Experimental Determination of Material Properties for Inflatable Aeroshell Structures

As part of a deployable aeroshell development effort, system design, materials evaluation, and analysis methods are under investigation. One specific objective is to validate finite element analysis techniques used to predict the deformation and stress fields of aeroshell inflatable structures under aerodynamic loads. In this paper, we discuss the results of an experimental mechanics study conducted to ensure that the material inputs to the finite element models accurately predict the load elongation characteristics of the coated woven fabric materials used in deployable aeroshells. These coated woven fabrics exhibit some unique behaviors under load that make the establishment of a common set of test protocols difficult. The stiffness of a woven fabric material will be influenced by its biaxial load state. Uniaxial strip tensile testing although quick and informative may not accurately capture the needed structural model inputs. Woven fabrics, when loaded in the bias direction relative to the warp and fill axes, have a resultant stiffness that is quite low as compared with the warp and fill directional stiffness. We evaluate the experimental results from two load versus elongation test devices. Test method recommendations are made based on the relevance and accuracy of these devices. Experimental work is conducted on a sample set of materials, consisting of four fabrics of varying stiffness and strength. The building blocks of a mechanical property database for future aeroshell design efforts are constructed. Nomenclature = Young's modulus = shear modulus = polar moment of inertia T = torque = internal pressure = radius = thickness = length of rotation = strain = shear strain = rotation angle = stress = shear stress = Poisson's ratio Subscripts h = hoop direction l = longitudinal direction 1 = 1 st principal direction 2 = 2 nd principal direction Acronyms IAD = Inflatable Aerodynamic Decelerator IRVE = Inflatable ReEntry Vehicle Experiment PAIDAE = Program to Advance Inflatable Decelerators for Atmospheric Entry 3 I. Introduction LL previous missions to land on Mars have used supersonic parachutes for deceleration after descending through the hypersonic regime. Due to the desire to land higher mass systems, better technologies for supersonic and new technologies for hypersonic deceleration may be necessary. Inflatable Aerodynamic Decelerators (IADs) are a technology currently being researched for use on high mass missions with application to Martian entry. Accurate modeling is required of the deformation and stress fields of aeroshell inflatable structures under aerodynamic loads during descent. Because these …