Investigation of creases in ultra - thin membranes

Abstract

The use of thin membranes is widespread in a variety of applications in a range of industries owing to the lightweight nature and small packaged volume attainable by them. When facilitating the storage of large areas of membranes by folding􀀀speci cally in aerospace applications, the resulting creases alter the physical state and material properties of the overall membrane structure. Even though numerical modelling is preferred as a viable tool in replicating space environments on earth in the form of reduced gravity and air drag, the idealisations utilised in these analyses require validation via small-scale experiments. The signi cance of this process is highlighted due to past endeavours which failed to idealise the crease mechanics accurately, leading to inaccurate predictions and eventual failure in complete missions. Moreover, the use of virtual testing in this regard is limited by the unavailability of accurate experimental data. In this research, an attempt has been made to characterise the crease mechanics of multiple creased thin Kapton 100 HN poyimide membranes using an experimental study.A combination of specimens consisting of two and three creases have been analysed in this regard, and moment􀀀angle responses were plotted using results of physical experiments. The results indicated di erent crease sti nesses for each crease in a parallel-creased specimen, with the highest sti ness observed for a crease nearest to the pinned support. However, all the sti ness values obtained herein were observed to be of a lower order than the simulation and physical experimental results obtained by previous researchers for membranes with a single crease, which could be attributed to the precise measurements taken during the experimental study and the incorporation of the e ect of self-weight of the membrane into its moment􀀀rotation response, which was neglected in earlier studies. The time dependence of the opening behaviour ii was also studied, and it was identi ed that the membrane achieves a constant opening angle in a shorter time duration on being loaded. An improved experimental setup was designed and developed, on identifying the limitations and inaccuracies observed in the experimental setup devised by previous researchers. This ensured controlled displacement being o ered to the membrane for capturing its deployment behaviour over a wider regime of loading, along with precise force measurement. The setup included additional measures to facilitate its usage for specimens of a wider range of dimensions, and to ensure proper alignment of the membrane, thereby enhancing the accuracy of the results obtained via the physical experiments which would then be utilised for idealisation schemes of deployment simulations in virtual environment. Crease sti ness determined for single-creased membranes utilising the improved setup was implemented in Abaqus/Explicit nite element package for the purpose of predicting the deployment behaviour of membrane structures with multiple creases accordingly. The crease-line was represented with connector elements specifying the rotational elasticity, and was observed to have negligible e ect on the deployment which contradicts the experimental observations. Hence, further investigations are required for assessing the accuracy of this claim. A quasi-static simulation was carried out for a simple creased unit based on traditional \Waterbomb" base for predicting the deployment behaviour consisting of intersecting creases. The simulation developed in Abaqus/Explicit environment was able to capture the deployment response observed in the physical experiments, in terms of maximum deployment ratio and shape on incorporating the e ect of gravity to the simulation.