Experimental Determination of Inflatable, Braided Tube Constitutive Properties

Abstract

The primary objective of this study was to conduct constitutive tests of relatively large diameter inflatable, braided fabric tubes at different inflation pressures and braid angles in order to quantify the longitudinal modulus, in-plane shear modulus and effective lamina stiffness properties. The stiffness properties quantified here are of high interest because the same braided fabric tubes have been used in the construction of test articles for a major, multi-year, ground based test campaign led by the United States National Aeronautics and Space Administration. These properties are also input directly into high-fidelity yet computationally intensive 3D shell-based finite-element simulations of the large, inflatable structures. Experimental methods employed during this study included tension-torsion testing, uniaxial tension testing of individual fibre tows, and uniaxial tension testing of gas bladder coupons. Digital image correlation was used to measure all of the geometric information that is necessary to perform netting theory calculations. The test results indicate that fabric in-plane shear modulus is highly dependent on both braid angle and inflation pressure, but that longitudinal stiffness is quite small and relatively unaffected by braid angle and pressure. In addition to advancing the state-of-the art in experimental constitutive property determination of inflatable, braided fabric, this study includes the development of a method to back calculate lamina properties from the experimental results that are suitable for use as input to commonly used finite-element programmes. The digital image correlation data revealed spatial variation of shear strain that was important to consider when computing the gross shear stiffness. Digital image correlation data also captured the braid surface flattening with increasing inflation pressure, which supports the fibre de-crimping theory.