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 degrees], (ii) Astroquartz/PMT-F7 [+/- 45 degrees], (iii) Flexlam [+/- 45 degrees/0 degrees/+/- 45 degrees] and (iv) Flexlam [+/- 45 degrees/90 degrees/+/- 45 degrees]. 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 degrees C, 50 degrees C, and 70 degrees C. The relaxation results indicate that the fiber-dominated test configuration, UD IM7/PMT-F7 [0 degrees] lamina, shows no measurable relaxation. However, the matrix-dominated configurations, Astroquartz/PMT-F7 [+/- 45 degrees] lamina and the Flexlam laminates, show measurable relaxation. The Flexlam laminates show less relaxation than the Astroquartz/PMT-F7 [+/- 45 degrees] 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.