Elsevier Vernet, N.; Ruiz, E.; Advani, S.; Alms, JB.; Aubert, M.; Barburski, M.; Barari, B.... (2014) Abstract:In this second international permeability benchmark, the in-plane permeability values of a carbon fabric were determined by 12 participants worldwide. One other participant also investigated the deformation of this fabric. The aim of this work was to obtain comparable results in order to make a step towards standardization of permeability measurements of fibrous reinforcements. The procedures used by most participants were according to the guidelines defined for this exercise after the first benchmark. Unidirectional injections in three in-plane directions of the fabric were conducted to determine the unsaturated in-plane permeability tensor. Parameters such as fiber volume fraction, injection pressure and fluid viscosity have been fixed in order to minimize sources of scatter. The comparison of the results from each participant was encouraging. The scatter between data obtained while respecting the test guidelines was close to the scatter of the setups themselves. A slightly 2 higher dispersion was observed when some parameters differed from the recommendations.Overall, a good correlation is observed between all the results of this exercise.

Although good progress was made by two international benchmark exercises on in-plane permeability, existing methods have not yet been standardized. This paper presents the results of a third benchmark exercise using in-plane permeability measurement, based on systems applying the radial unsaturated injection method. 19 participants using 20 systems characterized a non-crimp and a woven fabric at three different fiber volume contents, using a commercially available silicone oil as impregnating fluid. They followed a detailed characterization procedure and also completed a questionnaire on their setup and analysis methods. Excluding outliers (2 of 20), the average coefficient of variation (c v) between the participant's results was 32% and 44% (non-crimp and woven fabric), while the average c v for individual participants was 8% and 12%, respectively. This indicates statistically significant variations between the measurement systems. Cavity deformation was identified as a major influence, besides fluid pressure/viscosity measurement, textile variations, and data analysis.

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Textile permeability is one of the dictating factors in the fabrication of fibre-reinforced polymer composites. However, reproducibility of experimental in-plane permeability characterization is still a challenging task due to the lack of standardized test and evaluation procedures. The paper at hand addresses two major sources for discrepancies when characterizing in-plane permeability through optical observation of radial flow experiments: digital image processing and data evaluation algorithms. A digital image processing strategy is presented, which robustly handles varying lightning conditions, optical properties of the materials under test and image occlusions caused by mechanical elements of the test setup. The strategy is of universal validity and independent of the choice of reinforcing material and impregnating fluid. An experimental analysis compares two approaches for fitting elliptic geometry models to data points detected along the fluid flow front. The study reveals the impact of the fitting strategy on the resulting permeability data and the benefit of forcing the ellipse centre to that of the injection opening. The computation algorithm of Chan and Hwang, widely used for calculating in-plane permeability values from experimental data, is critically discussed. A correction of the algorithm is proposed which avoids a violation of isotropic data characteristics while adding robustness to the data reduction. An experimental analysis compares anisotropic in-plane permeability values obtained with different evaluation algorithms. The study highlights the impact of the computational algorithm on the permeability data and reveals discrepancies of up to 6%, which is considerable compared to the scatter typically reported for in-plane permeability data.

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b department of Polymer Materials and Plastics engineering, clausthal university of technology, clausthal-Zellerfeld, germany; c institut für verbundwerkstoffe gmbH, Kaiserslautern, germany ABSTRACT A novel approach is presented for modeling the temporally advancing fluid flow front in radial flow experiments for in-plane permeability characterization of reinforcing fabrics. The method is based on fitting an elliptic paraboloid to the flow front data collected throughout such an experiment. This "paraboloid" approach is compared to the conventional "ellipse" method and validated by means of data sets of optically tracked experiments from two different research institutions. A detailed discussion of the results reveals the benefits of the "paraboloid" method in terms of numerical efficiency as well robustness against temporal or local data variations. The "paraboloid" method is tested on temporally and spatially limited data sets from a testrig involving linear capacitive sensors. There, the method shows advantages over the conventional approach as it incorporates the entirety of available measurement data, particularly in the last stages of the experiments which are most characteristic for the material under test.

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