing the phase inversion process with interfacial polymerization on the lumen side of the fiber. A pulsation module in the bore fluid channel pulsates the bore fluid flow and leads to hollow fiber membranes with sinusoidal geometry. The fabrication of composite hollow fiber membranes with sinusoidal geometry is focus of this work. The sinusoidal geometry leads to reduced concentration polarization effects in membrane applications.     

Hydrothermal Dissolution of Low‐Quartz in Sodium Hydroxide Lyes: Kinetics and Equilibrium

Quartz flours were dissolved at temperatures between 100 and 220 °C for up to 7 d in NaOH lyes. The starting materials were characterized by particle size analysis. Liquid and solid reaction products were characterized chemically and structurally. Sodium water glasses with silica concentrations of up to 20 wt % were obtained. Radius reduction of size distributed particle ensembles was calculated to evaluate the concentration control of the dissolution rate which is due to saturation effects. An equilibrium between low‐quartz and the resulting water glass explains the obtained results.     

Accurate event-driven motion compensation in high-resolution PET incorporating scattered and random events

With continuing improvements in spatial resolution of positron emission tomography (PET) scanners, small patient movements during PET imaging become a significant source of resolution degradation. This work develops and investigates a comprehensive formalism for accurate motion-compensated reconstruction which at the same time is very feasible in the context of high-resolution PET. In particular, this paper proposes an effective method to incorporate presence of scattered and random coincidences in the context of motion (which is similarly applicable to various other motion correction schemes). The overall reconstruction framework takes into consideration missing projection data which are not detected due to motion, and additionally, incorporates information from all detected events, including those which fall outside the field-of-view following motion correction. The proposed approach has been extensively validated using phantom experiments as well as realistic simulations of a new mathematical brain phantom developed in this work, and the results for a dynamic patient study are also presented.