Governing the dispersion and the orientation of fibres in concrete through a suitably balanced set of fresh state properties and a carefully designed casting procedure, is a feasible and cost-effective way to achieve a superior mechanical performance of fibre reinforced cementitious composites, which may be required by the intended application, even keeping the fibre content at relatively low values (e.g. around 1% by volume). In this paper the possibility of pursuing the above said “integrated” approach has been addressed in the framework of larger project focused on developing a deflection-hardening FRCC (DHFRCC), reinforced with 100 kg/m3 (1.27% by volume) of short steel fibres (13 mm long and 0.16 mm in diameter). The material has to be employed to manufacture thin (30 mm) roof elements, without any kind of conventional reinforcement, which have been anticipated to work, as simply supported beams, over a 2.5 m span. The study hence paves the way to the possibility of exploiting at an industrial level the correlation among fresh state performance, fibre dispersion and hardened state properties of self consolidating steel fibre reinforced concrete to achieve enhanced structural performance tailored to the specific application.

High mechanical performance of fibre reinforced cementitious composites: the role of “casting-flow induced” fibre orientation

Recent experiments suggest that short fibers in concentrated suspensions align more slowly as a function of strain than models based on Jeffery’s equation predict. We develop an objective model that captures the slow orientation kinetics exhibited by short-fiber suspensions. The standard moment-tensor equation of fiber orientation is used to find equations for the change rates of the eigenvalues and eigenvectors of the orientation tensor. As a phenomenological assumption, the growth rates of the eigenvalues are reduced by a constant scalar factor, while the rotation rate expressions for the eigenvectors are unchanged. The eigenvalue/eigenvector equations are then reassembled as a tensor equation. An equivalent kinetic theory is also developed. The new model is tested in a variety of flows, and found to exhibit slower kinetics than the standard model but similar steady-state orientations. The model provides an excellent fit to the shear stress transient in a shear reversal experiment with a 30% glass fiber filled polybutylene terephthalate resin melt, and we show how this experiment can be used to determine the parameters of the model.

An objective model for slow orientation kinetics in concentrated fiber suspensions: Theory and rheological evidence

X-ray computed tomography provides an opportunity for a detailed examination of the inner structure of fibre reinforced composites. Three-dimensional images, obtained with micro-CT, can be used for a realistic modelling of composite materials. All modelling objectives imply the knowledge of the orientations of the fibres inside the composite, which determine the local (anisotropic) properties. This paper investigates application of the structure tensor, a concept from the image processing field, to the determination of the orientations of fibres and to segment the image into the material’s components, for the purpose of an automatic generation of a voxel-based description of the representative volume element. The segmentation of the images of CFRP materials into its components is performed by thresholding, or by clustering in a two-dimensional parameter space of the degree of anisotropy and average grey value or one of the components of the orientation vector. Clustering allows not only separating the matrix from the yarns, but also distinguishing the yarns of different primary orientations.

Quantification of the internal structure and automatic generation of voxel models of textile composites from X-ray computed tomography data

http://www.mate.tue.nl/~peters/Inj_Mould_Lectures/MVincent_Fibres.pdf

Description and modeling of fiber orientation in injection molding of fiber reinforced thermoplastics

This study forms part of a literature survey on residual stresses in continuous fibre reinforced thermoplastic composites. After identification of the factors responsible for the thermal residual stress build-up in Part I [Parlevliet PP, Bersee HEN, Beukers A. Residual stresses in thermoplastic composites – A study of the literature – Part I: Formation of residual stresses. Composites Part A – Appl Sci Manuf, in press, doi:10.1016/j.compositesa.2005.12.025 ], experimental techniques to determine the magnitude and distribution of these stresses on various mechanical levels are outlined. The techniques are placed in the following categories: techniques utilising the intrinsic composite constituents’ material properties; techniques employing embedded “foreign” stress sensors; techniques based on in-plane and out-of-plane deformations, and destructive techniques.

Residual stresses in thermoplastic composites—A study of the literature—Part II: Experimental techniques

Fibre-orientation measurements in short-glass-fibre composites—II: a quantitative error estimate of the 2D image analysis technique

Slow crack propagation in polyethylene under fatigue at controlled stress intensity

Dans un premier temps, les phenomenes a l'origine des contraintes residuelles en injection de thermoplastique sont decrits, avec les specificites liees aux polymeres renforces de fibres. Le refroidissement heterogene associe a une variation de comportement mecanique conduit aux contraintes d'origine thermique, auxquelles il faut ajouter les contraintes dues a la pression dans la phase fluide figee par la solidification. Les fibres de renfort introduisent une heterogeneite et une anisotropie de comportement mecanique et de retrait. Dans un deuxieme temps, une methode de mesure des contraintes residuelles basee sur l'enlevement de couches et la mesure des moments de flexion est presentee. Elle permet la mesure des contraintes residuelles pour des materiaux anisotropes comme les thermoplastiques renforces de fibres. En general, on obtient des contraintes de traction en cœur, et de compression en peau. L'effet du fraisage pour enlever les couches est evalue. La pression et le temps de maintien, la temperature de regulation du moule, le taux de fibres influencent les profils de contraintes.

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Évaluation des contraintes résiduelles dans les pièces injectées en thermoplastique renforcé de fibres courtes

Residual stresses in molded thermoplastics are caused by heterogeneous cooling and by different pressures frozen in at different time of the cycle. This usually leads to a core in tension, surface regions in compression, and sometime thin extreme surface layers in tension again. In a first part, the mechanisms of residual stresses formation are presented. In a second part, a technique to measure residual stresses, based on the measurement of the bending moments necessary to lay-flat a specimen after removal of a thin layer, is applied to unfilled and fiber reinforced materials with success. The technique, unlike the curvature measurement based method, eliminates the necessity of the assumption of constant material properties. Finally, we show the dependence of the solid polymer viscoelastic data on the thermomechanical history of the test specimen, and the consequences on the computed residual stresses, which are compared to the measurements.

Thomas Giroud

Papers

Description and modeling of fiber orientation in injection molding of fiber reinforced thermoplastics

Fibre-orientation measurements in short-glass-fibre composites—II: a quantitative error estimate of the 2D image analysis technique

Slow crack propagation in polyethylene under fatigue at controlled stress intensity

Évaluation des contraintes résiduelles dans les pièces injectées en thermoplastique renforcé de fibres courtes

Residual stresses in injection molded thermoplastics