Predicting bending behaviour of deployable booms made of thin woven fibre composites

Abstract

Design of advanced space structures like solar sails and reflectors are limited by the volume and payload capacity of launch vehicles. Thus, there is a trend to utilize deployable structures made of ultra-thin fibre composite materials over traditional mechanical hinges. Use of thin woven fibre composites enables them to self-deploy using stored strain energy and hence unfolds several benefits such as high strength to weight ratio, less complexity, negligible frictional effects during deployment. Booms made of thin fibre composite with epoxy matrix have been widely used in space structures since 1980s. Even though the deformable booms with ultra-thin composites conquer the aforementioned limitations, folding of such structures are limited to their elastic regime. Once the folding is extended beyond the elastic region, these composites are either subjected to fibre failure or to plastic deformation of matrix. Thus, now scientists are investigating the possibility of using more flexible elastomers, i.e. silicone which allows the fibres to micro-buckle and hence survive under extreme curvatures. However, use of soft elastomers in space structures can lead to poor structural performance after deployment. Also the composites like Carbon Fibre Reinforced Silicone (CFRS) are unable to store enough strain energy to provide required force for self-deployment when released. Dual matrix fibre composites were invented to solve that problem. Dual matrix fibre composites contain a continuous fibre reinforcement with soft elastomeric matrix like silicon in specified hinge regions and traditional epoxy matrix elsewhere to stabilize the deploying behaviour. Thus, the dual-matrix composites can entertain the high curvatures up to 1800 without failures in the deployable structures. As this matrix medium allows the fibres to micro-buckle (stress relief mechanism for the fibres in the compression zone) that enhance the folding mechanism to achieve higher curvatures without showing significant damage to the fibres in nonlinear region. It has been observed that these woven fibre-silicone composites have a highly non-linear moment-curvature relationship while there is no significant variation in iii axial stiffness. Further it has been shown that the classical lamination theory is over predicting the bending stiffness by 2 – 4 times when it comes to woven composites made of one to three plies. This research is focussed on understanding the influence of varying bending stiffness with the degree of deformation in predicting quasi-static deployment behaviour of dual-matrix composite booms. A case-study of a three-ply dual-matrix composite boom made of thin woven glass fibre has been selected and simulated with a commercial finite element package. It has been shown that bending stiffness of the soft-elastomer region needs to be varied with the degree of deformation for accurate predictions. Change of bending stiffness is attempted in three different methods. First the analysis has been performed with a series of independent simulations with specified bending stiffness for each model. Secondly the possibility of using import analysis where stress and material state is imported from a previous step. Finally an attempt is made to develop user-subroutine where the bending stiffness properties of the structure can be concurrently updated with degree of deformation.