High-cycle folding fatigue mechanics of a bistable composite tape-spring

A bistable composite tape-spring (CTS) is a thin-walled open slit tube, which can be coiled or folded into two stable configurations. The CTS has been applied to roll-out-solar-array and successfully launched to space station and micro-satellites based on their one-time deployment performance. There is growing interest on CTS to be applied in reversible deployable structures and foldable mechanical hinges; however, its high-cycle fatigue under large shape folding is still unknown. Here, we device a novel folding fatigue setup to investigate the foldingunfolding cyclic behaviour of the CTS. This is achieved by using a bespoke folding fatigue rig, where both the tape ends of the CTS were clamped separately on rotatable shafts to enable folding under cyclic axial displacements. Since stress concentration is more significant in the snapping fold region, analysis is focused on the peak fatigue stress. It is found that the folding peak stress decreases with the folding cycle: although progressive local damage is observed during 3000 to 100,000 cycles, the CTS is still functional and tends to be stablised after 300,000 folding cycles. The Basquin's law is applied to predict the fatigue life of the CTS, indicating a fatigue life of 1.4E11 folding cycles with a 40% decrease in peak folding stress. These findings are expected to facilitate the structural designs and applications of the CTS to flexible composite hinges.