Experimental evaluation of gas permeability through damaged composite laminates for cryogenic tank T. Yokozeki a, * , T. Ogasawara b , T. Aoki a , T. Ishikawa b a Department of Aeronautics and Astronautics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Aerospace Research and Development Directorate, Japan Aerospace Exploration Agency, Japan article info Article history: Received 3 December 2007 Received in revised form 21 April 2008 Accepted 26 May 2008 Available online 4 June 2008 Keywords: A. Polymer–matrix composites B. Matrix cracking C. Laminates Gas permeability abstract This paper describes some recent experimental techniques for leakage assessment in conjunction with permeability results of composite laminates with matrix cracks in relation to the application of compos- ite laminates to cryogenic fuel tank structures. Tensile loadings were applied to CFRP tubular specimens utilizing a cryogenic loading system for leak measurement at room temperature (RT) and at liquid nitro- gen temperature (LN 2 T). Helium gas permeability through damaged CFRP under both RT and LN 2 T con- ditions was compared. Furthermore, an experimental method for evaluating the effect of crack intersecting angle on the gas leakage was proposed, and measured results were presented. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Carbon fibre reinforced plastic (CFRP) laminates are attractive structural materials for use in aerospace structures because of their high performance (e.g., the specific stiffness/strength, the damage tolerance). To achieve radical structural weight reduction of space launch vehicles, the application of CFRP laminates to the cryogenic propellant tank structures has been widely attempted (e.g., liquid hydrogen and liquid oxygen tanks). Some recent studies of the fea- sibility of composite cryogenic propellant tanks indicate that ma- trix crack onset and its accumulation are inevitable because of severe thermomechanical loads when applying conventional high performance composites to cryogenic tanks [1,2]. Such accumu- lated multilayer matrix cracks might engender detrimental propel- lant leakage, as shown in Fig. 1. Therefore, fuel leakage or gas leakage through a damage network in CFRP laminates has come to be a major problem that dominates the feasibility of composite cryogenic propellant tanks. Adequate guidelines for the reliable application of CFRP laminates to cryogenic propellant tanks are necessary from both leakage and damage tolerance perspectives. After the failure of the X-33 composite cryogenic tank test [3], many studies of the evaluation of mechanical properties and matrix cracking behaviors under cryogenic conditions or thermal cycling conditions have been undertaken [4,5]. Several materials were sub- jected to screening for cryogenic tank application based on mechanical properties, cryogenic cracking trends, etc. Leak tests or permeability tests of the damaged composite laminates have also been attempted. Kumazawa et al. [1,2] investigated helium gas leakage through cross-ply laminates with matrix cracks in all con- stituent plies at RT using cruciform specimens under biaxial load- ings. Their results indicated a clear correlation between biaxial loadings and gas leak rates. McManus et al. [6] observed that the leakage is dependent on loading conditions, crack densities, and stacking sequence. Robinson et al. [7] developed a leak method that provides the capability to continuously measure gas leakage in a small coupon specimen under uniaxial loading. Their study also examined another method for leak measurement using CFRP panels under biaxial loadings. Gates et al. [8] measured through-edge hydrogen leakage using coupon specimens. Bechel [9] reported he- lium gas leakage through cryogenically cycled CFRP laminates. Kumazawa et al. investigated the leakage mechanism through a damaged network of composite laminates [1,2] and thereby devel- oped a simple semi-analytical model based on the leak conduc- tance of crack intersections. In this model, the leak conductance is proportional to the damage opening area at the intersecting ma- trix cracks, which is assumed to be proportional to the product of the average crack opening displacements (CODs) in the adjoining plies. Roy and Benjamin [10] extended the above-mentioned model by introducing delaminated crack opening displacement (DCOD). Although the predictive methodology of leakage through the damage network in composite laminates has been well studied, quantitative evaluation of leakage and comparative verification be- tween prediction and experimental measurement remain to be solved. Thus, basic studies of the leakage mechanisms must be pursued further. 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.05.019 * Corresponding author. Tel.: +81 3 5841 7023; fax: +81 3 5841 6598. E-mail address: yokozeki@aastr.t.u-tokyo.ac.jp (T. Yokozeki). Composites Science and Technology 69 (2009) 1334–1340 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech