Assessment of the fibre orientation factor in SFRC slabs

The design of steel fibre reinforced concrete (SFRC) structures is evolving towards a new approach that uses correction factors to consider differences between the small-scale characterisation specimens and the real-scale elements. Recently, the Model Code 2010 proposed an orientation factor (K) that accounts for the effects of the orientation in the structural response of elements. The present study focuses on the identification of this factor in SFRC slabs with different dimensions. For that, flexural tests on real-scale slabs were conducted and the fibre orientation was assessed with an inductive method. A finite element analysis showed the differences between the experimental curves and the prediction of the Model Code without considering K. Based on the results obtained, a range of values is proposed for K and validated. This study sheds light on possible modifications that this philosophy of design might require to better reproduce the behaviour of slabs.     

Dynamic compressive behaviour of unidirectional GFRP for various fibre orientations

The dynamic stress-strain behaviour of unidirectional glass-epoxy composite has been studied at an average strain rate of 265 s⁻¹ for fibre orientations of 0°, 10°, 30°, 45°, 60° and 90° with respect to loading axis, using the Kolsky pressure bars technique. GFRP (glass fibre reinforced plastic) is found to be strain rate sensitive for all fibre orientations. Compared to quasi-static, the dynamic ultimate strength increases almost 100% for 0°, 80% for 10° fibre orientations and about 45% for all other orientations. Failure occurs predominantly by tensile splitting in 0° specimens and by shear for all other orientations preserving the fibre direction in each case.     

Hyperelastic modelling of arterial layers with distributed collagen fibre orientations.

Constitutive relations are fundamental to the solution of problems in continuum mechanics, and are required in the study of, for example, mechanically dominated clinical interventions involving soft biological tissues. Structural continuum constitutive models of arterial layers integrate information about the tissue morphology and therefore allow investigation of the interrelation between structure and function in response to mechanical loading. Collagen fibres are key ingredients in the structure of arteries. In the media (the middle layer of the artery wall) they are arranged in two helically distributed families with a small pitch and very little dispersion in their orientation (i.e. they are aligned quite close to the circumferential direction). By contrast, in the adventitial and intimal layers, the orientation of the collagen fibres is dispersed, as shown by polarized light microscopy of stained arterial tissue. As a result, continuum models that do not account for the dispersion are not able to capture accurately the stress-strain response of these layers. The purpose of this paper, therefore, is to develop a structural continuum framework that is able to represent the dispersion of the collagen fibre orientation. This then allows the development of a new hyperelastic free-energy function that is particularly suited for representing the anisotropic elastic properties of adventitial and intimal layers of arterial walls, and is a generalization of the fibre-reinforced structural model introduced by Holzapfel & Gasser (Holzapfel & Gasser 2001 Comput. Meth. Appl. Mech. Eng. 190, 4379-4403) and Holzapfel et al. (Holzapfel et al. 2000 J. Elast. 61, 1-48). The model incorporates an additional scalar structure parameter that characterizes the dispersed collagen orientation. An efficient finite element implementation of the model is then presented and numerical examples show that the dispersion of the orientation of collagen fibres in the adventitia of human iliac arteries has a significant effect on their mechanical response.     

A novel image analysis procedure for measuring fibre misalignment in unidirectional fibre composites

A novel image analysis procedure named Fourier transform misalignment analysis (FTMA) for measuring fibre misalignment in unidirectional fibre composites is presented. Existing methods are briefly illustrated and evaluated. The FTMA-method is presented, describing the specimen preparation and elaborating how the image analysis algorithm uses Fourier transformation and a least squares method to compute single fibre orientations. On the basis of parameter investigations the robustness of the FTMA-method is investigated. Software generated micrographs with known fibre misalignment are used to determine the precision of the method. The precision is used, along with computation time and memory usage, to benchmark the FTMA-method against the existing multiple field image analysis (MFIA) method. It is found that the FTMA-method is at least as accurate as existing methods. Furthermore, the FTMA-method is much faster than the existing methods, completing a typical analysis in approximately 1 min. Overall, it is concluded that the FTMA-method is a robust, precise and time efficient tool for determining fibre misalignment in unidirectional fibre composites, offering a higher degree of detail than the existing MFIA-method.     

A non-destructive X-ray microtomography approach for measuring fibre length in short-fibre composites

An improved method based on X-ray microtomography is developed for estimating fibre length distribution of short-fibre composite materials. In particular, a new method is proposed for correcting the biasing effects caused by the finite sample size as defined by the limited field of view of the tomographic devices. The method is first tested for computer generated fibre data and then applied in analyzing the fibre length distribution in three different types of wood fibre reinforced composite materials. The results were compared with those obtained by an independent method based on manual registration of fibres in images from a light microscope. The method can be applied in quality control and in verifying the effects of processing parameters on the fibre length and on the relevant mechanical properties of short fibre composite materials, e.g. stiffness, strength and fracture toughness.     

Control of molecular orientations in mesophase pitch-based carbon fibre by blending PVC pitch

Coal tar-derived mesophase pitch and its blends with PVC pitch in 5 or 10 wt% were spun at temperatures from 340 to 390° C by applying pressurized nitrogen. The parent mesophase pitch and the blended pitch showed an excellent spinnability at temperatures from 360 to 380° C and from 350 to 380° C, respectively, to give a thin pitch fibre of 10m diameter. The transverse texture of the fibres from the parent mesophase pitch showed the radial orientation regardless of a higher spinning temperature of 390° C. In contrast, those from the blended pitches showed random orientation even at the lower spinning temperature of 350° C. The amounts of the blend extruded by spinning at each temperature under 0.2 kg cm^–2 G^–1 were always larger than those of the mesophase pitch. It is clarified in the present study that blending PVC pitch can realize stable spinning at lower temperatures, where the molecular orientation in the transverse section of the resultant carbon fibre was controlled through decreasing the viscosity of the whole mesophase pitch.     

Unifying measurements in testing measuring instruments

Structural and parametric identification are considered for the error probability distribution for a particular type of measuring instrument. Translated from Izmeritel’naya Tekhnika, No. 10, pp. 13–17, October, 2008.