Indian Journal of Fibre & Textile Research Vol. 34, September 2009, pp. 239-244 Determination of woven fabric impact permeability index M Tokarska a & K Gniotek Department for Automation of Textile Processes, Technical University of Lodz, Zeromskiego 116, 90-924 Lodz, Poland Received 7 July 2008; revised received and accepted 6 February 2009 An uncertainty in measuring a feature of technical fabrics called ‘impact permeability’ has been assessed in the form of impact permeability index. Impact permeability describes the difference in the behaviour of a fabric in dynamic airflow conditions in relation to its behaviour in static conditions. A mathematical model of pressure drop in woven fabric, which is formed as a result of impact airflow through the product, has been presented. For the assessment of inaccuracy in impact permeability index determination, the procedures described in ‘guide to the expression of uncertainty in measurement’ are used. An example of calculating uncertainty budgets of the parameters analysed has also been reported. It is observed that the method of determination of impact permeability index is useful. The index is a good measure of impact permeability for the technical fabrics with respect to their flow properties. Keywords: Air permeability, Impact permeability index, Uncertainty, Woven fabric 1 Introduction Woven fabrics are often used as canvas covers, parachutes, airbags, tents or protective clothing. The behaviour of textile products depends on the conditions that they are used in. During the use of such products, the fabrics are subjected to static and dynamic air pressures which cause air streams to move in a skew manner through their structure 1,2 . Static conditions 3 are assumed to be a reference point since the steady airflow through a porous membrane (a fabric) takes place. The sizes and shapes of pores do not change in this state. In the conditions of transient air flows, all the quantities are in fact time dependent; this refers both to air flow parameters and the fibrous structure manifesting all rheological features 2,4 . The studies of differences in the behaviour of fabrics during steady and transient airflows is based on the impact permeability of flat textile products 2,5 . The impact permeability of fabrics is greatly affected by their structural parameters. A neural network has been used to model this property 6 . In the present study, an uncertainty in measuring a feature of technical fabrics called ‘impact permeability’ has been assessed in the form of impact permeability index. A mathematical model of pressure drop in woven fabric, which is formed as a result of impact airflow through the product, has been presented. 2 Materials and Methods Ten specimens (n = 10) of polyester woven fabric were used to determine the air impact permeability. The fabric parameters are given in Table 1. _____________ a To whom all the correspondence should be addressed. E-mail: emagda@p.lodz.pl Table 1—Woven fabric parameters Fabric parameter Value Raw material Polyester Weave Plain Thickness 0.28 mm Surface mass 119 g/m 2 Apparent density 425 kg/m 3 Number of threads warp 48/cm weft 26/cm Linear density warp 19.7 tex weft 8.1 tex Breaking force warp 611 cN weft 214 cN Crimp warp 3.2 % weft 0.3 % Twist of threads Twistless