Relaxation behavior of thin-ply high strain composites using a dynamic mechanical analyzer based column bending test

Deployable space structures made from thin-ply high-strain composite (HSC) laminates can be compactly stowed, but prolonged stowage under high curvature can alter their deployed shape due to relaxation. In this research, a dynamic mechanical analyzer-based column bending test (CBT) method, with a custom-developed fixture was used to characterize the relaxation behavior of thin-ply HSCs. Four laminate configurations were prepared from thin-ply unidirectional IM7/PMT-F7 and Astroquartz/PMT-F7 prepregs: (i) IM7/PMT-F7 [0°], (ii) Astroquartz/PMT-F7 [±45°], (iii) Flexlam [±45°/0°/±45°] and (iv) Flexlam [±45°/90°/±45°]. Surface strains of 0.5, 1.0, 1.5, and 2.0% were sequentially applied to the specimen for 100-min durations, each separated by a 1000-min recovery period. This stepped strain approach was performed at 30°C, 50°C, and 70°C. The relaxation results indicate that the fiber-dominated test configuration, UD IM7/PMT-F7 [0°] lamina, shows no measurable relaxation. However, the matrix-dominated configurations, Astroquartz/PMT-F7 [±45°] lamina and the Flexlam laminates, show measurable relaxation. The Flexlam laminates show less relaxation than the Astroquartz/PMT-F7 [±45°] lamina due to the inclusion of unidirectional IM7 carbon fiber. The result also indicates that the relaxation behavior is time and strain-dependent, not temperature-dependent.