Capilaridad

Composites Science and Technology 70 (2010) 1628–1636

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Composites Science and Technology
journal homepage: www.elsevier.com/locate/compscitech

Dynamic capillary impact on longitudinal micro-flow in vacuum assisted impregnation and the unsaturated permeability of inner fiber tows
Min Li a,*, Shaokai Wang a, Yizhuo Gu a, Zuoguang Zhang a, Yanxia Lia, Kevin Potter b
a b

Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China Advanced Composites Centre for Innovation and Science, University of Bristol, University Walk, Bristol BS8 1TR, UK

a r t i c l e

i n f o

a b s t r a c t
This paper addresses issues of the synergeticdynamic effect of capillary force on the longitudinal impregnation driven by external pressures, especially under vacuum assistance. An apparatus was designed to detect the axial infiltration along unidirectional fiber bundles which were all aligned closely to give a representation of micro-flow channel of inner fiber tows. The external driving pressures were controlled sufficiently low, 20–60 kPa, onthe order of capillary pressures. Based on the analysis of infiltration velocities under different external pressures, dynamic capillary pressures can be determined experimentally. The results showed that capillary pressures, the most important force of microscopic flow through inner fiber yarns, acted as a drag force on the infiltration flow for vacuum assisted penetration into unidirectional fiberbundles. This unique drag effect is very different from traditional unsaturated infiltration, different from the compressed air driving permeation and the theoretical calculated data in this paper. Moreover, values and even signs of the dynamic capillary pressures varied with the fiber fraction of the assemblies as well as the fluid types. Further analysis demonstrated that the function of capillarypressure was closely related to the capillary number (Ca), acting as drag force when Ca larger than a critical value, and as a promotive force with smaller Ca. Consequently, unsaturated permeabilities of the unidirectional fiber bundles were estimated by taking consideration of both dynamic and quasi-static capillary pressures. Ó 2010 Elsevier Ltd. All rights reserved.

Article history: Received 23November 2009 Received in revised form 26 May 2010 Accepted 4 June 2010 Available online 9 June 2010 Keywords: A. Polymer-matrix composites (PMCs) E. Resin transfer moulding (RTM) D. Permeability B. Porosity/Voids

1. Introduction As one of the cost-effective liquid composite moulding (LCM) processes, vacuum assisted resin transfer moulding (VARTM) process has been recognized as a viable optionto manufacture large composite structures with complicated shapes [1]. In the process, a polymeric resin is introduced into the mould cavity by evacuating the mould cavity ahead of the infiltration front, so that resin saturates the dry fibrous reinforcement. Hence, complete filling of the reinforcement, both inter- and inner-fiber tows wetting becomes a major issue, which controls both the qualityof the product and the processing time. In the LCM process, resin impregnation is highly dependent on the permeability of fibrous reinforcement. As an important material parameter, permeability represents the degree of difficulty for a fluid to penetrate a porous medium, determined by the geometry of the fibrous preform, which can be predicted according to empirical equations, such as Carman–Kozenyequation [2], Gebart model [3] and Westhuizen model [4]. However, discrepancies are always observed between data from models and typical experi-

* Corresponding author. Tel./fax: +86 10 82339800. E-mail address: leemy@buaa.edu.cn (M. Li). 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.06.004

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