Theoretical and Experimental Investigation on Helium Leakage Characterization of Flexible Film-Fabric Laminated Composites

Transcription

1 Theoretical and Experimental Investigation on Helium Leakage Characterization of Flexible Film-Fabric Laminated Composites Theoretical and Experimental Investigation on Helium Leakage Characterization of Flexible Film-Fabric Laminated Composites Xue Feng Yao 1*, Qing Wu 1, Chao Xiong 1, Pin Jing Wen 1, and Hui Feng Tan 2 1 Department of Engineering Mechanics, AML, Tsinghua University, Beijing , China 2 Center for Composite Materials and Structures, Harbin Institute of Technology, ,China This paper was presented at Compo 2010, July 2010, China Summary Flexible film-fabric laminated composites with low-density and high-performance are important envelope materials in stratosphere aerostats. In this paper, a multilayer leak model is used to predict the helium leak rate of the virgin samples by the Fick s law, which shows a good agreement with experimental data. The influences of mechanical damage, ultraviolet radiation and intense ozone corrosion on the Young s modulus and the permeability of envelop materials are investigated in experiment, which are confirmed by means of damage-induced helium leakage models. Finally, helium leakage mechanisms of flexible film-fabric laminated composites are analyzed. 1. Introduction The flexible film-fabric laminated composites with multifunctional functions, such as light weight, high strength, high tear resistance, high barrier and anti-aging, are ideal envelope materials of stratospheric airship. However, few studies on helium permeability and leakage mechanism under the impact of ozone, UV radiation and mechanical loading are reported. Wanggu Kang et al. 1 studied the tensile strength and the tensile modulus of flexible film-fabric laminated composites under different temperatures; Yao et al. 2,3 experimentally studied the helium leakage mechanisms of flexible filmfabric laminated composites under different damage models. In this paper, the helium permeation mechanisms of a thin film fabric laminated envelope material are studied *Corresponding address: Xue Feng Yao yxf@mail.tsinghua.edu.cn using a multilayer permeation model. The damage induced helium mechanism about envelop materials are investigated by means of meso-damage mechanics and space environmental simulated experiments including UV radiation, ozone and mechanical loading. 2. Thin Film Fabric Laminated Envelope Material Film fabric laminated envelope material consisted of three different parts, i.e. protected layer, load carrier, and helium barrier, which uses fiber textile as its reinforced material and thermal-plastic polymer as its matrix material. The main compositions of helium barrier and load carrier are Mylar film and Vectran/PU adhesive, respectively. The major materials of protected layer are Tedlar film intermingling with inorganic nanoparticles TiO 2. Figure 1 shows the detail structural characterizations of envelope materials. 3. A Multilayer Permeation Model 3.1 Multilayer Permeation Process of Helium Helium atoms move from internal space of the envelope material into outside space, which will produce a pressure drop when the helium atoms pass through each layer of the flexible film-fabric laminated composites, respectively. In fact, helium diffusion process can be described by Fick s law 4.After certain permeation time, the steady-state is reached 5 : (1) Where J is the flux of helium in the direction of the flow; P is the permeability coefficient; p 0 p 1 is the pressure difference between two sides; h is the thickness of the flexible film-fabric laminated composite. Therefore, the helium flux for each layer of unit area of the flexible filmfabric laminated composite can be expressed as follows: Smithers Rapra Technology, 2011 Polymers & Polymer Composites, Vol. 19, No. 7,

2 Xue Feng Yao, Qing Wu, Chao Xiong, Pin Jing Wen, and Hui Feng Tan Figure 1. Flexible film-fabric laminated composites. (a) Geometrical configuration; (b) Microstructure (2) Where q L, q H and q B are the helium flux of unit time of the load layer, barrier layer and protective layer, respectively. h L, h H and h B are the thickness of each layer. P L ef f, P H and P B are the permeability coefficient of each layer, respectively. P 0, P H and P B are the pressure of each layer, respectively. According to the continuity theorem, the helium atoms passing the upper layer must pass the adjacent layer of the structure when ignoring the flow in horizontal direction between layers. q L = q H = q B (3) 3.2 Helium Permeability of Different Layers The helium permeability coefficient of Tedlar film for protective layer and Mylar film for barrier layer is m 2 /(s Pa) and m 2 /(s Pa), respectively, which are provided by the Technical data from DuPont company. For the existence of defect between the filaments and fiber bundles, the Fick s law can not be applied directly to describe helium diffusion through the load layer. The size of defect is much larger than the mean free path of helium at room temperature. According to Hagen- Poiseuille s law 6, helium flux can be expressed: (4) Where e is the porosity; d is the average diameter of pore; h is the viscosity coefficient of helium at room temperature; t is the tortuosity ratio; Dp is the pressure difference at two sides of materials. According to the calculated method in literature 7,8, the helium permeability of load layer is m 2 /(s Pa). Then the helium flux of whole flexible film-fabric laminated composite can be obtained using Eq.(3) and (4) as m/s, which is 22.4% higher than experimental data about m/s. 4. Experimental Investigation of Damage Induced Leakage Characterization 4.1 Meso-damage Model Meso-damage model is based on a representative volume element (RVE) in the material. Composite damages can be divided into three modes: surface damage, enclosed damage and through damage 8. For the enclosed cracks, there will be no concentration gradient along the diffusion direction in the closed area or the pressure is constant in the region. The equivalent effect of the enclosed cracks is that the structure is divided into two thinner sub-parallel structures from the original single layer as shown in Figure 2. Then the helium permeability P in the enclosed micro-crack region can be obtained: (5) The equivalent helium permeability coefficient P of the whole structure is: 620 Polymers & Polymer Composites, Vol. 19, No. 7, 2011

