Effect of yarn cross-sectional shape on resin flow through inter-yarn gaps in textile reinforcements

Axial flow through gaps between aligned straight yarns with realistic cross-sectional shapes, described by power-ellipses, was analysed numerically. At a given fibre volume fraction, equivalent gap permeabilities have a maximum at minimum size of elongated tapering parts of the gap cross-section and a ratio of gap width to height near 1. When the yarn spacing is given in addition to the fibre volume fraction, calculated maximum and minimum values for the equivalent permeability of inter-yarn gaps, which occur at near-rectangular and lenticular cross-sections, differ by factors of up to 3.3. Novel approximations for the shape factor and the hydraulic diameter in Poiseuille flow were derived as a function of the fibre volume fraction, the yarn cross-sectional aspect ratio and the exponent describing the shape of the power-elliptical yarn cross-section. This allows the equivalent gap permeability to be predicted with good accuracy for any fibre volume fraction and yarn cross-section

Effect of specimen history on structure and in-plane permeability of woven fabrics

Before being processed into composites, reinforcement fabrics may undergo repeated involuntary deformation, the complete sequence of which is here referred to as specimen history. To mimic its effect, fabric specimens were subjected to sequences of defined shear operations. For single fabric layers with unconstrained thickness, quantitative evaluation of photographic image data indicated that repeated shear deformation results in a residual increase in inter-yarn gap width. This translates into an increase in measured fabric permeabilities in multi-layer lay-ups at given compaction levels. The extent of both interrelated effects increases with increasing yarn density in the fabric and with increasing maximum angle in the shear history. Additional numerical permeability predictions indicated that the increase in permeability may be partially reversed by through-thickness fabric compression. The observations suggest that the effect of involuntary deformation of the fabric structure can result in variations in the principal permeability values by factors of up to 2

Influence of the micro-structure on saturated transverse flow in fibre arrays

This study analyses the influence of the random filament arrangement in fibre bundles on the resin flow behaviour. Transverse steady-state resin flow which occurs behind a liquid resin flow front was simulated numerically through statistically equivalent micro-structures at high fibre volume fractions, Vf >0.6, as observed in fibre bundles. The need of applying a minimum gap distance between neighbouring filaments was overcome by automated local mesh refinement. The derived permeability values showed significant scatter. Convergence of these values was determined at a ratio of flow length to filament radius greater than 20 for all three analysed fibre volume fractions. Mean permeabilities were between 6 and 10 times lower than those predicted for a hexagonal fibre array. A statistical model is proposed which is able to predict the scatter of observed permeabilities based on simple micro-structural descriptors

Konzepte zur Erfassung der kulturellen Marginalität von Minderheiten

Einleitend werden die Begriffe Minderheit und Marginalität so definiert, daß von allgemeinen Begriffen ausgegangen wird. Zugleich wird vorgeschlagen, Minderheit und Marginalität deutlich zu trennen. Anschließend werden drei Typen von kultureller Marginalität beschrieben: die statische Marginalität; die dynamisch-zentrifugale Marginalität; die dynamisch-zentripetale Marginalität. Im fünften Abschnitt werden dann die Bedingungen und Funktionen der kulturellen Marginalität von Minderheiten skizziert, um abschließend Möglichkeiten der Überwindung zu diskutieren. Dabei werden im Hinblick auf die Kulturelle Integration vier Gesellschaftstypen unterschieden: demokratisch-integriert; oligarchisch-integriert; dissoziativ-desintegriert; förderalistisch-desintegriert. (RW

Multiscale modeling of combined deterministic and stochastic fabric non-uniformity for realistic resin injection simulation

The local fiber arrangement in a bi-directional fabric formed to a complex shape was modeled considering the stochastic arrangement of filaments within yarns, which determines axial and transverse yarn permeabilities, and the stochastic arrangement of yarns in a fabric, which determines the dimensions of interyarn gap spaces locally. To mimic the uncertainty in fabric forming, drape simulation was randomized in terms of start point and yarn start orientations. From yarn permeabilities and simulated local yarn spacing distributions, local fiber volume fractions and fabric permeabilities were approximated. This allowed resin injection into a deformed fabric to be simulated for different drape scenarios with different probabilities and different degrees of fabric randomness. The results indicate that variability in fabric properties and the forming process affects flow front shapes and times for complete impregnation of the reinforcement

Advanced geometry modelling of 3D woven reinforcements in polymer composites: processing and performance analysis

Numerical methods have become increasingly effective tools for analysis and design of composite materials. This study investigates how the inclusion of geometrical variations in modelling 3D woven fabrics affects the accuracy of numerical predictions. Based on micro-Computed Tomography data of 3D orthogonal woven composites, unit cell models were generated in TexGen at different levels of geometrical detail. Two types of analysis were implemented: (a) computational fluid dynamics (CFD) simulates resin flow during fabric impregnation in composites processing to predict permeability; (b) implicit static finite element analysis predicts in-plane tensile strength of the composites. By comparison with experimental data, the numerical predictions indicate that local geometrical variations, particularly in yarn cross-section, surface crimp and binder yarn path, have significant influence on both permeability and material strength. It is important to model the precise geometry in certain locations while the overall geometry can be simplified in order to maintain the practicality of model generation

Through-thickness permeability study of orthogonal and angle-interlock woven fabrics

Three-dimensional (3D) woven textiles, including orthogonal and angle-interlock woven fabrics, exhibit high inter-laminar strength in addition to good in-plane mechanical properties and are particularly suitable for lightweight structural applications. Resin transfer moulding (RTM) is a cost-effective manufacturing process for composites with 3D-woven reinforcement. With increasing preform thickness, the influence of through-thickness permeability on RTM processing of composites becomes increasingly significant. This study proposes an analytical model for prediction of the through-thickness permeability, based on Poiseuille’s law for hydraulic ducts approximating realistic flow channel geometries in woven fabrics. The model is applied to four 3D-woven fabrics and three 2D-woven fabrics. The geometrical parameters of the fabrics were characterized by employing optical microscopy. For validation, the through-thickness permeability was determined experimentally. The equivalent permeability of inter-yarn gaps was found to account for approximately 90 % of the through-thickness permeability for the analysed fabrics. The analytical predictions agree well with the experimental data of the seven fabrics

A voz que aproxima: o leitor em tempos de reclusão

Resumo: Partindo de considerações sobre por que ler os clássicos, refletimos sobre o lugar do leitor nos tempos atuais, buscando problematizar o seguinte: quem é o leitor de hoje? Partimos da definição de leitor incomum, sustentada por parâmetros inscritos no século XVIII, confrontando-a com as características do leitor do século XXI, atingido por uma pandemia que o isola. Usamos a experiência do grupo de leitura intitulado “Lendo os clássicos em voz alta” e a análise de relatos dos participantes para entender em que medida a leitura em voz alta é capaz de construir um novo leitor, um leitor de 2020.

Quantification of micro-scale variability in fibre bundles

Local variations in the random filament arrangement in carbon fibre bundles were determined by optical microscopy and automated image analysis. Successive steps of abrading, polishing and acquiring micro¬graphs of the sample surface made it feasible to analyse the micro-structure over a series of cross-sections along the fibre bundle path. Random and systematic changes in local filament arrangements were determined. Systematic changes were related to the interaction of a fibre bundle with an intersect¬ing binder thread leading to a local increase of the fibre volume fraction at the interface. Random clus¬tering of filaments in areas of high or low fibre volume fractions within the fibre bundles were found to be unaffected by the relative position of the bundle