3 Theoretical and Experimental Investigation on Helium Leakage Characterization of Flexible Film-Fabric Laminated Composites (6) Where the damage variable d is defined as the ratio of the damaged surface area to the whole surface area, a is the average open displacement of cracks, P 0 is the helium permeability of the original material. Both t 1 and t 2 are the thickness of two thinner sub-parallel structures as shown in Figure 2. Figure 2. Enclosed crack 4.2 Ozone Corrosion Experiment The running speed of ozone generator is set at 5 g/h. After a certain time, ozone is imported into the bottom of the container which hold the test samples. The samples are removed after 1 to 6 hours and the residual ozone is exhausted into the atmosphere by a fume hood. The surfaces of test samples can be corroded uniformly during the whole ozone corrosion process. Figure 3. Influences of ozone corrosion on on flexible composite. (a) Helium permeability coefficients (b) Tensile modulus 4.3 UV Radiation Experiment 500w high-power light with 360 nm center wavelength and 2 w/cm 2 light intensity is applied in UV radiation experiment. The specimens are under the UV lamp with a 150 mm distance, and are removed after 12, 24, 36 and 60 hours, respectively. 4.4 Tensile Cycle Loading The sample is a rectangle specimen of 50 mm 200 mm according to standard ASTM D751. First, the specimens are stretched by the tensile machine at a lower speed to the strain of 2.5% and 5%, respectively. And then unloading and reloading to 2.5% and 5% strain. After 10 times unloading and reloading, a certain amount of the enclosed damage can be introduced into the structure. 5. Results and discussions Figure 4a shows that the helium permeability coefficient of the flexible film-fabric laminated composite decreases with the increasing of the ozone corrosion time. The linear relationship between helium permeability coefficient and time is P = 2.26( t). Figure 4b shows the tensile modulus of the flexible film-fabric laminated composite, which increases gradually with the increase of ozone corrosion time. A linear relationship can also be fitted from the experimental data between the tensile modulus and Polymers & Polymer Composites, Vol. 19, No. 7,

4 Xue Feng Yao, Qing Wu, Chao Xiong, Pin Jing Wen, and Hui Feng Tan Figure 3. Continued: (c) Solubility coefficients (d) diffusion coefficients Figure 4. Influences of UV radiation on flexible composite ozone corrosion time in the form of E = 4.136( t). Of course, the linear relationship of the flexible film-fabric laminated composite between helium permeability and its tensile modulus is P = E. Figure 3c and Figure 3d show the helium solubility coefficient and the diffusion coefficient of the flexible film-fabric laminated composite, respectively. There are no significant changes at initial 4 hours, and the solubility coefficient increases rapidly after 4 hours of ozone corrosion, while the diffusion coefficient turns to be an opposite trend under ozone corrosion. Figure 4 shows the change of helium permeability and tensile modulus of the flexible film-fabric laminated composite with the variation of UV radiation time. The results reflect that the helium permeability of the envelope material remains unchanged when the UV radiation time increase, so does the tensile modulus. Figure 5 shows the equivalent diffusion coefficient of the flexible film-fabric laminated composite, which increases with the increased tensile strain. The helium diffusion coefficient of virgin envelope sample is cm 2 /s. It increases to cm 2 /s with tensile strain of 0.025, which is 35.7% higher than that of virgin sample. The relationship between the diffusion coefficient and the damage is in good agreement with the prediction in Eq. (5). 6. Conclusions 1. Helium permeability of the flexible film-fabric laminated composite is predicted using multilayer permeability model, which shows a good agreement with experimental data. 2. Ozone corrosion can reduce helium permeability of the flexible filmfabric laminated composite and UV radiation has little effect on 622 Polymers & Polymer Composites, Vol. 19, No. 7, 2011

5 Theoretical and Experimental Investigation on Helium Leakage Characterization of Flexible Film-Fabric Laminated Composites Figure 5. Diffusion coefficients under tensile cycling References material properties for the excellent climate resistant performance of Tedlar film. 3. Tensile cycling can induce a certain amount of damage within the material, and increase the diffusion coefficient of the structure. Acknowledgements This research project was supported by China Nature Science Foundation (No ) and China Postdoctoral Research Foundation (No ). 1. Kang W., Suh Y., Woo K. and Lee I., Compos. Struct., 75 (2006) Yao X.F., Lei Y.M., Xiong C., Wang X.Q. and Wang Y.Q., J. Reinf. Plast. Compos., 29 (2010) Yao X.F., Lei Y.M., Xiong C., Wang X.Q. and Wang Y.Q., J. Appl. Polym. Sci., 116 (2010) Tremblay P., Savard M.M., Vermette J. and Paquin R., J. Membr.Sci., 282 (2006) Crank J. and Park G.S., London, Academic Press Inc, Uhihorn R.J.R., Keizer K., and Burggraaf A., J.Membr. Sci., 46 (1989) Tomohiro Y. and Takahira A., Takashi I, Adv. Composite Mater., 13 (2004) Gerlach A., Keller W., Schulz J. and Schumacher K., Microsystem Technologies, 7 (2001) Disdier S., Rey J.M., Pailler P. and Bunsell A.R., Cryogenics, 38 (1998) Polymers & Polymer Composites, Vol. 19, No. 7,

6 Xue Feng Yao, Qing Wu, Chao Xiong, Pin Jing Wen, and Hui Feng Tan 624 Polymers & Polymer Composites, Vol. 19, No. 7, 2011